The Fair Weather Boundary Layer in GATE: The Relationship of Subcloud Fluxes and Structure to the Distribution and Enhancement of Cumulus Clouds

Abstract

The structure of the convective atmospheric boundary layer and the characteristics of the associated turbulent mixing processes in undisturbed conditions over the tropical ocean are investigated using data collected during the GARP Atlantic Tropical Experiment (GATE). The data were obtained by a number of aircraft equipped with turbulence measuring instrumentation. The fluxes of momentum, sensible and latent heat throughout the subcloud layer are presented for four cases considered in detail. It is shown that the sensible and latent heat fluxes at the top of the mixed layer (and therefore the distribution of heating and moistening in the boundary layer) are strongly affected by the presence or absence of cumulus convection while the virtual heat flux remains unaffected. Features of this cloud-subcloud interaction are discussed in the light of Betts (1976) coupled cloud-subcloud layer model. An examination of the spectra (and cospectra) of subcloud-layer variables shows that with the exception of vertical velocity, the spectra are generally dominated by low-frequency fluctuations. This behavior is attributed to the effects of entrainment which may produce relatively large, long-lived excursions from the mixed-layer average in the weakly mixed GATE boundary layer and to the existence of mesoscale (∼10 km) variability. Wherever possible comparisons are drawn with previous measurements and corresponding situations over land, where mixing processes are usually much more energetic. Aircraft measurements of the sensible and latent heat fluxes are compared with those derived from tethered balloon probes and budget calculations which were employed concurrently during GATE. Aircraft and tethered balloon fluxes showed good agreement; however, the budget results differ, probably due to a different sampling strategy.