Q. Wang and B.A. Albrecht
Dept. of Meteorology
The Pennsylvania State University
University Park, PA 16802
814-863-1036 or 814-865-9500
Aircraft measurements made by tile NCAR Electra during the First ISCCP Regional Experiment (FIRE) are analyzed to study the updraft and downdraft properties in stratocumulus-topped boundary layers using a conditional sampling method. This study analyzes observations from two days of well-mixed boundary layers and two days of decoupled boundary layers.
The results show that the updrafts and downdrafts have the same geometric characteristics. They occupy nearly the same fractional area ranging from about 0.3 at the surface to about 0.5 below the cloud top. The average width of the events is about 0.2 of the boundary layer depth (z[exclamdown]) in the well-mixed cases and about 0.I5 z[exclamdown]; in the decoupled cases.
Updrafts and down drafts are found to play different roles in transporting heat
and moisture near the sea surface and close to the cloud top. Generally,
downdrafts have larger average perturbations of temperature and moisture and
therefore contribute more to the respective fluxes than updrafts in the upper
boundary layer and vise versa near the sea surface. However, the differences
in flux contribution between updrafts and downdrafts are much less significant
in the cloud layer than those close to the surface, which is especially true in
the decoupled cases. This indicates that the circulation driven by the
cloud-top radiative cooling may behave differently from that driven by surface
buoyancy. Using a top-hat flux parameterization the mass flux velocity [omega]
is found to be 0.4 of the convective velocity w*.