An Experimental Study of Turbulence Structure in the Fair-Weather Trade Wind Boundary Layer

Abstract

Measurements of air velocity, temperature and humidity were made from an aircraft in the fair-weather trade wind boundary layer. On the day of the experiment, the region studied was characterized by north-south bands of cloud-free and moderately clouded areas roughly 40 km in width. Mean winds, turbulence quantities, and thermodynamic variables were measured in both a clear and a partly cloudy region. Production of turbulence energy in the subcloud layer of both regions was mainly from wind shear. In the cloud-free region, the turbulence energy and momentum flux budgets were measured. One of the most striking features was the large region (covering almost two-thirds of the depth of the mixed layer) of negative production of turbulence energy by wind shear. Notwithstanding this unusual feature, the terms of the turbulence energy budget agree quite well with a model developed by Lenschow. In the cloudy area, a layer of strong wind shear was observed near cloud base. This, coupled with corresponding minima in the turbulence quantities, suggests a weak coupling, on the turbulence scale, between the cloud and subcloud layers.