Displays

Atmospheric aerosol (AA) influences cloud formation, lifetime and other properties; the processes between AA and clouds are source of uncertainty in weather and climate changes estimations [1]. Apart from airborne measurements, the processes in clouds can be also studied on fogs, or low clouds present at a station with high frequency of fog occurrence, such as at Milešovka, Czech Republic, where fog is present for almost 55 % of the time [2]. At the observatory located on the top of the mountain, with a professional meteorological station and measurements of fog/cloud characterization and vertical cloud profile, an additional measurement of aerosol particle number size distributions (PNSD) was done. PNSD from 10 nm to 20 µm was conducted using SMPS and APS spectrometers, measuring activated and interstitial particles. From the activated PNSD (aPNSD), the activated fraction (AF) was estimated [3] i.e. size dependent share of activated particles from all available ones. The AF was fitted with Sigmoidal function and the inflection point, D₅₀, a lower estimate for an activation diameter of fog [4], was calculated.

The changes in the aPNSD at the beginning of each fog episode have been studied. The largest changes in aPNSD and AF were found within the first two or three hours of the fog episode durations. During the episode, the D₅₀ shifted to the smaller particles, and the AF became steeper. For most episodes, 120 minutes after their beginning the size-dependent AF reached a steady-state. The exceptions  were observed during fog episodes preceded by another hydrometeor-related episode. Under such circumstances, the shift in the AF was not observed, as the steady state had been already reached during the preceding episode.

If the time evolution during whole episodes is taken into account, two main groups of AF behavior in time were also found, based on the meteorological situation prior episode beginning. For one group, there is a strong decrease in the D₅₀ in the first three hours, and later the D₅₀ reaches almost a constant value. The steady value is of about 200 nm for all the episodes, independently of the time of the fog occurrence (time of day, season). In the second group, part of a long-term hydrometeor-related situations, the decrease at the beginning of the episode is not visible and the D₅₀ only fluctuates around its original value. Depending on the air mass origin, it is either 90 or 200 nm.

This work was supported by the Czech Science Foundation under grant P209/18/15065Y.

[1]           IPCC, 2013. Cambridge Univ. Press, Cambridge, UK, and New York, 2013.

[2]           J. Fišák et al., Soil Water Res., vol. 196, pp. 273–285, 2009.

[3]           E. Asmi et al., Atmos. Chem. Phys., vol. 12, no. 23, pp. 11589–11607, 2012.

[4]           E. Hammer et al., Atmos. Chem. Phys., vol. 14, no. 19, pp. 10517–10533, 2014.

How to cite: Zíková, N., Pokorná, P., Pešice, P., Sedlák, P., and Ždímal, V.: Time evolution of activated aerosol particles in low clouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2630, https://doi.org/10.5194/egusphere-egu2020-2630, 2020.

Α considerable fraction of cloud condensation nuclei (CCN) originates from new particle formation (NPF). Because of this, NPF events themselves are thought to also increase CCN and cloud droplet number (CDN) and contribute to climate cooling. High resolution state-of-the-art simulations over Europe however portray a different view: radiatively important stratiform clouds influenced by NPF events experience a systematic and substantial decrease in droplet number during and after nucleation events. The drop in CDN occurs because particles present prior to the NPF experiences slower growth during and after each event (as the condensable material is consumed by the growth of the NPF particles that do not typically activate), leading to fewer CCN at the low supersaturation levels characteristic of stratiform clouds (~0.1%). Convective clouds, however, tend to experience a modest increase in cloud droplet number – consistent with established views on the NPF-cloud link. Our results are corroborated by published observational evidence and all together reshape our conceptual understanding of NPF events on clouds, where droplets in stratiform clouds tend to be reduced (leading to local warming from reductions in cloud albedo) but enhance in convection. Combined, these effects could bear important impacts on cloud structure following NPF events.

How to cite: Patoulias, D., Florou, K., N. Pandis, S., and Nenes, A.: New particle formation events persistently reduce cloud droplets in boundary layer clouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13194, https://doi.org/10.5194/egusphere-egu2020-13194, 2020.

It has been estimated that over 50% of the present-day global low-level cloud condensation nuclei (CCN) are formed from new particle formation (NPF), and that this process has a substantial effect on the radiative properties of shallow clouds (Gordon et al. 2017). In contrast, we have a very limited understanding of how NPF affects deep convective clouds. Deep clouds could interact strongly with NPF because they extend into the high free troposphere where most new particles are formed, and they are responsible for most of the vertical transport of the nucleating vapours. Andreae et al. (2018) hypothesised from ACRIDICON-CHUVA campaign that organic gas molecules are transported by deep convection to the upper troposphere where they are oxidised and produce new particles, which are then be entrained into the boundary layer and grow to CCN-relevent sizes.

Here we study the interaction of deep convection and NPF using the United Kingdom Chemistry and Aerosols (UKCA) model coupled with the Cloud-AeroSol Interacting Microphyics (CASIM) embedded in the regional configuration of UK Met Office Hadley Centre Global Environment Model (HadGEM3). We simulate several days over a 1000 km region of the Amazon at 4 km resolution. We then compare the regional model, which resolves cloud up- and downdrafts, with the global model with parameterised convection and low resolution.

Our simulations highlight three findings. Firstly, solely using a binary H₂SO₄-H₂O nucleation mechanism strongly underestimates total aerosol concentrations compared to observations by a factor of 1.5-8 below 3 km over the Amazon. This points to the potential role of an additional nucleation mechanism, most likely involving biogenic compounds that occurs throughout more of the free troposphere. Secondly, deep convection transports insoluble gases such as DMS and monoterpenes vertically but not SO₂ or H₂SO₄. The time scale for DMS oxidation (~ 1 day) is much longer than for monoterpene (1-2 hours), which points to the importance of simulating biogenic nucleation over the Amazon in a cloud-resolving model, while lower-resolution global models may adequately capture DMS effects on H₂SO₄ nucleation. Finally, we also examine the Andreae et al (2018) hypothesis of aerosol supply to the boundary layer by quantifying cloud-free and cloudy up- and downdraft transport. The transport of newly formed aerosols into the boundary layer is 8 times greater in cloud-free regions than in the clouds, but these transport processes are of similar magnitude for large aerosols.

How to cite: Wang, X., Grosvenor, D., Gordon, H., Andreae, M. O., and Carslaw, K.: Regional modelling of aerosol interaction with deep convection and the effect on new particle formation over Amazonia, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6244, https://doi.org/10.5194/egusphere-egu2020-6244, 2020.

The Aerosol Radiance Assimilation Study (ARAS) has created a new approach for the assimilation of visible/near-IR radiances into the ECMWF’s Integrated Forecast System (IFS) for the constraining aerosol properties within the model. The capability is based on a new observation operator, based on the forward model used in the Optimal Retrieval of Aerosol and Cloud (ORAC) retrieval scheme, which predicts top-of-atmosphere radiances based on the model's aerosol field with sufficient accuracy while being computationally efficient enough to run in a operational analysis system such as that run at ECMWF. The system has been tested in the full IFS assimilation system, replacing the currently operational assimilation of MODIS AOD products, using MODIS radiances.
This presentation will give an overview of the new operator, show example results of its impact on the model output and discuss its merits and disadvantages compared to the AOD assimilation. 

How to cite: Thomas, G., Benedetti, A., Quesada Ruiz, S., Letertre-Danczak, J., and Matricardi, M.: Assimilating visible radiances to constrain aerosol properties in the ECMWF Integrated Forecast System., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20358, https://doi.org/10.5194/egusphere-egu2020-20358, 2020.

The influence of clouds on the incoming solar and reflected thermal radiation remains one of the most important climate uncertainties. The global observation of vertical profiles of cloud ice and liquid water with simultaneous and collocated solar and thermal flux observation will provide crucial data to address this uncertainty. Furthermore, collocated global observation of vertical profiles of aerosol types are required to address the direct and indirect effects of aerosol.

In response to these needs, the European Space Agency (ESA), in cooperation with the Japan Aerospace Exploration Agency (JAXA), is implementing the Earth Cloud, Aerosol and Radiation Explorer Mission, EarthCARE.

Vertical profiles of cloud ice and liquid water, aerosol type, precipitation, heating rates, solar and thermal top-of-atmosphere radiances and flux profiles will be synergistically derived from the observations of the satellite’s four instruments.

Two active instruments are embarked, a cloud-aerosol lidar and a cloud Doppler radar. The Atmospheric Lidar (ATLID) operates at 355nm and is equipped with a high-spectral resolution receiver and depolarisation channel that separates molecular from particulate backscatter and distinguishes cloud and aerosol types. The Cloud Profiling Radar (CPR), provided by JAXA, is a highly sensitive W-band Doppler radar (94GHz) that measures cloud profiles, precipitation and vertical motion within clouds. The Doppler observation will measure vertical motion in clouds providing novel information on convection, precipitating ice particles and raindrop fall speed. Two passive instruments provide cloud and aerosol swath information and solar and thermal radiances and top-of-atmosphere fluxes. The Multi-Spectral Imager (MSI) has a 150km wide swath and seven channels in the visible, near-IR, short-wave IR, and thermal IR. The Broad-Band Radiometer (BBR) observes broad-band solar and thermal radiation reflected and emitted from the Earth, with three fixed fields of view: forward, nadir and backward.

In preparation for the science exploitation of the mission, complex data retrieval algorithms in the Ground Segment will exploit the synergy of the four instruments and deliver a range of cloud, aerosol and radiation related data products, including three-dimensional cloud-aerosol-precipitation scenes, with collocated broad-band heating rate and radiation data, over a mission lifetime of three years.

The presentation will provide an overview of the mission, main performances of the three ESA instruments and expected science data products.

How to cite: Wehr, T., Eisinger, M., Koopman, R., Lefebvre, A., Maeusli, D., Wallace, K., Pereira Do Carmo, J., Livschitz, J., and Maksym, S.: EarthCARE Mission Preparation Status: Performance and Science Processing, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3534, https://doi.org/10.5194/egusphere-egu2020-3534, 2020.

Liquid cloud droplets freeze homogeneously at -40°C. For temperature between -40 and 0°C, clouds can be either liquid, ice, or mixed-phase. Several variables determine the cloud phase: droplet size, ice nuclei concentration, meteorological parameters, etc. But, parameters which trigger, enhance or inhibit the phase transition are still poorly understood and disagreements remain between theory and observations. The phase transition is nonetheless important to determine cloud effects on climate.

In the present study, we analyse satellite observations from the geostationary passive instrument SEVIRI. We used the CLAAS-2 dataset to retrieve cloud top microphysical and optical properties from 2005 to 2015 over the Southern Ocean.

Cloud objects that contain liquid and ice pixels are identified for cloud top temperatures within specific temperature ranges: between -30 and -20°C, between -20 and -8°C, and between -8 to 0°C. The distributions of different cloud properties for mixed-phase, liquid or ice clouds are compared. For example, preliminary results show that cloud ice fraction increases with the cloud droplet size for cloud top temperature between -8 and 0°C. Indeed, ice fractions greater than 0.8 are associated with a median cloud droplet effective radius of 7 micrometers whereas ice fractions less than 0.2 are associated with a median cloud droplet effective radius of 12 micrometers. We hypothesize that this result can be associated to a secondary ice production process (e.g., the Hallet-Mossop process is the splinter production associated with riming process for temperature between -8 and -3°C and it can increase the ice particle concentration by several orders of magnitude). In line with our results, the Hallet-Mossop process is more efficient with larger cloud droplets. The spatial distribution of liquid and ice pixels within the cloud objects is also studied to better understand the phase partitioning of mixed-phase clouds.

How to cite: Coopman, Q., Hoose, C., and Stengel, M.: A space-based perspective on cloud phase partitioning over the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6819, https://doi.org/10.5194/egusphere-egu2020-6819, 2020.

The current generation of large-eddy models (e.g. the ICON-LEM) allows us to go beyond idealized simulations and to capture synoptic variability by including heterogeneous land surfaces as well as lateral boundary conditions. This would offer the possibility to compare simulations and observations of clouds on a detailed day-to-day basis. But while LEMs are able to reach resolutions that start to be comparable to state-of-the-art observations (e.g. Radar data), they are still facing the issue of different parameter spaces: either the model output has to be transfered to observable quantities, or the other way around. We will present examples from recent field campaigns (e.g. ACLOUD, EUREC4A), where we combined ICON-LEM simulations with remote sensing observations by applying the Passive and Active Microwave TRAnsfer simulator (PAMTRA). By the selection of examples, we will show the potential of this combination of high-resolution modeling, remote sensing observations and forward simulations at different places under different conditions (Arctic, European and Caribbean). While the general structure of clouds (e.g. timing, type, height) is often already captured quite well, the comparison to the remote sensing observations allows us to also get insights into the composition of clouds and to constrain microphysical parameterizations as well as the influence of the large-scale forcing on a more detailed level.

How to cite: Schemann, V. and Mech, M.: Confronting and combining high-resolution simulations (ICON-LEM) with remote sensing observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19201, https://doi.org/10.5194/egusphere-egu2020-19201, 2020.

Marine stratocumuli cover around a fifth of the worlds oceans and are a key contributor to Earth’s radiative balance at the surface. Their sensitivity to changes in anthropogenic aerosol concentrations remain a key uncertainty in the climate system. Our current understanding of their sensitivity and the plausible range of the aerosol-cloud radiative forcing is largely based on the process understanding obtained from field campaigns, high-resolution modelling, and satellite records of aerosol-induced phenomena such as volcano or ship tracks.

Yet, a large fraction of these records is only applicable to relatively shallow planetary boundary layers (PBLs). Ship tracks are only found in boundary layers up to a depth of 800m. Field campaigns and high-resolution modelling studies of aerosol-cloud-radiation interactions in marine stratocumuli have been restricted to a similar range of PBL depths in the past. Meanwhile over 70% of marine boundary layers reside in deeper PBLs.

The liquid water path (LWP) adjustment due to aerosol-cloud interactions in marine stratocumuli remains a considerable source of uncertainty for climate sensitivity estimates. An unequivocal attribution of LWP adjustments to changes in aerosol concentration from climatology remains difficult due to the considerable covariance between meteorological conditions alongside changes in aerosol concentrations.

Here, we combine a range of space-born remote sensing retrievals to investigate the relationship of cloud-radiative properties for different boundary layer depths and aerosol concentrations. As done in previous studies we utilise the susceptibility framework, i.e. the relative change in LWP scaled by the relative change in cloud droplet number concentration, to quantify the change in LWP adjustment with PBL depth. We show that the susceptibility of LWP adjustments triples in magnitude from values of -0.1 in PBLs shallower than 0.5 km to -0.33 in PBLs deeper than 1 km.

We further argue that LWP susceptibility estimates inferred from deep PBL climatologies are poorly constrained due to a lack of process-oriented observations. Meanwhile, susceptibilities inferred from climatology in shallow PBL regimes are consistent with estimates obtained from process modelling studies, but are overestimated as compared to pollution track estimates.

How to cite: Possner, A., Eastman, R., Bender, F., and Glassmeier, F.: Exploring the aerosol-cloud-radiation relationships in deep marine stratocumulus layers, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9470, https://doi.org/10.5194/egusphere-egu2020-9470, 2020.

Shallow cumulus clouds interact with their environment in myriad significant ways, and yet their behavour is still poorly understood, and is responsible for much uncertainty in climate models. Improving our understanding of these clouds is therefore an important part of improving our understanding of the climate system as a whole.

Modelling studies of shallow convection have traditionally made use of highly idealised simulations using large-eddy models, which allow for high resolution, detailed simulations. However, this idealised nature, with periodic boundaries and constant forcing, and the quasi-equilibrium cloud fields produced, means that they do not capture the effect of transient forcing and conditions found in the real atmosphere, which contains shallow cumulus cloud fields unlikely to be in equilibrium. 

Simulations with more realistic nested domains and forcings have previously been shown to have significant persistent responses differently to aerosol perturbations, in contrast to many large eddy simulations in which perturbed runs tend to reach a similar quasi-equilibrium. 

Here, we further this investigation by using a single model to present a comparison of familiar idealised simulations of trade wind cumuli in periodic domains, and simulations with a nested domain, whose boundary conditions are provided by a global driving model, able to simulate transient synoptic conditions. 

The simulations are carried out using the Met Office Unified Model (UM), and are based on a case study from the Rain In Cumulus over the Ocean (RICO) field campaign. Large domains of 500km are chosen in order to capture large scale cloud field behaviour. A double-moment interactive microphysics scheme is used, along with prescribed aerosol profiles based on RICO observations, which are then perturbed.

We find that the choice between realistic nested domains with transient forcing and idealised periodic domains with constant forcing does indeed affect the nature of the response to aerosol perturbations, with the realistic simulations displaying much larger persistent changes in domain mean fields such as liquid water path and precipitation rate. 

How to cite: Spill, G., Stier, P., Field, P., and Dagan, G.: Aerosol effects on shallow cumulus cloud fields in idealised and realistic simulations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18231, https://doi.org/10.5194/egusphere-egu2020-18231, 2020.

    Double-moment schemes with a constant shape parameter cannot describe the condensation process and the collision coalescence processes properly. Evolutions of cloud droplet spectra and raindrop spectra simulated with different current bulk microphysical schemes also showed big differences. The newly developed triple-moment scheme (IAP-LACS scheme) considered the variation of the shape parameters of water drop distributions by means of the radar reflectivities of cloud droplets and raindrops, respectively, during the condensation process and collision-coalescence processes. In order to evaluate the performance of our new scheme, we use large-eddy simulation in WRF to research the precipitation formation in Rain in Cumulus over the Ocean (RICO) observation study with new triple-moment warm cloud scheme. This paper will show the simulation results for the microphysical characteristic, specical for the evolution of warm raindrop size distribution in comparison with aircarft measurement. Our simulations show that our new triple-moment scheme can grasp the main characteristic of raindrop size distribution as observation and there must be difference exsiting in simulation results between new scheme and other microphysical bulk schemes.

How to cite: Deng, W.: Numerical Simulations for the Warm Rain Properties during RICO with a Newly Developed Triple-moment Warm Cloud Scheme, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4420, https://doi.org/10.5194/egusphere-egu2020-4420, 2020.

Cloud, aerosols and precipitation processes are perhaps the most critical uncertainties for weather and climate prediction. The complex nature of sub grid scale clouds makes traceable simulation of clouds and precipitation across scales difficult (or impossible). However, many observations and detailed simulations of clouds are available as input to larger scale models. Machine learning provides another potential tool to improve our empirical parameterizations of clouds. Here we present a comprehensive investigation of replacing the warm rain formation process in an earth system model with emulators that use detailed treatments from small scale and idealized models: specifically a stochastic collection kernel and a superdroplet approach. The emulators consist of multiple neural networks that predict whether specific tendencies will be nonzero and the magnitude of the nonzero tendencies. We describe the opportunity (massive speed up of cloud process calculations) and the risks of overfitting, extrapolation and linearization of a non-linear problem by using perfect model experiments with and without the emulators. The impacts on short term time tendencies of clouds and precipitation, as well as long term climatological means and important emergent properties of the climate system (like radiative forcing through aerosol-cloud interactions and cloud feedbacks to climate change) are assessed. 

How to cite: Gettelman, A., Chen, C., and Gagne, D.: Machine Learning Rain Formation and the Impacts on Clouds, Precipitation and Radiation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4636, https://doi.org/10.5194/egusphere-egu2020-4636, 2020.

Southwestern France is an important wine region where hail-producing storms could cause considerable economic loss. To study the initiation and growth of hailstone, a new microphysical scheme based on the LIMA (Liquid, Ice, Multiple Aerosols, Vié et al., 2016) has been developed. The original LIMA only contains two-moment scheme for rain water, cloud water, and ice crystal. Whereas, the other ice hydrometeors are described by a single-moment scheme. The new scheme adds a full two-moment framework to snow, graupel, and hailstone, thus allowing a better representation of the microphysical processes than the original partial two-moment approach could offer. A large area of southwestern France is actually covered by hailpad network. Results from this network alongside other data thus have been used to evaluate the performance of the single-moment ICE3 scheme, the partial two-moment LIMA scheme, and the new full two-moment scheme in reproducing the evolution of observed hail-producing storm cases. The difference as well as similarity in modeled structures of the storms including hailstone development by different microphysics schemes are examined and will be presented.

How to cite: Taufour, M. and Wang, C.: A new 2-moment microphysical scheme for studying hail initiation and growth: model-observation comparison over Southweest of France, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3548, https://doi.org/10.5194/egusphere-egu2020-3548, 2020.

Intentional release of large hygroscopic particles in a cloud, i.e. cloud seeding, is potentially capable of increasing rain formation. In this work, we focus on convective clouds of moderate intensity observed over the United Arab Emirates, where we use a large eddy simulator coupled with detailed bin aerosol-cloud microphysics module (UCLALES-SALSA) to study the processes controlling the seeding efficacy. Despite numerous field experiments, the conditions that favor efficient seeding induced rain enhancement are not well characterized. Models such as UCLALES-SALSA provide the means to study the microphysical effects in varying ambient conditions in a controlled setting. The clouds targeted by our simulations have a mixed-phase component and rain is primarily produced by the cold precipitation process. The results show that the fast growing droplets formed by the relatively large hygroscopic seeding aerosol affect the riming process in the mixed-phase region inside the clouds. The collision rate between the hydrometeors in the mixed-phase region is enhanced, producing larger frozen particles. Consequently, the ice tends to get more heavily rimed, as indicated by the fraction of rimed ice from the total ice mass, which promotes increased particle fall velocities.

However, the impact of seeding on the riming process depends on the state of the cloud and its environment. In conditions already favoring high rime fraction, often associated with relatively strong surface precipitation events, the effect of seeding is hindered, at least in terms of relative difference. Nevertheless, even if the effect of seeding on the total precipitation yield is small, it may still affect the timing of the precipitation onset, a topic currently under investigation. Work is also in progress to characterize the dependence between ambient conditions (in terms of aerosol and the thermodynamic properties of the atmospheric column) and the susceptibility of the mixed-phase clouds to seeding injection.

How to cite: Tonttila, J., Kokkola, H., Raatikainen, T., Ahola, J., Korhonen, H., and Romakkaniemi, S.: Model investigation into rain enhancement by hygroscopic seeding in mixed-phase convective clouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12065, https://doi.org/10.5194/egusphere-egu2020-12065, 2020.

Cloud seeding has been used as one water management strategy to overcome the increasing demand for water despite decades of inconclusive results on the efficacy of cloud seeding. In this study snowfall accumulation from glaciogenic cloud seeding is quantified based on snow gauge and radar observations from three days in January 2017, when orographic clouds in the absent of natural precipitation were seeded with silver iodide (AgI) in the Payette basin of Idaho during the Seeded and Natural Orographic Wintertime Clouds: The Idaho Experiment (SNOWIE). On each day, a seeding aircraft equipped with AgI flares flew back and forth on a straight-line flight track producing a zig-zag pattern representing two to eight lines of clouds visible through enhancements in radar reflectivity. As these seeding lines started to form precipitation, they passed over several snow gauges and through the radar observational domain. For the three cases presented here, precipitation gauges measured increases between 0.05-0.3 mm as precipitation generated by cloud seeding pass over the instruments. A variety of relationships between radar reflectivity factor and liquid equivalent snowfall rate were used to quantify snowfall within the radar observation domain. For the three cases, snowfall occurred within the radar observational domain between 25 -160 min producing a total amount of water generated by cloud seeding ranging from 123,220 to 339,540 m3 using the best-match Ze-S relationship. Uncertainties in radar reflectivity estimated snowfall are provided by considering not only the best-match Ze-S relationship but also an ensemble of Ze-S relationships based on the range of coefficients published from previous studies and then examining the percentile of snowfall estimates based on all of the Ze-S relationships within the ensemble. Considering the interquartile range and 5^th/95^th percentiles, uncertainties in total amount of water generated by cloud seeding can range between 20-45% compared to the best-math estimates. These results provide new insights towards understanding how cloud seeding impacts precipitation and its distribution across a region.

How to cite: Friedrich, K., Ikeda, K., Tessendorf, S., French, J., Rauber, R., Geerts, B., Xue, L., Rasmussen, R., Blestrud, D., Kunkel, M., Dawson, N., and Parkinson, S.: Quantifying snowfall from orographic cloud seeding , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19517, https://doi.org/10.5194/egusphere-egu2020-19517, 2020.

Aerosol radiative forcing plays an important role in the attribution of past climate changes, estimates of future allowable carbon emissions, and the assessment of potential geoengineering solutions. Substantial progress made over the past 40 years in observing, understanding, and modelling aerosol processes helped quantify aerosol radiative forcing, but uncertainties remain large.

In spring 2018, under the auspices of the World Climate Research Programme's Grand Science Challenge on Clouds, Circulation and Climate Sensitivity, thirty-six experts gathered to take a fresh and comprehensive look at present understanding of aerosol radiative forcing and identify prospects for progress on some of the most pressing open questions. The outcome of that meeting is a review paper, Bellouin et al. (2019), accepted for publication in Reviews of Geophysics. This review provides a new range of aerosol radiative forcing over the industrial era based on multiple, traceable and arguable lines of evidence, including modelling approaches, theoretical considerations, and observations. A substantial achievement is to focus on lines of evidence rather than a survey of past results or expert judgement, and to make the open questions much more specific.

This talk will present the key messages and arguments of the review and identify work that show promise for improving the quantification of aerosol radiative forcing.

How to cite: Bellouin, N. and the Ringberg 2018 review team: Bounding global aerosol radiative forcing of climate change, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7745, https://doi.org/10.5194/egusphere-egu2020-7745, 2020.

Clouds play a critical role in the hydrological cycle and modulating the Earth’s climate via precipitation and radiative forcing. Aerosol particles acting as cloud condensation nuclei and ice nucleating particles aid in cloud formation, shaping their microphysical structure. Previously thought to be unimportant for cloud formation, soot particles that undergo oxidation by ozone and/or aging with aqueous sulfuric acid result in being both good centers for cloud droplets and ice crystals formation. However, the associated changes in cloud radiative properties and the consequences for Earth’s climate remain uncertain, because these processes have not been considered in global climate models. Here we present both past and future global climate simulations, which for the first time consider the effect of such aged soot particles as cloud condensation nuclei and ice nucleating particles. Our results constitute the first evidence that aging of soot particles produce a 0.2 to 0.25 Wm^-2 less negative shortwave indirect aerosol forcing compared to previous estimates. We also conducted equilibrium climate sensitivity simulations representing a future warmer climate in which the carbon dioxide concentration is doubled compared to pre-industrial levels. Accounting for these soot aging processes significantly exacerbates the global mean surface temperature increase by 0.4 to 0.5 K. Thus, reducing emissions of soot particles will be beneficial for many aspects including air pollution and future climate.

How to cite: Lohmann, U., Friebel, F., Kanji, Z. A., Mahrt, F., Mensah, A. A., and Neubauer, D.: New evidence of soot particles affecting past and future cloud formation and climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3635, https://doi.org/10.5194/egusphere-egu2020-3635, 2020.

We present an analysis of the fast responses on pre-industrial climate due to present-day aerosols in a multi-model study based on Coupled Model Intercomparison Project Phase 6 (CMIP6) simulations from 10 Earth System Models (ESMs) and General Circulation Models (GCMs). The aforementioned simulations were implemented within the framework of the Aerosol Chemistry Model Intercomparison Project (AerChemMIP). All models carried out two sets of simulations; a control experiment with all forcings set to the year 1850 and a perturbation experiment with all forcings identical to the control, except for aerosols with precursor emissions set to the year 2014. The perturbation by the present-day aerosols indicates negative top of the atmosphere (TOA) effective radiative forcing (ERF) values around the globe, especially over continental regions of the Northern Hemisphere in summer, with the largest negative values appearing over East Asia. Simulations in 3 models (CNRM-ESM2-1, MRI-ESM2-0 and NorESM2-LM) with individual perturbation experiments using present day SO₂, BC and OC emissions show the dominating role of sulfates in all-aerosols ERF. In response to the pattern of all aerosols ERF, the fast temperature responses are characterised by cooling over the continental areas, especially in the Northern Hemisphere, with the largest cooling over East Asia and India and sulfate being the dominant aerosol surface temperature driver for present-day emissions. The largest fast precipitation responses are seen in the tropical belt regions, generally characterized by  a reduction over continental regions and a southward shift of the tropical rain belt. This is a characteristic and robust feature among most models in this study, associated with a southward shift of the Intertropical convergence zone (ITCZ) and a weakening of the monsoon systems around the globe (Asia, Africa and America) in response to hemispherically asymmetric cooling from a Northern Hemisphere aerosol perturbation. An interesting feature in aerosol induced circulation changes is a characteristic dipole pattern with intensification of the Icelandic Low and an anticyclonic anomaly over Southeastern Europe, inducing warm air advection towards the northern polar latitudes in winter.

This research was funded by the project "PANhellenic infrastructure for Atmospheric Composition and climatE change" (MIS 5021516) which is implemented under the Action "Reinforcement of the Research and Innovation Infrastructure", funded by the Operational Programme "Competitiveness, Entrepreneurship and Innovation" (NSRF 2014-2020) and co-financed by Greece and the European Union (European Regional Development Fund).

How to cite: Zanis, P., Akritidis, D., Georgoulias, A. K., Allen, R. J., Bauer, S. E., Boucher, O., Cole, J., Johnson, B., Deushi, M., Michou, M., Mulcahy, J., Nabat, P., Olivie, D., Oshima, N., Sima, A., Schulz, M., and Takemura, T.: A CMIP6 multi-model study of fast responses on pre-industrial climate due to present-day aerosols , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4920, https://doi.org/10.5194/egusphere-egu2020-4920, 2020.

Aerosols are a major climate forcer, but their historical effect has the largest uncertainty of any forcing, and so their mechanisms and impacts must be better understood. Due to their short lifetime, aerosols have large impacts near their emission region, but they also have effects on the climate in remote locations. In recent years, studies have investigated the influences of regional aerosols on global and regional climate, and the mechanisms that lead to remote responses to their inhomogeneous forcing. However, there has been little work on the influence of emissions from the tropics, as the aforementioned studies typically focused only on northern mid-latitude pollution effects. This work uses the new UK Earth System Model (UKESM1) to investigate the atmospheric composition and climate effects of tropical aerosols and aerosol precursor emissions. We performed three idealised perturbation experiments in which a) tropical SO₂ emissions were multiplied by a factor of 10; b) tropical biomass burning carbonaceous aerosol emissions were multiplied by 10; and c) tropical biomass burning carbonaceous aerosol emissions were entirely removed. Impacts on radiation fluxes, temperature, circulation and precipitation are investigated, both over the emission regions, where microphysical effects dominate, and remotely, where dynamical influences become more relevant. Increasing tropical SO₂ emissions causes a global cooling, and the asymmetric forcing (stronger negative forcing in the Northern Hemisphere Tropics) drives a southward shift of the intertropical convergence zone (ITCZ). The experiment with the large increase in tropical biomass burning organic carbon (OC) and black carbon (BC) features a net warming globally, and a local cooling in locations where the aerosol load increases the most, since OC and BC reduce radiation at the surface locally, causing cooling. However, whereas OC scatters radiation with a negative forcing, BC has a warming effect since it reduces the planetary albedo, and this warming wins out on the global scale. The forcing is asymmetric, but changes sign between seasons as biomass burning in Africa shifts across the Equator, driving a more complex response of the ITCZ. The removal of biomass burning OC and BC leads to opposite effects to the 10x increase, but with a smaller magnitude, and with dynamical changes playing a more important role than microphysical ones, relative to the larger perturbations. Using the Shared Socioeconomic Pathway scenarios (SSPs), transient future experiments have also been performed, testing the effect of Africa following a relatively more polluting route (SSP3-RCP7.0) to the rest of the world (SSP1-RCP1.9), relative to a global SSP1-RCP1.9 control. Preliminary results from this analysis will also be presented.

How to cite: Wells, C., Voulgarakis, A., and Kasoar, M.: The impact of perturbations to tropical aerosols and their precursors on local and remote climates, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7537, https://doi.org/10.5194/egusphere-egu2020-7537, 2020.

How to cite: Wilcox, L., Liu, Z., Samset, B., Hawkins, E., Lund, M., Nordling, K., Undorf, S., Bollasina, M., Ekman, A., Kirshnan, S., Merikanto, J., and Turner, A.: Accelerated increases in global and Asian summer monsoon precipitation from future aerosol reductions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9457, https://doi.org/10.5194/egusphere-egu2020-9457, 2020.

In this study, a new technique to determine distinct cloud regimes and their variation in space and time is proposed, evaluated, and applied to two satellite products over the Maritime Continent (MC). Compared to previous methods, the method presented here allows different types of cloud to co-exist in the same grid at the same time, giving rise to physically explainable and spatially continuous patterns in cloud regimes. Similar results generated by ISCCP – H and Himawari 8 data suggests that the method is robust. The 4 cloud regimes determined using this method are associated with shallow, mid-level, deep convective and high level clouds respectively. The analysis shows that he MJO–induced variation in total cloud fraction is dominated by day-time high–level clouds, while the diurnal MJO variability is mostly demonstrated by low–level cumulus. Spatial and temporal rainfall variability over the MC during austral summer is dominated by high–level clouds, while most local signatures and land–sea differences are attributed to deep convective clouds. Using an artificial neural network, the cloud patterns over the MC can be classified into nine categories, largely dominated by the MJO-phase. Active MJO activity is shown by systematic propagation around the cloud categories, with one category associated with the inactive MJO phase. The inhomogenous propagation of the MJO can partially be revealed in the generated patterns, which can be physically explained by the enhanced/suppressed convection over the Indian Ocean. This work has implications for understanding the MJO-scale variation in precipitation and diabatic heating associated with different cloud regimes, and its representation in mesoscale and climate scale modelling systems.

How to cite: Zhao, Z., Vincent, C., and Lane, T.: Cloud regimes and associated MJO variability over the Maritime Continent in austral summer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12998, https://doi.org/10.5194/egusphere-egu2020-12998, 2020.

There is growing evidence that the amount of solar radiation at the Earth’s surface is not stable over time but undergoes substantial multidecadal variations. Particularly, a decrease in surface solar radiation has been noted from the 1950s to the 1980s at widespread observation sites, a phenomenon popularly known as “global (solar) dimming”, followed by a partial recovery known as “brightening”. An interesting hotspot in this context is China, where surface solar radiation (SSR) underwent particularly large changes over the past decades.

Here we discuss our latest studies, which shed new light on the magnitude, causes and implications of this phenomenon in China. The focus is on recent developments, which indicate, that after decades of decline in surface solar radiation, some recovery can be noted since the mid-2000s in the SSR records observed by the Chinese Meteorological Agency. This recovery is not seen in satellite derived records, which assume a constant aerosol climatology in their retrieval algorithm, suggesting the necessity for a decrease in aerosol to reconcile the diverging trends (Wang et al., 2019). This is independently supported by an analysis of SSR trends specifically in the cloud-free atmosphere, which show a turn into increase since around 2006, also suggesting a reduction of aerosol over China in recent years (Yang et al., 2019).

In a further study, the combination of the Chinese SSR observations with collocated space-based measurements  of the net solar exchanges at the Top of Atmosphere from CERES enabled the determination of changes in solar absorption within the atmospheric column as a residual over recent decades. The results suggest that the recent brightening in China is predominately caused by a weakening of the solar absorption within the atmosphere. This indicates that a reduction of particularly the absorbing aerosol must have taken place in recent years (Schwarz et al., 2020).

In summary, all these studies provide independent evidence that air pollution mitigation efforts in China have successfully induced a trend reversal in the amount of solar radiation reaching the Earth’s surface, with some recovery in recent years after decades of dimming.

We further estimated that, if such a recovery could persist and air pollution levels could eventually be reduced down to the pristine 1960s levels in China, this would have major benefits for Chinese photovoltaic (PV) solar power production, which could be enhanced by as much as 13 %. With the PV capacity currently installed in China, and as projected for the year 2030, this would correspond to a yearly economic benefit of 2 and 6 billion US dollars, respectively, assuming current electricity prices (Sweerts et al., 2019).

References

Yang, S., Wang, X.L., Wild, M. (2018) J. Climate 31, 4529-4541.

Yang, S., Wang, X.L., Wild, M. (2019) J. Climate 32, 5901-5913.

Sweerts, B., Pfenninger, S., Yang, S., Folini, D., vanderZwaan, B., Wild, M. (2019) Nature Energy 4, 657-663.

Wang, Y., Trentmann, Y., Pfeifroth, U., Yuan, W., Wild, M. (2019) Remote Sens. 11, 2910.

Schwarz, M., Folini, D., Yang, S., Allan, R.P., and Wild, M. (2020) Nature Geoscience (in press)

How to cite: Wild, M., Schwarz, M., Wang, Y., Yang, S., Sweerts, B., Folini, D., and Trentmann, J.: Solar Dimming and Brightening: Recent Developments in China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2417, https://doi.org/10.5194/egusphere-egu2020-2417, 2020.

    In situ measurements are performed to study the size-resolved hygroscopic behaviour of submicron aerosols during pollution and fireworks episodes in winter from late January to February 2019 in Chengdu, a megacity in Sichuan Basin, using a humidity tandem differential mobility analyser (H-TDMA). The H-TDMA is operated at a relative humidity of 90% with dry aerosol diameters between 40 and 200 nm. Three modes of aerosol particles, including nearly hydrophobic mode (NH), less hygroscopic mode (LH), and more hygroscopic mode (MH), are found in the probability distributions of the growth factor (GF-PDF) during the campaign. The GF-PDF shows that aerosol particles are usually externally mixed. The average ensemble mean hygroscopicity parameter values (κ_Mean) over the entire sampling period are 0.16, 0.19, 0.21, 0.23, and 0.26 for aerosols with diameters of 40, 80, 110, 150, and 200 nm, respectively. These averages are lower than those in Shanghai and Nanjing. κ_Mean for aerosols larger than 110 nm, however, are higher than those in Beijing and Guangzhou during winter. Distinct diurnal patterns for all measured sizes are observed for the number fractions of the NH (NF_NH) and MH (NF_MH) modes as well as κ-PDF and κ_Mean. The NF_NH values are lower, but κ_Mean exhibits peak values during daytime. More aerosols are internally mixed because of photochemical ageing during daytime. The number fraction of LH (NF_LH) for the 40-nm diameter aerosols in clean periods (CPs) is larger than that in the pollution episode (PEs) because of the increasing amount of SOA formation. More aerosols of diameters larger than 80 nm are internally mixed during CPs and stage of contaminant accumulation, resulting in higher κ_Mean values compared to those in PEs. The aerosol emissions of fireworks that accumulate during the Chinese New Year's Eve contribute to the slow and continuous increasing trend in κ_Mean with average values of 0.19, 0.19, 0.21,0.23, and 0.27 for the 40, 80, 110, 150, and 200-nm diameter aerosols, respectively. These values are higher than those during the pre- and post-fireworks days. The hygroscopic properties of submicron aerosols in Chengdu are essential for understanding the formation and evolution of severe haze events in Sichuan Basin.

How to cite: Yuan, L.: Size-resolved hygroscopic behaviour and mixing state of submicron aerosols in a megacity of Sichuan Basin during pollution and fireworks episodesn during pollution and fireworks episodes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2499, https://doi.org/10.5194/egusphere-egu2020-2499, 2020.

Precipitation over Southern China for the month of April, which is largely associated with mesoscale convective systems (MCSs), has declined significantly in recent decades. It is unclear how this decline in precipitation may be related to the concurrent increase in anthropogenic aerosols in the atmosphere over this region. Using observation analyses and model simulations, we showed that anthropogenic aerosols significantly reduced MCS occurrences by 21% to 32% over Southern China in April, leading to less and weaker rainfall. Half of this MCS occurrence reduction was due to the direct radiative scattering and the indirect enhancement of non-MCS liquid cloud reflectance by aerosols, which stabilized the regional atmosphere. The other half of the MCS occurrence reduction was due to the microphysical and dynamical responses of the MCS to aerosols. The model simulations showed that the higher levels of aerosols and the resulting increase in liquid cloud droplets both enhance the scattering of sunlight, cool the surface, and stabilize the lower atmosphere. As a result, the occurrence of strong convective systems is suppressed, leading to decreased rainfall in April over Southern China. Our results demonstrated the complex effects of aerosols on MCSs via impacts on both convective systems and non-convective cloud systems in the regional atmosphere.

How to cite: Zhang, L. and Fu, T.-M.: Impacts of Anthropogenic Aerosols on Springtime Mesoscale Convective Systems over Southern China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4400, https://doi.org/10.5194/egusphere-egu2020-4400, 2020.

The Indian subcontinent is greatly vulnerable to air pollution, especially during the dry winter season. Here, we use 15 years (2003-2017) of satellite and model reanalysis datasets over India and adjoining Seas to estimate the trends in the number of days with high aerosol loading (i.e. hazy days) from October to February. The number of days with high aerosol loading in recent years (2013-2017) is increasing at the rate of ~2.6 days/year over Central India, which is surprisingly higher than the more polluted Indo-Gangetic Plain (~1.7 days/year).  Similar increment in absorbing aerosols is also visible in recent years.  As a result, the estimated atmospheric warming over Central India is two-fold higher than that over Indo-Gangetic Plain. This anomalous increase in hazy days over Central India is attributed to a relatively higher increase in biomass burning over this region. The number of days with high aerosol loading in recent years are also higher over the Arabian Seas, which is located downwind to Central India, as compared to the Bay of Bengal. Thus, our findings not only draw attention to deteriorating air quality over Central India but also underlines the significance of enhanced biomass burning activities under recent climate change.

How to cite: Kanawade, V., Thomas, A., and Sarangi, C.: Recent increase in winter hazy days over Central India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-329, https://doi.org/10.5194/egusphere-egu2020-329, 2020.

Anthropogenic and natural aerosols are important atmospheric constituents that can significantly reduce, by scattering and absorption, the solar radiation reaching the Earth’s surface. This impact depends on the aerosols properties, namely the optical thickness (τ), the exponent (α) and the coefficient (β) of Angström. These three parameters are first estimated by fitting the direct solar radiation measurements recorded on clear days with the Iqbal C model. The retrieval of τ and β using data collected in Tamanrasset, Southern Algeria, are in good agreement with those of retrieved by AERONET at the same time and location. However, α exponent comparison is not satisfactory, we have therefore developed an Artificial Neural Network method (ANN) to better estimate it. The ANN created was first learned from β and α obtained from AERONET. We then used β from the Iqbal C model with the ANN and obtain good estimate of α with R² of 60% compared to the Angstrom exponent from AERONET. We will first give in this presentation an overview of the Iqbal C model, then present the data used and the processing method, and finally discuss the main results of this study.

How to cite: Zaiani, M., Irbah, A., Djafer, D., and Delanoe, J.: Aerosol properties obtained on cloudless days through direct broadband solar radiation measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1542, https://doi.org/10.5194/egusphere-egu2020-1542, 2020.

Aerosol particles that act as cloud condensation nuclei (CCN) inuence cloud albedo and lifetime and thereby affect the planetary radiative balance. The indirect aerosol effect on climate is still one of the largest uncertainties and especially the role of biological particles is not yet well described. Pollen grains are primary biological particles that become airborne during the blooming season of plants. Pollen from wind pollinated plants represent a seasonally signifficant portion of the organic aerosol in the atmosphere. Intact pollen grains are rather large (10-100 µm) but under conditions of high humidity pollen grains have been shown to rupture and release cytoplasmic material including a large number of particles much smaller in size (0.5-5 µm).

In this study we extract soluble and insoluble material from several pollen samples (Phleum, Betula, Artimesia, Poa, Corylus and Ambrosia) and investigate the CCN activity of the extracts in a laboratory study. The main component of the experiment is the continuous-flow streamwise thermal-gradient cloud condensation nuclei counter (CCNC) from Droplet Measurement Technologies (DMT). The CCNC was calibrated with (NH₄)2SO₄. The activation behavior of (NH₄)2SO₄ is theoretically well described by Kohler equation. For particles which consist of a multitude of organic components it is convenient to represent the chemical composition through the hygroscopicity parameter κ. In the first part of the experiment, we determine the activation diameter at 5 different supersaturations and calculate the kappa parameter for all pollen samples. We find that the values fall in the range from 0.1-0.2. which is typical for particles composed of organic substances. Extracts from Betula pollen show the highest hygroscopicity (κ = 0.18), while extracts from Artimesia exhibit the lowest hygroscopicity (κ = 0.13). In the second part of the experiment we will also investigate the CCN activity of the insoluble material.

How to cite: Sonnenberg, S., Burkart, J., and Gratzl, J.: CCN properties of pollen from selected wind pollinated plants, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22087, https://doi.org/10.5194/egusphere-egu2020-22087, 2020.

Long term trends of various aerosol optical properties are observed over the city of Pune, the ninth most populated city in India using ground and satellite based instruments such as AERONET, MODIS (Aqua and Terra), MISR, CALIOP and reanalysis tool MERRA. Annually, the Aerosol Optical Depth is observed to be increasing over all the types of instruments (2004-17) with values of 0.01 to 0.006 yr^-1, whereas the Angstrom exponent has a negative slope (AERONET) which suggests that the fine aerosols are decreasing. Single scattering albedo (SSA) is also increasing (0.00657 yr^-1), which means the emission of smaller darker particles like soot has decreased over the years. MISR shows that the Absorbing AOD trend is decreasing in the overall study period (-0.0001237 yr^-1). All these annual trends are related to anthropogenic activities and show differing trends before and after 2008, the year when various pollution counter measures were introduced mainly in Pune and also in various nearby areas. After 2008, the AOD increasing slope reduces, and the AAOD reverses the trend from positive to a negative slope. The average height till various kinds of aerosols reach and their vertical profile is studied using CALIOP data. Monthly variations of AOD and their vertical distribution also observed and discussed. Aerosol characterization is done using the MERRA tool into dust, sea salt, sulfates, elementary carbon, and organic carbon. Their monthly variations are explained by source characterizations using the HySplit model. In summer, air from the Arabian sea brings in dust and sea salt into the city and in winter, aerosols come from central India dominantly as carbon and sulfates changing the air quality over there. This study lays its stress on the fact that even though aerosols cover over a city is mostly non-local, anthropogenic activities of that area do play a significant role and here the city of Pune is a role model to show how measures can be taken to improve air quality over any urban area.

How to cite: Das, S. and Pandithurai, G.: Variability of trends observed in Atmospheric Aerosol optical properties over Pune, India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21180, https://doi.org/10.5194/egusphere-egu2020-21180, 2020.

The particle size distribution function is one of the characteristics reflecting the composition of aerosol during sand lifting and removal in desert regions. This characteristic, in addition to known practical applications, is important in describing radiation processes during the exchange of heat fluxes and in forming cloud systems in the models of atmospheric dynamics. Fine dust-aerosol fractions (less than 2 µm in diameter) are especially important for the atmospheric radiation budget, because such fractions (having a significant lifetime) most efficiently interact with short-wave solar radiation. One of the central regularities in considering the size distributions of simulated dust-aerosol particles is the following formula based on the so-called fragmentation process and verified using a large amount of empirical data N (d) ~ d ^-2. Similar dependence for particles with size d > 1 µm is associated with the consideration of the fragmentation process as a particle splitting according to the log-normal distribution.

Results of field measurements taken in the near–Caspian (2002, 2003, 2007, 2009, 2010, 2011, 2013, 2014, 2016 years) and near–Aral-sea (1998) deserts under the conditions of weak winds (almost in the absence of saltation processes) and strong heating of the land surface are given. These results show that the fine mineral dust aerosol (0.1-1 µm) considerably contributes to the total aerosol content of the atmospheric surface layer under such conditions. The scaling of daytime mean size d distribution at a height of 2 m is close to d ^-5 in contrast to the law d ^-2 for fraction d >1 µm.

Different compositions of aerosol particles at 0.1 < d < 1 µm, and d >1 µm, including multicomponent fractions (less than 1 µm) may result in different probabilities of their integration and disintegration, which, finally, determine equilibrium particle size distributions. The simplest distribution approximations based on the Kolmogorov direct differential equation are given. 

This study was supported by the RFBR (19-05-50110) and the Presidium of the Russian Academy of Sciences (programs 12 and 20).

How to cite: Chkhetiani, O., Gledzer, E., and Vazaeva, N.: Size distribution functions of submicron aerosol and approximation based on the direct Kolmogorov equation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19928, https://doi.org/10.5194/egusphere-egu2020-19928, 2020.

Absorbing and non-absorbing aerosols have distinct effects on both global-mean and regional precipitation. Local changes of precipitation in response to aerosol perturbations are more complex than global-mean changes, which are strongly constrained by global energy budget. This work examines the changes of atmospheric energetic budget terms to study effects of large perturbations in black carbon (BC) and sulphate (SUL) on precipitation. Both cases show decrease of global-mean precipitation but with different geographical patterns. Decreased atmospheric radiative cooling contributes to the majority of decreased global-mean precipitation. It is caused by increased aerosols absorption in BC case but decreased cooling from clean-clear sky (without clouds and aerosols) in SUL case. Fast responses, which are independent of changes in sea surface temperature (SST), dominate the precipitation changes in the BC case, not only for global mean but also for regional patterns. Slow responses, which are mediated by changes in SST, dominate the precipitation responses in SUL case, both globally and regionally.

Relationships between temporal responses of local precipitation and diabatic cooling and precipitation are also examined for both BC and SUL perturbations. Both cases show remarkable similar pattern of correlations despite of essentially different patterns of changes in precipitation and diabatic cooling. Strong positive correlations are found over mid-latitude land and this is mainly due to the changes from surface sensible heat fluxes. Negative correlations are found over tropical oceans, mainly contributed by (longwave) radiative cooling from clouds and clean-clear sky. Further analysis shows this similarity is caused by the natural variability which is independent from external forcing. It indicates that the temporal relationship between changes in local precipitation and diabatic cooling is forcer-independent. This correlation is examined as a function of increasing spatial scales, which demonstrates the scale at which the dominating energetic term on regional precipitation shifts from energy transport to atmospheric diabatic cooling.

How to cite: Zhang, S., Stier, P., Watson-Parris, D., and Dagan, G.: Spatio-temporal Patterns of the Precipitation Response to Aerosol Perturbations from an Energetic Perspective, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18728, https://doi.org/10.5194/egusphere-egu2020-18728, 2020.

The influence of aerosols serving as cloud condensation nuclei (CCN) on the production of droplets in mixed-phase cloud systems is an ongoing research problem that influences their optical and microphysical properties. During February and March 2019, the Role of Aerosols and CLouds Enhanced by Topography on Snow (RACLETS) field campaign collected unique and detailed airborne and ground-based in-situ measurements of cloud and aerosol properties over the Swiss Alps. This study presents analysis of the observed CCN activity of the aerosol, which combined with observed aerosol size distributions, can be introduced into a cloud droplet activation parameterization to investigate the drivers of droplet variability in these clouds. The implications for secondary ice production are then discussed.

How to cite: Georgakaki, P., Bougiatioti, A., and Nenes, A.: The observed impact of aerosols on cloud droplet formation during the RACLETS campaign, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18123, https://doi.org/10.5194/egusphere-egu2020-18123, 2020.

The impact of aerosols on cloud properties is one of the largest uncertainties in the anthropogenic forcing of the climate system. As large, isolated sources of aerosol, ships provide the ideal opportunity to investigate aerosol-cloud interactions. However, their use for quantifying the aerosol impact on clouds has been limited by a lack on information on the aerosol perturbation generated by the ship.

In this work, satellite cloud observations are combined with ship emissions estimated from transponder data. Using over 17,000 shiptracks during the implementation of emission controls, the central role of sulphate aerosol in controlling shiptrack properties is demonstrated. Meteorological factors are shown to have a significant impact on shiptrack formation, particularly cloud-top relative humidity. Accounting for this meteorological variation, this work also demonstrates the potential for satellite retrievals of ship sulphate emissions, providing a pathway to the use of cloud observations for monitoring air pollution.

How to cite: Gryspeerdt, E., Smith, T., O'Keefe, E., Christensen, M., and Goldsworth, F.: Impact of ship emission controls recorded by cloud properties, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2324, https://doi.org/10.5194/egusphere-egu2020-2324, 2020.

We evaluate the impact of horizontal model resolution (~135, 60 and 25 km, respectively) and different levels of complexity of the aerosol-cloud interaction parameterization (interactive versus non-interactive) on springtime subtropical marine stratocumulus properties and stratocumulus-to-cumulus transition (SCT) using the atmosphere-only version of HadGEM-GC31. Higher resolution and non-interactive aerosols resulted in small, but significantly higher, liquid water contents and lower precipitation rates, in particular over the southern hemisphere. Higher resolution also resulted in a significantly stronger shortwave (SW) cloud radiative effect (CRE). Over the southern hemisphere, non-interactive aerosols also resulted in a stronger SW CRE, but over the northern hemisphere non-significant changes or a weaker SW CRE was obtained compared to the simulation using interactive aerosols. In general, no significant changes in the all-sky SW radiation was obtained. Only the model version with lowest resolution showed a weak tendency of a faster SCT than the other model versions. We conclude that a change in the complexity of the aerosol-cloud parameterization may significantly affect the SW CRE of marine stratocumuli, at least regionally, but the sign and magnitude of the impact will be dependent on the background level as well as the relative change in liquid water and the absolute change in cloud droplet number concentration of the specific model version.

How to cite: Ekman, A. M. L., Nygren, E., Svensson, G., and Bellouin, N.: Influence of horizontal resolution and complexity of aerosol-cloud interactions on marine stratocumulus and stratocumulus-to-cumulus transition in HadGEM-GC31, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4728, https://doi.org/10.5194/egusphere-egu2020-4728, 2020.

The maritime continent in Southeast Asia is characterized by the frequent convective activities on a wide range of scales, as well as by the seasonal emissions of biomass-burning particles. The emission of biomass-burning particles in this region typically peaks in September and October, whereas its intensity varies considerably from year to year. Since the atmospheric circulation over the region is heavily influenced by a range of meteorological and climatological variabilities, such as ENSO, it is important to quantitatively examine the impacts of biomass-burning particles on clouds while taking weather/climate regimes into account. We investigate the effects of biomass-burning particles on clouds, especially convective ones, with cloud-resolving simulations by the WRF-CHEM model. Instead of focusing on a particular case, our simulations cover an extended period of time in the month of September, allowing us to examine both individual convection and an ensemble of convective clouds developing under different weather/climate regimes and hence different aerosol abundance and distributions. Such long-term and high-resolution simulations over the region will give us an insight into the climate-regime dependent two-way interaction between aerosols and clouds.

How to cite: Takeishi, A. and Wang, C.: Interaction between biomass-burning aerosol and clouds under different climate/weather regimes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7433, https://doi.org/10.5194/egusphere-egu2020-7433, 2020.

The impact of aerosol on atmospheric composition and climate still represents one of the largest uncertainties in the quantification of anthropogenic climate change. This is particularly the case for modelling aerosol-cloud interactions, which requires a detailed knowledge of various processes acting on a wide range of spatial and temporal scales. While significant progress has been made in developing parameterizations for describing the aerosol activation process in liquid clouds in the framework of global models, the aerosol-induced formation of ice crystals in cirrus clouds is still poorly understood and only a few global models include explicit representations of aerosol-cloud interactions in the ice phase. This is due the high complexity of the freezing processes occurring in the ice phase, the uncertain properties of ice nucleating particles, and the competition between homogeneous and heterogeneous freezing at cirrus conditions. To tackle this issue, this study documents the implementation of a new cloud microphysical scheme, including a detailed parameterization for aerosol-driven ice formation in cirrus clouds, in the global chemistry climate model EMAC, coupled to the aerosol submodel MADE3. The new scheme is able to consistently simulate three regimes of stratiform clouds (liquid, mixed- and ice-phase/cirrus clouds), considering the activation of aerosol particles to form cloud droplets and the nucleation of ice crystals. In the cirrus regime, it allows for the competition between homogeneous and heterogeneous freezing for the available supersaturated water vapor, taking into account different types of ice-nucleating particles, whose specific ice-nucleating properties can be flexibly varied in the model setup. The new model configuration is tuned to find the optimal set of parameters that minimizes the model deviations with respect to observations. A detailed evaluation is performed comparing the model results for standard cloud and radiation variables with a comprehensive set of observations from satellite retrievals and in-situ measurements. The performance of EMAC-MADE3 in this new coupled configuration is in line with similar global coupled models and with other global aerosol models featuring ice cloud parameterizations. Some remaining discrepancies, namely a high positive bias in liquid water path in the northern hemisphere and overestimated (underestimated) cloud droplet number concentrations over the tropical oceans (in the extra-tropical regions), which are both a common problem of this kind of models, need to be taken into account in future applications of the model. To further demonstrate the readiness of the new model system for application studies, an estimate of the anthropogenic aerosol effective radiative forcing (ERF) is provided, showing that EMAC-MADE3 simulates a relatively strong aerosol-induced cooling, but within the range reported in the IPCC AR5 and in other, more recent, assessments.

How to cite: Righi, M., Hendricks, J., Lohmann, U., Beer, C. G., Hahn, V., Heinold, B., Heller, R., Krämer, M., Ponater, M., Rolf, C., Tegen, I., and Voigt, C.: Coupling aerosols to (cirrus) clouds in a global aerosol-climate model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7485, https://doi.org/10.5194/egusphere-egu2020-7485, 2020.

The cooling effect of anthropogenic aerosols on Earth's climate offsets part of the greenhouse gas warming effect. To reduce the uncertainty in the aerosol cooling effect on climate, aerosol impact on clouds needs to be better understood. In this research, we extend satellite observations of polluted cloud tracks from Toll et al. (2019, Nature, https://doi.org/10.1038/s41586-019-1423-9) with analysis of temporal evolution of anthropogenic cloud perturbations using satellite data from SEVIRI instrument onboard geostationary Meteosat satellite. Study area is concentrated to European part of Russia as we have found strong contrast between properties of polluted and unpolluted clouds in this area. We analyze the properties of polluted clouds at pollution hot spots and compare these to the properties of the nearby unpolluted clouds. We compare the temporal evolution of the properties of the polluted and unpolluted clouds to study the importance of diurnal cycle in aerosol-cloud interactions.

How to cite: Rahu, J., Toll, V., and Post, P.: Diurnal cycle in anthropogenic aerosol impacts on clouds, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8822, https://doi.org/10.5194/egusphere-egu2020-8822, 2020.

The influence of aerosol on lightning is very dependent on environmental factors, including thermal factors, humidity factors, and terrain factors. ADTD cloud-to-ground lightning data, ERA5 reanalysis data, and MERRA2 reanalysis data were applied to discuss the influence of aerosol on lightning activity in Sichuan basin. Thermodynamic factors were the main reasons for the difference in lightning density between the plateau and the basin. The results showed that the influence of aerosol on lightning activity in the basin and the plateau regions showed a significant difference, showing a positive correlation on the plateau and a negative correlation on the basin. In the plateau area, the aerosol concentration was relatively low, and the aerosol stimulated the lightning activity by influencing the microphysical processes. In the basin area, the aerosol load was very high, and the aerosol showed a more significant radiation effect. By reducing the solar radiation reaching the ground, the convective energy on the ground was reduced, and the intensity of lightning activity was finally suppressed.

How to cite: Zhao, P.: Effects of aerosol on lightning in Sichuan Basin, Southwest China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6056, https://doi.org/10.5194/egusphere-egu2020-6056, 2020.

Absorbing aerosols affect the climate system (radiative forcing, cloud formation, precipitation and more) by strongly absorbing solar radiation, particularly at ultraviolet and visible wavelengths. The environmental impacts of an absorbing aerosol layer are influenced by its single scattering albedo (SSA), the albedo of the underlying surface, and also by the atmospheric residence time and column concentration of the aerosols.

Black-carbon (BC), the collective term used for strongly absorbing, carbonaceous aerosols, emitted by incomplete combustion of fossil fuel, biofuel and biomass, is a significant contributor to atmospheric absorption and probably a main-driver in inter-model differences and large uncertainties in estimating the aerosol radiative forcing due to aerosol-radiation interaction (RFari). Estimates of BC direct radiative forcing suggest a positive effect of +0.71 Wm^-2 (Bond and Bergstrom (2006)) with large uncertainties [+0.08, +1.27] Wm^-2. These uncertainties result from poor estimates of BC atmospheric burden (emissions and removal rates) and its radiative properties. The uncertainty in the burden is due to the uncertainty in emissions (7.5 [2, 29] Tg yr^-1) and lifetime (removal rates). In comparison with the available observations, global climate models (GCMs) tend to under-predict absorption near source (e.g. at AERONET stations), and over-predict concentrations in remote regions (e.g. as measured by aircraft campaigns). This may be due to GCM’s weak emissions at the source, but longer lifetime of aerosols in the atmosphere.

This study aims to address the parametric uncertainty of GCMs and constrain the direct radiative forcing using a perturbed parameter ensemble (PPE) and a collection of observations, from remote sensing to in-situ measurements. Total atmospheric aerosol extinction is quantified using satellite observations that provide aerosol optical depth (AOD), while the SSA is constrained by the use of high-temporal resolution aerosol absorption optical depth (AAOD) measured with AERONET sun-photometers (for near-source columnar information of aerosol absorption) and airborne black-carbon in-situ measurements collected and synthesised in the Global Aerosol Synthesis and Science Project (GASSP) (for properties of long-range transported aerosols). Measurements from the airborne campaigns ATOM and HIPPO are valuable for constraining aerosol absorption in remote areas, while CLARIFY and ORACLES, that were employed over Southeast Atlantic, are considered in our study for near source observations of biomass burning aerosols transported over the bright surface of stratocumulus clouds.

Using the PPE to explore the uncertainties in the aerosol absorption as well as the dominant emission and removal processes, and by comparing with a variety of observations we have confidence to better constrain the aerosol direct radiative forcing.

How to cite: Deaconu, L. T., Watson-Parris, D., Stier, P., and Lee, L.: Constraining direct aerosol radiative forcing using remote sensing and in-situ constraints, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17044, https://doi.org/10.5194/egusphere-egu2020-17044, 2020.

In situ aircraft measurements obtained during the RACORO field campaign are analyzed to study the aerosol effects on different cloud regimes. The results show that with increasing cloud condensation nuclei (CCN), cloud droplet number concentration (N_d) significantly increases in stratocumulus (Sc) while remains almost unchanged in cumulus (Cu). By using a new approach to strictly constrain the dynamics in Cu, we found that neither simultaneously changing cloud dynamics nor dilution of cloud water induced by entrainment-mixing can explain the observed insensitivity of N_d. The different degree of reduction in cloud supersaturation caused by increasing aerosols might be responsible for the observed different aerosol indirect effect between Sc and Cu.

How to cite: Jia, H., Ma, X., Yu, F., Liu, Y., and Yin, Y.: Distinct Impacts of Increased Aerosols on Cloud Droplet Number Concentration of Stratus/Stratocumulus and Cumulus, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-8998, https://doi.org/10.5194/egusphere-egu2020-8998, 2020.

On average stratocumulus clouds cover about 23% of the ocean surface and are important for Earth’s radiative balance. They typically self-organize into cellular patterns and thus are often referred to as mesoscale-cellular convective (MCC) cloud systems. In the Southern Ocean (SO), low-level clouds cover between 20% to 40% of the ocean surface in the mid-latitudes where they exert a substantial radiative cooling. In a previous study, McCoy et al (2017) demonstrated that different MCC regimes may be associated with different cloud albedos and thus different cloud radiative forcing.
Many of the MCC clouds in the SO are not pure liquid but contain a mixture of liquid and ice. Here we investigate whether the formation of ice within these mixed-phase clouds influences MCC organization and thus the cloud-radiative effect.
To investigate the cloud phase we use the raDAR-liDAR (DARDAR) data product (version 1) from Cloud-Aerosol-Water-Radiation Interactions (ICARE) Data and Services Center which provides collocated data from Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO), CloudSat and Moderate Resolution Imaging Spectroradiometer (MODIS). The “Simplified DARMASK Categorization Flag” of DARDAR is used to categorize the vertically resolved cloud phase into a single cloud phase per data point: clear, multi-layer, liquid, mixed or ice. In order to distinguish between open and
closed MCC regimes, we collocate the DARDAR product with an MCC classification data set from McCoy et al (2017) which is based on a neural network algorithm applied to MODIS Aqua data.
Our preliminary results confirm previous ground-based observations that most mixed-phase clouds are composed of a supercooled liquid top and ice underneath. Furthermore, our preliminary analysis suggests open MCCs occur more frequently as mixed-phase clouds (57% (DJF), 55% (JJA)) in the SO compared to liquid clouds (39% (DJF), 37% (JJA)) during both summer (DJF) and winter (JJA). In contrast, closed MCCs are more likely to appear as liquid clouds (58%) in comparison to mixed-phase clouds (40%) during winter, whereas during summer there seems to be no tendency for closed MCCs to be either liquid (51%) or mixed (49%).

How to cite: Danker, J., Sourdeval, O., McCoy, I. L., Wood, R., and Possner, A.: Satellite Observations of Organizational Regimes in Low-Level Mixed-Phase Clouds over the Southern Ocean, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13229, https://doi.org/10.5194/egusphere-egu2020-13229, 2020.

The cooling of the Earth’s climate through the effects of anthropogenic aerosols on clouds offsets an unknown fraction of greenhouse gas warming. We discuss how causal relationship between aerosols and clouds can be derived from contrast between clouds polluted by anthropogenic aerosols and nearby unpolluted clouds. Ship tracks have been long considered to be real-world laboratories of aerosol-cloud interactions. More recently, polluted cloud tracks induced by aerosols emitted from volcanoes and wildfires and various industrial sources - such as oil refineries, smelters, coal-fired power plants, and cities have been analysed (Toll et al. 2019; Nature, https://doi.org/10.1038/s41586-019-1423-9). In this research, we extend satellite observations of polluted cloud tracks from Toll et al. (2019) with analysis of smaller and larger scale polluted cloud areas detected in satellite images.

Polluted clouds are detected in MODIS and SEVIRI satellite images as areas with strongly increased cloud droplet number concentrations. Polluted cloud tracks can be utilized to study frequency and magnitude of anthropogenic cloud droplet number perturbations and subsequent cloud adjustments. Anthropogenic aerosol perturbations on liquid-water clouds are detected in various major global industrial areas. Both tens of kilometres wide ship-track-like polluted cloud tracks and hundreds by hundreds of kilometres wide polluted cloud areas show that cloud water can both increase and decrease in response to aerosols depending on meteorological conditions. On average, there is relatively weak decrease in cloud water. Polluted cloud tracks also show that cloud fraction can both increase and decrease compared to nearby less polluted clouds. Applicability of pollution tracks to study impact of absorbing aerosols situated above clouds on below-lying clouds is discussed. We expect that utilization of real-world laboratories of aerosol impacts on clouds will lead to improved physical parameterizations in global climate models and more reliable projections of the future climate.

How to cite: Toll, V., Trofimov, H., and Rahu, J.: Real-world laboratories for studying anthropogenic aerosol impacts on clouds and Earth’s climate, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4558, https://doi.org/10.5194/egusphere-egu2020-4558, 2020.

The Mount Lu (Lushan) observational station of cloud and fog in Jiujiang, China was restarted in 2015. The observational experiment of clouds/fog and precipitation was conducted from 2015 to 2018 in Mount Lu station. The observation dataset of clouds/fog on the Mount Lu were collected and established. The observational characteristics of clouds and precipitation were investigated from November 2015 to February 2018, including microphysics properties of clouds/fog and precipitation of 15 months in cold and warm seasons. The statistical results suggested that the heavy precipitation on the Mount Lu was frequent in summer with the maximal daily precipitation exceeding 100 mm. The maximal number of clouds and fogs days reached 25 days per month, with the lowest visibility about 20m. Due to radiative effect of clouds and fog in the (early) morning, the lowest temperature in the diurnal variation of temperature happened at about 9 o’clock, right before the dissipation of clouds and fog. Based on the analysis of radar data, stratiform precipitation, stratocumulus and convective precipitation in the autumn and winter respectively accounted for 29%, 44% and 27% of the total precipitation, and convective and stratocumulus precipitation in the spring and summer respectively accounted for 83% and 17% of the total precipitation. Compared with precipitation in urban areas, the small and medium raindrops were predominant in the precipitation processes on Mount Lu. Compared with fog in urban areas, the clouds and fog were characterized by smaller number concentration, the more significant bimodal and wider spectra. With the increase of precipitation within cloud, the more raindrops in number and larger raindrops in size were easier to initiate the coagulation mechanism, resulting in reduction of cloud droplets smaller than 11μm and larger than 30 μm. As a result, the peak at 11μm became more obvious. During the snowfall periods, the small cloud droplets were abundant, and the solid precipitation growth consumed large freezing cloud droplets through the rimming process.

How to cite: Guo, L., Guo, X., Lou, X., Lu, G., Lyu, K., Sun, H., Li, J., and Zhang, X.: An Observational Study of Macroscopic and Microphysical Characteristics of Clouds and Precipitation on Mount Lu, Jiangxi, China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3111, https://doi.org/10.5194/egusphere-egu2020-3111, 2020.

Atmospheric aerosols have an important role in global climate and weather by scattering and absorbing incoming shortwave radiation and absorbing outgoing longwave radiation that influences the Earth’s radiation budget. The aerosol indirect effect (AIE)  on the cloud microphysical properties has been studied over a high altitude site, Mahabaleshwar (17.92˚ N, 73.66˚ E; 1380m a.m.s.l.), Maharashtra, India, using ground-based in-situ measurements during monsoon season (June - August) of 2017. The AIE was estimated using cloud droplets number concentration (AIE_n) and cloud droplet effective radius (AIE_s) at different fixed liquid water contents (LWC). The AIE was varying in the range 0.01 – 0.13 when LWC was varying from 0.04 – 0.26 gm^-3. The maximum values of AIE_n and AIE_s (0.125 and 0.119) were found at the lower LWC bin (0.06 - 0.07 gm^-3). The calculated values of AIE_n and AIE_s showed that the values of AIE_n were overestimated due to the dispersion effect. The maximum dispersion offset observed was 17.4% at LWC bin 0.16 – 0.17 gm^-3. After dispersion correction, the offset was reduced and AIE_n became close to AIEs. So dispersion correction is necessary for the correct estimation of AIE using cloud droplet number concentration (CDNC). For the first time in India, cloud droplets are classified into smaller and medium size droplets to study their relative dispersion and their contribution to the total dispersion of cloud droplet size distribution. The contribution of smaller and medium-size droplets on dispersion at a lower and higher LWC region was studied. In lower LWC (high AIE), the concentration of smaller size droplets are higher (71%) than medium size droplets, but medium size droplets are the major contributor (61%)  of dispersion compared to the contribution by smaller size droplets. When LWC is higher (low AIE), the number concentration of smaller size droplets was reduced and the concentration of medium size droplets increased, compared to the case of lower LWC. However, the dispersion contribution by smaller size droplets was increased and the dispersion contribution by medium size droplets was reduced. An inverse relation between CDNC at a particular size class (small/medium) and their contribution to dispersion in CDSD was observed. 

How to cite: Pandithurai, G.: Aerosol-cloud interactions as observed over Western Ghats, India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21444, https://doi.org/10.5194/egusphere-egu2020-21444, 2020.

The effects of natural aerosols on microphysical processes in clouds are quite important for their development and evolution and still pose some unresolved questions on the impact they have in the atmosphere and climate. The processes where they interfere, can lead to an uncertainty in the intensity of precipitation and the hydrometeor species as well as the temporal and spatial extent of the affected areas. Apart from the scientific interest of such studies, the outcome highly affects applications and early warning systems associated to water management,  food security and agriculture.

For the needs of the study, the state of the art atmospheric modeling system RAMS-ICLAMS was used to investigate the effects of desert dust concentrations on microphysical processes in clouds. The model is able to run in very high resolutions in order to resolve cloud processes explicitly. Extreme case studies were selected, simulated and the model performance was evaluated showing satisfactory results. Sensitivity tests were performed in order to quantify the direct, indirect and semi-direct impact of CCN and IN concentrations. These tests showed notable effects on the cloud microphysical processes, as well as on hydrometeors. This further enhances the need for a more accurate description of aerosol feedbacks in regional and climate atmospheric models.

How to cite: Chaniotis, I., Patlakas, P., and Kallos, G.: A numerical study of dust particle effects on cloud microphysical processes and hail/precipitation impacts, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19108, https://doi.org/10.5194/egusphere-egu2020-19108, 2020.

Himawari-8 is the new geostationary satellite of the Japan Meteorological Agency (JMA) and carries the Advanced Himawari Imager (AHI), which is greatly improved over past imagers in terms of its number of bands and its temporal/spatial resolution. In this work, two different methods for the detection of the different levels of fog involving heavy pollutants by using the Himawari-8 were developed in China. The two different methods are the method of the difference between the 11.2 mm and 3.9 mm brightness temperatures (BTD_3.9-11.2) and the method of 3.9 mm Pseudo-Emissivity (ems_3.9).  The 3.9 mm Pseudo-Emissivity is the ratio of the observed 3.9 mm radiance and the 3.9 mm blackbody radiance calculated using the 11.2 mm brightness temperature. We identified the parameters optimal threshold at the 2400 stations and the grid points using the BTD_3.9-11.2 and ems_3.9 for different levels of fog involving heavy pollutants. Results on land and sea from the two methods were compared with surface observations from 2400 weather stations in China and CALIPSO (Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observation) VFM (Vertical Feature Mask) products. The results show that both the method of BTD_3.9-11.2 and the method of ems_3.9 can accurately identify the different levels of fog involving heavy pollutants and the accuracy of ems_3.9 method is slightly better than the BTD_3.9-11.2. The accuracy of two methods has increased significantly and the false alarm rate has significantly decreased with the decrease of the visibility. When the visibility is less than 50 m, the HR, FAR and KSS of the BTD_3.9-11.2 method (the ems_3.9 method) were 0.89 (0.90), 0.15 (0.15) and 0.74 (0.75), respectively. When mid- or high-level clouds were removed using surface temperature of the ground observations, the HR and KSS of two methods for the different levels of fog has increased significantly, and the FAR has significantly decreased. When the visibility is less than 1000 m, the HR of the BTD_3.9-11.2 method (the ems_3.9 method) is increased to 0.81(0.85) from 0.71 (0.74), the FAR is decreased to 0.12 (0.13) from 0.27 (0.28), and the KSS is increased to 0.69 (0.72) from 0.44 (0.46). The KSS of two method increase by 0.23 and 0.26, respectively. Three cases analysis show that the fog area can be clearly identified by using the BTD_3.9-11.2, ems_3.9 and RGB composite image. The results of the detection of sea fog by using Himawari-8 data and using CALIPSO VFM products have consistency.

How to cite: Wang, H., Zhang, Z., and Liu, D.: Detection of Fog Involving Heavy Pollutants by Using the New Geostationary satellite Himawari-8, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3929, https://doi.org/10.5194/egusphere-egu2020-3929, 2020.

Numerical models in atmospheric sciences do not only need to approximate the flow equations on a suitable computational grid, they also need to include subgrid effects of many non-resolved physical processes. Among others, the formation and evolution of cloud particles is an example of such subgrid processes. Moreover, to date there is no universal mathematical description of a cloud, hence many cloud schemes were proposed and these schemes typically contain several uncertain parameters. In this study, we propose the use of algorithmic differentiation (AD) as a method to identify parameters within the cloud scheme, to which the output of the cloud scheme is most sensitive. We illustrate the methodology by analyzing a scheme for liquid clouds, incorporated into a parcel model framework. Since the occurrence of uncertain parameters is not limited to cloud schemes, the AD methodology may help to identify the most sensitive uncertain parameters in any subgrid scheme and therefore help limiting the application of Uncertainty Quantification to the most crucial parameters.

How to cite: Baumgartner, M., Sagebaum, M., Gauger, N. R., Spichtinger, P., and Brinkmann, A.: Algorithmic Differentiation for Cloud Schemes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1401, https://doi.org/10.5194/egusphere-egu2020-1401, 2020.

The warming effect of cirrus clouds is well-known. In recent years, in order to mitigate global warming, cirrus cloud thinning as a newly emerging method of geoengineering has been studied based on climate modeling. Adding a few (~10 L^–1) INPs (ice nucleating particles including ice crystals) might hinder homogeneous ice nucleation, which can produce a large number of ice crystals (~1000 L^–1), and then reduce cirrus clouds. On the other hand, the cirrus clouds might increase if too much INPs were added. Therefore, the effectiveness of cirrus seeding on cooling our earth is still in debate. In this study, we developed a method (optimal seeding scheme) to calculate the minimum concentration of seeding INPs, which is just enough to prevent homogeneous nucleation from happening. Simulation with the Community Atmosphere Model version 5(CAM5) using the optimal seeding scheme shows a significant cooling effect (–1.4 W/m²), which is equal to two-thirds of the cooling potential (–2.1 W/m²) derived from the pure heterogeneous simulation (i.e., homogeneous ice nucleation is artificially switched off). Seeding fixed 20 L^-1 and 200 L^-1 concentrations of INPs show the global average radiative effect at –0.5 W m^-2 (cooling) and 0.1 W m^-2 (warming), respectively. The cooling effect of seeding fixed number concentration of INPs is not obvious, which is consistent with previous studies. Furthermore, using the optimal seeding scheme, the sensitivities of cooling effects to seeding area, ice nucleation parameterizations and homogeneous ice nucleation occurrence frequency are also investigated.

How to cite: Liu, J. and Shi, X.: A comprehensive estimate of the global cooling effect from hindering homogeneous ice nucleation under cirrus conditions with CAM5, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4939, https://doi.org/10.5194/egusphere-egu2020-4939, 2020.

Qilian mountains, located in the arid and semi-arid region of Northwest China, has more amount of natural precipitation than that on both north and south sides, with unique geographical environment and abundant water vapor supply. It is a very important water resource for the surrounding areas. To deeper understand the features of cloud over the areas is significant for the utilization of cloud water resources and sustainable development in this region. In this article, based on MOD08-M3 data, grid ground precipitation data and FY-2 series satellite cloud parameter inversion products, the spatial and temporal features of cloud macro/micro physical parameters, such as Cloud Amount（CA）, Cloud Water Path（CWP）, Cloud Top Temperature（CTT）, Cloud Top Pressure（CTP）, Cloud Optical Depth（COD） and Cloud Particle Effective Radius (CPER) over Qilian Mountains area were analyzed, as well as the relationship between the precipitation and cloud parameters. The results are as follows:

• (1) The regional average values of CA, CWP, CTP, COD and CPER in Qilian Mountains area are 55.50 %, 148.95 g/m², -21.13 ℃, 456.56 hPa, 12.64 and 21.04 μm, respectively. From 2006 to 2015, CA, CWP, COD and CPER decreased by 2.3 %, 21 g/m², 0.68 and 0.51 μm, respectively. CTT and CTP increased by 1.9 ℃ and 65.2 hPa, respectively. Cloud water resources over the area are abundant.
• (2) There is the richest cloud water resource over the main area of Qilian Mountains, and the cloud parameter condition in Wushaoling area is the best for precipitation. The high value areas of CA in four seasons are distributed in Xining and surroundings, main and south part of mountain range, and Lenghu area, respectively. The high value areas of CWP in four seasons are located in the northeast, north-middle the main part of mountain area and the eastern side of Subei, respectively. The high value areas of COD are located in the east of Subei in winter and in the southeast of the study area in other seasons. The high value areas of CPER in spring are located in the region except Hexi Corridor. In other seasons they are located between Lenghu and Subei, Subei and Tuole, and in the northeast of range, respectively.
• (3) The monthly precipitation is positively correlated with CA , CWP, COD, but negatively correlated with CTT and CTP. The relationship between CPERs and precipitation is positive in January, April, July, November and December, but negative in other months. CA and CPERs are most correlated with precipitation in May and September, respectively. while the correlation between other cloud parameters and precipitation are the highest in January.
• (4) When the values of COD and CPER are too small or too large, the actual precipitation will be limited.

Key words: Cloud physical parameters; Precipitation; Water resource; Qilian Mountains

How to cite: Zhang, W., Wang, Y., Zhao, Q., Pu, C., and Chen, Y.: Analysis of the Temporal-Spatial Characteristics of Cloud Parameters and the Relationship with the Precipitation over Qilian Mountains Area in Northwest China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5226, https://doi.org/10.5194/egusphere-egu2020-5226, 2020.

Macro- and micro-physical properties of summer convective clouds and precipitation over the central Tibetan Plateau (TP) were investigated using the in-situ observations during the Third Tibetan Plateau Atmospheric Sciences Experiment (TIPEX-Ⅲ) in 2014. The advanced aircraft and radar observational systems were employed during the experiment.
Results show that the convective events over the central TP were characterized as frequently weak precipitation with a significant daily variation. The convections were generally initiated in the late morning and peaked in the late afternoon, and the convective clouds were turned into stratiform clouds in the nighttime. The average heights of cloud top and cloud base were 11.62 ± 2.45 km and 6.89 ± 1.58 km, respectively. The average rain rate was ≈ 1.2 mm/h, and compared to M-P distribution, the Γ distribution was more suitable in describing the raindrop size distribution of precipitation over the central TP.
Aircraft observations show that the clouds over the central TP were normally in a mixed-phase state, and had lower concentrations of cloud particles and weaker updraft, but more larger particles than over plains. The particle size distributions (PSDs) of cloud droplets were mainly bimodal, and the large cloud particles (> 50 μm) had an exponential PSD type. The aircraft observed convective clouds were mainly singular newly born or developing convective cells, in which ice processes happened at early stage, quick and massive glaciation happened at higher altitude, coalescence and rimming contributed to the formation of precipitation.

How to cite: Chang, Y., Guo, X., Tang, J., Lu, G., and Qi, P.: The observed properties of summer convective clouds and precipitation over the central Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1884, https://doi.org/10.5194/egusphere-egu2020-1884, 2020.

Clouds play a key role in Earth’s energy and water budgets. This study examines the cloud cover changes in two nearer and distinct locations in tropical south India, one location is a near-coastal region in Thiruvananthapuram (8.52^oN, 76.90^oE) and another one in Western Ghats mountain ranges (10.15^oN, 77.01^oE). The study validated the following reanalysis product with Lufft CHM 15k ceilometer observations in both locations during 2017: ERA5, ERA-Interim and MERRA-2 reanalysis. ERA5 daily cloud cover data has the lowest RMSE (20 %) than other reanalysis datasets in these tropical locations. Correlation between daily ceilometer cloud cover observation and reanalysis datasets shows that ERA5 data has better temporal cloud cover anomaly (0.8) in the mountain location. All reanalysis datasets show significant correlation (0.01 level) with observation. RMSE (correlation) is higher (lower) in coastal region. Further, a long period cloud cover trend (1985-2016) in both locations are calculated from multiple reanalysis and ISCCP datasets. All datasets show a consistent and significant (0.01 level) increasing cloud cover in both locations.  ISCCP Mean IR cloud amount (marginal) shows the highest increasing trend compared to reanalysis datasets (5.9 %). In the coastal location, the cloud cover increasing trend is much higher and all datasets agree well on it. A long period correlation analysis is performed between cloud cover variability in the study region and north Indian ocean SST to study their relation. Bay of Bengal SST is highly positively correlated with the cloud cover in the study region (significant at 0.01 level). This suggests that the observed increase in cloud cover has a strong relationship with the north Indian Ocean warming.

How to cite: Chirikandath Kalath, U.: A study on cloud cover in reanalysis datasets in tropical south India, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7911, https://doi.org/10.5194/egusphere-egu2020-7911, 2020.

There are conditions between cloudy and cloud-free air at which it is hard to define the suspended particles in the atmosphere either as a cloud or an atmospheric aerosol; it is called twilight or transition zone. This occurs when characteristics of the suspended particles are between those corresponding to a pure cloud and those corresponding to a pure atmospheric aerosol. However, in most meteorological and climate studies the condition of sky is assumed to be either cloudy (fully developed cloud) or cloud-free (dry aerosol), neglecting the transition zone. The present communication aims to show the uncertainties introduced by this simplified assumption in modeling longwave radiation. For this purpose, the parameterizations RRTMG, NewGoddard and FLG included in the Weather Research and Forecasting Model (WRF) version 4.0 were isolated from the whole model. These parameterizations were then used to perform a number of simulations under ideal “cloud” and “aerosol” modes, for different values of (i) cloud optical thicknesses resulting from different sizes of ice crystals or liquid droplets, cloud height, mixing ratios; and (ii) different aerosol optical thicknesses combined with various aerosol types. The differences in the resulting longwave radiative effects (RE) at the top of the atmosphere and at the Earth surface were analyzed. The primary results show: (1) the parameterization RRTMG is not capable of simulating the REs of the aerosols in the longwave region, (2) different assumptions of a situation corresponding to the transition zone lead to a mean relative uncertainty of about 170% in the estimated longwave irradiance at both top of the atmosphere and surface, (3) the absolute uncertainties observed in the surface downwelling irradiances are substantially greater than those relating to the upwelling irradiances at top of the atmosphere.

How to cite: Jahani, B., Calbó, J., and González, J.-A.: Longwave Radiation at the Cloud-Aerosol Transition Zone from Radiative Parameterizations in Weather Research and Forecasting Model (WRF) , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10344, https://doi.org/10.5194/egusphere-egu2020-10344, 2020.

The conditions between cloudy and cloud-free air, named “Transition (or twilight) Zone”, are a major source of uncertainty in the climate and meteorological studies. The transition zone involves microphysical and radiative characteristics which lay on the border between those corresponding to a pure cloud and those corresponding to pure atmospheric aerosols. Several studies show that a notable proportion of cloudless sky at any time may correspond to this phase. However, as the information available about radiative effects of this phase is still very limited in most meteorological and climate studies the condition of sky is assumed to be either cloudy (fully developed cloud) or cloud-free (dry aerosol), neglecting the transition zone. This implies that these models consider the area/layer corresponding to the transition zone as either cloud or aerosol. The authors of the current communication have shown in a previous work that there are substantial uncertainties associated with modeling the surface shortwave irradiances under this assumption [Jahani et al. (2019) JGR: Atmospheres, 124. https://doi.org/10.1029/2019JD031064]. The present communication aims to show the uncertainties in modeling the heating rate in the atmosphere (due to shortwave solar radiation) driven from different treatments of the transition zone. For this purpose, the relatively detailed shortwave radiation parameterizations included in the Weather Research and Forecasting model (WRF) version 4.0, which allow users to consider different treatments of aerosols and clouds (RRTMG, NewGoddard and FLG), were isolated from the whole model. These parameterizations were then utilized to perform a number of simulations under ideal “cloud” and “aerosol” modes, for different values of (i) cloud optical thicknesses resulting from different sizes of ice crystals or liquid droplets, cloud height, mixing ratios; and (ii) different aerosol optical thicknesses combined with various aerosol types. The optical thickness under both aerosol and cloud modes was considered to vary between 0.01 and 2.00. The differences in the resulting atmosphere column averaged heating rate were analyzed. The results showed (i) the simplified assumption about the state of the sky leads to a large difference among the atmospheric shortwave heating rate, (ii) magnitude of these uncertainties is higher when parameterizations which cope with the Radiative Transfer Equation in more detail (RRTMG and NewGoddard) are used.

How to cite: Calbó, J., Jahani, B., and González, J.-A.: Shortwave Heating Rate at the Cloud-Aerosol Transition Zone from Radiative Parameterizations in the Weather Research and Forecasting Model (WRF), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11326, https://doi.org/10.5194/egusphere-egu2020-11326, 2020.

Aerosol particles can serve as cloud condensation nuclei (CCN) to influence orographic clouds. Autoconversion, which describes the initial formation of raindrops from the collision of cloud droplets, is an important process for aerosol–cloud–precipitation systems. In this study, seven autoconversion schemes are used to investigate the impact of CCN on orographic warm-phase clouds. As the initial cloud droplet concentration is increased from 100 cm⁻³ to 1000 cm⁻³ (to represent an increase in CCN), the cloud water increases and then the rainwater is suppressed due to a decrease in the autoconversion rate, leading to a spatial shift in surface precipitation. Intercomparison of the results from the autoconversion schemes show that the sensitivity of cloud water, rainwater, and surface precipitation to a change in the concentration of CCN is different from scheme to scheme. In particular, the decrease in orographic precipitation due to increasing CCN is found to range from −87% to −10% depending on the autoconversion scheme. Moreover, the surface precipitation distribution also changes significantly by scheme or CCN concentration, and the increase in the spillover (ratio of precipitation on the leeward side to total precipitation) induced by increased CCN ranges from 10% to 55% under different autoconversion schemes. The simulations suggest that autoconversion parameterization schemes should not be ignored in the interaction of aerosol and orographic cloud.

How to cite: Xiao, H.: Effect of Aerosol Particles on Orographic Clouds: Sensitivity to Autoconversion Schemes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12070, https://doi.org/10.5194/egusphere-egu2020-12070, 2020.

Stratocumulus clouds are low-level boundary layer clouds that cover 23% of the ocean surface on a global average, with a mean coverage of 25% to 40% in the mid-latitude oceans. These clouds affect Earth's radiative balance due to their strong radiative cooling effect. Many climate models underestimate the reflection of short wave radiation over the Southern Ocean (SO) which results in a positive mean bias of 2K in the annual mean SST in the mid-latitudes of the southern hemisphere. The organization, cloud field properties and the cloud radiative effects of these clouds occur at the lee of cold front in the SO are analyzed in this study. At this conference, we will present preliminary results.
Real case simulations are performed in this study by using ICON - LAM (Icosahedral Nonhydrostatic - Limited Area Model) with two-way nesting domains of resolutions 4.9 km to 2.4 km to 1.2 km. The initial and lateral boundary conditions for the model are derived from IFS meteorological data. CAPRICORN (Clouds, Aerosols, Precipitation, Radiation, and Atmospheric Composition over the Southern Ocean) field campaign that took place during March and April 2016 has continuously observed the open-cell and stratocumuli using shipborne radars and lidars on 26 and 27 March 2016 at the lee of a cold front between 47ºS 144ºE and 45ºS 146ºE (South of Tasmania). The results are evaluated quantitatively and qualitatively with the shipborne observations and HIMAWARI satellite retrievals respectively.

How to cite: Ramadoss, V., Protat, A., Huang, Y., Siems, S., and Possner, A.: Simulating mixed-phase cloud properties with ICON around the CAPRICORN field campaign at the kilometre scale, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12573, https://doi.org/10.5194/egusphere-egu2020-12573, 2020.

Marine and coastal fog forms mainly from the cooling of warm and moist air advected over a colder sea surface. Atlantic Canada is one of the foggiest regions of the world due to the strong temperature contrast between the two oceanic currents in the vicinity. Recurring periods of low visibility notably disrupt off-shore operations and marine traffic, but also land and air transportation. On longer time-scales, marine fog variability also has a significant impact on the global radiative budget. Clouds, including fog, are the greatest source of uncertainty in the current climate projections because of their complex feedback mechanisms. Meteorological records indicate a significant negative trend in the occurrence of foggy conditions over the past six decades at most airports in Atlantic Canada, with large internal variability, including interannual and interdecadal variations. Using the airport observations, reanalysis data and climate model outputs, we investigated the various variabilities on the trend, at interannual and interdecadal scales, and attempted to address what caused these changes in fog frequency. Our results show that the strength and position of the North Atlantic Subtropical High as well as the sea-surface temperature of the cold and warm waters near Atlantic Canada were highly correlated with fog occurrence. We applied the derived fog indices on climate model outputs and projected the fog trends and variability in the different future climate scenarios. The results from this study will be compared with those obtained from other methods and the implications will be discussed.

How to cite: Duplessis, P., Zhang, M., Perrie, W., Isaac, G. A., and Chang, R. Y. W.: Linking marine fog variability in Atlantic Canada to changes in large-scale atmospheric and marine features, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12591, https://doi.org/10.5194/egusphere-egu2020-12591, 2020.

Climate sensitivity is a measure for the global mean temperature change of the earth in response to a given radiative forcing. In an experiment with an instantaneous forcing by e.g. a doubling of the atmospheric CO₂ content the radiative imbalance at the top of the atmosphere can be regarded as a function of the global mean temperature change. In such an experiment the climate sensitivity can be approximated by linearly extrapolating to zero the TOA imbalance where equilibrium is obtained. The thus derived value is usually referred to as effective climate sensitivity. It has been established however, that the effective climate sensitivity changes over time. While the reason for this change is not clear, most recent investigations of the abrupt4xCO2 experiments of multiple members of the CMIP5 archive point to a delay in warming of the eastern tropical Pacific region relative to the global average in the multi model mean. Due to high stability in this region the heat is trapped there close to the surface which reduces the local lower tropospheric stability. The trapping of the warming close to the surface implies that the longwave cooling is less efficient in this region and its delayed warming relative to the global average increases global climate sensitivity over time. The decrease in lower tropospheric stability furthermore reduces low cloud cover leading to less negative low cloud feedback which causes additional warming.

We investigate the delayed warming in the eastern Pacific region in more detail in terms of its effects on stability as well as clouds for individual members and multi model means of both the CMIP5 and CMIP6 archives. We find that in the multi model mean, the CMIP6 members show an even larger delayed warming than the CMIP5 members. Furthermore, the individual members of both archives generally exhibit the same pattern of delayed eastern tropical Pacific warming and a corresponding decrease in lower tropospheric stability in the same region, which indicates robustness of the earlier results based on the CMIP5 multi model mean. Additionally, there is a decrease in liquid water content in the lower atmospheric layers, confirming the influence of reduced lower tropospheric stability on low clouds. However, there are several further regions such as the Southern Ocean with a consistent delayed warming and reduced stability, which might influence climate sensitivity as well.

How to cite: Eiselt, K.-U., Graversen, R., and Fredriksen, H.-B.: Time dependence of climate sensitivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12950, https://doi.org/10.5194/egusphere-egu2020-12950, 2020.

Clouds and their interaction with aerosols are considered one of the major factors that are connected with uncertainties in predictions of climate change and are highly associated with earth radiative balance. Semi long term in-situ measurements of Arctic low-level clouds have been conducted during last 10 year (2009 - 2019) autumns at Sammaltunturi station (67◦58´N, 24◦07´E, and 560 m a.s.l.), the part of Pallas Atmosphere - Ecosystem Supersite and Global Atmosphere Watch (GAW) programme. During these years a unique data set of continuous and detailed ground-based cloud observations over the sub-Arctic area was obtained. The in-situ cloud measurements were made using two cloud probes that were installed on the roof of the station: the Cloud, Aerosol and Precipitation Spectrometer probe (CAPS) and the Forward Scattering Spectrometer Probe (FSSP), both made by droplet measurement technologies (DMT, Longmont, CO, USA). CAPS in­cludes three instruments: the Cloud Imaging Probe (CIP, 12.5 μm-1.55 mm), the Cloud and Aerosol Spectrometer (CAS-DPOL, 0.51-50 μm) with depolarization feature and the Hotwire Liquid Water Content Sensor (Hotwire LWC, 0 - 3 g/m³). Vaisala FD12P weather sensor was used to measure all the meteorological data. The essential cloud microphysical parameters we investigated during this work were the size distributions, the total number concentrations, the effective radius of cloud droplets and the cloud liquid water content. The year to year comparison and correlations among semi long term in situ cloud measurements and meteorology are presented.

How to cite: Doulgeris, K. and Brus, D.: In situ ground based measurements of low level clouds during 10 years of Pallas cloud experiments., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13138, https://doi.org/10.5194/egusphere-egu2020-13138, 2020.

Fog can be hazardous weather. Dense and polluted fog is especially known to impact transportation, air quality, and public health. Low visibilities on fog days threaten the safety of air, sea and land traffic, especially in strong dense fog (SDF) and extremely dense fog (EDF), which is the most likely to cause accidents such as car rear-end collisions and ship collisions. Throughout more than ten years of observations, strong dense fog (SDF) (visibility less than 200m) and extremely dense fog (EDF) (visibility less than 50m) often occurred in the central and eastern regions of China. This could lead to serious traffic accidents.

This research summarizes the research results of dense fog in China, including the burst reinforcement features of strong dense fog (SDF) formation, universal feature of SDF, the microphysical process of the fog body enhancement, the causes of burst reinforcement and the characteristics of the boundary layer structure. There are also remarks about fog dissipations. The research results show that there are still many important scientific problems to be solved about dense fog. Future directions for understanding Dense Fog Burst Reinforcement including that: (1) How fog expands to the surrounding areas, and what factors influence the spread of fog? (2) The physical mechanism of dense fog burst reinforcement. (3) It needs to be further observed to study the role of low-level jets in the formation of dense fog. How the low-level jet stream forms? (4) impact of air pollution on the dense fog formation.

How to cite: Liu, D., Li, Z., Yan, W., Wang, H., Zhu, C., Zhu, Y., and Zu, F.: Dense Fog Burst Reinforcement over Eastern China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2930, https://doi.org/10.5194/egusphere-egu2020-2930, 2020.

Atmospheric aerosols emitted from both natural and anthropogenic sources play a crucial role in the Earth’s radiation budget and regulating its climate. The mechanisms through which aerosols influence the radiation budget of the Earth is often classified as direct, semi-direct, and indirect effects of aerosols. It is important to understand the perturbation caused in the radiation budget of the Earth due to changing emissions of individual aerosol species and their precursors not only for estimating the responses of the climate system to such perturbations but also to be able to attribute these responses to changes in specific aerosol species and their sources for planning any mitigation or adaptation strategy to any undesirable consequences of climate change caused by aerosols. In the present study we use the Community Atmosphere Model version 5.3 (CAM5.3) to quantify the direct, semi-direct, and indirect aerosol radiative forcing due to changes in the emissions of individual aerosol species or their precursors from the pre-industrial (PI) to present day (PD) period following a new methodology proposed by Ghan et al. (2012) involving additional radiative diagnostics with neglected absorption and scattering of aerosols, whereas absorption and scattering of aerosols for the actual model setup remains unchanged. A series of systematically designed simulations with concentrations of individual aerosol species set to zero are conducted in order to estimate the direct, semi-direct, and indirect aerosol radiative forcing due to the corresponding aerosol species. Our preliminary results shows the global annual mean value of direct Short-Wave radiative forcing (DRF) at TOA due to all aerosols to be around -0.01W/m², while the Cloud radiative forcing (CRF) to be around -1.5W/m². The bias in the aerosol radiative forcing estimates as per the old conventional method are almost -0.55W/m² for DRF which is nearly 60 times the DRF estimated using the new approach and 0.23W/m² for CRF which is almost 15.43% of the total CRF at TOA respectively. Interestingly, for the South Asian region, the DRF based on the new approach is found to be positve in almost across south Asia (0.097 W/m²) thereby signifying a trapping of energy in the atmosphere due to aerosols, whereas according to the old conventional method the DRF is estimated to be around -0.59 W/m² signifying a loss of energy in the atmosphere due to aerosols. Similarly a difference of about 1 W/m² is noted in the estimates of CRF as per the new and the old methods of estimating radiative forcing. More results with greater details on the contribution of individual aerosols towards the total aerosol radiative forcing and other important meteorological parameters will be presented.

How to cite: Sharma, A. K. and Ganguly, D.: Investigating the sensitivity of direct, semi-direct and indirect effects of aerosols to changes in the emissions of individual aerosol species using a climate model., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18115, https://doi.org/10.5194/egusphere-egu2020-18115, 2020.

The ice phase in clouds determines many of their key properties and influences the water cycle, since most precipitation globally originates from the ice phase. Ice crystals falling as seeds from an ice cloud into a lower lying mixed-phase or liquid cloud can influence ice and precipitation formation. In the lower lying cloud, the ice crystals feed on the liquid, grow and enhance precipitation (seeder-feeder mechanism) or trigger glaciation (natural cloud seeding). The seeder-feeder mechanism has been associated with the intensification of extreme precipitation and flooding.

Even though there have been multiple case studies of the seeder-feeder mechanism and a few on natural cloud seeding, estimates of the occurrence frequency of these processes are lacking.

We derived the frequency of possible seeding situations (ice-layer above liquid or mixed-phase cloud layer) from radar/lidar satellite observations over Switzerland. We found an ice layer to be present above another cloud layer about 20% of the time. The distance between the cloud layers was uniformly distributed between 100m and 10km. In sublimation calculations we used the mean effective ice crystal radius from the satellite observations and calculated the crystals’ sublimation height, assuming a spherical shape. In a significant number of cases ice crystals would survive the fall between the two cloud layers. We investigated the effect of the falling ice crystals on the lower lying cloud layer and on precipitation formation in sensitivity studies of selected situations with the regional climate model COSMO.

The high occurrence frequency of seeding situations and the survival of the ice crystals indicate the seeder-feeder process and natural cloud seeding as widespread phenomena. To infer their importance, the magnitude of the seeding ice crystals’ effect on lower lying clouds and precipitation needs to be established.

How to cite: Proske, U., Bessenbacher, V., Dedekind, Z., Lohmann, U., and Neubauer, D.: Potential of natural seeding by ice clouds over Switzerland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18691, https://doi.org/10.5194/egusphere-egu2020-18691, 2020.

Automatic lidars and ceilometers (ALCs) are well-established instruments for remote sensing of the atmosphere, with a large network of instruments deployed globally. Even though they provide a wealth of information about clouds and aerosol, they have not been used extensively to evaluate models. They complement active satellite observations, which are often unable to accurately detect low clouds due to obscuration by mid and high-level clouds. ALCs cannot be used directly for atmospheric model cloud scheme evaluation due to the wavelength-dependent attenuation of the lidar signal by clouds. Therefore, a forward lidar simulator has to be used to transform model fields to simulated backscatter comparable to backscatter measured by ALCs. Here we describe the Automatic Lidar and Ceilometer Framework (ALCF), an open source lidar processing tool and forward ground-based lidar simulator capable of transforming widely-used reanalysis and model output into a data structure which can be directly compared with observations. It implements steps such as conversion, absolute calibration, resampling, noise removal, cloud detection, model data extraction, and forward lidar simulation. The simulator is based on the Cloud Feedback Model Intercomparison Project (CFMIP) Observation Simulator Package (COSP), previously used with spaceborne lidars, with extensions for several ground-based ALCs. The forward simulator is essential to get from raw ALC and model data to a one-to-one backscatter profile. It also allows statistical comparison of cloud between models and observations. Four common commercial ALCs (Vaisala CL31, CL51, Lufft CHM 15k and Sigma Space MiniMPL), three reanalyses (ERA5, JRA-55, and MERRA-2), and two NWP models and GCMs (AMPS and the Unified Model) are supported. We present case studies evaluating cloud in the supported reanalyses and models using multi-instrument observations at three sites in New Zealand. We show that at these sites the reanalyses and models generally underestimate cloud fraction and overestimate cloud albedo. We demonstrate that the ALCF can be used as a generic cloud evaluation tool. It can assist in improving model cloud simulation, which has been identified as a critical deficiency in contemporary models limiting the accuracy of future climate projections.

How to cite: Kuma, P., McDonald, A., Morgenstern, O., Querel, R., Silber, I., and Flynn, C.: Ground-based lidar processing and simulator framework for comparing models and observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21290, https://doi.org/10.5194/egusphere-egu2020-21290, 2020.

Marine stratocumulus clouds of the eastern Pacific play an essential role in the Earth's energy and radiation budget. Parts of these clouds off the west coast of South America form the major source of water to the hyper-arid area at the northern coast of Chile. Within the DFG collaborative research center 'Earth evolution at the dry limit', for the first time, a long-term study of the vertical structure of clouds and their environment governing the moisture supply to the coastal part of the Atacama is available.

Three state of the art ground based remote sensing instruments were installed for one year at the airport of Iquique/Chile (20.5°S, 70.2°W, 56m a.s.l.) in close cooperation with Centro del Desierto de Atacama (Pontificia Universidad Católica de Chile). The instruments provide vertical profiles of wind, turbulence and temperature, as well as integrated values of water vapor and liquid water. Instrument synergy provides vertical cloud structure information.

We observe a land-sea circulation with a super-imposed southerly wind component. Highest wind speeds can be found during the afternoon. Clouds show a distinct seasonal pattern with a maximum of cloud occurrence during winter (JJA) and a minimum during summer (DJF). Clouds are higher and vertically less extended in winter than in summer. Liquid water path shows a diurnal cycle with highest values during night and morning hours and lowest values during noon. Furthermore, the clouds contain much more liquid water in summer. The turbulent structure of the boundary layer, together with the temperature profile, can be used to characterize the mechanism driving the cloud life cycle.

How to cite: Schween, J. H., Westbrook, S., and Löhnert, U.: Stratocumulus Clouds at the West Coast of South America: Observations of Diurnal and Seasonal Cycle , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21980, https://doi.org/10.5194/egusphere-egu2020-21980, 2020.

There is much uncertainty about high concentrations of ice observed in clouds and their origins. In the literature, there have been previous experimental studies reported about the sublimation process of an ice crystal causes emission of fragments by breakup.   Such sublimational breakup is a type of secondary ice production, which in natural clouds can cause ice multiplication. 

To represent this process of sublimation breakup in any cloud model, the present study proposes a numerical formulation of the number of ice fragments generated by sublimation of pristine ice crystal. This is done by amalgamating laboratory observations from previous published studies. The number of ice fragments determined by relative humidity (RH) and initial size of the ice particle were measured in the published experiments, and by simulating them we are able to infer parameters of a sublimation breakup scheme.   At small initial sizes, the dependency on size prevails, whereas at larger sizes both dependencies are comparable. This formulation is compared with observations to see the behaviour of it.

How to cite: Deshmukh, A. and Phillips, V.: Empirical Formulation for number of ice crystal fragments by sublimational breakup, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22027, https://doi.org/10.5194/egusphere-egu2020-22027, 2020.