AS1.15

The net radiative effects of tropical clouds are determined by the evolution of thick, freshly detrained anvil clouds that cool the climate into thin anvil clouds that warm the climate. To determine the role of these clouds in climate change it is important to understand how their microphysical and macrophysical properties control their radiative properties. We use cloud resolving model simulations to study the small-scale processes that drive anvil evolution and determine a delicate balance between thick and thin anvil clouds. Tiny differences in how ice crystals form, grow, shrink, or interact with solar or terrestrial radiation can lead to large differences in the climatic role of anvils. In this talk, we highlight the large impact of the interaction between radiation and ice crystal nucleation on the climatic properties of anvils. Such processes are currently not well represented in models used for climate projections. Therefore it is also not surprising that the uncertainty in tropical anvil cloud feedback is the dominant contributor to the total cloud feedback uncertainty. In addition, we show evidence that the high cloud feedback depends on the description of ice nucleation and the environmental amount of ice nucleating particles and cloud droplet number concentration.

How to cite: Gasparini, B., Voigt, A., Hartmann, D. L., and Blossey, P. N.: From ice crystals to climate: clearing high clouds of uncertainty, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7524, https://doi.org/10.5194/egusphere-egu22-7524, 2022.

We present first results aiming at understanding the impact of gravity waves on homogeneous ice nucleation in a simplified microphysics set-up. We use a 1D model of homogeneous ice nucleation, growth, sedimentation, and mixing due to wind shear. This model describes the evolution of ice crystals number and mass with 36 bins of size, after freezing on particles of ammonium sulfate. 420 supersaturated air parcels of 20m thickness and 200m length, distributed in a 7 columns grid, are simulated simultaneously, allowing exchanges of ice crystals between the air masses by sedimentation and horizontal mixing.

A first simple set-up represents the large-scale ascending motion of air parcels in the tropics, with a vertical speed of 0.5 mm/s. Air parcels follow a typical tropical temperature profile, between 16400 and 17700 m of altitude. The maximum of nucleated ice crystals is found just below the cold point and nucleation occurs in a layer of 400m, whereas sedimented ice crystals are found down to the bottom of the columns of air masses, showing that the width of cirrus clouds is different from the nucleation layer. The majority of nucleated ice crystals is small enough to be carried up with the air parcels. Yet the fall streaks of a few bigger crystals deplete the humidity of air parcels beneath, preventing new nucleation events from happening. We find that more than half of the total ice mass in air parcels is from the sedimented crystals only, even though they represent less than a third of the number of crystals counted in our system. These few crystals are responsible for more than half of the diminution of the humidity within the air parcels.

A second experiment is designed to take into account smaller scale perturbations induced by gravity waves, by coupling the microphysics model with lagrangian temperature measurements from superpressure balloons of the first Stratéole-2 campaign. Gravity waves are found to create more nucleation events, in time and at all altitude levels of our experiment, expanding the nucleation layer up to one kilometer. The larger cooling rates create more small crystals, but the growth of ice is slowed down by the waves’s warming phases and the decreased humidity from more nucleation events. Furthermore, gravity waves prevent the biggest ice crystals from appearing. Last, the addition of gravity waves removes on average less humidity from the air parcels than the sedimentation of ice crystals nucleated during the slow unperturbed ascent.

How to cite: Corcos, M., Hertzog, A., Plougonven, R., and Podglajen, A.: A simplified microphysics model to assess the impact of gravity waves on homogeneous ice nucleation in the tropical tropopause layer, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8402, https://doi.org/10.5194/egusphere-egu22-8402, 2022.

Atmospheric aerosols can act as ice nucleating particles (INPs) and thereby influence the formation and the microphysical properties of cirrus clouds, resulting in distinct climate modifications. From laboratory experiments several types of aerosol particles have been identified as effective INPs at cirrus conditions. However, the understanding of the global atmospheric distribution of INPs in the cirrus regime is still highly uncertain as in situ observations are scarce and limited in space and time.

We perform global model simulations with the ECHAM/MESSy Atmospheric Chemistry (EMAC) general circulation model including the aerosol microphysics submodel MADE3 (Modal Aerosol Dynamics model for Europe, adapted for global applications, third generation) coupled to a two-moment cloud microphysical scheme and a parametrization of aerosol-induced ice formation in cirrus clouds. We present a global climatology of INPs in the cirrus regime, that includes, besides mineral dust and soot, also crystalline ammonium sulfate and glassy organics as INPs at cirrus conditions. The model representation of ammonium sulfate and organic ice nucleating particles includes a formulation of the particle phase state, as recent laboratory measurements suggest that only crystalline ammonium sulfate and glassy organics initiate ice nucleation.  

After implementing the different INP types into the microphysical cirrus cloud scheme, their ice nucleation potential at cirrus conditions is analysed, considering the possible competition mechanisms between different INPs. The simulated INP concentrations in the range of about 1 to 100 L⁻¹ agree well with in situ observations and other global model studies. Our model results suggest that glassy organic particles probably have only minor influences, as ambient conditions often inhibit the glassy phase. On the other hand, crystalline ammonium sulfate often shows large INP concentrations, has the potential to influence ice nucleation, and should therefore be taken into account in future model applications.

How to cite: Beer, C., Hendricks, J., and Righi, M.: A global climatology of ice nucleating particles derived from model simulations with EMAC-MADE3, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8134, https://doi.org/10.5194/egusphere-egu22-8134, 2022.

The representation of Arctic mixed-phase clouds (MPCs) in global climate models (GCMs) is becoming a widely acknowledged challenge, which highlights the necessity of revisiting the microphysical parameterizations associated with this type of clouds. The relatively sparse ice-nucleating particles (INPs) in the Arctic region (Wex et al., 2019) cannot always account for the high ice crystal number concentrations (ICNCs) found in Arctic MPCs. This indicates the presence of additional ice multiplication processes, known as secondary ice production (SIP), that can rapidly enhance the few primary ice crystals (e.g., Korolev and Leisner, 2020). All GCMs include parameterizations of primary ice production (PIP), but they still lack description of some important SIP processes.

In this study we propose a new approach towards parameterizing SIP in polar stratiform clouds. The new parameterization encompasses the use of the ice-enhancement factor (IEF), which is a multiplication factor applied to primary ice crystals, to consider the effect of the three most important SIP mechanisms, namely the Hallett-Mossop (HM), the ice-ice collisional break-up (BR) and the droplet-shattering (DS) process. The derivation of the IEF parameterization is based on two-year regional climate simulations over the Ny-Ålesund station performed by the mesoscale Weather Research and Forecasting model (WRF) with augmented cloud microphysics (Sotiropoulou et al., 2021; Georgakaki et al., 2021) to account for all the SIP mechanisms. The WRF simulations indicate that the mean production rates of SIP can be up to 5 orders of magnitude higher than PIP at warm subzero temperatures higher than -10 ˚C. The production of secondary ice particles in the simulated Arctic clouds is found to be dominated by the BR process, with the contribution of DS and HM being substantially smaller. Machine learning techniques are then used to automatically detect patterns in the WRF dataset and to extract a parameterized expression of the IEF as a function of key thermodynamic and microphysical parameters. The newly developed formulation can effectively be implemented in GCMs with double-moment representations of the ice hydrometeors, which is expected to improve the modeled liquid-ice phase partitioning and therefore, the representation of radiation patterns and precipitation processes.

Georgakaki, P., Sotiropoulou, G., Vignon, É., Billault-Roux, A.-C., Berne, A., and Nenes, A.: Secondary ice production processes in wintertime alpine mixed-phase clouds, Atmos. Chem. Phys. Discuss, https://doi.org/10.5194/acp-2021-760, in review, 2021.

Korolev, A. and Leisner, T.: Review of experimental studies of secondary ice production, Atmos. Chem. Phys., 20, 11767–11797, https://doi.org/10.5194/acp-20-11767-2020, 2020.

Sotiropoulou, G., Vignon, É., Young, G., Morrison, H., O'Shea, S. J., Lachlan-Cope, T., Berne, A., and Nenes, A.: Secondary ice production in summer clouds over the Antarctic coast: an underappreciated process in atmospheric models, Atmos. Chem. Phys., 21, 755–771, https://doi.org/10.5194/acp-21-755-2021, 2021.

Wex, H., Huang, L., Zhang, W., Hung, H., Traversi, R., Becagli, S., Sheesley, R. J., Moffett, C. E., Barrett, T. E., Bossi, R., Skov, H., Hünerbein, A., Lubitz, J., Löffler, M., Linke, O., Hartmann, M., Herenz, P., and Stratmann, F.: Annual variability of ice-nucleating particle concentrations at different Arctic locations, Atmos. Chem. Phys., 19, 5293–5311, https://doi.org/10.5194/acp-19-5293-2019, 2019.

How to cite: Georgakaki, P., Sotiropoulou, G., and Nenes, A.: Parameterizing secondary ice production in Arctic mixed-phase clouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11263, https://doi.org/10.5194/egusphere-egu22-11263, 2022.

Antarctic Peninsula climate is a very sensitive system that is strongly affected by the temperature increase, compared to other Antarctic regions. Moisture transport from lower latitudes influences this region indirectly through precipitation, radiative forcing, and heat advection. Our understanding of the factors responsible for the enhanced moisture transport and its impact on the surface and energy balance of the Antarctic Peninsula is incomplete, particularly, regarding cloud and precipitation microphysics and their temporal evolution.  

The goal of this study is to investigate the temporal and spatial evolution of cloud properties during high-intensity precipitation events with phase transition, associated with an atmospheric river event over the Antarctic Peninsula in April 2021. Our analysis is based on PolarWRF simulations, precipitation properties derived from MRR-Pro measurements, hourly observations at Vernadsky station, ERA-5 reanalysis and GFS forecast.

We run simulations with PolarWRF forced with ERA-5 reanalysis and compare the simulation results with ground-based meteorology observations and measurements, conducted during the seasonal expedition at Vernadsky station in April 2021. Polar-WRF configuration included 3 domains with 9, 3, and 1-km spatial resolution, centered over the Vernadsky station with two double-moment cloud microphysics parameterization schemes: Morrison and Thompson. From Polar-WRF simulations we  analyse the following characteristics: radar reflectivity, vertical and horizontal components of wind speed, temperature, cloud top temperature and water content, mixing ratio and number concentrations of ice, snow, and rain. We focus our analysis on two vertical cross-sections, which represent the properties of the main atmospheric river flow. “Perpendicular” to the flow cross-section passes over Anvers Island and Kyiv Peninsula. “Parallel” the flow cross-section passes over the Akademik Vernadsky station, the mountains of Antarctic Peninsula and the Larsen B ice shelf.   

We analyze two cases with observed intense precipitation with phase transition during the first days of April 2021. The first intense rain event was associated with a cyclone, centered over the Amundsen Sea and reaching up to tropopause (about 10km). The second intense precipitation event with precipitation phase transition was associated with moisture intrusion from extratropical latitudes, possibly atmospheric river, in combination with a shallow cyclone centered over 64.53° S, 76.25° W, and height up to about 3km. High precipitation intensity and temperature increase were observed during both events. 

Comparison with observation and measurements at Vernadsky station shows a good agreement in precipitation phase and the timing of its transitions. Polar-WRF simulations showed  development of strong updrafts and downdrafts due to the orographic effect during both precipitation events. Temperature and reflectivity profiles confirm that precipitation originated from  mixed-phase clouds.  High intensity of precipitation could be connected to the high intensity of the crystal growth due to the Findeisen-Bergeron process, while the temperature  was -10⁰.. -12 ⁰C up to 4 km high. This information is difficult to verify due to a lack of vertical measurements such as radiosounding, etc. However, it gives some understating about atmospheric flow transformation during intense precipitation events in the Northern Antarctic Peninsula.

How to cite: Chyhareva, A., Krakovska, S., Palamarchuk, L., and Gorodetskaya, I.: Mixed cloud properties during high-intensity precipitation events over Northern Antarctic Peninsula, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-10388, https://doi.org/10.5194/egusphere-egu22-10388, 2022.

Mixed-phase clouds can be found at temperatures between 0 and -40°C and consist of supercooled cloud droplets and ice crystals. Their formation is triggered by different processes forming or introducing ice crystals in a supercooled cloud. Once ice crystals are present they grow at the expense of the cloud droplets due to the Wegener-Bergeron-Findeisen process, which causes a partial or complete glaciation of the cloud. Secondary ice processes can accelerate the glaciation.

In the global climate model ECHAM-HAM there are three different trigger processes, which introduce initial ice crystals into a supercooled cloud: heterogeneous ice nucleation, sedimentation of ice crystals from upper cloud layers, e.g. cirrus clouds, and vertical transport (vertical diffusion) of ice crystals. The aim of our study is to analyze the importance of each process in ECHAM-HAM. We investigated the role of all processes by conducting an ensemble of simulations where individual or combinations of processes are turned on or off. The outcome was analyzed with the factorial method using the supercooled liquid fraction of a mixed-phase cloud as a tracer for the microphysical structure. The analysis shows that sedimentation of ice crystals is crucial for mixed-phase clouds in ECHAM-HAM. Ice nucleation seems only to be an important trigger process if there are no ice crystals sedimented from above. However, even then sedimentation is important to distribute the freshly nucleated ice crystals within the supercooled cloud.

How to cite: Ickes, L., Neubauer, D., and Lohmann, U.: What is triggering ice in mixed-phase clouds: A process analysison the importance of ice nucleation and sedimentation with ECHAM-HAM, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8879, https://doi.org/10.5194/egusphere-egu22-8879, 2022.

Tropical ice clouds play an important role in the energy balance of the tropical atmosphere, yet their modeling has been a challenge and feedback from high clouds in climate models is very uncertain. The new generation of Global Storm Resolving Models (GSRM) is capable of resolving convective and mesoscale motions globally, and therefore promises to greatly advance our understanding of tropical clouds. This new generation of model also creates new opportunities for comparison with measurements since their horizontal resolution is comparable to that of active sensor measurements. Here we show the value of metrics evaluating cloud fraction, ice mixing ratio and longwave cloud radiative heating to validate tropical ice clouds simulated by Global Storm Resolving Models using A-Train ice cloud retrieval products. For this purpose, we use the X-SHiELD experimental Cloud Resolving Model, developed at NOAA/GFDL, and observations based on the 2C-ICE and DARDAR products, with the addition of the 2B-FLXHR-LIDAR radiative transfer algorithm for the validation of broadband fluxes. We show that, by aggregating model output and measurements in ice water path – pressure space, biases in the ice distribution can be revealed, whereby the position of the anvils is too low in the model. Such biases point to deficiencies in the microphysics of cloud ice, are likely shared by other models using similar microphysics packages, and have important consequences on thermal emission properties.

How to cite: Bolot, M., Fueglistaler, S., Harris, L., Cheng, K.-Y., and Zhou, L.: Tropical ice clouds validation in Global Storm Resolving Models using active sensor retrievals, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11931, https://doi.org/10.5194/egusphere-egu22-11931, 2022.

Interactions between aerosols and clouds, as well as their radiative consequences, have been a long-standing problem to understand cloud physics as well as anthropogenic impacts on climate. Satellite-based investigations of the direct and indirect impact of aerosols on liquid clouds have led to significant progress in the understanding during the last decade. This is partly due to the emergence of adapted cloud properties provided by satellites, such as the droplet number concentration. Ice clouds have suffered from such adapted quantity for much longer, but solutions have recently been appearing.

This study investigates aerosol - ice clouds interactions using ice crystal number concentration (Ni) profiles from a lidar-radar dataset (DARDAR-Nice), used cojointly with with collocated aerosol information from the Copernicus Atmospheric Monitoring Service (CAMS) reanalyses. A multitude of cloud regimes, subdivided into seasonal and regional bins, are considered in order to disentangle meteorological effects from the aci signature. First results of joint-histograms between Ni and the aerosol mass show an overall positive sensitivity of Ni to the aerosols load. This response is particularly strong towards to cloud-top and flattens towards cloud-base, consistently with expectations for homogeneous nucleation processes. The response of the ice water content, in terms of adjustment to the initial aerosol perturbation as also quantified.

How to cite: Sourdeval, O., Gryspeerdt, E., Krämer, M., and Quaas, J.: Assessment of ice clouds - aerosol interactions in global satellite observations, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-12632, https://doi.org/10.5194/egusphere-egu22-12632, 2022.

Contrail cirrus represent the largest aviation radiative forcing (RF) component on climate. However, the evolution of individual contrails to embedded contrail cirrus and the difference of properties of contrail cirrus and natural cirrus clouds are still not completely resolved. The ML-CIRRUS (Mid Latitude Cirrus) campaign was motivated by these questions and deployed a comprehensive set of in situ and remote sensing instruments aboard the German HALO aircraft to investigate them.

This study shows findings concerning comparisons between contrail cirrus and natural cirrus through combining airborne in situ measurements and MSG (Meteosat Second Generation) satellite remote sensing as well as detailed radiative transfer model (RTM) simulations for one case over West of Ireland in the North Atlantic Region during the ML-CIRRUS experiment on 26 March 2014. CiPS (Cirrus Properties from SEVIRI) and APICS (Algorithm for the Physical Investigation of Clouds with SEVIRI) were developed to retrieve cloud properties using thermal and solar observations of MSG. Using the linear regression and a neural network, RRUMS (Rapid Retrieval of Upwelling irradiances from SEVIRI) is able to estimate outgoing longwave radiation (OLR) and reflected solar radiation (RSR) at top-of-atmosphere (TOA). Comparing remote sensing derived microphysical properties with airborne measurements, CiPS is sensitive to thin cirrus layers while APICS enhances the accuracy for higher optical thickness. As for radiative effects, a TOA RSR and OLR estimation method was developed based on RTM simulations exploiting in situ measurements, observations and ERA5 model atmospheric data for both cirrus and cirrus-free regions.

As the result we find, based on average values of in situ data along the HALO flight track, that the radii for contrail cirrus are about 27% smaller than those of natural cirrus. Particle sizes increase from contrails to embedded contrails and later decrease slightly in the subsaturated environment. The evolution of optical thickness from MSG appears to be controlled by ambient relative humidity, with higher values for embedded contrails than for contrails in supersaturated conditions and smaller values in subsaturated conditions. In general, TOA broadband irradiances estimated from our simulations compare well with RRUMS outputs and CERES/GERB products, indicating that our atmospheric models provide a good representation of reality and can thus be used to determine RF of the ice clouds probed during this flight. To this end, ice clouds are removed from the atmosphere input to the RTM to approximate the conditions unaffected by contrails, embedded contrails, and natural cirrus. The RF results indicate cirrus warming during the early morning period. Contrails net RF increases by a factor of 3.5 after evolving into embedded contrails. On average, the net RF of contrails and embedded contrails is more strongly warming than that of natural cirrus.

This study will possibly be of interest for related researches on assessing the climate impacts of natural cirrus and contrail cirrus and formulating mitigation options.

How to cite: Wang, Z., Bugliaro, L., Voigt, C., Schumann, U., Jurkat-Witschas, T., and Heller, R.: Observations of Microphysical Properties and Radiative Effects of Contrail Cirrus and Natural Cirrus over the North Atlantic, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6100, https://doi.org/10.5194/egusphere-egu22-6100, 2022.

Contrail cirrus, including line-shaped contrails, cause a net warming effect to the Earth’s climate. Of great importance to estimate their radiative effect is the coverage and mean optical thickness, which are closely associated with the conditions affecting the formation and microphysical properties of contrail cirrus.

This study focuses on contrail cirrus observations over central Europe and the Northeast Atlantic from the airborne ML-CIRRUS campaign in 2014. To identify contrail cirrus in the dataset of cloud observations, the following method is used: (1) the Schmidt-Appleman-Criterion is calculated, determining whether the environmental conditions are suitable for contrail formation, (2) an aircraft plume detection algorithm is adapted, identifying if a measured air mass originated from aircraft exhaust and (3) based on (1) and (2), statistical analyses are performed, resulting in a description of the general characteristics of contrail and natural cirrus.

Applying this method, not only are contrail cirrus and natural cirrus separated by their different microphysical properties (mean mass radius, ice crystal number and ice water content), but the favorable spatial occurrence conditions of contrail cirrus are also detected: Contrail cirrus occur with rather high frequency at the cruising altitude, where the atmospheric pressure ranges from 200 to 245 hPa (ambient temperature 207 – 218 K). Of particular interest is the occurrence of contrail cirrus in slightly ice-subsaturated environments, where the relative humidity with respect to ice (RH_ice) centers around 90 % instead of ice supersaturation as believed hitherto. This also differs from the in-cloud RH_ice centering at 100 % in natural cirrus. Inspecting the occurrence frequency of air masses with RH_ice > 90 % in comparison to RH_ice > 100 % from passenger aircraft observations above Europe and the North Atlantic during the IAGOS-MOZAIC period from 1995 to 2010, about 45 % of the air masses are prone to contrail cirrus formation instead of 30 % found in merely ice-supersaturated environments. Considering this finding in the routing of passenger flights, the avoidance of slightly ice-subsaturated to ice-supersaturated conditions might lead to a reduction of the occurrence of contrail cirrus and thus to a possible mitigation of their climate impact.

[Note: This work is carried out under the EU H2020 Research and Innovation Action “Advancing the Science for Aviation and Climate (ACACIA)”, funded by the European Union under the Grant Agreement No. 875036.]

How to cite: Li, Y., Mahnke, C., Rohs, S., Bundke, U., Spelten, N., Dekoutsidis, G., Groß, S., Voigt, C., Schumann, U., Petzold, A., and Krämer, M.: Observations of contrail cirrus in ice-subsaturated environments and implications for mitigating the climate impact of aviation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-7830, https://doi.org/10.5194/egusphere-egu22-7830, 2022.

Aviation outflow is the only anthropogenic source of pollution that is directly emitted into the upper troposphere. This emission has the potential to modify the cloudiness directly by forming linear contrails and indirectly by injecting aerosols, which can act as cloud condensation nuclei (CCN) and ice nucleating particles (INP). Contrail cirrus can persist either in cloud-free supersaturated air, increasing high-cloud cover or inside natural cirrus cloud, and therefore modifying the microphysical properties of already existing cirrus clouds. Even though the situation that an aircraft flies through a natural cirrus is one of the highly probable situations in the upper troposphere, its subsequent impact is unclear with the present state of knowledge. Quantifying such impact is necessary if we are to properly account for the influence of aviation on climate. One main limitation preventing us from better identifying these impacts is the lack of height resolved measurements inside the cirrus clouds.

In this study, we used new retrievals from combined satellite cloud radar and lidar (CloudSat/CALIPSO; DARDAR-Nice algorithm), which provide height resolved information of ice crystal number concentration, at intercepts between the CALIPSO ground track and the position of civil aircraft operating between the west coast of the continental United States (Seattle, San Francisco and Los Angeles) and Hawaii during 2010 and 2011 from an earlier study.

Comparing cloudy air behind the aircraft inside the flight track to the adjacent regions and to ahead of the aircraft revealed a notable difference in ice number concentration at 300 m to 540 m beneath the flight height. These differences are derived from the reduction of ice number concentrations as we proceed toward the cloud base in regions unaffected by aviation and the increase of ice crystals as we distance a few hundreds of meters beneath the flight level in the regions affected by aviation.

How to cite: Marjani, S., Tesche, M., Bräuer, P., Sourdeval, O., and Quaas, J.: Satellite observations of the impact of individual aircraft on ice crystal number in cirrus clouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-4967, https://doi.org/10.5194/egusphere-egu22-4967, 2022.

Climate model simulations of cloud radiative properties over the Southern Ocean (SO) show that clouds reflect too little solar radiation compared with observations. This results in large errors in the modelled sea surface temperature, atmospheric circulation and climate sensitivity. Low-level (LL) mixed-phase clouds (MPCs) in the cold sectors of extratropical cyclones are identified as the main contributor to the SO radiation bias.

In this study, LL clouds are investigated between 40°S and 82° S to provide a new insight into their geographical distribution, as well as their spatial and temporal variabilities. The methodology relies on DARDAR products which exploits the synergy of CALIPSO's lidar and CloudSat's radar space-borne remote sensing observations. Based on DARDAR cloud-type products, a cloud classification program was developed to establish cloud spatial and temporal distributions.  This study concerns all types of cloud, including MPCs and supercooled-water containing clouds. The mean seasonal LL cloud cover for 2007-2010 over oceans (including sea-ice) varies from 64.4% in winter to 68.4% in fall. Larger cloud covers are observed between 50°S and 65°S where clouds are present more than 80% of the time. Dividing the studied area into smaller regions allowed to extract homogeneous sectors in term of cloud coverage. This analysis draw attention on some regions, such as the Tasman Sea sector that undergoes the highest seasonal variations for MPC and USLC occurrence, and the Argentinian coasts that presents important differences with other regions at the same latitudes. Over the Southern Ocean, the Weddell Sea sector stands out with a relatively low LL cloud occurrence.

Statistical analyses were carried out to determine the influence of the meteorological and biological conditions on cloud occurrence. Even though air temperature drives all cloud-type occurrences, it was found that the lower-tropospheric stability (LTS) is a good predictor of ice-cloud occurrence between 40°S and 50°S, particularly. With biological activity, first results indicate strong correlations with cloud occurrence, where chlorophyll-a, nanophytoplankton and particulate organic carbon concentrations are investigated between 40°S and 60°S.

How to cite: Bazantay, C., Jourdan, O., Mioche, G., Delanoë, J., Cazenave, Q., Uitz, J., and Sellegri, K.: Variability of low-level clouds over the southern oceans, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9550, https://doi.org/10.5194/egusphere-egu22-9550, 2022.

Cirrus clouds have a large impact on the Earth’s climate system. Overall this impact is positive, but depending on their macrophysical and optical properties, the effect of single clouds can be quite different. Thus, cirrus clouds still introduce large uncertainties in climate change predictions. To gain better knowledge of the impact of cirrus clouds, it is of importance to study their macrophysical and optical properties and their dependence on formation processes, environmental conditions, and their evolution with time.
To improve our knowledge, the ML-Cirrus mission took place in March/April 2014. Research flights with the German research aircraft HALO; equipped with remote sensing and in-situ measurements, were performed over Central Europe and over the Northeast Atlantic Ocean. In this study we use measurements taken from the airborne LIDAR system WALES, which is a combined water vapor differential absorption and high spectral resolution lidar. Our main focus is on the humidity distribution within cirrus clouds and in the cloud-free air in their vicinity. For that we use Relative Humidity with respect to ice (RHi), calculated form the WALES water vapor measurements in a 2D field along the flight track together with ECMWF temperature data interpolated to the same grid. We identify cirrus clouds using the following criteria: a) backscatter ratio >= 3 b) linear depolarization ratio >= 20% and c) temperature < 235 K. We further split the cirrus clouds into two main categories according to their formation process: a) in-situ formed clouds and b) liquid-origin clouds. 
Overall, we find that, 34.1% of in-cloud data points are supersaturated with respect to ice. Supersaturation is also detected in 6.8% of the cloud-free data points. Regarding their vertical structure, most clouds have higher supersaturations close to cloud-top and become subsaturated near the cloud bottom. When the probability densities of RHi are calculated with respect to temperature, the in-cloud data points seem to have two peaks. One around 225K and close to saturation, RHi=100%. And a second one at colder temperatures around 215K and subsaturated, RHi = 90%. This means, that most cirrus clouds are measured either in a warmer saturated environment or a colder subsaturated environment. These two regions seem to represent the two cirrus cloud categories mentioned above. In-situ formed clouds are mostly cold and unsaturated, with RHi values below the lower threshold for heterogeneous nucleation. Liquid-origin clouds are usually warmer and supersaturated, with RHi values commonly up to the high threshold for heterogeneous nucleation. Finally, regarding the temporal evolution of cirrus clouds, we find that the vertical structure of RHi within the clouds is indicative of their life stage. RHi skewness tends to go from positive to negative values as the cloud ages. RHi modes are subsaturated in young clouds, supersaturated in mature clouds and return to subsaturated in dissipating clouds.

How to cite: Dekoutsidis, G., Groß, S., and Wirth, M.: A study of water vapor within and in the vicinity of cirrus clouds at mid-latitudes, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13370, https://doi.org/10.5194/egusphere-egu22-13370, 2022.

The sizes and number of cloud particles are crucial parameters that determine the physical and optical properties of clouds and with that their radiative feedback to climate. However, measurements of cloud particle size distributions (PSDs) are difficult to accomplish, because clouds are always located at a certain height in the atmosphere. In addition, the entire cloud particle size range cannot be covered with one instrument and also, an undisturbed sampling cloud particles across their entire size range has only been successful for about 15 years.
To build a larger data set of cloud PSDs, we have merged PSD measurements from 11 airborne field campaigns between 2008 and 2021 in tropical, mid-latitude and Arctic ice, mixed and liquid clouds, where we spend a total of 238 hours of measurement time in clouds during 163 flights, of which 131 hours in ice clouds, 62 hours in mixed clouds and 45 hours in liquid clouds. The cloud PSDs are from different instruments which do not
record particle sizes in equally sized intervals. Therefore, the cloud particle numbers are interpolated to a logarithmic equidistant size grid. From this synchronized data set it is now possible to derive not only averaged PSDs, but occurrence frequencies of particle sizes and numbers. We will present occurrence patterns of particle sizes and concentrations in mixed-phase and cirrus clouds in 10°C temperature intervals between -90 to 0°C.           

                                                                                      Cloud PSD heatmaps of cirrus and mixed phase clouds.

In this study we will also present  more detailed analyses of cirrus clouds by sorting the PSDs in three ranges of ice water content and temperatures, respectively. First results show that in thin cirrus - which are mostly of in-situ origin- the dominant ice particle size changes from small ice particles at low temperatures (~3-20μm diameter) to larger sizes in warmer cirrus (~20–200μm diameter). Thick cirrus, which are a mixture of in-situ and liquid origin, generally contain larger ice particles at all temperatures, the warmer the temperature, the larger ice particles appear in the PSDs.
These occurrence patterns of cloud particle sizes represent a valuable data set that can be used to validate and improve the representation of especially ice clouds in global climate models and in the  retrieval of satelllite-based remote sensing observations.

Accompanying presentations @ EGU 2022, AS 1.15:
– Spang, R., Krämer, M. and Spelten, N.: A database of microphysical and optical properties of
   thin to thick cirrus clouds derived from bimodal particle size distributions.
– Bartolome Garcia, I., O. Sourdeval, M. Krämer, R. Spang: Parametrization of in-situ cloud particle
   size distributions including small particles.

How to cite: Krämer, M., Nicole, S., Armin, A., and Reinhold, S.: Occurrence patterns of cloud particles sizes in cirrus and mixed-phase clouds, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5119, https://doi.org/10.5194/egusphere-egu22-5119, 2022.

The detailed information on the particle size distributions (PSDs) of ice clouds is essential for various topics of radiative transport in a cloudy atmosphere. However, retrieval of microphysical and optical properties from PSDs from remote sensing instruments are affected by a lack in the information content of the measurement quantities, which does not allow to retrieve all parameter of a PSD. Usually, cirrus PSD parameterizations based on in situ measurements are used to reduce the number of unknowns in the retrieval process. The same arguments are applicable for model calculation on the radiative impact of cirrus clouds, where cirrus can have a warming or cooling effect depending on their microphysical (size, number, and shape) and macrophysical  (thickness and height) properties. Detailed information on the PSD shape are essential to improve the retrievals with forward models, where usually a priori information on the shape of the PSD are required, and for radiative transfer calculation for the quantification of the cloud radiative effect of cirrus. 

Here, we will present a more detailed analysis of the PSD measurements compiled in a recent large database (see Krämer et al., 2022, EGU, AS 1.15). With 11 campaigns and 238 flight hours in cloud conditions the database is currently the most comprehensive datasets for studying PSD parameters and the potential importance of the bimodality of ice cloud PSDs. The PSDs are not affected by the so-called shattering effect and cover for all campaigns particle diameters down to 3 microns. 

The procedure to derive microphysical and optical properties from the measured PSDs is to select predefined ice water content (IWC) and temperature grids for computing mean conditions. The database covers IWC from 10^-6 to 1 g/m³ and is especially well-suited to investigate optically thinnest clouds hitherto not included in PSD data bases. Other gridding parameters have been also investigated, for example number density. An iterative approach for fitting bimodal lognormal functions to the measured PSD by minimizing a cost function have been applied to the data with overall good fitting results. Characteristics of the fitted PSDs and the corresponding microphysical and optical properties will be presented.    

How to cite: Spang, R., Spelten, N., and Krämer, M.: A database of microphysical and optical properties of thin to thick cirrus clouds derived from bimodal particle size distributions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-5382, https://doi.org/10.5194/egusphere-egu22-5382, 2022.

The Cirrus-HL field campaign with the German research aircraft HALO took place out of Oberpfaffenhofen, Germany, in the summer month June and July 2021. The main objective was to probe cirrus clouds in the mid-latitudes and arctic upper troposphere. We operated the cloud spectrometer NIXE-CAPS (Krämer et al., 2016, 2020) to measure size distributions and number concentrations (N_ice) of ice particles. In addition, the hygrometer FISH (Meyer et al., 2015) was installed to obtain water vapor mixing ratio outside of clouds and derive ice water content (IWC) from total water inside of clouds. As the IWC measurement from the total water instruments can only used as indicator aboard HALO (Afchine et al. 2018), we derive the IWC from the NIXE-CAPS particle size distributions in the size range 3 to 937 µm. Gas-phase water vapor concentration inside of clouds is provided by the SHARC hygrometer and additionally converted into relative humidity wrt. ice (RHi) for the analysis. In total, 28.2 hours (18.9 hours in Mid-latitudes (< 60°N) and 9.2 hours Arctic (> 60°N)) of measurements inside of cirrus clouds in the temperature range between 208-240K during the 23 science flights were conducted.

In this study, we analyze the humidity conditions inside and outside of cirrus clouds as well as the cirrus cloud properties in the two different geographical regions (Mid-latitude and Arctic) with a special focus on the appearing supersaturations (RHi > 100 %). Especially in the Arctic region we find higher supersaturations inside but also outside of cirrus clouds in contrast to the mid-latitudes. As the RHi and also the Nice inside of clouds depends on the vertical updraft we correlate these quantities with the measured vertical velocity and can find only a vertical updraft effect in the Mid-latitudes but not in the Arctic. However, the particle size distributions in the two regimes exhibit a clear difference with generally less and larger ice particles in the Arctic cirrus clouds. Homogeneous ice nucleation occurs typically at higher supersaturation compared to heterogeneous nucleation which means freezing of ice nucleating particles (INP). The observations could indicate the dominant role of homogeneous nucleation in the Arctic under low updraft unpolluted conditions (low INP concentration). The high supersaturations found outside of clouds further confirm this hypothesis, as heterogeneous nucleation typically occurs at lower supersaturations. In summary, we present in-situ observations with higher supersaturations in- and outside of cirrus clouds as well as small and large ice particles indicating a clean summer Arctic upper troposphere in contrast to the Mid-latitudes.

Acknowledgement: We would like to thank the two coordinators of the HALO missions Cirrus-HL, Christiane Voigt (DLR) and Tina Jurkat (DLR), for their efforts.

References:

• Afchine et al., AMT, doi.org/10.5194/amt-11-4015-2018.

• Krämer et al., ACP, doi:10.5194/acp-16-3463-2016.

• Krämer et al., ACP, doi.org/10.5194/acp-20-12569-2020.

• Meyer et al., ACP, doi:10.5194/acp-15-8521-2015.

How to cite: Rolf, C., Krämer, M., Spelten, N., Afchine, A., and Zöger, M.: Mid-latitude and Arctic supersaturations observed during Cirrus-HL, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11087, https://doi.org/10.5194/egusphere-egu22-11087, 2022.

The radiative energy budget in the Arctic undergoes a rapid transformation compared to global mean changes. Understanding the role of cirrus in this system is vital, as they interact with short- and long-wave radiation at the top of the tropopause, aside other indirect radiative effects through heterogeneous processes and interaction with humidity. Between autumn and spring, the presence of cirrus can be decisive as to a net gain or loss of radiative energy in the polar atmosphere. To improve modelling capabilities with respect to cirrus, their well observable radiative effect needs to be linked to the occupied atmospheric volume and microphysical properties, accessible through in-situ measurements.

In an effort to derive radiative properties of cirrus in a real scenario in this sensitive region, we use in-situ measurements of ice water content (IWC) performed during the POLSTRACC aircraft campaign in the boreal winter and spring 2015/2016 employing the German research aircraft HALO. A large dataset of IWC measurements of mostly thin cirrus at high northern latitudes was collected in the upper troposphere and also frequently in the lowermost stratosphere. From this dataset we selected vertical profiles that sampled the complete vertical extent of cirrus cloud layers. These profiles exhibit a vertical IWC structure that will be shown to control the instantaneous radiative effect both in the long and short wavelength regimes.

We perform radiative transfer calculations with the UVSPEC model from the libRadtran program package in a one-dimensional column between the surface and the top of the atmosphere (TOA), taking as input the IWC profiles, as well as the state of the atmospheric column (temperature, humidity, trace gases) at the time of measurement, as given by ECMWF IFS and CAMS products. In parameter studies, we vary the surface albedo and solar zenith angle in ranges typical for the Arctic region, we find the strongest (positive) radiative forcing of cirrus over bright snow, whereas the forcing is mostly weaker and even ambiguous over the open ocean in winter and spring. The vertical IWC structure over several kilometres in the vertical affects the irradiance at the TOA, at times by means of symmetrically or asymmetrically distributed effective radiative layers. A strong heating rate profile within the cloud drives dynamical processes and may contribute to the thermal stratification at the tropopause.

Our case studies highlight the importance of a detailed treatment of cirrus clouds for estimations of the radiative energy budget in the Arctic. Furthermore, as they still rely on various assumptions regarding ice crystal microphysics, they provide a path to further substantiate the results using recent observations from the dedicated DLR lead HALO mission CIRRUS-HL on cirrus in high latitudes.

How to cite: Marsing, A., Meerkötter, R., Heller, R., Jurkat-Witschas, T., Kaufmann, S., and Voigt, C.: Using in-situ measurements of ice water content to characterize the cloud radiative effect of Arctic cirrus, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-8114, https://doi.org/10.5194/egusphere-egu22-8114, 2022.