Conditional Sampling of Updrafts and Downdrafts in the Marine Atmospheric Boundary Layer

Abstract

The properties of updrafts and downdrafts in the lower third of the marine atmospheric boundary layer (MABL) over the central Pacific Ocean are investigated using a conditional sampling technique. When the drafts are classified according to their heat and moisture content, the properties of the major classes (moist updrafts and dry downdrafts) are in agreement with a parcel displacement model of vertical mixing. The minor class events appear to be the result of the reversal of motion of the major class events. Drafts that consume turbulent kinetic energy (TKE), although small in number in the lower third of the MABL, have spatial scales comparable with drafts that produce TKE. At the lowest level, the area occupied by positively buoyant downdraft exceeds the area occupied by negatively buoyant updrafts by a factor of 2. Updrafts and downdrafts produce a large fraction of the total fluxes of heat moisture and momentum, increasing from 75% at 0.07zi to 85% at 0.32zi. Of the draft contribution to the fluxes, 95% is due to the mean properties of the events and only 5% is due to the correlated fluctuations within the events. A convective man flux parameterization, based on the mean conditions within updraft and downdrafts, is obtained for the lower third of the mixed layer.