Louise Nuijens - Invited speaker

Despite playing a key role in the atmospheric circulation, the representation of momentum transport by moist convection (cumulus clouds) has been largely overlooked by the model development community over the past decade, at least compared with diabatic and radiative effects of clouds. In particular, how shallow convection may influence surface and boundary layer winds is not thoroughly investigated. In this talk, we discuss the role of convective momentum transport (CMT) in relation to wind and circulation biases in the ECMWF IFS model and its reanalysis products, and we use high-frequency wind profiling measurements to inform our understanding of convectively driven wind variability.
Our focus is on the trades, where at short leadtimes the model produces stronger than observed easterly flow near the surface and thus a weaker meridional (Hadley) circulation. By turning off the process of CMT by shallow convection in the model, and zooming in on a few grid points close to Barbados, we find that the modeled biases in surface winds are related to an overestimation of easterly flow near cloud base, which are felt at the surface via turbulent and convective flows. The cause of this bias is investigated by studying the modeled wind tendencies for selected periods where we have observations of boundary layer winds. The observed winds are sampled on convection to test the idea that wind turning biases are caused by erroneous convectively driven variability or additionally by turbulent mixing under clear skies.
13 Dec 2019

ID: 70024864