Momentum transport in organized precipitating shallow convection

Speaker: Alessandro Savazzi, TU Delft

Accurate wind prediction depends crucially on the representation of atmospheric flows that range from small-scale turbulence to coherent convective structures and meso-scales circulations in the presence of organized precipitating convection. Little is known on how parameterizations should be changing as grid meshes are refined nor how they should be behaving in cases of organized shallow convection. With the aid of multi-day large-eddy simulation hindcasts of trade-wind convection on a domain of 150 x 150 km2 this study analyses how flows of different scales contribute to momentum transport and modulate the trade-winds under a realistically varying large-scale flow. The Dutch Atmospheric Large Eddy Simulation (DALES), using a horizontal grid mesh of 100 m, is forced with output from the mesoscale weather model HARMONIE-AROME (using a 2.5 km grid mesh). Between February 1st and Feb 10th during the EUREC4A campaign east of Barbados, the simulation subjectively captures different cloud patterns observed, including sugar, gravel, and flowers. The momentum flux has sharp horizontal gradients, with fluxes even changing sign, throughout the domain. At 200 m, turbulence dominates the momentum flux even in the presence of cold pools, while at upper levels scales O (1-20 km) become important. The momentum flux carried by the mesoscales increases also with the degree of organization, where scales larger than 2 km carry up to 50% of the total flux in the cloud layer.  The simulations reveal the pronounced diurnal cycle in zonal and meridional wind and suggest that more organized convection helps set the diurnality in wind that is otherwise driven primarily by the imposed dynamics. More clustered cloud fields are frequent in the morning, have larger tendencies near the surface, and carry nonzero momentum flux through higher levels. Our results suggest the need to account for spatial distribution to improve scale adaptive parameterizations of the momentum flux.

Bio:
I am a PhD candidate at TU Delft under the supervision of Louise Nuijens since November 2021. I work on the CMTRACE project founded by the Dutch Research Council (NWO) as part of the VIDI programme.
Before my PhD I visited ECMWF for 9 months where I analyzed trade wind biases in IFS forecast and reanalysis.
The focus of my PhD is on identifying mechanisms underlying Convective Momentum Transport (CMT), in particular with respect to the role of CMT in the momentum budget.
The EUREC4A field campaign is largely exploited in my work to emphasize cloud organization and variability of winds in realistic settings. I try to combine unique measurements with fine-scale simulations and regional weather models.