On the structure of the trade wind moist layer

Abstract

The trade-wind moist layer is itself subdivided in the vertical into two superposed layers of different convective regime, because of the occurrence of water vapor condensation at about 600-700 m above the tropical oceans. Below the condensation level, in the so-called "subcloud" layer, unsaturated convective turbulence predominates. Eddies 50-150 m across are characteristic and recent studies suggest that larger scales of motion with dimensions 10-50 km (size of cloud groups) are also significant. No evidence of cloud-scale motions below cloud base have been found, except in precipitating downdrafts. Above the condensation level, cumulus convection is the major transport process; small-scale turbulence is confined to the neighborhood of clouds, which form in bunches separated by wider, weakly subsiding clear areas. The lower four-fifths of the subcloud layer is well-stirred and has been christened the "mixed layer". The lapse rate is close to dry adiabatic and the moisture content of the air is nearly constant with height, decreasing only 3-6% from 15m above the sea to its top at about 550 m. The thickness of the mixed layer commonly shows variations of 20% in space and as much as 100% in time, with extreme day-to-day variations of about 300-700 m. Recent evidence suggests that its space variations on a 10-50 km scale are associated with the bunchy grouping of trade cumuli. It appears that the clouds are grouped in places where the mixed layer is thickened, reaching close to the condensation level of the air within it. The trade-wind mixed layer thus plays the crucial role of a "valve" in the earliest phases of the atmosphere's energy supply. Its structure regulates both the input from the sea below through evaporation and the output aloft through cumulus formation. The present study has therefore been divided into three parts. Part I examines the structure of air below cloud; Part II is concerned with the features of the cloud layer and cumulus convection; and Part III attempts to construct a physical model of the operation of the moist layer as a whole.