AS1.22

How to cite: Mackie, A. and Byrne, M. P.: Impact of circulation on tropical cloud feedbacks in cloud resolving models, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2669, https://doi.org/10.5194/egusphere-egu21-2669, 2021.

This work examines the representation of convection-circulation coupling over tropical West Africa in convection-permitting models. Tropical West Africa is not only a region characterised by extremely high-impact weather, in the form of intense and frequent organised convection, but it is also a region of strong baroclinicity and wind shear, and therefore an excellent natural laboratory for examining the connections between mesoscale convection and synoptic circulations. Developing understanding ofconvection-circulation coupling is crucial to informing development of convection parameterisations and improving regional forecasts of high-impact weather.

We evaluate output from the CP4-Africa configuration of the Met Office Unified Model to investigate links between convective activity and synoptic motions. To illustrate its strengths in representing convection-circulation feedbacks, CP4 output is compared to that from a similar UM configuration which uses a convection parameterisation.

We examine the mean diurnal cycle of circulation during the storm season. Distinct diurnal patterns in circulation tendency are compared to patterns in updraughts and precipitation, which illustrate different forms of convection which can be observed at different points during the day. A “congestus” mode convects up to around the freezing level from morning until early evening, while deep organised convection triggers in the mid-to-late afternoon and persists overnight. The two forms of convection appear to cause characteristically different responses in the synoptic circulation.

To confirm which physical processes cause changes to circulation in the region, we calculate terms in the circulation tendency equation. Separating these terms into mean and eddy-flux contributions allows us to establish the extent to which mesoscale systems and synoptic structures each influence the diurnal changes to circulation.

How to cite: Morris, F., Schwendike, J., Parker, D., and Bain, C.: Convection-circulation interactions over West Africa in simulations with explicit and parameterised convection, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2088, https://doi.org/10.5194/egusphere-egu21-2088, 2021.

Intrusions of dry upper-level extratropical air into the tropics play an important role in shaping the synoptic time-scale variability of the low-level cloud cover over the tropical oceans. In this study, we present a detailed Lagrangian analysis of an extratropical dry intrusion in the western North Atlantic, which occurred in January-February 2018. During this period, the easterly trade winds were interrupted for several days by coherent packages of rapidly descending air parcels reaching from the mid-latitude jet stream region into the sub-cloud layer close to Barbados. As those air parcels are anomalously dry and cold, they have a notable impact on diabatic processes in the vicinity of the trade wind cloud tops such as longwave cooling and cloud evaporation and sublimation. To quantify the Lagrangian heat budget along the dry intrusion, we performed a simulation with the Integrated Forecasting System (IFS, 0.4° horizontal resolution, 137 vertical levels) from the European Centre for Medium Range Weather Forecasts (ECMWF) with diabatic heating rate (DHR) output. We calculated back-trajectories based on hourly three-dimensional wind fields and analysed the DHR along the dry intrusion air parcels. In the first part of their descent from the mid-tropospheric jet stream region, the dry intrusion air parcels’ heat budget is dominated by adiabatic warming. In the second part of their descent, they experience strong diabatic cooling at cloud tops, due to microphysical and radiative processes. This leads to cross-isentropic flow, which allows these air parcels to pass through the inversion and to penetrate into the boundary layer. Thereafter they experience strong diabatic warming by turbulent fluxes. The presented detailed case study thus illustrates, how the rapidly subsiding extratropical dry intrusion air interacts with the parametrised subgrid-scale processes at cloud top in the model, thereby affecting the thermodynamic conditions in the boundary layer.

How to cite: Müller, S., Boettcher, M., Villiger, L., and Aemisegger, F.: Diabatic processes associated with an extratropical dry intrusion reaching into the western North Atlantic trade wind region, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2260, https://doi.org/10.5194/egusphere-egu21-2260, 2021.

We use measurements from the Elucidating the role of clouds-circulation coupling in climate (EUREC⁴A) campaign to characterise the variability in the meso-scale divergence and vertical motion (pressure velocity, 𝜔) ranging across time-scales from a few hours to a month (the entire campaign period from 19^th January - 15^th February, 2020). The area-averaged divergence is estimated using measurements of horizontal winds from dropsondes launched in a circular flight path (~200 km diameter), something that was carried out extensively during EUREC⁴A – 85 circles over 19 flight-days in the North Atlantic trade-wind region.

From these estimates, we characterise the vertical structure and variability of divergence and 𝜔 in the trades. We find that 𝜔 above the sub-cloud layer is quite consistent vertically when averaged over long periods. The value stays around 1-1.5 hPa/h, which agrees well with the roughly 1.5 K/day cooling rate of the trades. However, significant intra- and inter-day variability can be found between 𝜔 profiles, in terms of the magnitudes, ranging from -7 hPa/h to 6 hPa/h as well as in terms of the vertical structure of these profiles. Daily mean sub-cloud layer divergence varies significantly from that of the cloud-layer in magnitude, and for most flight days, we also observe a sign change between the two. Changes in the vertical structure over different days suggest that a local maximum of either divergence or convergence is usually seen near the inversion layer. Our findings can provide insight into how the atmospheric state varies over short time-scales, as well as their impact on cloudiness, thus providing clues about a predominantly important question in climate science — the clouds-circulation coupling.

How to cite: George, G., Stevens, B., Bony, S., and Vogel, R.: The vertical structure and variability of the meso-scale motion field in the trades, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-16069, https://doi.org/10.5194/egusphere-egu21-16069, 2021.

During the 2020 Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) and ElUcidating the Role of Cloud- Circulation Coupling in ClimAte (EUREC4A) field campaigns, a team from the University of Colorado Boulder deployed the RAAVEN Remotely-Piloted Aircraft System (RPAS). The RAAVEN RPAS was equipped with the miniFlux measurement system to observe the marine boundary layer upwind of Morgan Lewis, Barbados.  Over the course of 23 days, the team completed 39 flights covering nearly 80 flight hours.  Flights were conducted in and just above the boundary layer at altitudes between 10 and 1000 m, with a focus on capturing regular thermodynamic and kinematic profiles of the lower atmosphere, along with statistics on vertical transport and spatial variability.  In this presentation, we will give initial details on the observed state of the lower atmosphere.  This includes information on the structure and internal variability of thermodynamic and kinematic properties, turbulence intensity, turbulent surface fluxes and their variability, and details on the structure of vertical velocities in the lower atmosphere.

How to cite: de Boer, G., Calmer, R., Borenstein, S., Choate, C., Rhodes, M., Hamilton, J., Cox, C., Argrow, B., and Intrieri, J.: In situ observations of the near-shore atmospheric boundary layer during ATOMIC/EUREC4A from small Uncrewed Aircraft Systems , EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12635, https://doi.org/10.5194/egusphere-egu21-12635, 2021.

Tradewind clouds can exhibit a wide diversity of mesoscale organizations, and the turbulence of marine atmospheric boundary layer (MABL) can exhibit coherent structures and mesoscale circulations. One of the objectives of the EUREC4A (Elucidating the role of cloud-circulation coupling in climate) field experiment was to better understand the tight interplay between the mesoscale organization of clouds, boundary-layer processes, and the large-scale environment.

During the experiment, that took place East of Barbados over the Western Tropical Atlantic Ocean in Jan-Feb 2020, the French ATR-42 research aircraft was devoted to the characterization of the cloud amount and of the subcoud layer structure. During its 17 research flights, it sampled a large diversity of large scale conditions and cloud patterns. Multiple sensors onboard the aircraft measured high-frequency fluctuations of potential temperature, water vapour mixing ratio and wind , allowing for an extensive characterization of the turbulence within the subcloud layer. A quality-controled and calibrated turbulence dataset was produced on the basis of these measurements, which is now available on the EUREC4A AERIS data portal.

The MABL turbulent structure is studied using this dataset, through a spectral analysis of the vertical velocity. Vertical profiles of characteristic length scales reveal a non-isotropic structure with a stretching of the eddies along the mean wind. The organization strength of the turbulent field is also explored by defining a diagnostic based on the shape of the vertical velocity spectrum. The structure and the degree of organization of the subcloud layer are characterized for different types of mesoscale convective pattern and as a function of the large-scale environment, including near-surface wind and lower-tropospheric stability conditions.

How to cite: Brilouet, P.-E., Lothon, M., and Bony, S.: How is the marine atmospheric boundary layer turbulence organized in the trades ?, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12339, https://doi.org/10.5194/egusphere-egu21-12339, 2021.

A main aim of the EUREC4A project is to better understand the interaction clouds and convection with changes in the circulation. A key part of this uncertainty in models is the response of the convection parametrization to changes in the grid-scale forcing. This uncertainty can be difficult to isolate due to the complexity of models leading to many competing errors. The comprehensive observations taken during the EUREC4A field campaign give us the opportunity to run convection parametrizations directly from observations. I will show the response of the Met Office's new convection parametrization (CoMorph) to profiles derived directly from EUREC4A observations. Initial tests with the dropsonde dataset (JOANNE), show that CoMorph can produce realistic forcing within the observational uncertainty. The aim is to include more observations into this framework to identify area in which the convection parametrization can be improved.

How to cite: Saffin, L., Denby, L., and Blyth, A.: Driving a Convection Parametrization with EUREC4A Observations, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14886, https://doi.org/10.5194/egusphere-egu21-14886, 2021.

Local impact of a stochastic shallow convection scheme on clouds and precipitation is tested in a case study over the tropical Atlantic on 20th December 2013 using the Icosahedral Nonhydrostatic Model (ICON) of the German Weather Service. ICON is used at a grid resolution of 2.5 km and is tested in several configurations that differ in their treatment of shallow convection. Two versions of a scale-aware stochastic shallow convection scheme are compared to the operational deterministic scheme and a case with no representation of shallow convection. The model is evaluated by comparing synthetically generated irradiance data for both visible and infrared wavelengths against actual satellite observations. The experimental approach is designed to distinguish the local effects of parameterized shallow convection (or lack thereof) within the trades versus the ITCZ. 
The stochastic cases prove to be superior in reproducing low-level cloud cover, deep convection and its organization, as well as the distribution of precipitation in the tropical Atlantic ITCZ. In these cases, convective heating in the subcloud layer is substantial, boundary layer depth is increased as a result of the heating, while evaporation is enhanced at the expense of sensible heat flux at the ocean’s surface. The stochastic case where subgrid shallow convection is deactivated below the resolved deep updrafts shows that local boundary-layer convection is crucial for a better representation of deep convection. Based on these results, our study points to a necessity to further develop parameterizations of shallow convection for the use at the convection-permitting resolutions and to assuredly include them in weather and climate modelling efforts. 

How to cite: Sakradzija, M., Senf, F., Scheck, L., Ahlgrimm, M., and Klocke, D.: Local Impact of Stochastic Shallow Convection on Clouds and Precipitation in the Tropical Atlantic, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13321, https://doi.org/10.5194/egusphere-egu21-13321, 2021.

The global surface temperature response to CO2 doubling (Equilibrium Climate Sensitivity or ECS) is a key uncertain parameter determining the extent of future climate change. Sherwood et al. (2020) estimated the ECS to be within [2.6K - 4.5K], but in the Coupled Model Intercomparison Project phase 6 (CMIP6), 1/3 of the General Circulation Models (GCMs) show ECS exceeding 4.5K (Zelinka et al., 2020). CNRM-CM6-1 is one of these models, with an ECS of 4.9K. In this paper, we sampled 30 atmospheric parameters of CNRM-CM6-1 and produced a Perturbed Physics Ensemble (PPE) of atmospheric-only simulations to explore the feedback parameters diversity and the climatological plausibility of the members. This PPE showed a comparable  range of feedback parameters to the multi-model archive, from 0.8 W.m-2/K to 1.8 W.m-2/K. Emulators of climatological performance and feedback parameters were used together with  observational datasets to search for optimal model configurations conditional on different net climate feedbacks. The climatological constraints considered here did not themselves rule out the higher end ECS values of 5K and above. An optimal subset of parameter configurations were chosen to sample the range of ECS allowing the assessment of feedback constraints in future fully coupled experiments.

References :

Sherwood, S. C., Webb, M. J., Annan, J. D., Armour, K. C., Forster, P. M., Hargreaves, J. C., ... & Zelinka, M. D. (2020). An assessment of Earth's climate sensitivity using multiple lines of evidence. Reviews of Geophysics, 58(4), e2019RG000678.

Zelinka, M. D., Myers, T. A., McCoy, D. T., Po‐Chedley, S., Caldwell, P. M., Ceppi, P., ... & Taylor, K. E. (2020). Causes of higher climate sensitivity in CMIP6 models. Geophysical Research Letters, 47(1), e2019GL085782.

How to cite: Peatier, S., Sanderson, B., and Terray, L.: Evaluating parametric sensitivity of climate feedbacks in CNRM-CM6, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15730, https://doi.org/10.5194/egusphere-egu21-15730, 2021.

Mesoscale cellular convective (MCC) clouds occur in large-scale patterns over the ocean, are prevalent in sub-tropical cloud regions and mid-latitudes, and have important radiative impacts on the climate system. On average, closed MCC clouds have higher albedos than open or disorganized MCC clouds for the same cloud fraction which suggests differences in micro- and macro-physical characteristics between MCC morphologies. Marine cold air outbreaks (MCAOs) influence the development of open MCC clouds and the transition from closed to open MCC clouds in the mid-latitudes. A MCAO index, M, combines atmospheric surface forcing and static stability and can be used to examine global MCC morphology dependencies. MCC cloud morphology occurrence is also expected to shift with sea surface temperature (SST) changes as the climate warms. Analysis of MCC identifications (derived from a neural network classifier applied to MODIS satellite collection 6 liquid water path retrievals) and ECMWF ERA5 reanalysis data shows that closed MCC cloud occurrence shifts to open or disorganized MCC within an M-SST space. Global climate models (GCMs) predict that M will change regionally in strength as SSTs increase. Based on our derived MCC-M-SST relationship in the current climate, closed MCC occurrence frequency is expected to increase with a weakening of M but decrease with an increase in SSTs. This results in a shift to cloud morphologies with lower albedos. Cloud controlling factor analysis is used to estimate the resulting low cloud morphology feedback which is found to be spatially varied and between ±0.15 W m^-2 K^-1. Because the morphology feedback is estimated to be positive in the extra-tropics and is not currently represented in GCMs, this implies a higher climate sensitivity than GCMs currently estimate.

How to cite: McCoy, I. L., McCoy, D. T., Wood, R., Zuidema, P., and Bender, F. A.-M.: The Role of Mesoscale Cellular Convective Cloud Morphologies in Low Cloud Feedbacks, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8516, https://doi.org/10.5194/egusphere-egu21-8516, 2021.

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 unprecented oppertunity to study the ambient conditions (e.g. windshear, horizontal convergence, subsidence) while simultaneously measuring the cloud properties. Using an unsupervised neural network able to autonomously discover discover different patterns of convective organisation this work quantifies the ambient and cloud-properties present in differently organised regimes and in 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 2021, online, 19–30 Apr 2021, EGU21-15962, https://doi.org/10.5194/egusphere-egu21-15962, 2021.

We conducted radiative convective equilibrium (RCE) experiments with varying domain size and sea surface temperature (SST) using the global cloud-system-resolving model NICAM (Satoh et al. 2014) to investigate the dependence of the maximum horizontal scale of the convective cluster on SST.

Convective self-aggregation in RCE simulations are widely studied, where convections spontaneously organize into a humid convective cluster even in the absence of inhomogeneities in boundary conditions and forcing. Previous studies show that convective self-organization does not occur when the domain size is too small, and that convective region become single-connected regions within a certain domain size, whereas when the domain size is large enough, multiple convective clusters are generated. In a previous study, although the maximum horizontal scale of the convective cluster was estimated to be about 4000 km, but the domain size of the simulation was smaller than the Earth surface, so it is not certain whether the preferable size of the convective aggregation exists over the realistic domain of the Earth. Moreover, it is now well understood how the horizontal size of the aggregation depends on SST; this aspect is relevant to understanding of the climate sensitivity.

The experiments were conducted with the NICAM simulations with switching off convective parameterization over a non-rotating spherical domain over the area of the region by varying the radius (the Earth radius R, R/2, R/4, R/8, and R/16). The horizontal uniform constant SST was changed as 295, 300, and 305K. The results show that there was a single convective cluster at a radius of R/4 or less, while there were multiple convective clusters at a radius of R/2 or more. The threshold for the transition between multiple convective clusters and a single convective cluster is found to be between R/4 and R/2. Physical variables such as vertical profiles of temperature and humidity gradually changes as the radius becomes larger, and converged at the radius R/2. For the SST dependency, the result robustly indicates that the maximum horizontal scale of the convection cluster is not monotonic with SST and it was largest for SST 300K.

As the domain size increases, the domain average moistens, and the boundary layer wind speed increases. Because the diabatic radiative cooling is constrained by the temperature and humidity structure, the surface evaporation and thus the surface wind speed must also be constrained with an upper limit; this is why the maximum horizontal scale exists and there are multiple convective clusters for the domain size larger than R/2. We also found that the moist static energy transport from the convective region decreases as the domain becomes larger, as pointed out by Patrizio and Randall (2019). The horizontal scale dependence of the convective cluster is related to two factors: the effect of the horizontal pressure difference in the boundary layer and the circulation structure of free troposphere. The energy budget analysis also explains the SST dependence of the maximum horizontal scale of the convective clusters.

How to cite: Matsugishi, S. and Satoh, M.: Horizontal scale of large-scale convective self-aggregation and their sensitivity to SST, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13685, https://doi.org/10.5194/egusphere-egu21-13685, 2021.

This study investigates the role of radiative processes in shaping the spatial distribution of shallow clouds, using in-situ measurements retrieved during the EUREC4A field campaign. Horizontal gradients in atmospheric radiative cooling above the boundary layer had been advanced as important drivers of shallow circulation and low-level winds, through their effect on surface pressure gradients. Modeling studies first recognized their importance in idealized simulations of deep convection in radiative-convective equilibrium, then found a weaker role for idealized cases of very shallow convection; but recent work using remote-sensing data argued for their importance in strengthening the circulation close to the margin between dry and moist regions, on synoptic scales, arguing for a possible significance for these radiative effects on observed cloud structures.

Here we investigate cases of intermediate scale, observed during the EUREC⁴A field campaign, where shallow convection extends vertically up to 4 km, and whose spatial organization can be described on mesoscales as “fish” or “flower” patterns. We perform careful radiative transfer calculations, using state-of-the-art spectroscopic data and over two thousand of dropsondes and radiosondes launched, to capture the fine details of radiative cooling profiles usually missed by satellite measurements. The large number of sondes allows us to sample radiative cooling information for the organization pattern of interest and analyze it in conjunction with the direct wind and humidity measurements. We also use geostationary estimates of precipitable water in clear-sky in order to cross-check the sonde data, and connect them to the organization pattern and to the position of the moist margin.

Our results target the following relationships previously identified in idealized simulations: (a) between horizontal gradients in moisture and in top-of-the-boundary-layer radiative cooling, (b) between these radiative cooling gradients and surface wind anomalies across the moist margin, and (c) between the strength of surface winds as a function of the distance from the moist margin. These results will allow us to test the importance of radiative transfer processes in a real case of shallow convective organization.

How to cite: Fildier, B., Muller, C., Touze-Peiffer, L., and Albright, A. L.: In-situ estimates of the role of radiative cooling for shallow convective organization, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-4953, https://doi.org/10.5194/egusphere-egu21-4953, 2021.

IWV data were retrieved from a network of nearly fifty Global Navigation Satellite System (GNSS) stations distributed over the Caribbean arc for the period 1 January-29 February 2020 encompassing the EUREC4A field campaign. Two of the stations had been installed at the Barbados Cloud Observatory (BCO) during fall 2019 in the framework of the project and are still running. All other stations are permanent stations operated routinely from various geodetic and geophysical organisations in the region. High spatial and temporal Integrated Water Vapour (IWV) observations will be used to investigate the atmospheric environment during the life cycle of convection and its feedback on the large-scale circulation and energy budget.

This paper describes the ground-based GNSS data processing details and assesses the quality of the GNSS IWV retrievals as well as the IWV estimates from radiosoundings, microwave radiometer measurements and ERA5 reanalysis.

The GNSS results from five different processing streams run by IGN and ENSTA-B/IPGP are first intercompared. Four of the streams were run operationally, among one was in near-real time, and one was run after the campaign in a reprocessing mode. The uncertainties associated with each of the data sets, including the zenith tropospheric delay to IWV conversion methods and auxiliary data, are quantified and discussed. The IWV estimates from the reprocessed data set are compared to the Vaisala RS41 radiosonde measurements operated from the BCO and to the measurements from the operational radiosonde station at Grantley Adams international airport (GAIA). A significant dry bias is found in the GAIA humidity observations with respect to the BCO sondes (-2.9 kg/m2) and the GNSS results (-1.2 kg/m2). A systematic bias between the BCO sondes and GNSS is also observed (1.7 kg/m2) where the Vaisala RS41 measurements are moister than the GNSS retrievals. The HATPRO IWV estimates agree with the BCO soundings after an instrumental update on 27 January, while they exhibit a dry bias compared to GNSS and BCO sondes before that date. ERA5 IWV estimates are overall close to the GAIA observations, probably due to the assimilation of these observations in the reanalysis. However, during several events where strong peaks in IWV occurred, ERA5 is shown to significantly underestimate the IWV peaks. Two successive peaks are observed on 22 January and 23/24 January which were associated with heavy rain and deep moist layers extending from the surface up to altitudes of 3.5 and 5 km, respectively. ERA5 significantly underestimates the moisture content in the upper part of these layers. The origins of the various moisture biases are currently being investigated.

How to cite: Bock, O., Bosser, P., Flamant, C., Doerflinger, E., Jansen, F., Fages, R., Bony, S., and Schnitt, S.: IWV observations from a network of ground-based GNSS receivers during EUREC4A, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-14481, https://doi.org/10.5194/egusphere-egu21-14481, 2021.

In the framework of the EUREC4A campaign, integrated water vapour (IWV) contents were retrieved over the open Tropical Atlantic Ocean using Global Navigation Satellite System (GNSS) data acquired from three research vessels : R/V Atalante, R/V Maria S. Merian, and R/V Meteor. This study describes the GNSS processing method and compares the GNSS IWV retrievals with IWV estimates from the ECMWF fifth ReAnalysis (ERA5), from the MODIS infra-red products, and from terrestrial GNSS stations located along the tracks of the ships. The ship-borne GNSS IWVs retrievals from R/V Atalante and R/V Meteor compare well with ERA5, with small biases (-1.62 kg/m2 for R/V Atalante and +0.65 kg/m2 for R/V Meteor) and a RMS difference about ~2.3 kg/m2. The results for the R/V Maria S. Merian are found  to be of poorer quality, with RMS difference of about 6 kg/m2 which are very likely due to the location of the GNSS antenna on this R/V prone to multipath effects. The comparisons with ground-based GNSS data confirm these results. The comparisons of all three R/V IWV retrievals with MODIS infra-red product show large RMS differences of 5-7 kg/m2, reflecting the enhanced uncertainties of this satellite product in the tropics. These ship-borne IWV retrievals are intended to be used for the description and understanding of meteorological phenomena that occurred during the campaign, east of Barbados, Guyana and northern Brazil.

How to cite: Bosser, P., Bock, O., Flamant, C., Bony, S., and Speich, S.: Integrated water vapour content retrievals from ship-borne GNSS receivers during EUREC4A, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-10322, https://doi.org/10.5194/egusphere-egu21-10322, 2021.

The new airborne thermal infrared imager VELOX (Video airbornE Longwave Observations within siX channels) is introduced. It covers six spectral bands in the thermal infrared wavelength range from 7.7 μm to 12 μm and is operated on board of the German High Altitude and Long Range Research Aircraft (HALO) of the German Aerospace Center (Deutsches Luft und Raumfahrtzentrum, DLR). The imager measures two-dimensional (2D) fields of the upward terrestrial radiance within a field of view of 35.5° by 28.7° with 640 by 512 spatial pixels. These 2D radiance fields can be converted into 2D fields of brightness temperature. With a horizontal resolution of better than 10 m VELOX extends the HALO remote sensing instrument suite to observe clouds and surface properties. The calibration and correction procedures for VELOX are presented. First measurements, collected during the ElUcidating the RolE of Cloud-Circulation Coupling in ClimAte (EUREC⁴A) campaign are shown, including analysis of the cloud top brightness temperature, cloud mask/fraction calculations, cloud top altitude estimates, and Sea Surface Temperature (SST) analysis. The investigations reveal that the cloud top temperature can be resolved with a resolution of about 0.1 K, which translates into a vertical resolution of about 10 m with respect to cloud top altitude.

How to cite: Schäfer, M., Wolf, K., Ehrlich, A., Hallbauer, C., Jäkel, E., Röschenthaler, T., Stevens, B., and Wendisch, M.: VELOX - A new thermal infrared imager for airborne remote sensing of cloud and surface properties, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1214, https://doi.org/10.5194/egusphere-egu21-1214, 2021.

We investigate the abundance and radiative effect of small and optically thin clouds in trade wind cumulus cloud fields from high-resolution satellite imagery. Using radiative transfer calculations to simulate clear-sky observations, we can identify optically thin cloud areas in ASTER images, a signal that is undetected by the satellite products that are commonly used for cloud radiative effect and cloud feedback analysis. Results from the analysis within the EUREC4A campaign suggest that the area covered by optically thin clouds is approximately as big as the area covered by clouds that are detected by common cloud masking algorithms. Compared to clear-sky ocean observations, the enhanced radiance from optically thin clouds leads to a high-bias in clear-sky estimates and hence a low-bias in the estimated radiative effect of trade wind cumuli. Next to the radiative effect, we discuss further implications that a broad cloud optical depth distribution might have on modelling results of a perturbed climate.

How to cite: Mieslinger, T., Kölling, T., Brath, M., Stevens, B., and Buehler, S. A.: Optically thin clouds in the winter trades, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-12663, https://doi.org/10.5194/egusphere-egu21-12663, 2021.

The evolution of clouds and their impact on weather and climate is closely related to the cloud droplet size distribution, which is often represented by two parameters: the cloud droplet effective radius (reff) and the effective variance (veff). The droplet radius (reff) determines the radiative effect of clouds on climate. The effective variance is a measure of the width of the size distribution which is, for instance, important to understand the formation of precipitation or entrainment and mixing processes. We present an airborne remote-sensing technique to determine reff and veff from high-resolution polarimetric imaging observations of the LMU cloud camera system specMACS.

Recently the spectral camera system has been upgraded by a wide-field polarization resolving RGB camera which was operated for the first time on the HALO aircraft during the EUREC⁴A campaign. The new polarimeter is ideally suited for observing the cloudbow - an optical phenomenon which forms by scattering of sunlight by liquid water cloud droplets at cloud top. The cloudbow is dominated by single scattering which has two implications: Its visibility is significantly enhanced in polarized measurements and its structure is sensitive to the cloud droplet size distribution at cloud top. This allows the retrieval of reff and veff by fitting the observed polarized cloudbow reflectances against a look-up table of pre-computed scattering phase functions.

The characteristics of the polarimeter are optimized for the measurement of the cloudbow. The wide field-of-view is key for observing the cloudbow (scattering angle 135° -165°) for a wide range of solar positions. Another advantage is the high spatial and temporal resolution which allows the study of small-scale variability of cloud microphysics at cloud top with a horizontal resolution of up to 20 m. Combining the polarimetric cloudbow technique with an existing stereographic retrieval of cloud geometry allows to derive vertical profiles of the droplet size distribution at cloud top. Observations of different EUREC⁴A cloud fields are used to demonstrate the retrieval technique and to present first spatial distributions and vertical profiles of cloud droplet size distributions.

How to cite: Pörtge, V., Kölling, T., Zinner, T., Forster, L., Emde, C., and Mayer, B.: Spatial distributions of cloud droplet size distributions from cloudbow observations measured with specMACS, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-8862, https://doi.org/10.5194/egusphere-egu21-8862, 2021.

Cloud free skies are rare in the trades.  We analyze conditions in which cloud-free conditions prevail.  For this purpose Raman water vapor measurements from the Barbados Cloud Observatory, complemented by ship-based measurements during EUREC4A are used to explore water vapor variability in the marine boundary layer.   We explore the consistency of the inferred cloud base height with estimates of temperature and water vapor from the lidar signal, and examine the co-variability of these quantities.  After having established the properties of these measurements, we seek to use them as well as others, to explain in what ways periods of cloud-free conditions are maintained, investigating the hypothesis that only when the wind stills is it simply sunny.

How to cite: Stevens, B., Serikov, I., Albright, A. L., Bony, S., George, G., Hirsch, L., Jansen, F., Kölling, T., Schulz, H., Vogel, R., and Worbes, L.: Why sometimes its simply sunny (in the trades), EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-15408, https://doi.org/10.5194/egusphere-egu21-15408, 2021.

Fields of shallow convection exhibit a rich spatial variability forming patterns of various shape, size and arrangement, commonly denoted as organization and often associated with precipitation. To understand either might require understanding both. However, the distribution and patterns of precipitation in shallow convection have received little attention so far.  

We investigate whether spatial patterning matters for the amount or intensity of precipitation in a scene. Are details of the spatial distribution important? Therefore, we analyse if and how the number, size and spatial arrangement of rain objects vary with scene precipitation rates. To do so, we exploit observational data from the C-band radar PoldiRad installed during the EUREC⁴A measurement campaign scanning a sector with approximately 200 km range east of Barbados in the western tropical Atlantic and compare to storm resolving simulations with ICON.

Our analyses suggest that it is mostly the precipitating area, which is determined by the number and size of rain objects, that regulates scene rainfall amounts. Especially the tail of large objects increases widening the spread in rain object sizes with increasing scene rainfall. While ICON captures this behaviour qualitatively, it overall simulates too small objects that rain too intense. We conclude that the extent of precipitation objects is more relevant for scene precipitation rates than a close spacing of objects.

How to cite: Radtke, J., Naumann, A. K., Ament, F., and Hagen, M.: Spatial patterns of precipitation in shallow convection during EUREC4A, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-2722, https://doi.org/10.5194/egusphere-egu21-2722, 2021.

We develop a novel method to detect cold pools from atmospheric soundings over tropical oceans and apply it to sounding data from EUREC⁴A. The proposed method exploits the fact that the air in a cold pool is denser than the air above it. It leads us to define cold pool soundings as those for which the mixed-layer height is smaller than 400 m. We first test this criterion by verifying its consistency with surface temperature and precipitation in a realistic high-resolution simulation over the western tropical Atlantic. Applying to EUREC⁴A data, we then identify 7 % of EUREC⁴A dropsondes and radiosondes as cold pool soundings. In two selected case studies, we find that cold pool soundings coincide with mesoscale cloud arcs and temperature drops in the surface time series. Statistics for the entire campaign further characterize the signature of cold pools in temperature, humidity and wind profiles. In the presence of wind shear, we show in particular that the spreading of cold pools is favored downshear, suggesting downward momentum transport by unsaturated downdrafts. These results support the robustness of our simple method in different environmental conditions and illustrate the new insights it offers for the characterization of cold pools and their environment. 

How to cite: Touzé-Peiffer, L., Vogel, R., and Rochetin, N.: Detecting cold pools from soundings during EUREC4A, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-1038, https://doi.org/10.5194/egusphere-egu21-1038, 2021.

Convective cold pools (CPs) have been recognised as an important ingredient in the organization of convective cloud fields and the formation of intense rain events (Feng et al. 2015, Torria and Kuang 2019). To understand the life cycle of CPs and their mutual interaction, idealised large-eddy simulations (LES) of isolated or colliding CPs have become an important tool to develop simple theories about the propagation speed, the dissipation rate and the moisture distribution within the CP and the surrounding environment (Rooney 2015; Langhans et al. 2015; Romps and Jeevanjee 2016; Grant et al. 2016, 2018). On the contrary, the formation of CPs and specifically their relation to their parent rain events has so far not gained much attention in idealised studies. This is surprising, as the relation between the generating rain event and the CP strength is relevant for the theoretical understanding of the ‘rain – CP – rain’ cycle and the parameterization of CPs, which aims at adjusting for the enhanced convective triggering under the presence of CPs, where the triggering scales with the CP strength. 

In this study we thus examine the relation between rain intensity, duration and CP strength in an idealised setting. To this end, we include the temporal extent of the rain event that forms the CP through evaporative cooling by varying the duration, intensity and area of the air volume that is cooled and moistened to simulate the generation of a CP. This finite duration of the CP forcing has been neglected by most studies that initialise the CP by an instantaneous forcing alone (e.g., Rooney 2015, Grant 2016). Our simulations show that a continuous cooling, imitating persistent rainfall, affects the generated CP only over a period of approximately ten minutes. Shorter cooling leads to smaller and weaker CPs, while cooling occurring after 10mins does not substantially affect the CP properties, such as its radius and propagation speed, the internal circulation in the CP head. Consequently, the CP’s effect on the environment as measured in terms of the updraft strength ahead of the CP, increases for cooling times up to 10mins and converges thereafter. To imitate a change in precipitation intensity, we vary the cooling amplitude. As expected, stronger cooling leads to stronger CPs. However, this effect is surprisingly small and does not substantially alter the CPs’ internal structure. 

To test the extent to which these results can be translated to ‘real’ CPs generated by evaporative cooling of rainfall, we study the relation between rain intensity and CP strength in comprehensive LES of deep convection and observational data. Hereby, we hopefully can improve our understanding , how best to characterise rain events, e.g. by their instantaneous or time-integrated precipitation statistics, to determine the CP strength.

How to cite: Meyer, B., Fiévet, R., and Haerter, J. O.: Predicting cold pool strength as a function of rain duration and intensity, EGU General Assembly 2021, online, 19–30 Apr 2021, EGU21-13390, https://doi.org/10.5194/egusphere-egu21-13390, 2021.