The role of convection in the momentum budget of ICON-LEM hindcasts over the North Atlantic

Motivated by the uncertain role of convective momentum transport from low clouds in setting patterns of wind in the trades, we discuss the impact of shallow convection on boundary-layer winds and its role in the overall momentum budget in the trades from large-domain large-eddy simulations. To this end, we analyse ICON-LEM hindcast simulations over the (sub)tropical North Atlantic during the NARVAL1 and NARVAL2 flight campaigns.

We describe that the character of the momentum flux profile differs significantly in regimes of shallow and deep convection and thus its influence on cloud-layer and near-surface winds. In particular, we establish that the momentum transport tendency is of similar importance as other terms in the momentum budget, and though the shape of the profile is remarkably insensitive to the horizontal resolution of the simulation, the relative role of subgrid and resolved fluxes changes with resolution. Furthermore, we find that counter-gradient transport occurs even in the absence of organisation, namely in the lower cloud layer, where cloudy updrafts carry slow momentum air upwards, which locally accelerates winds and may play a role at maintaining the cloud-base wind maximum.