Print Email Facebook Twitter The role of shallow convection in the momentum budget of the trades from large-eddy-simulation hindcasts Title The role of shallow convection in the momentum budget of the trades from large-eddy-simulation hindcasts Author Helfer, K.C. (TU Delft Atmospheric Remote Sensing) Nuijens, Louise (TU Delft Atmospheric Remote Sensing) Vijay Dixit, V. (TU Delft Atmospheric Remote Sensing) Date 2021 Abstract 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. Subject convective momentum transportK-diffusionlarge-eddy simulationmomentum budgetshallow convectiontrade winds To reference this document use: http://resolver.tudelft.nl/uuid:15be65ab-4fdc-4adf-b565-dab6842dd284 DOI https://doi.org/10.1002/qj.4035 ISSN 0035-9009 Source Royal Meteorological Society. Quarterly Journal (online), 147 (737), 2490-2505 Part of collection Institutional Repository Document type journal article Rights © 2021 K.C. Helfer, Louise Nuijens, V. Vijay Dixit Files PDF qj.4035_1_.pdf 23.6 MB Close viewer /islandora/object/uuid:15be65ab-4fdc-4adf-b565-dab6842dd284/datastream/OBJ/view