A Parameterization of Mesoscale Enhancement of Surface Fluxes for Large-Scale Models

The paper investigates the enhancement of surface fluxes by atmospheric mesoscale motions. The authors show that horizontal wind variabilities induced by these motions (i.e., gustiness) need to be considered in the parameterization of surface fluxes used in general circulation models (GCMs), as they always occur at subgrid scale. It is argued that there are two different sources of gustiness: deep convection and boundary layer free convection. The respective scales (time and length) and the convective patterns are very different for each of these sources. A general parameterization of the gustiness distinguishing these two effects is proposed. For boundary layer free convection, the gustiness is related to the free convection velocity. To establish this relationship, both observations and numerical simulations are used. Revisiting the Coupled Ocean–Atmosphere Response Experiment data, the authors propose a new value of the proportionality coefficient that links the free convection velocity and the gustiness. For deep convection, the dominant source of gustiness is the occurence of downdrafts and updrafts generated by convective cells. It is shown that these motions produce large enhancement of surface fluxes and should be parameterized in GCMs. Results indicate that the gustiness can be related either to the precipitation or to the updraft and downdraft mass fluxes.