The uncertain response of clouds to global warming is a major contributor to the spread in climate sensitivity across climate models. Cloud feedback uncertainty is related to a limited understanding of the coupling between clouds, convection and the large-scale circulation. This session focuses on efforts to advance our understanding of the cloud-circulation coupling and its role in climate change. Contributions from dedicated field campaigns such as EUREC4A, from various observing platforms like ground-based and satellite remote sensing or in situ measurements, as well as modelling and theoretical studies are welcomed. We also invite abstracts focusing on the role of convective organization and precipitation in modulating the cloud-circulation coupling and cloud feedbacks.

Convener: Raphaela VogelECSECS | Co-conveners: Claudia AcquistapaceECSECS, Leif DenbyECSECS, Florian EwaldECSECS, Marcus KlingebielECSECS
| Attendance Thu, 07 May, 08:30–10:15 (CEST)

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Chat time: Thursday, 7 May 2020, 08:30–10:15

D3105 |
Bjorn Stevens, Sandrine Bony, David Farrell, Alan Blyth, Chris Fairall, Johannes Karstensen, Trish Quinn, Sabrina Speich, and Team Eurec4a

The EUREC4A field campaign took place during January and February 2020, in the lower trades of the northern tropical Atlantic, over and in the seas windward of Barbados.  The initial purpose of the campaign was to test hypothesized cloud responses underpinning large positive radiative feedbacks from the desiccation of marine shallow convection with warming. To do so EUREC4A built on a long-standing cooperation with the Caribbean Institute for Meteorology and Hydrology to collect long-term cloud observations. Its scope was subsequently expanded by the addition of many partners, with funding from a variety of additional EU and UK projects, and US participants through ATOMIC, to address many additional questions. These ranged from the role of fine-scale eddies and fronts on air-sea coupling, to the effects of meso-scale organization on cloud radiative effects, to the strength of aerosol cloud interactions, among others. Hundreds of scientists from nearly a dozen nations -- incorporating measurements from four large Research Vessels and five Research Aircraft, an advanced remote sensing ground station and a large number of autonomous vehicles in the air and sea -- combined their expertise  to develop an unusually comprehensive picture of the processes relevant to the lower atmosphere and the upper ocean in the lower trades. We share our first impressions from EUREC4A, its surprises, and its prospects for answering some of the riddles that motivated this tremendous and coordinated effort.

How to cite: Stevens, B., Bony, S., Farrell, D., Blyth, A., Fairall, C., Karstensen, J., Quinn, T., Speich, S., and Eurec4a, T.: EUREC4A: First Impressions, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6116, https://doi.org/10.5194/egusphere-egu2020-6116, 2020.

D3106 |
Geet George, Bjorn Stevens, Sandrine Bony, and Marcus Klingebiel

This study uses measurements from the Elucidating the Role of Clouds-Circulation Coupling in Climate, EUREC4A and the second Next-Generation Aircraft Remote Sensing for Validation, NARVAL2 campaigns to investigate the influence of large-scale environmental conditions on cloudiness. For the first time, these campaigns provide divergence measurements, making it possible to explore the impact of large-scale vertical motions on clouds. We attempt to explain the cloudiness through the varying thermodynamics and dynamics in the different environments.  For most of the NARVAL2 case-studies, cloudiness is poorly related to thermodynamical factors such as sea-surface temperature and lower tropospheric stability. Factors such as integrated water vapour and pressure velocity (ω) at 500 hPa and 700 hPa can be used to distinguish between actively convecting and suppressed regions, but they are not useful in determining the variation in cloudiness among suppressed cases. We find that ω in the boundary layer (up to ∼2 km) has a more direct control on the low-level cloudiness in these regions, than that in the upper layers. We use a simplistic method to show that ω at the lifting condensation level can be used to determine the cloud cover of shallow cumulus clouds. Thus, we argue that cloud schemes in models should not rely only on thermodynamical information, but also on dynamical predictors.

How to cite: George, G., Stevens, B., Bony, S., and Klingebiel, M.: Large-scale vertical motion and its influence on cloudiness, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4379, https://doi.org/10.5194/egusphere-egu2020-4379, 2020.

D3107 |
Transition of low clouds in the East China Sea and Kuroshio region in winter: A regional atmospheric model study
Jingchao Long, Yuqing Wang, Suping Zhang, and Jingwu Liu
D3108 |
Louise Nuijens, Irina Sandu, Beatrice Saggiorato, Hauke Schulz, Mariska Koning, Kevin Helfer, and Vishal Dixit

Despite playing a key role in the atmospheric circulation, the representation of momentum transport by moist convection (cumulus clouds) has been largely overlooked by the model development community over the past decade, at least compared with diabatic and radiative effects of clouds. In particular, how shallow convection may influence surface and boundary layer winds is not thoroughly investigated. In this talk, we discuss the role of convective momentum transport (CMT) in setting low-level wind speed and its variability and evaluate its role in long-standing wind biases in the ECMWF IFS model.

We use high-frequency wind profiling measurements and high-resolution large-eddy simulations to inform our understanding of convectively driven wind variability. We do this at two locations: in the trades, using wind lidar and radiosonde measurements from the Barbados Cloud Observatory and the intensive EUREC4A field campaign, and over the Netherlands, using an observationally constrained reanalysis wind dataset and large-eddy simulation hindcasts.

At both locations we use the data and model output to investigate whether CMT can be responsible for a missing drag near the surface in the IFS model. Namely, at short leadtimes, the model produces stronger than observed easterly/westerly flow near the surface, while “a missing drag” produces weaker than observed wind turning. Consequently, the meridional overturning circulation in both the tropics and midlatitudes is weaker in the IFS and in ERA-Interim and ERA5 reanalysis products.

Comparing simulated and IFS wind tendencies at selected grid points at the above locations, and by turning off the process of CMT by shallow convection in the model, we gain insight in the role of CMT in explaining wind biases. We find that CMT alone does not explain a missing drag near the surface. CMT often acts to accelerate winds near the surface. But CMT plays a role in communicating biases in cloud base wind speeds towards the surface. In the trades, a strong jet near cloud base is determined by thermal wind and a strong flux of zonal momentum through cloud base, where “cumulus friction” minimizes. Near this jet, the presence of (counter-gradient) turbulent momentum fluxes produces most of the drag. Implications of these findings for CMT parameterization are discussed.

How to cite: Nuijens, L., Sandu, I., Saggiorato, B., Schulz, H., Koning, M., Helfer, K., and Dixit, V.: Convectively driven wind variability in connection to wind biases in the ECMWF operational weather model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21249, https://doi.org/10.5194/egusphere-egu2020-21249, 2020.

D3109 |
Sabrina Schnitt, Ulrich Löhnert, and Rene Preusker

Understanding atmospheric processes, such as e.g. cloud and precipitation formation, requires high-resolution water vapor and temperature profile observations particularly in the cloudy boundary-layer. As current observation techniques are limited by low spatial or temporal resolution, the potential of combining microwave radiometer (MWR) with differential absorption radar is investigated by analysing the retrieval information content and retrieval uncertainty. Two radar frequency combinations are analyzed: Ka- and W-band (KaW), available at e.g. Barbados Cloud Observatory, as well as a synthetic combination of G-band frequencies (167 and 175 GHz, G2), simulated using the Passive and Active Microwave TRAnsfer model PAMTRA.

The novel synergistic retrieval approach is based on an optimal estimation retrieval scheme. The absolute humidity profile is retrieved from the MWR K-band brightness temperatures, as well as the Dual-Wavelength Ratio (DWR) signal of the two radars. Evaluating a suite of radiosonde profiles measured at Barbados from 2018, adding the active KaW combination to K-band MWR brightness temperatures increases the information content for the retrieved profile from 3.2 to 3.4 degrees of freedom for signal (DoF). The usage of the higher G2 radar frequencies leads to higher Dual-Wavelength Ratios (DWRs), and, in combination with the MWR, to increased DoF (4.5), decreased retrieval errors, and a more realistic retrieved profile within the cloud layer. Information partitioning among MWR and the radars makes the synergy particularly beneficial: the profile below and within the cloud is restricted by the radar observations, whereas the water vapor above cloud top and the LWP are constrained by the MWR.

Based on selected case studies with single- as well as multi-layered clouds from the EUREC4A campaign, different retrieval configurations will be evaluated based on the resulting retrieval error, as well as the Degrees of Freedom. Tools for customizing the retrieval to the trade wind driven atmosphere will be analyzed by e.g. constraining the humidity profile to saturation within the cloud layer, or making use of a direct inversion approach of the differential attenuation signals.

How to cite: Schnitt, S., Löhnert, U., and Preusker, R.: Combining Differential Absorption Radar and Microwave Radiometer for Water Vapor Profiling in the Cloudy Trade-Wind Environment, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12107, https://doi.org/10.5194/egusphere-egu2020-12107, 2020.

D3110 |
Victor Mayta and Angel Adames

In this work, the tropical wave precipitation-buoyancy relationship is revisited by analyzing 4-times daily wave-filtered brightness temperature, reanalysis, and radiosonde datasets over tropical South America during the wet season. Prior studies demonstrated that an integrated measure of buoyancy well-diagnoses precipitation over land and ocean. However, it is an open question whether the buoyancy-based approach can yield a robust relation to precipitation for equatorial wave disturbances. To advance our understanding of this relationship, a comprehensive analysis of their vertical thermodynamic structure and potential interactions with the basic state is also presented. An emphasis is placed on understanding the convection coupling mechanism in convectively coupled Kelvin and inertia-gravity waves. It will be shown that buoyancy is a better predictor of convection for these disturbances than the often-used moist static energy (MSE). Examination of this discrepancy reveals that a cooling of the lower troposphere by gravity wave motions, which reduces MSE, is key to the production of precipitation in these disturbances.

How to cite: Mayta, V. and Adames, A.: Tropical precipitation–buoyancy relationship in Convectively Coupled Waves over South America region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11137, https://doi.org/10.5194/egusphere-egu2020-11137, 2020.

D3111 |
Bithi De and George Tselioudis

Recent analyses of Coupled Model Intercomparison Project phase 6 (CMIP6) models have shown higher climate sensitivities than previously reported, and this increase has been preliminary attributed to the simulation of anomalous Shortwave Cloud Radiative Effect (SWCRE) over the southern midlatitude regions. In this work, we further explore how the seasonal and annual SWCRE over different regions of the globe influence the model climate sensitivities. Our study suggests a significant contribution of SWCRE on climate sensitivities in both northern and southern midlatitudes; and the relationship remains robust across the seasons. Additionally, we assess the underlying physics of the inter-model spread to diagnose model biases. The results will contribute to quantify the severity of the Equilibrium Climate Sensitivity, as simulated by the CMIP6 models.

How to cite: De, B. and Tselioudis, G.: Regional and Seasonal Influence of Cloud Radiative Effects on CMIP6 Model Climate Sensitivity, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12467, https://doi.org/10.5194/egusphere-egu2020-12467, 2020.

D3112 |
Sandrine Bony, Hauke Schulz, Jessica Vial, and Bjorn Stevens and the EUREC4A team

Trade-wind clouds exhibit a large diversity of spatial organizations at the mesoscale. Over the tropical western Atlantic, a recent study has visually identified four prominent mesoscale patterns of shallow convection, referred to as Flowers, Fish, Gravel and Sugar. By using 19 years of satellite and meteorological data, we show that these four patterns can be identified objectively from satellite observations, and that on daily and interannual timescales, the near-surface wind speed and the strength of the lower-tropospheric stability discriminate the occurrence of the different organization patterns. Moreover, we point out a tight relationship between cloud patterns, low-level cloud amount and cloud-radiative effects. The EUREC4A field study taking place upwind of Barbados in Jan-Feb 2020 offers an opportunity to investigate these relationships from an in-situ and process-oriented perspective. Preliminary results will be discussed.

How to cite: Bony, S., Schulz, H., Vial, J., and Stevens, B. and the EUREC4A team: Dependence of mesoscale patterns of Trade-wind clouds on environmental conditions: an investigation using satellite and in-situ observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4622, https://doi.org/10.5194/egusphere-egu2020-4622, 2020.

D3113 |
Jessica Vial, Hauke Schulz, and Raphaela Vogel

Oceanic shallow convective clouds, which prevail in the trade-wind regions, have long been of great interest, because they strongly impact climate on a wide range of scales and they are critical in the estimation of the magnitude and pace of global warming. But surprisingly, the most fundamental mode of tropical variability, that is the daily cycle, has received very little attention for this cloud category, so that our knowledge of the diurnal processes in this oceanic shallow cumulus regime and their influence on climate at broader scales remains extremely limited. We recently relaunched the exploration of this topic. New investigating tools have been used, including large-eddy simulations run over large domains in realistic configurations and in-situ observations from the Barbados Cloud Observatory, which have helped study this daily cycle in the North Atlantic trade-wind region with a lot more details than was possible 40 years ago when it was first documented. Important features of this daily cycle have been found, which can have far reaching implications for climate change studies. Our hypothesis is that understanding the processes that control trade-wind cumuli on the diurnal timescale will benefit to our understanding of the mechanisms that are involved in the tropical marine low-level cloud feedbacks. In this regard, the wealth of observational data that will be collected during the EUREC4A campaign is unprecedented and offers a tremendous opportunity to enrich the characterisation and understanding of the mechanisms of the trade-wind daily cycle. Preliminary results will be discussed with a focus on the role of the shallow convective mixing and mesoscale organization in the daily cycle of trade-wind cumuli.

How to cite: Vial, J., Schulz, H., and Vogel, R.: On the understanding of the trade-wind cumuli daily cycle: the role of convective mixing and mesoscale organization of convection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9807, https://doi.org/10.5194/egusphere-egu2020-9807, 2020.

D3114 |
Christoph Heim, Laureline Hentgen, Nikolina Ban, and Christoph Schär

Even though the complexity and resolution of global climate models (GCMs) has increased over the last decades, the inter-model spread of equilibrium climate sensitivity has not narrowed. The representation of subtropical low-level clouds and their associated radiative feedbacks in climate models still poses a major challenge. A fundamental problem underlying the simulation of such clouds is their multiscale nature. On the one hand, current GCMs allow to capture the large-scale processes but are too coarse to represent the mesoscale and microscale dynamical processes governing their formation and dissipation. On the other hand, large eddy simulations (LES) resolving the micro scale are bound to small domains and thus lack a robust representation of the large-scale flow and the mesoscale organisation of the clouds. Convection-resolving models (CRMs) are an attractive compromise between the former two since they allow for simulations at much higher resolution than in conventional GCMs and on larger domains than in LES.

Here we analyse how CRMs simulate stratocumulus decks and investigate causes for inter-model differences. We consider a set of ten CRMs (nine GCMs that are run at convection-resolving resolution during a short time period as part of the pioneering DYAMOND initiative, and the limited area model COSMO run by ourselves) used to simulate stratocumulus clouds over the South-East Atlantic during a 40 day period. The simulations cover a range of horizontal grid spacings between 5 and 1 km.

We find pronounced differences in the mean cloud cover among the analysed CRMs. In comparison to observed radiation (CERES), most of them underestimate cloud cover, in particular the low-lying stratus decks close to the African coast. Nevertheless, the simulated mesoscale cloud organisation is realistic and similar in the set of CRMs, with few exceptions showing organisation on larger scales than in the other models. In general, the simulated cloud field appears to be more sensitive to the model choice than to the horizontal resolution.

Despite the differences in the cloud cover, most models capture the subtropical inversion and its spatial structure relatively well. Therefore, differences in the inversion strengths do not suffice to explain variability in the simulated cloud cover fraction between models. However, we find a relation between the mean height of the stratocumulus layer (or inversion layer) and its cloud cover fraction: Models with higher inversions tend to simulate a higher cloud cover fraction, bringing them closer to observations. Similarly, stronger vertical mixing within the boundary layer and enhanced surface latent heat fluxes appear to be related to higher cloud cover. Such relations may help to determine the physical processes responsible for the differences among CRMs in the simulated stratocumulus field.

How to cite: Heim, C., Hentgen, L., Ban, N., and Schär, C.: Simulation of subtropical marine stratocumulus clouds in convection-resolving models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9123, https://doi.org/10.5194/egusphere-egu2020-9123, 2020.

D3115 |
Eyal Ilotoviz, Shira Raveh-Rubin, and Virendra Ghate

Intrusions of dry air from the upper troposphere were recently suggested to reach the boundary layer and cause its significant deepening. Dry intrusions (DIs) are synoptic-scale slantwise descending airstreams from the midlatitude upper tropospheric jet towards the boundary layer at lower latitudes, thus acting as a circulation type potentially key for understanding boundary-layer cloud occurrence and regime transition. DIs occur mainly during winter over the mid-latitude oceanic storm track regions behind cold fronts trailing from cyclones. These regions are also home to marine boundary clouds that are an important component of the Earth’s radiation budget as they reflect much higher radiation back to the space compared to the ocean surface thereby cooling the Earth’s surface. Although subsidence is generally an inherent feature of the subtropical marine boundary layer, it is unclear how the marine boundary layer reacts to the transient, dynamically distinct DI, differently from the nominal subtropical subsidence resulting from the descending branch of Hadley circulation.

In this study we use the observations made at the Atmospheric Radiation Measurement (ARM) Eastern North Atlantic (ENA) site (39N, 28W) to characterize the impact of dry intrusions on Marine Boundary Layer (MBL) characteristics such as surface fluxes, thermodynamic stabilities and winds. Our analyses are based on measurements from the campaign: radiosondes, surface station data, polarimetric radar, lidar, radar wind profiler, ceilometer among others. Using all identified DI trajectories during the winters of 2016-2018 based on European Center for Medium-range Weather Forecasts (ECMWF) ERA Interim reanalysis data, we distinguish DI days from those before and following DIs, as well as periods with no DIs at all (with and without the occurrence of cold fronts for comparison). We find that during DI events the well-mixed MBL deepens and its vertical structure changes dramatically. Namely, the lower troposphere cools and dries substantially, inducing strong surface sensible and latent heat fluxes, while a strong inversion builds up at the MBL top, all affecting cloud occurrence. Finally, we used the numerical weather prediction (NWP) model COSMO at 2.2 km horizontal resolution to understand the detailed flows and structure in the MBL during DI events.

How to cite: Ilotoviz, E., Raveh-Rubin, S., and Ghate, V.: Impact of Dry Intrusions on the Marine Boundary Layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13042, https://doi.org/10.5194/egusphere-egu2020-13042, 2020.

D3116 |
Veronika Pörtge, Tobias Kölling, Tobias Zinner, Linda Forster, and Bernhard Mayer

The cloud droplet size distribution determines the evolution of clouds and their impact on weather and climate. First, droplet size determines
the cloud radiative effect. Second, evolution of clouds and formation of precipitation are determined by droplet size and the shape of the size distribution. Therefore, measurements of the size distribution are important to further our understanding of clouds and their role in the earth system. We present a remote sensing technique for droplet size and width of the size distribution based on polarized observations of the glory and the cloudbow.
Glory and cloudbow are caused by backscattering of sunlight by spherical droplets in liquid clouds. This backscattering results in colorful concentric rings surrounding the observer’s shadow; the formation is described quantitatively by Mie theory. The rings of the glory appear in an angular range of 170° – 180° scattering angle and the larger cloudbow rings in a range of about 130° – 160° . The angular radius of the rings is the most accurate and direct measure of the droplet size at cloud edge. In addition, the sharpness of the rings conveys information about the width of the droplet size distribution. The visibility of glory and cloudbow is significantly enhanced by the use of polarized observations which reduce the contribution of multiple scattering.
The specMACS sensor of LMU Munich has been upgraded recently by a polarization-sensitive wide-angle imager which was operated for the first time on the HALO aircraft during the EUREC4A campaign. The newly installed sensor offers a high spatial and temporal resolution, allowing to study small-scale variability of cloud microphysics at cloud top with a resolution of about 20 m. specMACS measurements and first retrieval results using the glory-cloudbow technique are presented.

How to cite: Pörtge, V., Kölling, T., Zinner, T., Forster, L., and Mayer, B.: Cloud Droplet Size Distributions from Observations of Glory and Cloudbow during the EUREC4A Campaign, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-18813, https://doi.org/10.5194/egusphere-egu2020-18813, 2020.

D3117 |
Tom Lachlan-Cope, Alan Blyth, Steven Boeing, Philip Rosenberg, Paul Barrett, Keith Bower, Michael Flynn, James Dorsey, Gary Lloyd, and Leif Denby

The EUREC4A project took place during January and February of 2020 and involved aircraft and ships from Germany, France, the United States of America and the United Kingdom. The aim of the project is to advance the understanding of the interplay between clouds, convection and circulation and their role in climate change. The Twin Otter belonging to the British Antarctic Survey (BAS) has been used to take observations of clouds and aerosols to the East of Barbados in conjugation with the French ATR-42 and the German Halo aircraft. Here we report the preliminary results of the observations made by the British aircraft. These observations will include aerosols from 10nm to 10micron and numbers of cloud condensation nuclei as well as detailed in-situ measurements of clouds microphysical properties. The observations have been taken over a one month period and taken as a whole can be used to provide a statistical view of the aerosols and clouds observed during EUREC4A by the BAS Twin Otter Aircraft.

How to cite: Lachlan-Cope, T., Blyth, A., Boeing, S., Rosenberg, P., Barrett, P., Bower, K., Flynn, M., Dorsey, J., Lloyd, G., and Denby, L.: Clouds and Aerosols observed during EUREC4A by the UK Twin Otter aircraft., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3667, https://doi.org/10.5194/egusphere-egu2020-3667, 2020.

D3118 |
Tran Vu La, Christophe Messager, Rémi Sahl, and Marc Honnorat

Convective Systems (CS) are dangerous weather events since they are associated with intense precipitation (up to 50 mm/hr) and strong surface winds (exceeding 20 m/s), for instance over the sea surface. Furthermore, they happen suddenly and evolve quickly, and thereby their effects on the sea surface are difficult to track and predict. Thanks to the geostationary meteorological satellites of METEOSAT (Europe), GOES (USA), and Himawari (Japan), the CS detection and tracking can be performed in most of the world with a 5-15-minute observation time sampling and about 2.8-km spatial resolution (up to about 1-km for the new–generation satellites). Indeed, the instruments onboard these satellites perform the CS detection based on the identification of deep convective clouds. The deeper the convective clouds, the lower the brightness temperature is. The highest (coldest) clouds have the lowest brightness temperature (200 K–205 K).

While the CS detection has been significantly improved for recent years thanks to the infrared images, the investigation of strong winds (or wind gusts) produced by the CS downdrafts hitting the sea surface did not progress a lot. It is mainly due to the lack of in-situ data and (especially) high-resolution remote sensing images. Some studies proposed the use of ASCAT scatterometers for the detection of surface wind patterns associated with the CS. However, the ASCAT only identified the mesoscale patterns (100–300 km) and failed to detect the convective-scale gust fronts (5–20 km), due to their large spatial resolution (12.5–25 km wind grid). To be able to observe both small- and large-scale surface wind patterns, Synthetic Aperture Radar (SAR) images are used in this study thanks to their high spatial resolution, wide swath, and availability in most weather conditions. Indeed, the obtained results in (La et al., 2018, 2020) illustrate that Sentinel-1 (C-band SAR) may detect surface wind patterns in shapes of a mesoscale squall line and sub-mesoscale convection cells. The associated wind intensity with the patterns exceeds 10–25 m/s.

To strengthen the assumption that the detected wind patterns on SAR images are produced by the CS downdrafts hitting the sea surface, we use the corresponding METEOSAT images for the detection of deep convective clouds (200 K–205 K brightness temperature). The comparisons between Sentinel-1 and METEOSAT images illustrate that surface wind patterns and deep convective clouds have a matching in spatial location (and sometimes in shape). In particular, the coldest spots of deep convective clouds correspond to the one with high wind intensity (15–25 m/s) of the patterns. This result thus permits to highlight a strong relationship between the detected wind patterns on the sea surface and the CS aloft.

How to cite: La, T. V., Messager, C., Sahl, R., and Honnorat, M.: Remote Sensing for Convective System Tracking and Associated Sea Surface Wind Pattern Detection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14102, https://doi.org/10.5194/egusphere-egu2020-14102, 2020.

D3119 |
Raphaela Vogel and Sandrine Bony

Most uncertainty in the warming response of trade-wind cumuli in climate models occurs near cloud base and is associated with model diversity in the strength of shallow convective mixing. In contrast to climate models, cloud-base cloudiness in large-eddy simulations (LES) and in observations is relatively insensitive to changes in the environment. The cumulus-valve mechanism provides a conceptual framework for understanding changes in cloud-base cloudiness in response to changes in the shallow-convective mass flux (M)—an important measure for convective mixing. The mechanism assumes that M keeps the mixed-layer top close to the lifting condensation level, which could explain a larger cloud-base cloudiness with larger M if the increase in M was mostly due to an increasing area fraction of cumuli. Here we use real-case LES over the tropical Atlantic to understand if cloud-base cloudiness increases with increasing M.

We find that M explains a lot of the variations in cloud-base cloudiness (correlation coefficient R=0.86), but the maximum relative humidity at the mixed-layer top (RHmax) needs to be considered additionally to explain the nighttime behavior of cloud-base cloudiness (R=0.95). The coupling of M and RHmax through adjustments in the sub-cloud layer depth is crucial for regulating cloud-base cloudiness. Inability of GCMs to adjust the sub-cloud layer depth in response to a change in M may likely contribute to their overestimated trade-cumulus cloud feedback. The simulated relationships will be compared to measurements from the EUREC4A field campaign.

How to cite: Vogel, R. and Bony, S.: A detailed look at the cumulus-valve mechanism and its potential implications for cloud-base cloudiness, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5975, https://doi.org/10.5194/egusphere-egu2020-5975, 2020.

D3120 |
Anna Lea Albright, Sandrine Bony, Jean-Louis Dufresne, and Jessica Vial

How will low-level clouds respond to global warming? We approach this question by first investigating the spread of climate sensitivity and cloud feedbacks in CMIP6 models. We stratify the cloud response by circulation regime and focus in greater detail on the cloud response in tropical regimes of subsidence and weak ascent  (i.e., their vertical structure in the present-day and future climate, how cloud profile changes relate to changes in cloud-controlling factors). This CMIP6 model analysis dovetails with an observational analysis of low cloud responses from the EUREC4A field campaign. We seek to employ a simple model of low cloud behavior, constrained with observations from EUREC4A and longer time series from the Barbados Cloud Observatory, to better constrain the range of low cloud behavior spanned by CMIP6 models. 

How to cite: Albright, A. L., Bony, S., Dufresne, J.-L., and Vial, J.: Low-level cloud feedbacks in CMIP6 models and EUREC4A observations , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4077, https://doi.org/10.5194/egusphere-egu2020-4077, 2020.

D3121 |
Marcus Klingebiel, Heike Konow, and Bjorn Stevens

Mass flux is a key parameter to represent shallow convection in global circulation models. To estimate the shallow convective mass flux as accurately as possible, observations of this parameter are necessary. Prior studies from Ghate et al. (2011) and Lamer et al. (2015) used Doppler radar measurements over a few months to identify a typical shallow convective mass flux profile based on cloud fraction and vertical velocity. In this study, we extend their observations by using long term remote sensing measurements at the Barbados Cloud Observatory (13° 09’ N, 59° 25’ W) over a time period of 30 months and check a hypothesis by Grant (2001), who proposed that the cloud base mass flux is just proportional to the sub-cloud convective velocity scale. Therefore, we analyze Doppler radar and Doppler lidar measurements to identify the variation of the vertical velocity in the cloud and sub-cloud layer, respectively. Furthermore, we show that the in-cloud mass flux is mainly influenced by the cloud fraction and provide a linear equation, which can be used to roughly calculate the mass flux in the trade wind region based on the cloud fraction.


Ghate,  V.  P.,  M.  A.  Miller,  and  L.  DiPretore,  2011:   Vertical  velocity structure of marine boundary layer trade wind cumulus clouds. Journal  of  Geophysical  Research: Atmospheres, 116  (D16), doi:10.1029/2010JD015344.

Grant,  A.  L.  M.,  2001:   Cloud-base  fluxes  in  the  cumulus-capped boundary layer. Quarterly Journal of the Royal Meteorological Society, 127 (572), 407–421, doi:10.1002/qj.49712757209.

Lamer, K., P. Kollias, and L. Nuijens, 2015:  Observations of the variability  of  shallow  trade  wind  cumulus  cloudiness  and  mass  flux. Journal of Geophysical Research: Atmospheres, 120  (12), 6161–6178, doi:10.1002/2014JD022950.

How to cite: Klingebiel, M., Konow, H., and Stevens, B.: Measuring the variability of the shallow convective mass flux profiles in the tropical trade wind region, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4780, https://doi.org/10.5194/egusphere-egu2020-4780, 2020.

D3122 |
Leonie Villiger, Franziska Aemisegger, Maxi Boettcher, and Heini Wernli

In the tropical winter trades of the North Atlantic in the vicinity of Barbados four different mesoscale organisation patterns of clouds – sugar, gravel, flower, fish - are observed regularly. Each pattern is associated with a distinct cloud amount and radiative footprint. Therefore, the relative occurrence frequency of these patterns affects the global radiative budget. As shown by a recent study (Bony et al. 2019, Geophysical Research Letter), the occurrence of the four patterns is controlled by the near-surface wind speed and the strength of lower tropospheric instability. It is however not yet clear, whether these cloud patterns occur preferably in specific larger-scale flow configurations. These can be associated for example with upper-level wave breaking in the extratropics and different positions and strengths of low-level subtropical anticyclones.

Lower tropospheric air parcels at different altitudes in the trades are expected to have different transport histories associated with distinct diabatic processes such as radiative effects, phase changes within and below clouds and turbulent mixing. The diabatic processes encountered during transport modulate the thermodynamic properties of the air parcels and therefore influence the vertical thermodynamic structure of the atmosphere in the trades.

In this study, the impact of large-scale air mass advection on the thermodynamic profiles over Barbados is analysed for each of the four mesoscale organisation patterns observed during EUREC4A. The airmass transport history is characterised for different homogenous atmospheric layers. These layers are identified based on vertical pseudo-soundings above the Barbados Cloud Observatory (BCO) using ECMWF analysis data for cases where profiles agree well with independent observations from balloon soundings. The large-scale circulation within the 10 days prior to the sounding is considered for computing the trajectories of the air masses arriving in these layers. Backward trajectories are calculated with three-dimensional analysis wind fields. Thereby, the thermodynamic history and large-scale circulation configuration associated with the four cloud organisation patterns is described from a Lagrangian perspective. In addition, composites of the sea level pressure field provide information whether the four patterns co-occur with systematically differing positions and/or intensities of subtropical anticyclones. In future work, stable water isotopes will be used as observational tracers to find supportive evidence of the characterised transport history.

How to cite: Villiger, L., Aemisegger, F., Boettcher, M., and Wernli, H.: The influence of the large-scale circulation on the thermodynamic profiles in the trades from a Lagrangian perspective , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4859, https://doi.org/10.5194/egusphere-egu2020-4859, 2020.

D3123 |
Tim Bates and Patricia Quinn

The fair-weather cumulus clouds, that cover much of the low-latitude oceans, affect the radiation balance of the planet by reflecting incoming solar radiation and absorbing outgoing longwave radiation.  These clouds also drive atmospheric circulation by mixing the lower atmosphere in a process called shallow convection.  This mixing, in turn, affects sea surface temperature and salinity by moderating the air-sea exchange of energy and moisture.  Marine boundary layer (MBL) atmospheric aerosols play a role in the processes described above by scattering and absorbing solar radiation and by serving as cloud condensation nuclei (CCN) thereby influencing cloud droplet concentrations and size; the extent, lifetime, and albedo of clouds; and the frequency and intensity of precipitation. Quantifying the role of aerosols over the Northwest Tropical Atlantic is critical to advance understanding of shallow convection and air-sea interactions.

MBL aerosol properties were measured aboard the RV Ronald H. Brown during the EUREC4A and ATOMIC field studies in January/February 2020.  Aerosols encountered during the study include background sulfate/sea spray particles and African dust/biomass burning particles.  Aerosol physical, chemical, optical and cloud condensation nuclei properties will be presented and their interaction with local and regional circulation.

How to cite: Bates, T. and Quinn, P.: Shipboard Measurements of Aerosol Properties in the Coupled Ocean-Atmosphere System of the Northwest Tropical Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5901, https://doi.org/10.5194/egusphere-egu2020-5901, 2020.

D3124 |
Leif Denby

The representation of shallow tradewind cumulus clouds in climate models accounts for majority of inter-model spread in climate projections, highlighting an urgent need to understand these clouds better. In particular their spatial organisation appears to cause a strong impact of their radiative properties and dynamical evolution. The precise mechanisms driving different forms of convective organisation which arise both in nature and in simulations are however currently unknown.

The EUREC4A field campaign presents an unprecedented opportunity to study these clouds by measuring simultaneously the ambient conditions (e.g. windshear, horizontal convergence, subsidence) and the cloud properties. Using an unsupervised neural network able to autonomously discover different patterns of convective organisation this work quantifies the ambient and cloud-properties present in differently organised regimes and in the transitions between these regimes.

The model is trained on GOES-R imagery of the tropical Atlantic. Spatial maps of convective organisation and temporal evolution of these will be presented together with large-scale influences on their development, helping unpick the dynamics of convective clouds in this region.

How to cite: Denby, L.: Unsupervised Classification of Convective Organisation in EUREC4A with Deep Learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20551, https://doi.org/10.5194/egusphere-egu2020-20551, 2020.

D3125 |
Claudia Acquistapace and Tobias Boeck

Trade wind cumulus clouds play a vital role in the Earth's radiation budget and produce up to 20% of the total precipitation in the tropics. However, we still don't know how they will respond to global warming. Precipitation from trade wind cumuli can alter cloud macroscopic properties and the boundary layer structure and dynamics.

Precipitation development in models is very uncertain, being dependent on simulation setup and microphysics. In particular, the autoconversion scheme dramatically affects precipitation flux, cloud structure, and organization. Currently, no evaluations of the different autoconversion schemes with observations reduced the uncertainties in rain processes. Precipitation can impact convection organization and circulation intensity with massive effects on climate sensitivity and its evaporation determines the intensity of cold pools and influences the cloud field organization. It is hence key to quantify evaporation rates and their spatiotemporal variability. Parametrizations of evaporation below cloud base are available but strongly depend on the drop size distribution of raindrops. Also, in the observations, evaporation rates are hard to observe directly.

Here, we would like to present the potential given by the observations collected on the Maria S. Merian ship during the EUREC4A campaign to estimate evaporation rates and provide advanced multi-sensor observations of rain onset and development. The synergy of multiple in-situ and remote-sensing from the ship as well as aircraft observations available will allow to constrain the autoconversion scheme in LES models and reduce the uncertainties connected to rain processes. Moreover, quantification of evaporation rates will clarify the role of precipitation in moisturizing the boundary layer in trade wind regions.


How to cite: Acquistapace, C. and Boeck, T.: Precipitation within EUREC4A: a multi-sensor ship-based approach to tackle warm rain processes, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6265, https://doi.org/10.5194/egusphere-egu2020-6265, 2020.

D3126 |
Yangze Ren and Huiwen Xue

Cloud feedback in mid-latitude marine stratocumulus is not clearly understood due to few reliable observations. Stratocumulus cloud is the most frequent and extensive cloud type over mid-latitude marine areas and has strong short-wave radiative effect. In this study, large eddy simulation (LES) is used to resolve the vertical structure of mid-latitude marine stratocumulus. We find that, in the wintertime over North Pacific, stratocumulus cloud often forms in regions of high pressure and large-scale sinking motion, and can remain in steady-state for a couple of days. We then choose two typical cases to do LES simulation: One has a lower cloud top height and a stronger temperature inversion (case l), without mesoscale cellular structure; the other has a higher cloud top height and a weaker temperature inversion (case h), with closed-cell cellular structure. The liquid water content profiles are adiabatic, and the boundary layer is well-mixed for both cases. In case l, the main source of turbulent kinetic energy (TKE) is from cloud top long-wave radiative cooling for the entire boundary layer. In case h, TKE production due to cloud-top longwave cooling is only significant in the cloud layer, and the subcloud layer TKE is mainly from surface processes.

How to cite: Ren, Y. and Xue, H.: The vertical structure of mid-latitude marine stratocumulus simulated by large eddy simulation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6361, https://doi.org/10.5194/egusphere-egu2020-6361, 2020.

D3127 |
Heike Konow, Marcus Klingebiel, and Felix Ament

Trade wind cumulus clouds are the predominant cloud type over the tropical Atlantic east of the island of Barbados. Parameters describing their macroscopic shape can help characterizing and comparing general features of clouds. This characterizing will indirectly help to constrain estimates of climate sensitivity, because models with different structures of trade wind cumuli feature different response to increased CO2 contents.

Two aircraft campaigns with the HALO (High Altitude LOng range) aircraft took place in the recent past in this region: NARVAL-South (Next-generation Aircraft Remote-Sensing for VALidation studies) in December 2013, during the dry season, and NARVAL2 in August 2016, during the wet season. During these two campaigns, a wide range of cloud regimes from shallow to deep convection were sampled. This past observations are now extended with observations from this year’s measurement campaign EUREC4A, again during the dry season. EUREC4A is endorsed as WCRP capstone experiment and the synergy of four research aircraft, four research vessels and numerous additional observations will provide comprehensive characterizations of trade wind clouds and their environment.

Part of the NARVAL payload on HALO is a 35 GHz cloud radar, which has been deployed on HALO on several missions since 2013. These cloud radar measurements are used to segment individual clouds entities by applying connected component analysis to the radar cloud mask. From these segmented individual clouds, macrophysical parameters are derived to characterize each individual cloud.

This presentation will give an overview of the cloud macrophysics observed from HALO during EUREC4A. Typical macrophysical parameters, i.e. cloud depth, cloud length, cloud fraction, are analyzed. We will relate these to observations from past campaigns and assess the representativeness of EUREC4A. As special focus of the EUREC4A campaign, measurements will be performed during different times of the day to detect diurnal cycles. Macrophysical parameters can be used to characterize changes over the day and cloud scenes of similar clouds types can be identified.

How to cite: Konow, H., Klingebiel, M., and Ament, F.: Cloud macrophysical properties from airborne observations during EUREC4A, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11106, https://doi.org/10.5194/egusphere-egu2020-11106, 2020.

D3128 |
Eleni Tetoni, Florian Ewald, Gregor Möller, Martin Hagen, Tobias Zinner, Christoph Knote, Bernhard Mayer, Qiang Li, and Silke Groß

Many studies have shown that multi-wavelength radar measurements can be valuable in inferring information about the size of observed hydrometeors in the atmosphere. Dual-wavelength radar method is widely known in such retrievals as it takes advantage of the different scattering behavior of hydrometeors in Rayleigh and MIE regime. Hydrometeors with sizes much smaller than the radar wavelength, act like Rayleigh scatterers and their radar reflectivity Z is proportional to the sixth power of their size. While these particles become larger due to riming or aggregation processes, with sizes comparable or larger than the radar wavelength, MIE effects can occur and thus, Z is proportional to the second power of their size. In the framework of IcePolCKa (Investigation of the initiation of Convection and the Evolution of Precipitation using simulatiOns and poLarimetric radar observations at C- and Ka-band) project, the evolution of ice in the precipitation formation will be studied exploiting these differences in both scattering regimes. Except for the logarithmic radar reflectivity difference, known as Dual-Wavelength Ratio (DWR) or Dual-Frequency Ratio (DFR), between C-band POLDIRAD weather radar from German Aerospace Center (DLR) in Oberpfaffenhofen and the Ka-band MIRA-35 cloud radar from Ludwig Maximilian University of Munich (LMU), other measured polarimetric variables from both radars, i.e. Differential Reflectivity (ZDR), Reflectivity Difference (ZDP), Linear Depolarization Ratio (LDR) will be also used. In addition to observations, scattering algorithms, i.e. T-matrix, will provide scattering simulations for a variety of ice particles shapes, sizes and mass-size relations. Combining DWR, polarimetric measurements and simulations the shape and/or the density of the observed ice particles will be retrieved. In this presentation, we will describe the instrumentation setup as well as the measuring methods in detail. Furthermore, we will present preliminary results of the retrieval approach using T-matrix calculations and measurements. Our first dataset consist of observations during snow events over Munich in January 2019 in order to avoid strong attenuation effects in the Ka-band.

How to cite: Tetoni, E., Ewald, F., Möller, G., Hagen, M., Zinner, T., Knote, C., Mayer, B., Li, Q., and Groß, S.: Investigating the contribution of polarimetry in retrieving ice microphysical properties using Dual-Wavelength radar observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13930, https://doi.org/10.5194/egusphere-egu2020-13930, 2020.

D3129 |
Theresa Mieslinger, Manfred Brath, Stefan A. Buehler, and Bjorn Stevens

The uncertain radiative effect of shallow cumulus clouds over tropical ocean significantly contributes to the high uncertainty in climate sensitivity estimates. Radiances corresponding to clear-sky and cloudy areas can be observed in moderate resolution satellite images. To observe the radiance originating from very small clouds and from the transition zone surrounding shallow cumulus clouds, the ‘twilight zone’, high-resolution data is required. Twilight zone radiances can be higher than clear-sky radiances due to unresolved cloud fragments and/or humidified aerosols. The area of the twilight zone depends on the resolution of the underlying data. If we think of the twilight zone in terms of partially cloudy pixels, such an area results in a high uncertainty in cloud and aerosol retrievals, as they are based on cloudy and clear-sky assumptions respectively. A precise knowledge of radiances from clouds and their twilight zone is decisive in terms of the total cloud reflectance and subsequently the shallow cumulus cloud radiative effect, which climate models struggle to properly simulate.

We therefore investigate the abundance and importance of such a twilight zone from high-resolution satellite images from ASTER recoded previously and during the EUREC4A field campaign. We use radiative transfer simulations to model the contribution of background clear-sky radiances including aerosols. Subtracting known clear-sky radiances from observed cloud field radiances leaves us with a precise knowledge of non-clear-sky radiances originating from shallow cumulus clouds and their surrounding twilight zone.

How to cite: Mieslinger, T., Brath, M., Buehler, S. A., and Stevens, B.: Identifying the radiative ‘twilight zone’ surrounding shallow cumulus clouds from high-resolution ASTER observations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19086, https://doi.org/10.5194/egusphere-egu2020-19086, 2020.

D3130 |
Tobias Kölling, Veronika Pörtge, Linda Forster, Tobias Zinner, Claudia Emde, and Bernhard Mayer

The EUREC4A field campaign, which takes place in January and February 2020 in the trade wind region east of Barbados, aims to Elucidate the Couplings Between Clouds, Convection and Circulation (Bony et al. 2017). For this field campaign, the hyperspectral imaging system specMACS (Ewald et al. 2016) has been equipped with additional color and polarization resolving cameras. The system is operated in downwards looking perspective on board the HALO research aircraft during this field campaign, aiming at the observation and characterization of clouds.

The combination of push-broom type spectral imaging sensors with two dimensional polarization resolving cameras offers new possibilities for cloud remote sensing. Using two dimensional images and stereographic techniques, the three dimensional structure of the cloud scene can be reconstructed (Kölling et al. 2019). The availability of a 3D model of the observed scene then allows to properly fuse passive observations from multiple sensors into a common data base. Additional information like cloud top height, cloud surface orientation, and an estimate of shadowed regions can aid previously available retrieval methods. Furthermore, the availability of polarization resolving images allows to strongly amplify the signal of single scattering processes. This and the large field of view of the two dimensional cameras largely improves the ability to derive cloud droplet size and width of the size distribution from the observation of cloudbows and glories (Mayer et. al. 2004, Pörtge 2019).

The poster will give an overview about the current instrument configuration and show data and first results from the EUREC4A field campaign.

Bony, S., Stevens, B., Ament, F. et al.: EUREC4A: A Field Campaign to Elucidate the Couplings Between Clouds, Convection and Circulation, Surv Geophys (2017) 38: 1529. https://doi.org/10.1007/s10712-017-9428-0

Ewald, F., Kölling, T., Baumgartner, A., Zinner, T., and Mayer, B.: Design and characterization of specMACS, a multipurpose hyperspectral cloud and sky imager, Atmos. Meas. Tech., 9, 2015–2042, https://doi.org/10.5194/amt-9-2015-2016, 2016.

Kölling, T., T. Zinner, B. Mayer, 2019, Aircraft-based stereographic reconstruction of 3-D cloud geometry, Atmos. Meas. Tech., 12, 1155-1166, https://doi.org/10.5194/amt-12-1155-2019, 2019.

Mayer, B., Schröder, M., Preusker, R., and Schüller, L.: Remote sensing of water cloud droplet size distributions using the backscatter glory: a case study, Atmos. Chem. Phys., 4, 1255–1263, https://doi.org/10.5194/acp-4-1255-2004, 2004.

Veronika Pörtge. Cloud Droplet Size Distributions from Observations of Glory and Cloudbow. Master’s thesis, Ludwig-Maximilians-Universität München, 11 2019.

How to cite: Kölling, T., Pörtge, V., Forster, L., Zinner, T., Emde, C., and Mayer, B.: The hyperspectral and polarization resolving imager specMACS during EUREC4A, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20300, https://doi.org/10.5194/egusphere-egu2020-20300, 2020.

D3131 |
Florian Ewald, Silke Groß, Martin Hagen, Tobias Kölling, and Bernhard Mayer

Clouds play an important role in the climate system since they have a profound influence on Earth’s radiation budget and the water cycle. Uncertainties in current climate models arise from a limited understanding of the coupling between cloud dynamics, cloud microphysics and, in turn, cloud radiative properties. Over decades, radiative properties of cloud tops were extensively studied using passive observations from multiple satellite missions. In recent years, our understanding of the inner workings of clouds has been greatly advanced by the deployment of cloud profiling microwave radars from low-earth orbit like CloudSat or the upcoming EarthCARE satellite mission. In order to exploit the future synergy between the cloud radar and the passive imager on EarthCARE, the scientific community is in dire need of collocated and spatially highly resolved measurements in advance of future spaceborne missions.  

In this context, the German research aircraft HALO is equipped with the high-power (30kW) cloud radar HAMP MIRA operating at 35 GHz and the hyperspectral imager specMACS (400 nm – 2500 nm). During the EUREC4A campaign, a number of flights were conducted over shallow marine boundary clouds in the vicinity of Barbados to collect simultaneous measurements with both instruments. For the first time, the spatial resolution of the Doppler velocity measurements from HALO now better match (<100 meter) the spatial resolution of the radiance imager, allowing for a more detailed separation of small up- und down-drafts.

In this presentation, we will give first impressions of these collocated, highly resolved radar-imager measurements of shallow marine boundary clouds during EUREC4A. On the basis of this data set we will try to answer the question if a correlation between the vertical Doppler velocity and the upwelling solar radiance for this kind of clouds can be observed. Such a relationship could prove valuable to assist synergistic retrievals (e.g. radar-lidar) in narrowing down the microphysical assumptions on which these retrievals rely upon. Furthermore, this data set could serve as a benchmark for cloud resolving modeling by constraining the relationship between cloud dynamics and radiation.

How to cite: Ewald, F., Groß, S., Hagen, M., Kölling, T., and Mayer, B.: Can we observe a correlation between vertical Doppler velocity and upwelling solar radiance for shallow marine boundary clouds?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13451, https://doi.org/10.5194/egusphere-egu2020-13451, 2020.

D3132 |
Higher climate sensitivity and cloud feedback in ECHAM6.3 with a prognostic cloud cover scheme due to changes in sub-grid scale dynamics
Steffen Muench and Ulrike Lohmann
D3133 |
Kumar Roy, Parthasarathi Mukhopadhyay, Ravuri Phani Murali Krishna, Bidyut Bikhash Goswami, and Boualem Khouider

Marine boundary layer (MBL) cloud is one of the major sources of uncertainty in the climate models and they have been identified in the Intergovernmental Panel on Climate Change’s (IPCC’s) fourth assessment as a primary source of uncertainty in determining the sensitivity of climate models. Further simulating it realistically is a huge challenge. To better represent organized convection in the Climate Forecast System version 2 (CFSv2), a stochastic multicloud model (SMCM) parameterization is adopted and it has showed promising improvement in different features of tropical convection. But the simulation of marine boundary cloud in CFSv2 SMCM (EXP1) is yet to be ascertained. We have calibrated the model by using radar observations and followed Markov-chain process to generate key parameters like transition probability, required for EXP1. This paper describes climate simulations of the EXP1 and 25 year run is made and last 20years are analysed. It replaces pre-existing convection scheme (CTL) and shows improvement in many aspects of climate compared to CTL. In addition, global distribution of MBL cloud is also improved and it is also with better agreement with observational analysis, which is inaccurate in CTL. Further, the transition from stratocumulus to trade cumulus is well simulated in EXP1. These results are also supported also by quantitative analyses like Root Mean Square Error (RMSE) etc. The improvement seen in EXP1 can be largely attributed to the general improved in the representation of shallow and cumulus clouds compared to CTL.

How to cite: Roy, K., Mukhopadhyay, P., Krishna, R. P. M., Goswami, B. B., and Khouider, B.: Evaluation of Marine Boundary layer cloud in the NCEP Climate Forecast System (Version 2) via Stochastic Multicloud Model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4067, https://doi.org/10.5194/egusphere-egu2020-4067, 2020.

D3134 |
Kevin Helfer, Louise Nuijens, Vishal Dixit, and Pier Siebesma

Motivated by the uncertain role of convective momentum transport from low clouds in setting patterns of wind in the trades, we discuss the impact of shallow convection on boundary-layer winds and its role in the overall momentum budget in the trades from large-domain large-eddy simulations. To this end, we analyse ICON-LEM hindcast simulations over the (sub)tropical North Atlantic during the NARVAL1 and NARVAL2 flight campaigns.

We describe that the character of the momentum flux profile differs significantly in regimes of shallow and deep convection and thus its influence on cloud-layer and near-surface winds. In particular, we establish that the momentum transport tendency is of similar importance as other terms in the momentum budget, and though the shape of the profile is remarkably insensitive to the horizontal resolution of the simulation, the relative role of subgrid and resolved fluxes changes with resolution. Furthermore, we find that counter-gradient transport occurs even in the absence of organisation, namely in the lower cloud layer, where cloudy updrafts carry slow momentum air upwards, which locally accelerates winds and may play a role at maintaining the cloud-base wind maximum.

How to cite: Helfer, K., Nuijens, L., Dixit, V., and Siebesma, P.: The role of convection in the momentum budget of ICON-LEM hindcasts over the North Atlantic, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4701, https://doi.org/10.5194/egusphere-egu2020-4701, 2020.

D3135 |
Hauke Schulz, Ryan Eastman, and Bjorn Stevens

Uncertainty in the response of clouds to warming is the leading source of uncertainty in projections of future warming. To a large fraction the frequently occurring shallow cumulus clouds in the trade wind region contribute to this uncertainty. In symbiosis with thin clouds of stratiform extent they often create various cloud patterns.

We introduce a neural network that is able to detect the mesoscale organization from GOES16 and MODIS satellite imagery in order to put eight years of ground-based measurements of the Barbados Cloud Observatory into the context of mesoscale organization. With this combination of long-term ground-based measurements from the trade-wind region and satellite image classifications, we overcome the common resolution limitations of satellite derived cloud products of shallow cumuli and are able to present the characteristics of shallow convection depending on the mesoscale organization with great detail.

By using back-trajectories and EUREC4A field campaign data, we show that differences in the atmospheric environment are not only present at the time of pronounced mesoscale organization, but are already distinguishable days ahead in LTS, wind speed and SST.

How to cite: Schulz, H., Eastman, R., and Stevens, B.: Evolution of Organized Shallow Convection, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6326, https://doi.org/10.5194/egusphere-egu2020-6326, 2020.

D3136 |
Nicole Albern, Aiko Voigt, David W. J. Thompson, and Joaquim G. Pinto

Clouds and the midlatitude circulation are strongly coupled via radiation. Previous studies showed that global cloud-radiative changes contribute significantly to the global warming response of the midlatitude circulation. Here, we investigate the impact of regional cloud-radiative changes and identify which regional cloud-radiative changes are most important for the impact of global cloud-radiative changes. We show how tropical, midlatitude and polar cloud-radiative changes modify the annual-mean, wintertime and summertime jet stream response to global warming across ocean basins. To this end, we perform global simulations with the atmospheric component of the ICOsahedral Nonhydrostatic (ICON) model. We prescribe sea surface temperatures (SST) to isolate the impact of cloud-radiative changes via the atmospheric pathway, i.e. changes in atmospheric cloud-radiative heating, and mimic global warming by a uniform 4K SST increase. We apply the cloud-locking method to break the cloud-radiation-circulation coupling and to decompose the circulation response into contributions from cloud-radiative changes and from the SST increase.

In response to global warming, the North Atlantic, North Pacific, Northern Hemisphere and Southern Hemisphere jet streams shift poleward and the North Atlantic, Northern Hemisphere and Southern Hemisphere jets strengthen. Global cloud-radiative changes contribute to these jet responses in all ocean basins. In the annual-mean and DJF, tropical and midlatitude cloud-radiative changes contribute significantly to the poleward jet shift in all ocean basins. Polar cloud-radiative changes shift the jet streams poleward in the northern hemispheric ocean basins but equatorward in the Southern Hemisphere. In JJA, the poleward jet shift is small in all ocean basins. In contrast to the jet shift, the global cloud-radiative impacts on the 850hPa zonal wind and jet strength responses result predominantly from tropical cloud-radiative changes.

The cloud-radiative impact on the jet shift can be related to changes in upper-tropospheric baroclinicity via increases in upper-tropospheric meridional temperature gradients, enhanced wave activity and increased eddy momentum fluxes. However, the response of the atmospheric temperature to cloud-radiative heating is more difficult to understand because it is modulated by other small-scale processes such as convection and the circulation. Our results help to understand the jet stream response to global warming and highlight the importance of regional cloud-radiative changes for this response, in particular those in the tropics.

How to cite: Albern, N., Voigt, A., Thompson, D. W. J., and Pinto, J. G.: Which regional cloud-radiative changes are most important for the global warming response of the midlatitude jet streams?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6982, https://doi.org/10.5194/egusphere-egu2020-6982, 2020.

D3137 |
Martin Hagen, Florian Ewald, Silke Groß, Qiang Li, Lothar Oswald, and Eleni Tetoni

Low-level clouds in the trade regions play an important role in the Earth’s climate system since they have a considerable influence on the Earth’s radiation budget. However, the understanding of the coupling between cloud dynamics, cloud microphysics, and mesoscale organization is limited. This results in a large uncertainty in current climate predictions. Despite the importance, observations in these regions are limited. Geostationary satellites cannot provide high resolution three-dimensional details of clouds and precipitation. Polar orbiting satellites like the A-Train satellites Cloudsat and Calipso or the upcoming EarthCARE satellite do provide detailed profiles of cloud properties, but the temporal evolution cannot be observed. On the other hand, long range weather radar observations can provide both, high spatial and temporal observations, however not many weather radar do cover the trades.

During the Eurec4a campaign DLRs C-band polarimetric weather radar POLDIRAD was installed on the island of Barbados. The scope of the radar measurements is manifold:

- POLDIRAD will provide high resolution observations of the different mesoscale cloud patterns as observed from satellites: Flowers, Gravel, Fish, and Sugar. Will the mesoscale organization have an influence on observable microphysical properties?

- POLDIRAD will put the detailed measurements by aircraft (in situ and remote sensing) into a greater context. How are the aircraft measurements related to the spatial distribution of the precipitation pattern? How are the aircraft measurements related to the temporal evolution of the precipitation pattern?

- POLDIRAD will put the observed profiles of clouds and precipitation at the Barbados Cloud Observatory BCO at Deebles Point into a greater context. How are the profile measurements related to the spatial distribution of the precipitation pattern? How are the profile measurements related to the temporal evolution of the precipitation pattern?

How to cite: Hagen, M., Ewald, F., Groß, S., Li, Q., Oswald, L., and Tetoni, E.: Characterizing precipitation in convective cloud regimes in the trades with the polarimetric C-band radar POLDIRAD, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12434, https://doi.org/10.5194/egusphere-egu2020-12434, 2020.

D3138 |
Ludovic Touzé-Peiffer, Nicolas Rochetin, and Raphaela Vogel

A considerable amount of literature has been devoted to the study of strong convective squall line. In particular, many studies have noted the role of cold pools on the persistence of these squall lines. Observations and simulations have shown that squall lines are often associated with pools of air cooled by partial rain evaporation. Such cold pools spread at the surface and may initiate new convective cells at their edges, thus contributing to the maintenance of a squall line. Under which environmental conditions the lifting at the edges of cold pools is most efficient has been subject to many debates. Yet, it is generally acknowledged that the environmental wind shear is a critical factor in this process. 

Recent observations and realistic simulations over the trade-wind region have revealed persistent structures of shallow cumuli associated with surface cold pools. We will call these structures shallow convective squall lines, due to their similarity with strong convective squall lines. Based on simulations from the German model ICON and on recent observations from the field campaign EUREC4A, we will study the characteristics of these shallow convective squall lines and their lifecycle. Similarly to strong convective squall lines, shallow convective squall lines organized around a leading edge composed by many updrafts and downdrafts feeding the surface cold pools. We will see that the environmental wind shear plays a key role in the persistence of these shallow convective squall line, and we will compare our findings with classical theories for strong convective squall lines.

How to cite: Touzé-Peiffer, L., Rochetin, N., and Vogel, R.: On the persistence of tropical shallow convective squall lines - the role of cold pools, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13370, https://doi.org/10.5194/egusphere-egu2020-13370, 2020.

D3139 |
Marcel Schröder, Freja Nordsiek, Oliver Schlenczek, Antonio Ibanez Landeta, Eberhard Bodenschatz, and Gholamhossein Bagheri

Clouds play a key role in the energy balance of the Earth's atmosphere and its radiation budget. The lack of detailed understanding of clouds is one of the reasons for the uncertainties in weather forecasting and climate modelling. The dynamics of clouds extend over a wide range of spatial and temporal scales from micrometers to km and milliseconds to hours. Besides Plinian volcanic eruptions, clouds show the highest turbulence level on earth. The multiscale properties of the turbulent flow in combination with moisture and temperature transport, phase transitions, and inertial particle dynamics present a challenge for modelling and parameterization.  Here we use a specially developed airborne platform, the Mini-Max-Planck-CloudKite (Mini-MPCK), to measure meteorological and cloud microphysical properties with high spatial and temporal resolution. The mini-MPCK is a 75 qm helium-filled balloon kite carrying a tether-mounted instrument for measuring atmospheric state parameters, and the density and size distribution of cloud particles. We will report on measurements from the trade wind region obtained during the EUREC4A campaign in Jan-Feb 2020.

How to cite: Schröder, M., Nordsiek, F., Schlenczek, O., Ibanez Landeta, A., Bodenschatz, E., and Bagheri, G.: Airborne Atmospheric Measurements with the mini Max Planck CloudKite, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22375, https://doi.org/10.5194/egusphere-egu2020-22375, 2020.