AS1.8
Orals: Wed, 26 Apr | Room M1
The responses of tropical anvil cloud and low-level cloud to a warming climate are among the largest sources of uncertainty in our estimates of climate sensitivity. However, most research on cloud feedbacks relies on either global climate models with parameterized convection, which do not explicitly represent small-scale convective processes, or small-domain models, which cannot directly simulate large-scale circulations. We investigate how self-aggregation, the spontaneous clumping of convection in idealized numerical models, depends on cloud-radiative interactions with different cloud types, sea surface temperatures (SSTs), and stages of aggregation in simulations that form part of RCEMIP (the Radiative-Convective Equilibrium Model Intercomparison Project). Analysis shows that the presence of anvil cloud, which tends to enhance aggregation when collocated with anomalously moist environments, is reduced in nearly all models when SSTs are increased, leading to a corresponding reduction in the aggregating influence of cloud-longwave interactions. We also find that cloud-longwave radiation interactions are stronger in the majority of General Circulation Models (GCMs), typically resulting in faster aggregation compared to Cloud-system Resolving Models (CRMs). GCMs that have stronger cloud-longwave interactions tend to aggregate faster, whereas the influence of circulations is the main factor affecting the aggregation rate in CRMs.
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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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 38o 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, 24–28 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, 24–28 Apr 2023, EGU23-2791, https://doi.org/10.5194/egusphere-egu23-2791, 2023.
We present novel remote sensing observations of cloud droplet size distributions retrieved from polarimetric observations of the wide-field airborne imaging system specMACS. The measurements were collected during the EUREC4A field campaign which took place in January and February 2020 in the trade wind region east of Barbados. We focus on observations of the cloudbow which is an optical phenomenon that results from single scattering of sunlight by liquid droplets close to the cloud top. The cloudbow signal strongly depends on the cloud droplet size distribution. By fitting model simulations (stored in a look-up table) to the cloudbow observations, both the effective radius and the effective variance (i.e., the width) of the droplet size distribution are retrieved. Traditional retrieval techniques based on total reflectance measurements are able to determine the effective radius but do not provide information on the effective variance. However, to fully understand cloud growth processes and the interaction between clouds and solar radiation, both parameters must be known. Furthermore, the cloudbow is only weakly affected by 3-D radiative transfer effects which is beneficial since these are usually a problem for traditional methods.
High-resolution maps of the cloud droplet size distribution with a spatial resolution of 100 m by 100 m are presented. The maps reveal patterns within the cloud droplet size distribution at cloud top that could originate from mechanisms like entrainment or mixing processes. We further show first results of an application of the retrieval to simulated specMACS observations. The images were generated using the 3-D radiative transfer model MYSTIC and are based on realistic LES cloud field simulations. We will investigate limitations and uncertainties of the retrieval using the simulated dataset.
How to cite: Pörtge, V., Kölling, T., Weber, A., Volkmer, L., Emde, C., Zinner, T., Forster, L., and Mayer, B.: High spatial resolution retrieval of cloud droplet size distribution from polarimetric specMACS observations and application to simulated data, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-11662, https://doi.org/10.5194/egusphere-egu23-11662, 2023.
Orals: Thu, 27 Apr | Room M1
A shallow cumulus cloud transition from a sugar to flower type of organization occurred under a layer of mineral dust on 2nd 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 5th 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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, 24–28 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.
Motivated by an intriguing observation regarding the universality of continental shallow Cu fields regardless of their geographical location, we explore their similarities. We combine satellite observations, along with machine learning classification and numerical modelling to show that these cloud fields share many important properties, such as the patterns they form and their tendency to form over and near forests and vegetated lands, thus termed greenCu.
Moreover, we show that in spite of their occurrence in different climatic regions, from the tropics to mid- and high-latitudes, greenCu fields are associated with similar large-scale meteorological conditions.
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, 24–28 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, 24–28 Apr 2023, EGU23-11082, https://doi.org/10.5194/egusphere-egu23-11082, 2023.
Posters on site: Wed, 26 Apr, 14:00–15:45 | Hall X5
The dominant cloud type in the subtropical Atlantic is the trade wind cumulus with a cloud base located near the lifting condensation level (LCL) below 1 km. Other common clouds in this region with their base above 1 km are stratiform cloud layers or cloud edges near the trade wind inversion at 2-3 km. Precipitation in all these clouds mainly forms at temperatures above freezing point by collision and coalescence. Therefore, precipitation generally occurs as light rain/drizzle from stratiform cloud layers or as showers from well-developed trade wind cumuli. Precipitation underneath a cloud base is often visible as fall streaks. If the precipitation evaporates before reaching the ground, these fall streaks are called virga.
Combined continuous long-term ground-based remote-sensing observations with vertically pointing cloud radar and ceilometer are well-suited to identify these precipitation evaporation fall streaks. Here we show the first application of a new open-source tool, the Virga-Sniffer which was developed within the frame of RV Meteor observations during the ElUcidating the RolE of Cloud–Circulation Coupling in ClimAte (EUREC4A) field experiment in Jan–Feb 2020 in the Tropical Western Atlantic. In the simplest approach, it detects virga from time-height fields of cloud radar reflectivity and time series of ceilometer cloud base height. The Virga Sniffer was applied to RV Meteor observations during EUREC4A and statistical results as well as an evaporation case study are presented. Spectral W-band radar data from a fall streak, identified as virga by the Virga-Sniffer, was used to calculate evaporative cooling rates. Sensitivity studies were performed to investigate the influence of vertical wind and relative humidity uncertainties. Possible future applications of the Virga-Sniffer within the frame of EUREC4A include detailed studies of precipitation evaporation with a focus on cold pools or cloud organization, or distinguishing moist processes based on water vapor isotopic observations.
How to cite: Kalesse-Los, H., Kötsche, A., Foth, A., Röttenbacher, J., Vogl, T., and Witthuhn, J.: First applications of the Virga-Sniffer – a new tool to identify precipitation evaporation using ground-based remote-sensing observations, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12306, https://doi.org/10.5194/egusphere-egu23-12306, 2023.
Tracking clouds has multiple applications. It is used for short-term weather forecasting as well as long-term weather and climate analyses. Our long-term goal is to investigate cloud life cycles under different conditions, such as marine or continental areas, over deserts, or in areas with increased anthropogenic aerosols. This is a key element in understanding cloud radiation effects and the human influence on the cloud life cycle.
To identify clouds and their trajectories, we are using Particle Image Velocimetry [1] which is well-known for measuring velocities in fluid dynamics. These velocity fields are used to predict the positions of clouds in the next timestep. The predicted positions are then compared to the observed positions to match clouds across timesteps. The algorithm can work on any geostationary satellite data set or equivalent model data [2].
Currently we are comparing satellite data from the EUREC4A campaign [3] (observed by the Advanced Baseline Image onboard the GOES-16 satellite) and model output from ICON-LEM [4]. Both datasets are located east of Barbados in the Caribbean Sea. This is done to benchmark the model settings and to identify which of the three model resolution best captures the satellite data. The cloud tracking allows us to look at the lifetimes of the clouds and the development of cloud physical properties over the lifetime of a cloud. This leads to a more refined investigation into the cloud behavior.
The presented results are twofold. Firstly, we will show a direct comparison of individual cloud trajectories between observed and model data to establish a deeper understanding of the methodology and datasets. Secondly, we will look at the distributions of clouds sizes and lifetimes to compare different resolutions of model data to the observed satellite data.
References:
[1] Raffel et al. (2007) "Particle Image Velocimetry - A Practical Guide", Springer Verlag, doi: 10.1007/978-3-540-72308-0
[2] Seelig et al. (2021) "Life cycle of shallow marine cumulus clouds from geostationary satellite observations", in JGR: Atmospheres, doi: 10.1029/2021JD035577
[3] EUREC4A campaign: www.eurec4a.eu
[4] Dipankar et al. (2015) “Large eddy simulation using the general circulation model ICON”, in Journal of Advances in Modeling Earth Systems 7(3): 963-986, doi: 10.1002/2015MS000431
How to cite: Müller, F., Seelig, T., and Tesche, M.: Tracking Clouds: Comparing Geostationary Satellite Observations and Model Data, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-5796, https://doi.org/10.5194/egusphere-egu23-5796, 2023.
Boundary layer cloud transitions at high latitudes play a key role in Arctic climate change, and are partially controlled by large-scale dynamics such as subsidence. While measuring large- and mesoscale divergence has proven notoriously difficult, the recent NARVAL and EUREC4A airborne campaigns in the subtropics have finally achieved this goal using multiple dropsondes releases in circular patterns. If this method also works at high latitudes is still an open research question, given the considerable differences in atmospheric dynamics. Answering this question was one of the main objectives of the recent HALO-(AC)^3 field campaign near Svalbard in Spring 2022. Circular dropsonde patterns were realized during various research flights by two airplanes, independently sampling Cold Air Outbreaks (CAO) in the Fram Strait with multiple dropsondes. This study presents a first overview of the results. We find that the method indeed yields reliable estimates of mesoscale gradients in the Arctic, yeilding robust vertical profiles of both subsidence and vorticity. Sensitivity to aspects of the method is investigated, including dependence on sampling area and the divergence calculation. Ongoing work to drive targeted Lagrangian high resolution simulations of the observed CAOs exclusively with HALO-(AC)³ data will be briefly discussed.
How to cite: Paulus, F., Neggers, R., Svensson, G., and Karalis, M.: Measuring meso-scale gradients in the Arctic during HALO-(AC)³, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-17058, https://doi.org/10.5194/egusphere-egu23-17058, 2023.
The representation of shallow tradewind cumulus clouds in climate models accounts for the 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.
We show how the continuum of convective organisation states can be analysed as an emergent property of the embedding space representation learnt by a neural network through unsupervised learning. Specifically we will use a technique to extract an estimate of the manifold in a high-dimensional space on which possible states of convective organisation lie. Through composition of reanalysis and observations onto this manifold we are able to extract the characteristics of the atmosphere which coincide with different forms of convective organisation, and further we are able to map transitions between different states of organisation and study how these develop.
We will show results from analysing: a) what the radiative properties of different forms of organisation are, b) what atmospheric characteristics coincide with different forms of organisation and c) what transitions occur when following air-masses along Lagrangian trajectories. Specifically, we find: a) net radiation changes significantly between different forms of organisation, b) agreement with previous studies on the importance of boundary layer wind-speed and to some degree atmospheric stability, and c) we are able to succinctly capture what transitions occur between regimes.
How to cite: Denby, L.: Charting the realms of Convective Cloud Organisation using Unsupervised Learning, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-12508, https://doi.org/10.5194/egusphere-egu23-12508, 2023.
Determining the shallow-convective mass flux at cloud base is the principle closure needed in convective parameterizations. Closure methods are usually developed and tested based on large-eddy simulations of a limited number of idealized cases. Here we evaluate how well common closures reproduce the magnitude and variability of the cloud-base mass flux observed during the recent EUREC4A field campaign upstream Barbados. The true observed mass flux is estimated as a residual of the sub-cloud layer mass budget from the circular dropsonde arrays at the 200km scale. To assess the closures, we diagnose all parameters of the chosen closures from the same dropsonde data or from coincident turbulence and cloud measurements of a second aircraft. Preliminary results suggest that (i) both variability in the area fraction and vertical velocity scales should be accounted for to reproduce the observed mass flux variability, (ii) methods using the turbulence kinetic energy to approximate the vertical velocity scale outperform methods based on the subcloud convective velocity scale, and (iii) the closure formulations should be general enough to remain useful when applied to other data and regimes.
How to cite: Vogel, R. and Mellado, J. P.: Evaluation of mass flux closures using EUREC4A observations, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-8842, https://doi.org/10.5194/egusphere-egu23-8842, 2023.
Posters virtual: Wed, 26 Apr, 14:00–15:45 | vHall AS
Sailors have long used cumulus clouds to guide their ships into areas of favourable winds. On the upwind side of cumulus clouds, the clouds’ thermal circulation would add momentum to the prevailing flow, while on the downwind side, the opposing branch of the circulation would reduce the flow. In this study, we take a simple approach to evaluating this sailors’ theorem and visualise the winds in the sub-cloud layer in the vicinity of shallow cumulus clouds. This is done by collocating cloud radar and wind lidar profiling measurements during EUREC4A on board the RV Meteor and at the BCO for a six-month period. Is there evidence for a thermal circulation in the wind surrounding clouds, or for plume-like structures that support a mass-flux approach? We will discuss our findings for clouds of increasing depth, for which clustered convection and cold pool gustiness become increasingly important.
How to cite: Nuijens, L. and Koning, M.: Sub-cloud layer winds in the vicinity of trade-wind cumulus, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-7513, https://doi.org/10.5194/egusphere-egu23-7513, 2023.
The cloud fraction of shallow non-precipitating cumulus residing at the lifting condensation level (LCL) increases in the afternoon, most evident in airborne lidar observations from EUREC4A. Observations from the HALO platform and from the R/V Ronald H. Brown are used to articulate the responsible process. Three hypotheses are investigated: 1) afternoon increases of the ocean sea surface temperature help support buoyancy fluxes that lift air parcels to saturation, as seen in tropical regions under low wind speeds; 2) shortwave absorption of the sub-cloud layer helps deepen the sub-cloud layer, so that its mixed-layer height can reach the LCL; 3) clouds form where the cloud layer is already moist. We invite the reader to take a moment here to choose which hypothesis you think is correct.
Analysis to date suggests #3 is the correct explanation. If so, then the next question is to identify why the daytime cloud layer is more or less moist in some places and times. Ideas for this can either be moisture redistribution from shallow circulations occurring at scales of approximately 200 km, or, moisture transport occurring at larger scales. These will be explored prior to the meeting, as well as ramifications for the diurnal cycle.
How to cite: Zuidema, P., McCoy, I., Perez, M., and Baidar, S.: What explains the population of daytime, optically-thin clouds below one km in the marine trade wind region?, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4689, https://doi.org/10.5194/egusphere-egu23-4689, 2023.
The Sun becomes brighter with time, but Earth's climate is roughly temperate for life during its long-term history; for early Earth, this is known as the faint young Sun problem (FYSP). Besides the carbonate-silicate feedback, recent researches suggest that a long-term cloud feedback may partially solve the FYSP. However, the general circulation models they used cannot resolve convection and clouds explicitly. This study re-investigates the clouds using a near-global cloud-permitting model without cumulus convection parameterization. Our results confirm that a stabilizing shortwave cloud feedback does exist, and its magnitude is ≈6 W m−2 or 14% of the energy required to offset a 20% fainter Sun than today, or ≈10 W m−2 or 16% for a 30% fainter Sun. When insolation increases and meanwhile CO2 concentration decreases, low-level clouds increase, acting to stabilize the climate by raising planetary albedo, and vice versa.
How to cite: Yan, M., Yang, J., Zhang, Y., and Huang, H.: Cloud Feedback on Earth's Long-Term Climate Simulated by a Near-Global Cloud-Permitting Model, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-4127, https://doi.org/10.5194/egusphere-egu23-4127, 2023.
Mesoscale cloud morphology patterns in the trade-winds can be grouped by their distinct appearance, size, and radiative properties into four categories: Sugar, Gravel, Flowers, and the synoptically driven Fish. Two occurrence pathways for the larger boundary-layer cloud organization structures were observed during the wintertime 2020 EUREC4A-ATOMIC joint campaign: i) regional Gravel persistence and ii) transitions to Gravel and Flowers from smaller Sugar clouds. Understanding the contributions to larger cloud structure occurrence under pathways of persistence vs. transitions from smaller clouds has utility in predicting their occurrence under climate change. Two EUREC4A-ATOMIC case studies are developed for these respective pathways during multi-day periods when observational platforms were longitudinally distributed across the ocean in parallel with Barbados. A Lagrangian analysis framework is developed by using for/backward 30-hr boundary layer trajectories initialized every 3-hr from the RV Ronald H. Brown (i.e., the mid-evolution reference platform) to connect upwind (e.g., the Northwest Tropical Atlantic Station buoy) and downwind (e.g., the Barbados Cloud Observatory) platforms. This synergistic, multi-platform campaign dataset is supplemented with satellite observations and reanalysis. Motion-stabilized Doppler-lidar observations at the RV Ronald H. Brown and the Barbados Cloud Observatory allow us to examine characteristics of cloud and plume dynamics in addition to the impact of environmental conditions expected to influence cloud organization and development (e.g., surface wind speeds, energy and moisture fluxes, stability, entrainment, large- and meso-scale subsidence, and aerosols). Eulerian differences between key platforms over the campaign period are evaluated and campaign findings are further extended using multi-year Lagrangian analysis.
How to cite: McCoy, I. L., Zuidema, P., Baidar, S., Vogel, R., Eastman, R., Schulz, H., and Brewer, A.: Evaluation of Trade Wind Mesoscale Morphology Evolution and Transitions, EGU General Assembly 2023, Vienna, Austria, 24–28 Apr 2023, EGU23-10781, https://doi.org/10.5194/egusphere-egu23-10781, 2023.
