How to cite: Holloway, C., Pope, K., Stein, T., and Jones, T.: Radiation, Clouds, and Self-Aggregation in RCEMIP Simulations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-1071, https://doi.org/10.5194/egusphere-egu23-1071, 2023.

Most climate models show a precipitation increase with warming that is smaller than the increase in moisture, which requires a weakening of the convective mass flux and a slowing of the overturning circulation. In this study we use global-storm resolving models (DYAMOND models) to identify the systematic relationships between the precipitation, the vertical velocity and the overturning circulation in the tropics. The cloud-resolving simulations that are 40-day long in winter allow us to study the dynamical response of precipitation over a wide range of spatial scales. A data reduction and inference method, δ-MAPS, provides an efficient way to reduce the complexity and dimensionality of high-resolution simulations. We use the domains identified in 2d fields of atmosphere mass content of water vapor – interpreted as regions of homogeneous precipitable water – as preferential domains to derive the isentropic distribution of vertical mass transport and the isentropic streamfunction. The isentropic analysis consists in sorting the air parcels in terms of equivalent potential temperature, which offers a simple representation of the convective overturning. A multiscale decomposition allows us to quantify the contribution of the mesoscale circulation in comparison to the large-scale overturning circulation. Finally, the results are compared between the different DYAMOND models to evaluate the intermodel spread. By doing so, we evaluate to what extent the spread in precipitation in model ensemble may arise from the differences in representation of the overturning circulation at different scales.

How to cite: Ricard, L., Nenes, A., Stephan, C., and Falasca, F.: Reponse of precipitation to dynamics in global-storm resolving models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14787, https://doi.org/10.5194/egusphere-egu23-14787, 2023.

Cloud cover products from multiple satellite projects are long enough to provide a robust evaluation of climate models. Using global atmosphere models forced by observed sea-surface temperature and employing satellite simulator software, three generations of the Community Atmosphere Model are evaluated. This inter-generational comparison shows how the cloud radiative effect has improved through time but cloud cover has shown only modest improvements over the past decade. Diagnostics are introduced that allow a decomposition of spatial biases to separately evaluate systematic errors in the mean from the spatial variability. Errors in cloud properties are evaluated using a dynamical regimes analysis to connect the climatological errors to the large-scale circulation. Two closely related, current-generation models, CESM2 and E3SM, are compared to show how slightly different model development and tuning decisions can impact the the cloud climatology. Leveraging multiple long-term satellite data sets suggest that despite improvements through time, there remain significant systematic errors in cloud cover. It is suggested that simultaneously constraining cloud radiative effect and cloud cover, and therefore reducing the longstanding "too few, too bright" bias, is feasible and could improve climate projections. 

How to cite: Medeiros, B.: Satellite-based evaluation of cloud cover through three generations of global atmosphere models, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10934, https://doi.org/10.5194/egusphere-egu23-10934, 2023.

In January and February 2020, the joint EUREC⁴A/ATOMIC field campaign took place in the Tropical Atlantic near Barbados, with the goal to advance our understanding of the interplay between clouds, convection and circulation including their role in climate change. Within the scope of this campaign, several unique satellite-based datasets have been collected, including very high-spatial resolution multispectral images with 10x10m² pixel size acquired by polar-orbiting Copernicus Sentinel-2 satellites. In this presentation, the first analysis of these high-resolution observations focused on tropical trade cumuli is given. This cloud type is characterized by small-scale spatiotemporal variability that is unresolved at the spatial resolution of current meteorological satellite imagers.

Using the high-resolution Sentinel-2 observations, we will show the clouds can be considered as individual objects with associated properties, such as shape and size parameters, but also their mean radiative properties. A novel technique will be presented for deriving cloud height from Sentinel-2 observations, which exploits the geometric relationship between cloud objects and their shadows. Furthermore, the cloud fraction and cloud size distribution are calculated for various trade cumulus scenes. The uncertainties arising from choices in our cloud detection scheme will be discussed.

We show that a substantial fraction of clouds has equivalent diameters below the pixel size of commonly used meteorological satellite instruments. Consequently, and consistent with previous studies, the cloud size distribution and domain-average cloud fraction from coarse-resolution satellite imagers are shown to be biased and highly sensitive to pixel resolution. In addition, a large fraction of pixels identified as cloudy contains significant clear-sky contributions, and it is no longer possible to characterize clouds as objects. We will discuss how this affects the accuracy of cloud property determination and biases estimates of cloud radiative forcing.

How to cite: Ritter, O., Bley, S., and Deneke, H.: Object-based characterization of Tropical Trade Cumuli During the EUREC4A/ATOMIC Field Campaign using Sentinel-2 observations, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-13751, https://doi.org/10.5194/egusphere-egu23-13751, 2023.

In 2020, the EUREC⁴A (ElUcidating the RolE of Clouds-Circulation Coupling in ClimAte) field mission set out to investigate the role of shallow convective mixing in regulating trade cumulus and their influence on global climate. Recent results from this mission refute the idea that shallow convective mixing reduces cloudiness, as previous studies had argued. Instead, they suggest that shallow convective mixing is positively correlated with cloudiness when both are modulated by mesoscale circulations. Here, we provide independent evidence that further substantiates these findings. Using the unprecedented collection of water isotopic data sampled during EUREC⁴A, we derive estimates of total moisture exported from the sub-cloud layer by shallow convective mixing. We also derive vertical profiles of exported sub-cloud layer moisture, which allow us to investigate how shallow convective mixing alters the vertical structure of thermodynamic quantities and clouds. We show a strong association between the amount of moisture exported, the top-heaviness of the exported-moisture profile, the trade wind inversion height, and the average cloud top altitude. All increase when large cloud decks are present, indicating a role for mesoscale convective organization. We extend the analysis with remotely sensed isotope ratios in order to investigate the associations between mixing, moisture export, and cloudiness on larger scales (in both time and space) and to examine the conditions that favor convective organization at the mesoscale.

How to cite: Bailey, A., Noone, D., and Henze, D.: Moisture export by shallow convective mixing during EUREC4A, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10638, https://doi.org/10.5194/egusphere-egu23-10638, 2023.

Measurements over the north-Atlantic trade wind regions from the recent EUREC⁴A field campaign (ElUcidating the RolE of Cloud–Circulation Coupling in ClimAte) have revealed a large variability in mesoscale (ca. 200 km) vertical velocity. This variability is primarily attributable to shallow mesoscale overturning circulations (SMOCs) , which have also been shown to influence mesoscale moisture variability. From EUREC⁴A observations, it is found that the mesoscale horizontal divergence (D) averaged over the mixed layer covaries strongly with surface D. We exploit this finding by using satellite observations of surface divergence to understand SMOCs further. We mainly use WindSAT measurements for surface divergence due to its larger swath compared to scatterometers and its superior performance under precipitating conditions. In WindSAT observations, surface divergence shows a negative correlation with integrated water vapour (IWV), but also shows the large variance when IWV is large, indicating that there might be two regimes in the divergence-moisture relationship. To further investigate the divergence-moisture relationship, we perform object-identification in the IWV field and characterize surface divergence therein. A synergy with geostationary cloudiness fields from GOES-16 helps us further interpret these characteristics in the context of the spatial organization of clouds. As satellite observations go beyond the space-time coverage of field campaigns, we are able to document (a) statistics of the spatial scales of SMOCs as well as (b) some consistent relationships between SMOCs and atmospheric moisture.

How to cite: George, G., Bouniol, D., and Couvreux, F.: How shallow circulations couple to moisture in the trades - A perspective from satellites, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10014, https://doi.org/10.5194/egusphere-egu23-10014, 2023.

Mesoscale vertical velocity is a key element to understand links between clouds, radiation and circulation. However, in situ measurements of this variable remain sparse and costly.

Here, we present a method to estimate clear-sky free tropospheric mesoscale vertical motion from rapid-scan geostationary satellite radiance measurements in the water vapour absorption band. Subsidence is indeed associated with drying and a decrease of emission level height (and conversely for ascendance). Under basic physical assumptions, the associated temporal changes in brightness temperature can be quantitatively related to vertical velocity.

The retrieval method is evaluated against in situ observations from EUREC4A and OTREC field campaigns that sampled respectively the winter trades and the intertropical convergence zone. Although suffering from significant error bars (+/-4hPa/hr), retrievals are able to reproduce the general temporal evolution and spatial patterns of mid-tropospheric mesoscale vertical motion.

The retrieval method is further evaluated using kilometer-scale simulations associated with a radiative transfer code. Basic climatological features are captured such as the distribution of mesoscale vertical velocity or its influence on cloud cover.

Despite notable drawbacks, the method is able to provide time-continuous estimations of vertical velocity in clear sky regions of the whole tropical belt at the scale of the pixel of the satellite imager. First results suggest that mesoscale (20-200km) vertical velocity structures are ubiquitous in the tropical free troposphere.

These observations could prove valuable for studying dynamical links between deep convection and its environment, especially in association with the new generation of satellites, that will provide measurements of in-cloud convective mass flux and clear-sky time-continuous water vapour and temperature profiles.

How to cite: Poujol, B. and Bony, S.: A method to estimate clear sky mesoscale vertical motion from geostationary satellite imagery, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5009, https://doi.org/10.5194/egusphere-egu23-5009, 2023.

The WIVERN (WInd VElocity Radar Nephoscope) concept, now in Phase 0 of the ESA Earth Explorer program, promises to complement Doppler wind lidar by globally observing, for the first time, the vertical profiles of winds in cloudy areas. The mission will also strengthen the cloud and precipitation observation capability of the Global Observing System by providing unprecedented revisit time of cloud and precipitation vertical profiles. The mission hinges upon a single instrument, i.e., a dual-polarization Doppler W-band scanning cloud radar with a 3 m circular aperture non-deployable main reflector. The WIVERN antenna conically scans a large swath (of about 800 km) around nadir at an off-nadir angle of about 38^o at 12 rpm (revolutions per minute). This viewing geometry allows daily revisits poleward of 50°, 50-km horizontal resolution, and approximately 1-km vertical resolution. A key element is the use of closely spaced pulse pairs one of which is H polarised the other V polarised, so that the target does not have time to reshuffle, and, providing there is no significant cross-talk between the two returns, the high velocities associated with wind storms can be retrieved. 

In this paper we will discuss the scientific objectives of the mission and will outline some of the technical challenges of the measuring technique. In particular we will discuss how to correct for wind biases introduced by the satellite motion and wind shear across the beam, how to account for cross-talk between the H and V returns due to depolarisation by meteorological targets, how to calibrate the instrument and how to identify mis-pointing of the antenna that could affect Doppler accuracy. We will also present examples of Level 1 products via an end to end simulations applied to high resolution cloud resolving models and expected performances of the instrument in terms of cloud/precipitation and wind coverage.

How to cite: Battaglia, A., Illingworth, A., Tridon, F., Rizik, A., Martire, P., and Scarsi, F. E.: The WInd VElocity Radar Nephoscope (WIVERN): a candidate mission for the ESA Earth Explorer 11, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14814, https://doi.org/10.5194/egusphere-egu23-14814, 2023.

Shallow trade-wind cumulus clouds originate from thermals which rise from the turbulent subcloud layer and penetrate high enough to reach their lifting condensation level. Those thermals transport heat and moisture into the cloud layer. Analogously, the subject of such a transport can be the small-scale turbulence.

Turbulence measurements near the cloud base and in the subcloud layer were performed in the course of the EUREC4A field campaign by the ATR research aircraft in a large number of repeatable flight segments (Brilouet et al., 2021). In this study, we exploit this extensive dataset to derive properties of turbulence corresponding to short 'local' domains, of the size of the order of 100 m, e.g. turbulence kinetic energy dissipation rate, anisotropy and inertial range scaling. Taking advantage of the substantial amount of data provided by the EUREC4A measurements, we compare the statistics of those parameters between the areas inside cumulus clouds, outside them at the same altitude and at three levels inside the subcloud layer.

Such a comparison indicates that the character of small-scale turbulence inside cumulus clouds can be considered comparable to the one observed in the subcloud layer but significantly differs from that observed at the same level outside the clouds. As the cloud fraction at cloud base is typically rather a small number (about 4% during EUREC4A), it is in consequence inherently difficult for large scale models to accurately parameterize the intensity of turbulence and mixing in the trade-wind regime.

How to cite: Nowak, J., Wacławczyk, M., and Malinowski, S.: Turbulence properties inside and outside trade-wind cumulus clouds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-2791, https://doi.org/10.5194/egusphere-egu23-2791, 2023.

A shallow cumulus cloud transition from a sugar to flower type of organization occurred under a layer of mineral dust on 2^nd February 2020, during the multinational Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) and the Elucidating the Role of Clouds-Circulation Coupling in Climate (EUREC⁴A) campaigns. Lagrangian large eddy simulations following an airmass trajectory along the tradewinds are used to explore radiative impacts of the diel cycle and mineral dust on the sugar-to-flower (S2F) cloud transition. The large-scale meteorological forcing is derived from the ECMWF Reanalysis 5^th Generation (ERA5) and based on aerosol measurements from the U.S. Ronald H. Brown Research Vessel and the French ATR-42 Research Aircraft during the field campaigns. A 12-hr delay in the diel cycle accelerates the S2F transition at night, leading to more cloud liquid water and precipitation. The aggregated clouds generate more and stronger cold pools, which alter the original mechanism responsible for the organization. Although there is still mesoscale moisture convergence in the cloud layer, the near-surface divergence associated with cold pools transports the subcloud moisture to the drier surrounding regions. New convection forms along the cold-pool edges, generating new flower clouds. The modulation of the surface radiative budget by free-tropospheric mineral dust poses a less dramatic effect on the S2F transition. Mineral dust releases longwave radiation, reducing the cloud amount at night, and absorbs shortwave radiation during the day, cooling the boundary-layer temperature and increasing the overall cloud amount. Cloud-top radiative heating because of more clouds strengthens the mesoscale organization, enlarging the aggregate areas, and increasing the cloud amount further.

How to cite: Narenpitak, P., Kazil, J., Yamaguchi, T., Quinn, P., and Feingold, G.: The Sugar-To-Flower Shallow Cumulus Transition Under the Influences of Diel Cycle and Free-Tropospheric Mineral Dust, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4307, https://doi.org/10.5194/egusphere-egu23-4307, 2023.

Mineral dust is one of the major contributors to the global aerosol load with the Sahara being its largest source. Dust particles can be transported over many days and thousands of kilometers. The main transport route spans from Africa over the Atlantic Ocean towards the Caribbean. Most of the time dust-transport takes place in the so-called Saharan Air Layer (SAL).  During its transport the SAL affects the Earth’s atmosphere by scattering and absorption of solar and terrestrial radiation, and by changing cloud evolution and cloud properties. The main season for the transatlantic dust transport is during the boreal summer months. However, dust can be transported towards the Caribbean also during wintertime, although this happens with less frequency.

Airborne lidar measurements with the combined water vapor differential absorption and high spectral resolution lidar system WALES provide simultaneous measurements of the water vapor mixing ratio and of aerosol properties. We use the measurements during the NARVAL-II experiment in August 2016 and during the EUREC4A experiment in January/February 2020 to characterize the long-range transported SAL in summer- and in wintertime, and to investigate its radiative effect and its impact on the subjacent shallow marine trade wind convection. We found, that a small amount of water vapor embedded in the SAL has a strong impact on the radiative heating effect of this layer and consequently also on the atmosphere’s stability. During summertime, when the SAL is well separated from the marine boundary layer, the radiative effect of the SAL dominates. The evolution of shallow marine clouds below the SAL is suppressed. In wintertime, the SAL is transported at lower altitudes and the dust layer is frequently mixed into the marine boundary layer. During this time of the year the effect of the SAL on the evolution and lifetime of marine trade wind convection is much more complex, as the dust particles within the SAL might additionally act as cloud or ice nuclei.

In our presentation we will give an overview of the performed measurements and the radiative transfer calculations. We will present the radiative effects of the separated summertime SAL, and show first results of the impact of the wintertime SAL on the atmosphere’s stability and cloud properties.

How to cite: Gross, S., Gutleben, M., and Wirth, M.: Impact of elevated Saharan Air layer on shallow marine convection, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-5585, https://doi.org/10.5194/egusphere-egu23-5585, 2023.

The subtropical marine stratocumulus-to-cumulus cloud transition and associated cloud size distributions are studied using Large-Eddy Simulations based on EUREC4A data. The simulations with the DALES code follow a Lagrangian trajectory from initially overcast stratocumulus to the tropical shallow cumulus region at the HALO flight site near Barbados, covering four days and three complete diurnal cycles. Mean state and bulk properties for different domain sizes are evaluated against aircraft data. In addition, time-continuous high-frequency data from Geostationary Operational Environmental Satellite (GOES) are used to investigate the evolution of cloud size distributions, focusing on the diurnal evolution of mesoscale cloud features. TOA brightness temperature data from GOES is used at a spatial resolution of 2x2 km² to characterize cloud populations and cloud morphology. These are compared to TOA brightness temperatures calculated from DALES output using the RRTOV simulator, as applied to subdomain-averages of similar dimensions. We find that the simulation with the largest domain size (100x100 km²) best reproduces the observed boundary layer and cloudy states at the HALO target site. The same applies to the amplitude and evolution of cloud cover as detected by GOES. The upstream nocturnal flower cloud organization is also reproduced, albeit with a slight time delay. 

How to cite: Ghazayel, S. and Neggers, R.: Evaluation of the diurnal evolution of flower cloud organization in multi-day Lagrangian large-eddy simulations based on EUREC4A against GOES satellite data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-4907, https://doi.org/10.5194/egusphere-egu23-4907, 2023.

Cumuli clouds in the trade winds are a great source of uncertainty for the future climate as their net radiative effect is hardly represented in global models. The spatial organization of these clouds, that drives their radiative effect, has been categorized into 4 major patterns: Sugar, Flower, Gravel and Fish (Bony et al. 2020). The processes governing their spatial organization and the relationships with the environmental properties remain however unclear. This study investigates the sensitivity of the Flower organization to the environmental mesoscale heterogeneities in water vapor and winds. A case of Flower organization, producing 100-km wide cloud clusters, is selected from the EUREC⁴A-ATOMIC campaign that took place east of Barbados in January-February 2020. A Large-Eddy Simulations using the Meso-NH model and a 100-m horizontal grid-spacing has been extensively validated by satellite and aircraft high-resolution observations (Dauhut et al., 2022) and serves as a reference. By removing alternatively the humidity or the wind heterogeneities, we show that mesoscale humidity anomalies play a critical role in driving cloud organisation into Flower. Further investigations indicate that humidity heterogeneities in the cloud layer influence the development of a shallow mesoscale circulation and have a larger impact than the heterogeneities in the sub-cloud layer. Different chains of processes are proposed to explain such a sensitivity.

How to cite: Dauhut, T., Couvreux, F., and Bouniol, D.: How do environmental mesoscale heterogeneities influence the trade-wind cloud organization?, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-8935, https://doi.org/10.5194/egusphere-egu23-8935, 2023.

Recent observations revealed that meso-scale patterns of shallow convection in the downwind trades can be connected to specific atmospheric environments whose characteristics are not solely from within the trades but have traces from tropical or mid-latitudinal origin depending on the pattern. As a consequence of this co-variability of patterns and air-mass characteristics, a different feedback to a changing climate is anticipated and will be modulated by the observed, pattern-dependent net cloud radiative effects. By conducting large-eddy simulations we evaluate how well current climate models reproduce this co-variability in cloudiness and its environment and whether the meso-scale patterns are represented due to the observed mechanisms. To capture the full range of patterns and its processes these simulations are done on large-scale domains with grid-spacings of 625m, 312m and 156m and focus on the EUREC4A field campaign time period. By repeating the simulation with an increased aerosol load, we reveal pattern-dependent sensitivities. With frequently raining patterns showing the largest response, the importance of different processes depending on the meso-scale organization is emphasized.

How to cite: Schulz, H., Bjorn, S., and Wood, R.: Evaluating hm-scale simulations of trade wind clouds using EUREC4A data, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-10856, https://doi.org/10.5194/egusphere-egu23-10856, 2023.

The cloud systems of the North Atlantic trades (NAT) have been a topic of curiosity due to significant uncertainty in their physical characteristics, physical process understanding across various spatial and temporal scales, and their impact on the regional climate system. Initial research has provided the causal link for cloud systems having distinct organizational aspects (formerly described as Sugar, Gravel, Fish, and Flower) with the net radiative flux over the region. Questions have been raised about how the changing climate will influence the frequency of occurrence of these cloud regimes and how the net radiative impact will change the regional climate system.

However, cloud systems represent a continuous spectrum where not-so-visually distinct systems also occur. Existing clustering mechanisms sort organizations into separate classes. Yet, in reality, the organization often does not align with those pre-defined classes but transitions amongst them or simply occurs in a continuous spectrum. Using two complementary neural networks in self-supervision (without human interference), we investigate the representation learning of cloud systems both in a continuous space describing a cloud system spectrum and in a discreet space aiming to identify distinct cloud systems. 50,000 GOES-ABI cloud optical depth NAT images from 2017 – 2021 covering the EUREC⁴A study area are randomly cropped to 256 x 256 pixels and sorted/labeled by the machine.

We study the climatological representation of EUREC⁴A’s cloud patterns in the continuous embedding space. We follow the Maria S. Merian ship track inside the feature space matching the ship-based atmospheric remote sensing and ERA5 profiles with the K-nearest satellite images. This analysis examines the consistency of the environmental conditions for cloud systems identified as close to each other in the continuous feature space. We investigate the relationship between the net cloud radiative effect and the radiative flux characteristics in the continuous space, finding a strong functional relationship with the cloud system’s pattern and distributions.

In the discrete space, we aim to identify the optimal number of classes that could represent the continuous space. We also aim to understand if these discrete classes correspond to the categories identified in the physical and visual space. Moreover, to better understand the decision of the neural network for a particular cloud pattern, we visualize the network’s focus in the activation space. We find that different self-attention heads of the neural network learn to focus on different semantics of the cloud system distribution.

Finally, we found that different regularizations applied during the training of the network directly impact the representation learning of the cloud system, and we show how to use such regularizations to improve the understanding of cloud systems.

How to cite: Chatterjee, D., schnitt, S., Bigalke, P., Acquistapace, C., and Crewell, S.: Complementary approaches in self-supervision to exploit EUREC4A measurements and satellite observations for cloud systems over North Atlantic trades, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-12257, https://doi.org/10.5194/egusphere-egu23-12257, 2023.

Cloud resolving models run to equilibrium in idealized simulations of radiative-convective equilibrium often show the deep convection spontaneously transitioning from random organization to a state where convection in aggregated into clusters.  This results in a drier mean state and aggregation could be important for climate sensitivity, and is missing in classical parameterization schemes.  However, the onset and nature of the equilibrium, and the sensitivity to lower boundary conditions, differs dramatically between models that use different representations of moist physics and diffusion parameterizations, and varying dynamical cores.  In order to shed light on this, we develop a highly idealized, spatially explicit, stochastic reactive-diffusive model for the column relative humidity in the tropics.

The model is run to equilibrium and it is found that, depending on the model parameter settings and experimental framework, it can produce equilibrium states where the convection remains random, or states where the convection is highly aggregated. Many results of the full-physics models are reproduced, such as their sensitivity to model resolution and domain size, with aggregation more likely using coarse grid sizes and larger domains. The simple model thus allows to explain these sensitivities of the full physics models, with convective nearest-neighbor distances constrained to decrease with smaller domains or higher resolution, which reduces the spatial heterogeneity of column humidity and makes aggregation less likely.  Expanding on these arguments, we use dimensional analysis to combine the model parameters that describe how sensitive convection is to humidity, the subsidence drying rate, and the spreading of humidity by advection and diffusion processes, along with the domain size and resolution.  Using large ensembles of over 1000 simulations, we demonstrate that aggregation occurs at a precise critical value of the resulting dimensionless parameter, which will refer to as the aggregation number.   We suggest that the aggregation number could prove useful to diagnose the differences between full physics models of the atmosphere. 

How to cite: Tompkins, A. and Biagioli, G.: A dimensionless parameter to predict spontaneous convective aggregation onset in a idealized stochastic-diffusive model of radiative-convective equilibrium, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-14609, https://doi.org/10.5194/egusphere-egu23-14609, 2023.

Prevalent over the world’s oceans and continents, shallow clouds still comprise a main aspect of the uncertainty related to cloud feedback and climate sensitivity. Compared to shallow clouds over the ocean, confined to specific marine environments, shallow cumulus (Cu) over land occur in diverse locations throughout the globe.

How to cite: Dror, T., Koren, I., Altaratz, O., Chekroun, M. D., and Silverman, V.: On the properties of greenCu: continental, organized shallow clouds, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-17350, https://doi.org/10.5194/egusphere-egu23-17350, 2023.

Cold-air outbreaks (CAOs) form marine boundary layer (MBL) clouds that undergo rapid overcast-to-broken cloud regime transitions, initiated by substantial rain. CAOs are usually accompanied by dry intrusions (DIs) that subside as free-tropospheric (FT) air into the postfrontal sector of mid-latitude storms. For an exemplary cold-air outbreak in the NW Atlantic that showed faster transitions (corresponding to reduced extents of overcast clouds) closer to the low-pressure system, we posit that varying transitions are caused by an uneven meteorological pattern imposed by the prevailing DI. We compile satellite observations, reanalysis fields, and Lagrangian large-eddy simulations (LES) translating along MERRA2-based trajectories to show that postfrontal trajectories closer to the low-pressure system are uniquely favorable to rain formation (and, thus, cloud transitions) as they show (1) weaker FT subsidence rates, (2) greater FT humidity, (3) greater MBL windspeeds, and (4) a colder MBL as well as reduced lower-tropospheric stability. We present an updated conceptual view of postfrontal cloud formation that may guide future investigations.

How to cite: Tselioudis, G., Tornow, F., Ackerman, A., and Fridlind, A.: The impact of dry intrusions on midlatitude cold-air outbreak cloud transitions, EGU General Assembly 2023, Vienna, Austria, 23–28 Apr 2023, EGU23-11082, https://doi.org/10.5194/egusphere-egu23-11082, 2023.