J.-L. Redelsperger, F. Guichard and S. Mondon
Centre National de Recherches Météorologiques/GAME (CNRS & Météo-France), Toulouse, France
Journal of Climate, 2000, vol. 13, pp. 402–421.
Abstract: The paper investigates the enhancement of surface fluxes
by atmospheric mesoscale motions. We 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 (GCM),
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 COARE (Coupled Ocean-Atmosphere Response
Experiment) data, we propose a new value of the proportionality coefficient
which 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.