It is well known that subtropical shallow convection transports heat and water vapour upwards from surface. It is less clear if it also transports horizontal momentum upwards to significantly affect the trade winds in which it is embedded. We utilize unique multi-day large eddy simulations run over the tropical Atlantic with ICON-LEM to investigate the character of convective momentum transport (CMT) by shallow convection.
For a typical trade wind profile during boreal winter, the convection acts like an apparent friction to decelerate the north-easterlies. This effect is maximum below the cloud base while in the cloud layer, the friction is minimum but is distributed over a relatively deeper layer. In the cloud layer, the zonal component of the momentum flux is counter-gradient and penetrates deeper than reported in traditional shallow cumulus LES cases. The transport through conditionally sampled convective updrafts and downdrafts explains the weak friction effect but not the counter-gradient flux near cloud tops.
The analysis of the momentum flux budget reveals that, in the cloud layer, the counter-gradient flux is driven by convectively triggered non-hydrostatic pressure-gradients and horizontal circulations surrounding the clouds. A model set-up with large domain size and realistic boundary conditions is necessary to resolve these effects.