The Momentum Budget during EUREC4A

What sets the low-level wind structure observed during EUREC4A? Motivated by this question, we derive the large-scale wind forcing from HALO circle dropsondes aided by ERA5 reanalysis data. We use these to derive the frictional force and constrain the profile of turbulent stresses making use of wind lidar measurements aboard the RV Meteor and in-situ momentum fluxes from autonomous measurement platforms. In particular, we are interested in the vertical extent, character and variability of the frictional layer. The measurements are compared with early studies of the momentum budget (e.g., ATEX and BOMEX) and cast in light of the momentum budget derived from ICON-LES hindcasts spanning the North-Atlantic during NARVAL. These reveal non-negligible zonal momentum flux within the cloud layer, but the portion of the flux carried by turbulent eddies with scales larger than 150 m hardly introduces a so-called "cumulus friction", especially not near the zonal wind jet, just above cloud base. Instead, these turbulent eddies - which we associate with convection and mesoscale circulations - are responsible for a region with maximum friction that is found below cloud base. This suggests that dry and moist convection help sustain a zonal wind jet. Near the surface, convection helps to reduce friction - a finding which the EUREC4A observations may confirm.