Eddy-fluxes and spectra in the GATE sub-cloud layer

Abstract

An investigation of the structure of the GATE sub-cloud layer carried out using turbulence sensors attached to a tethered balloon cable is described. The observations were made at one or two levels up to 400m near the centre of the C-scale ship array during the final phase of the experiment. The data were used to derive the vertical eddy fluxes of momentum, sensible and latent heat and the spectra and cospectra of the turbulence parameters, all over one-hour long periods. Surface fluxes were estimated using the bulk aerodynamic formulae. The results were classified according to the convective activity prevailing during each one-hour period. Marked changes in the vertical flux profiles with increasing convective activity are demonstrated. Spectra and cospectra in conditions of suppressed convection (small cumulus clouds only, no showers) show the increasing importance of larger scale mixing processes as heights increase to about 300m, but above this (near the top of the mixed layer) processes on a smaller scale are found to predominate. On these latter scales there is an upward flux of momentum which is a consequence of rather shallow vertical mixing in a region where wind speed decreases with height: in more disturbed conditions deeper convection, coupled with the reversed vertical gradient of wind, causes an upward momentum transfer on larger scales.