Momentum transport in organised shallow cumulus convection

Abstract

This study investigates momentum transport in shallow cumulus clouds as simulated with the Dutch Atmospheric Large Eddy Simulation (DALES) for a 150 x 150-km2 domain east of Barbados during nine days of EUREC4A. DALES is initialized and forced with the mesoscale weather model HARMONIE-AROME and subjectively reproduces observed cloud patterns. This study examines the evolution of momentum transport, which scales contribute to it, and how they modulate the trade-winds. Daily-mean momentum flux profiles show down-gradient zonal momentum transport in the sub-cloud layer, which turns counter-gradient in the cloud layer. The meridional momentum transport is non-trivial, with mostly down-gradient transport throughout the trade-wind layer except near the top of the surface layer and near cloud tops. Substantial spatial and temporal heterogeneity in momentum flux is observed with much stronger tendencies imposed in areas of organised convection. The study finds that while scales <2 km dominate momentum flux at 200 m in unorganized fields, sub-mesoscales O(2-20 km) carry up to 50% of the zonal momentum flux in the cloud layer in organised fields. For the meridional momentum flux, this fraction is even larger near the surface and in the sub-cloud layer. The scale-dependence of the momentum flux is not explained by changes in convective or boundary layer depth. Instead, the results suggest the importance of spatial heterogeneity, increasing horizontal length scales, and counter-gradient transport in the presence of organised convection.