The vertical profiles of the wind speed and direction in atmospheric boundary layers are strongly controlled by turbulent friction. Some global weather forecast and climate models parameterize the turbulent momentum fluxes by means of a downgradient eddy diffusion approach, in which the same stability-dependent eddy viscosity profile is applied to both horizontal wind components. In the present study we diagnose eddy viscosity profiles from large-eddy simulations of a stable, a neutral and six convective boundary layers. Each simulation was forced by the same geostrophic wind of 15 ms$^{-1}$, but with a different surface heat flux. The stably stratified boundary layer sustains the largest friction and largest ageostrophic wind turning, due to its shallow depth, which leads to a steep slope (large vertical divergence) of the momentum fluxes. For convective cases we find that the eddy viscosity profiles for the two horizontal wind components are very different, in particular, we find negative eddy viscosities for the cross-isobaric wind component, indicating that its turbulent transport is counter the mean gradient. This implies that a purely downgradient diffusion approach for turbulent momentum fluxes is inadequate. To assess the consequence of applying an anisotropic diffusion approach, a modified solution of the Ekman spiral is presented. It is found that an anisotropic diffusion approach allows for a different vertical profile of the wind in terms of the height of maximum wind speed and the turning of the wind.

A growing body of literature investigates convective organisation, 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 for cold-pool development, and another set inhibits evaporation of precipitation and thus cold-pool formation. Without (or with only weak) subcloud-layer shear, conditions are unfavourable 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, 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. Forward shear 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).

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 idealised 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 behaviour: 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.

It is well known that subtropical shallow convection transports heat and water vapour upwards from 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 multi-day large eddy simulations run over the tropical Atlantic with ICON-LEM to investigate the character of convective momentum transport (CMT) by shallow convection. For a typical trade wind profile during boreal winter, the convection acts like an apparent friction to decelerate the north-easterlies. This effect is maximum below the cloud base while in the cloud layer, the friction is minimum but is distributed over a relatively deeper 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 the weak friction effect but not the counter-gradient flux near cloud tops. The analysis of the momentum flux budget reveals that, in the cloud layer, the counter-gradient flux is driven by convectively triggered non-hydrostatic 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.

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 twelve 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 (≈100x100 km2) that have been run at resolutions of 150–600 m, which are subjected to a realistically varying flow that has developed in the outer domain. Combined, the resolved advection and the subgrid stresses decelerate the easterly flow from the surface up to about 2 km in winter and 1 km in summer, a result that is insensitive to horizontal resolution. The unresolved processes are strongest near surface and are well captured by traditional K-diffusion theory, but convective-scale motions that are not considered in K-diffusion theory contribute the most in the upper part of 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.

The science guiding the \EURECA campaign and its measurements are presented. \EURECA comprised roughly five weeks of measurements in the downstream winter trades of the North Atlantic — eastward and south-eastward of Barbados. Through its ability to characterize processes operating across a wide range of scales, \EURECA 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, 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 four hundred hours of flight time by four heavily instrumented research aircraft, four global-ocean class research vessels, an advanced ground-based cloud observatory, a flotilla of autonomous or tethered measurement devices operating in the upper ocean (nearly 10000 profiles), lower atmosphere (continuous profiling), and along the air-sea interface, a network of water stable isotopologue measurements, complemented by special programmes of satellite remote sensing and modeling with a new generation of weather/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 \EURECA explored — from Brazil Ring Current Eddies to turbulence induced clustering of cloud droplets and its influence on warm-rain formation — are presented along with an overview \EURECA’s outreach activities, environmental impact, and guidelines for scientific practice.

This study investigates how wind shear and momentum fluxes in the surface- and boundary layer vary across wind and cloud regimes. We use a nine-year-long data set from the Cabauw tower of the Ruisdael Observatory (NL) complemented by (8.2 x 8.2 km^2) daily LES hindcasts. An automated algorithm classifies observed and simulated days into different cloud regimes: 1) clear-sky days, 2) days with convective clouds (cumulus) rooted in the surface layer, with three ranges of cloud cover, and 3) days with clouds not rooted near the surface. Categorized days in observations and LES do not fully match, with a tendency of the LES to develop convective clouds on clear-sky days and less frequently produce non-rooted clouds, whose scales are far larger than the LES domain. Even so, the climatology and diurnal cycle of winds are for each regime very similar in LES and observations, strengthening our confidence in LES’ skill to reproduce certain clouds for an atmospheric state. Wind shear is smallest in clear-sky and cumulus regimes with limited cloud cover (CLCC), which also have the weakest 200 m wind speed and largest surface buoyancy flux. They have notably larger cross-wind fluxes, although along-wind momentum flux profiles are similar across all regimes. Cloudy days have larger momentum fluxes distributed over deeper layers, sustaining up to 20% of the surface flux value at cloud base. Compared to clear-sky, the CLCC regimes have stronger updrafts and deeper mixed-layers. At similar atmospheric stability, surface friction is larger and underestimated by Monin-Obukhov Similarity Theory.