Abstract
AbstractMotivated 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.
Highlights
The trade winds are easterly winds that prevail over the tropical and subtropical oceans north and south of the Equator
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
Earlier works investigating the vertical wind structure in the trades include studies by Riehl et al (1951) using WWII weather ship data collected in the North Pacific as well as the momentum budget reconstructions using radiosonde arrays from the Barbados Oceanographic and Meteorological Experiment (BOMEX; Holland and Rasmusson, 1973) and the Atlantic Tradewind Experiment (ATEX; Augstein et al, 1974; Brümmer et al, 1974)
Summary
Earlier works investigating the vertical wind structure in the trades include studies by Riehl et al (1951) using WWII weather ship data collected in the North Pacific as well as the momentum budget reconstructions using radiosonde arrays from the Barbados Oceanographic and Meteorological Experiment (BOMEX; Holland and Rasmusson, 1973) and the Atlantic Tradewind Experiment (ATEX; Augstein et al, 1974; Brümmer et al, 1974) In these studies, the frictional force arising from Reynolds stresses is derived as a residual from other terms in the momentum budget, for example the large-scale advection, pressure-gradient and Coriolis forces. Using general circulation model (GCM) simulations, Richter et al (2014) suggested that CMT is crucial for explaining the strength and direction of surface winds over the equatorial Atlantic In none of these studies, the relative influence of small-scale turbulence versus more coherent thermal plumes associated with convection on the total momentum transport within the boundary layer is analysed.
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