Atmospheric delay has a profound impact on synthetic aperture radar (SAR) interferometry, inducing a spatial signal that significantly devaluates interferometric products. While the wide-scale variability of the atmosphere can be adequately modeled with global or regional weather models, it is especially the turbulent and convective part of the atmosphere at smaller spatial and temporal scales that is typically poorly captured. Due to the high resolution and precision of InSAR, there is a need for a realistic modeling of the 3-D distribution of turbulent refractivity in the boundary layer. This would enable assessment of the impact of a temporal or spatial model misalignment on the interferometric products, and contribute to studying the impact for future SAR missions. Here we test the feasibility of an advanced large Eddy simulation (LES) model to simulate a time-series refractivity distribution with a high spatio-temporal resolution to show the spatio-temporal variability of the troposphere on short time scales. We found for a fair-weather situation that the LES model produces realistic atmospheric simulations that match stochastically with results found in interferometric studies and that tropospheric delay variation leads to significant phase gradients within several minutes. This implies that even when using an (unrealistic) perfect weather model with resolutions similar to the SAR image, realizations that are several minutes apart from the time of the SAR acquisition will lead to significant phase errors. We propose the use of LES models as a realistic instrument to perform InSAR quality-assessments and for the development and simulation for future missions.@en

The science guiding the EUREC4A campaign and its measurements is presented. EUREC4A comprised roughly 5 weeks of measurements in the downstream winter trades of the North Atlantic - eastward and southeastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, EUREC4A marked a turning point in our ability to observationally study factors influencing clouds in the trades, how they will respond to warming, and their link to other components of the earth system, such as upper-ocean processes or the life cycle of particulate matter. This characterization was made possible by thousands (2500) of sondes distributed to measure circulations on meso- (200 km) and larger (500 km) scales, roughly 400 h of flight time by four heavily instrumented research aircraft; four global-class research vessels; an advanced ground-based cloud observatory; scores of autonomous observing platforms operating in the upper ocean (nearly 10 000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface; a network of water stable isotopologue measurements; targeted tasking of satellite remote sensing; and modeling with a new generation of weather and climate models. In addition to providing an outline of the novel measurements and their composition into a unified and coordinated campaign, the six distinct scientific facets that EUREC4A explored - from North Brazil Current rings to turbulence-induced clustering of cloud droplets and its influence on warm-rain formation - are presented along with an overview of EUREC4A's outreach activities, environmental impact, and guidelines for scientific practice. Track data for all platforms are standardized and accessible at 10.25326/165 , and a film documenting the campaign is provided as a video supplement.@en

Cumulus clouds ('fair-weather clouds') form as a result of atmospheric convection and have vertical extents between a few hundred metres (humilis species) and several kilometres (congestus species). They are a major source of uncertainty in the estimation of climate sensitivity by climate models. In order to reach more agreement in cloud changes due to global warming as predicted by different climate models, a better understanding of the physics of these clouds is needed. Shallow cumulus clouds are particularly common over the oceans of Earth's trade-wind regions, which are situated roughly between the 10° and 30° parallels on both hemispheres and are characterised by steady easterly surface winds. These winds are part of the Hadley cell, a large-scale circulation system in which air flows away from the equator at high altitudes and towards the equator near the surface. As a consequence, vertical shear (i.e. vertical differences in wind speed and direction) is common in this region. While recent studies have shown that (surface) wind speed is an important predictor of cloudiness in this region, little work has been done to elucidate how shear affects clouds. Vice versa, clouds also affect the wind by vertically transporting it. While this convective momentum transport (CMT) undoubtedly plays an important role in the force balance that sets the trade winds, only little is known about the details of how CMT sets the vertical structure of the wind and of the spatial scales of the momentum-transporting eddies. In this thesis, more light is shed on the bidirectional interaction between shallow cumulus convection and the wind. Particular focus is put on the effect of wind shear on convection and on the different spatial scales (convective and turbulent) at which convection affects the wind at different heights. To this end, results from numerical large-eddy-simulation (LES) experiments are utilised in this thesis. Due to their fine horizontal resolution (of hundreds of metres and less), LES is able to numerically resolve clouds and the largest turbulent eddies explicitly. This leads to a high degree of realism in the simulation. Together with the possibility to artificially simplify the experimental set-up as well as the completeness of the output (in terms of time, space and quantities), this makes LES the ideal tool to understand physical mechanisms in the atmosphere. To identify and understand the effect of wind shear on cumulus convection, LES experiments were carried out in which typical conditions of the trades were simulated, while the amount of wind shear was systematically varied. In these idealised LES, vertical wind shear effectively limits the deepening of trade-wind convection. Several mechanisms are responsible for this, which depend on the direction of the shear vector (vertically decreasing or increasing wind speed) as well as the altitude at which shear is present. A situation with easterly surface winds that weaken with height and eventually turn westerly is referred to as backward shear, and the opposite situation with easterlies that strengthen with height is called forward shear. Different directions of wind shear cause different surface winds due to CMT, which in turn affect the surface evaporation: Faster surface winds occur in the presence of forward shear and lead to stronger evaporation of sea water, resulting in deeper convection. Forward shear in the subcloud layer also leads to a spatial separation of precipitative downdrafts and emerging updrafts, as clouds move faster than their subcloud-layer roots; this is favourable for convective development. Conversely, under backward shear, the surface evaporation is weakened and precipitative downdrafts interfere with updrafts, hindering convective deepening. However, once clouds grow to sufficient depths, they may produce precipitation so strong that the associated downdrafts spread out laterally near the surface, forming a distinct circular region of cold air, a cold pool. The spreading of this cold pool can cause uplift at its edges, triggering new convection. Backward shear limits the triggering of such secondary convection at cold-pool fronts, while forward shear facilitates it. Finally, shear of any direction in the cloud layer weakens cloud updrafts through an enhanced downward-oriented pressure perturbation force. The limiting effect of wind shear on cloud depth also affects the thermodynamic properties of the cloud layer: The relative humidity is larger and its decrease near the trade-wind inversion is more distinct if clouds are shallower. Large-domain LES hindcasts of specific days during the NARVAL measurement campaigns (which took place in December 2013 and August 2016 in the North Atlantic trades) give a uniquely realistic and complete view on the momentum balance of the trade winds. The combined effect of advection resolved by the model — which here is interpreted as CMT — and unresolved small-scale turbulence is to decelerate the wind in a layer that extends from the surface up to a height of about 2 km in winter and 1 km in summer. However, the role of each term in the balance depends on the altitude. CMT itself acts to accelerate near-surface winds, and only due to strong small-scale turbulence, there is still an overall frictious force at this height. Halfway into the subcloud layer, CMT starts to act as a frictious force. This friction is strengthened by small-scale turbulence from cloud base upwards and quickly diminishes with height. Thus, the cumulus clouds themselves do not introduce significant friction at the altitude where the zonal trade-wind jet resides, which coincides with cloud base. In fact, combined with momentum transport against the wind gradient (counter-gradient momentum transport), they may help to sustain this jet. Overall, wind shear appears to be an important player in setting the typical structure of the trade-wind atmosphere by modulating the depth of convection and may thus even affect cloud-radiative effects. Conversely, convection and turbulence give rise to an overall frictious force on the trade winds. CMT alone acts to accelerate the winds near the surface, which may weaken the Hadley circulation, while in the cloud layer, CMT hardly affects the wind. @en

Motivated by the abundance of low clouds in the subtropics, where the easterly trade winds prevail, we study the role of shallow convection in the momentum budget of the trades. To this end, we use ICON-LEM hindcasts run over the North Atlantic for 12 days corresponding to the NARVAL1 (winter) and NARVAL2 (summer) flight campaigns. The simulation protocol consists of several nested domains, and we focus on the inner domains (≈100 × 100 km²) which have been run at resolutions of 150–600 m and are forced by analysis data, thus exhibiting realistic conditions. Combined, the resolved advection and the subgrid stresses decelerate the easterly flow over a frictional layer that balances the prevailing geostrophic wind forcing. Irrespective of the horizontal resolution, this layer is about 2 km deep in the strong winter trades and 1 km in summer, as winds and geostrophic forcing weaken and cloudiness reduces. The unresolved processes are strongest near the surface and are well captured by traditional K-diffusion theory, but convective-scale motions which are not considered in K-diffusion theory contribute the most in the upper part of the mixed layer and are strongest just below cloud base. The results point out that convection in the mixed layer – the roots of trade-wind cumuli and subcloud-layer circulations – play an important role in slowing down easterly flow below cloud base (but little in the cloud layer itself), which helps make the zonal wind jet more distinct. Most of the friction within the clouds and near the wind jet stems from smaller-scale turbulence stresses.

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It is well known that subtropical shallow convection transports heat and water vapor upwards from the surface. It is less clear if it also transports horizontal momentum upwards to significantly affect the trade winds in which it is embedded. We utilize unique multiday large-eddy simulations run over the tropical Atlantic with ICON-LEM to investigate the character of shallow convective momentum transport (CMT). For a typical trade-wind profile during boreal winter, CMT acts as an apparent friction to decelerate the north-easterly flow. This effect maximizes below the cloud base while in the cloud layer, friction is very small, although present over a relatively deep layer. In the cloud layer, the zonal component of the momentum flux is counter-gradient and penetrates deeper than reported in traditional shallow cumulus LES cases. The transport through conditionally sampled convective updrafts and downdrafts explains a weak friction effect, but not the counter-gradient flux near the cloud tops. The analysis of the momentum flux budget reveals that, in the cloud layer, the counter-gradient flux is driven by convectively triggered nonhydrostatic pressure-gradients and horizontal circulations surrounding the clouds. A model set-up with large domain size and realistic boundary conditions is necessary to resolve these effects.

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To advance the understanding of the interplay among clouds, convection, and circulation, and its role in climate change, the Elucidating the role of clouds-circulation coupling in climate campaign (EUREC4A) and Atlantic Tradewind Ocean-Atmosphere Mesoscale Interaction Campaign (ATOMIC) collected measurements in the western tropical Atlantic during January and February 2020. Upper-air radiosondes were launched regularly (usually 4-hourly) from a network consisting of the Barbados Cloud Observatory (BCO) and four ships within 6-16°N, 51-60°W. From 8 January to 19 February, a total of 811 radiosondes measured wind, temperature, and relative humidity. In addition to the ascent, the descent was recorded for 82 % of the soundings. The soundings sampled changes in atmospheric pressure, winds, lifting condensation level, boundary layer depth, and vertical distribution of moisture associated with different ocean surface conditions, synoptic variability, and mesoscale convective organization. Raw (Level 0), quality-controlled 1 s (Level 1), and vertically gridded (Level 2) data in NetCDF (Stephan et al., 2020) are available to the public at AERIS (https://doi.org/10.25326/137 https://doi.org/10.25326/137). The methods of data collection and post-processing for the radiosonde data set are described here.

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A growing body of literature investigates convective organization, but few studies to date have sought to investigate how wind shear plays a role in the spatial organization of shallow (trade-wind) convection. The present study hence investigates the morphology of precipitating marine cumulus convection using large-eddy-simulation experiments with zonal forward and backward shear and without shear. One set of simulations includes evaporation of precipitation, promoting cold-pool development, and another set inhibits the evaporation of precipitation and thus cold-pool formation. Without (or with only weak) subcloud-layer shear, conditions are unfavorable for convective deepening, as clouds remain stationary relative to their subcloud-layer roots so that precipitative downdrafts interfere with emerging updrafts. Under subcloud-layer forward shear (FS), where the wind strengthens with height (a condition that is commonly found in the trades), clouds move at greater speed than their roots and precipitation falls downwind away from emerging updrafts. FS in the subcloud layer appears to promote the development of stronger subcloud circulations, with greater divergence in the cold-pool area downwind of the original cell and larger convergence and stronger uplift at the gust front boundary. As clouds shear forward, a larger fraction of precipitation falls outside of clouds, leading to more moistening within the cold pool (gust front).

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Motivated by an observed relationship between marine low cloud cover and surface wind speed, this study investigates how vertical wind shear affects trade-wind cumulus convection, including shallow cumulus and congestus with tops below the freezing level. We ran large-eddy simulations for an idealized case of trade-wind convection using different vertical shears in the zonal wind. Backward shear, whereby surface easterlies become upper westerlies, is effective at limiting vertical cloud development, which leads to a moister, shallower, and cloudier trade-wind layer. Without shear or with forward shear, shallow convection tends to deepen more, but clouds tops are still limited under forward shear. A number of mechanisms explain the observed behavior: First, shear leads to different surface wind speeds and, in turn, surface heat and moisture fluxes due to momentum transport, whereby the weakest surface wind speeds develop under backward shear. Second, a forward shear profile in the subcloud layer enhances moisture aggregation and leads to larger cloud clusters, but only on large domains that generally support cloud organization. Third, any absolute amount of shear across the cloud layer limits updraft speeds by enhancing the downward oriented pressure perturbation force. Backward shear—the most typical shear found in the winter trades—can thus be argued a key ingredient at setting the typical structure of the trade-wind layer.

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Convective momentum transport (CMT) is hypothesized to play an important role in explaining patterns of near-surface wind in the tropics and subtropics. In light of the uncertainties in (shallow) convective mixing in global models, and its link to the large-scale Hadley circulation, which is strongly defined by the trade-winds, our study focuses on how shallow cumulus convection and horizontal winds in the trades interact. In particular, we ask, what is the structure of CMT by shallow convection, and how does it depend on the background wind profile? Using the Dutch Atmospheric Large Eddy Simulation (DALES), we simulate idealized cases for shallow cumulus and congestus clouds typical for the subtropics. We impose idealized geostrophic wind forcing with varying shear – informed by reanalyses – and first show how shear influences convection and cloudiness. A robust and known signal is that shear increases cloud cover, just by slanting clouds, but for very large shear it can also increase cloud fraction at cloud base. Furthermore, in the presence of wind shear, clouds grow less deep despite the fact that they are more buoyant. Yet, this does not always imply larger vertical velocities. We also observe considerable heterogeneity in the horizontal winds related to the patterns of clouds. Cloudy volumes generally have lower (horizontal) wind velocities than clear sky volumes, as updrafts presumably carry air with lower momentum from near the surface upwards. But also, in clear sky volumes in the vicinity of clouds wind speeds tend to differ from the mean wind. In this presentation, we will explain these changes in horizontal winds with convective organization, and show the impact on total and convective momentum transport. To unravel the influence of changing thermodynamics and of the organization of clouds, we also repeat simulations with fixed surface fluxes and without precipitation, removing feedbacks that involve changes in the near-surface wind. @en