Vertical velocity structure of nonprecipitating continental boundary layer stratocumulus clouds

摘要

Continental boundary layer (BL) stratocumulus clouds affect the local weather by modulating the surface energy and moisture budgets and are also intimately tied to the diurnal cycle of the turbulence in the BL. Vertical velocity structure of these clouds is studied using data from the Atmospheric Radiation Measurement Program's Southern Great Plains observing facility located near Lamont, Oklahoma. Data from vertically pointing cloud radar from eleven cases of nonprecipitating BL stratocumulus clouds are used to obtain half-hourly values of in-cloud vertical velocity variance, skewness, updraft fraction, downdraft fraction, and mass flux. The variance showed a general decrease with height, while the skewness was weakly positive in the cloud layer and negative near cloud top on half-hourly time scales. Vertically coherent structures spanning through the entire cloud layer were responsible for ~40% of the observed vertical velocity variance, while horizontally (temporally) coherent structures lasting at least 20 s were responsible for ~77% of the vertical velocity variance. The half-hour periods were then classified on the basis of the surface buoyancy flux (B) as small B periods (B 60Wm~(-2)). Small B periods exhibited lower variance near cloud base compared to large B periods with the differences being reversed near cloud top. The skewness was positive in the cloud layer during large B periods and negative during small B periods, while the skewness was negative near cloud top in both scenarios. The small B periods had about ten times more updrafts greater than 1 m s~(-1) near cloud top compared to large B periods.