Structure and Heat, Moisture and Momentum Budgets of a Convective Mixed Layer during AMTEX'75

[摘要] Average structure and large-scale heat, moisture and momentum budgets of a convective mixed layer during cold air outbreaks are examined. The atmosphere is divided into a mixed layer, an inversion layer and a layer above which is not affected by the air-sea interaction processes. Potential temperature and winds are generally uniform in the mixed layer and have large increases in the inversion layer. Mixing ratio of the water vapor slightly decreases with height in the mixed layer and falls rapidly in the inversion layer.There exists divergence in the mixed layer and convergence above. Large-scale downward motions dominate the whole layer with a maximum in the inversion layer. Large apparent heat and moisture sources are found in the mixed layer. Net heat sinks and large moisture sources exist in the inversion layer mainly due to evaporation. There are large momentum sinks in the subcloud layer, but small sources and sinks in the cloud layer. Large downward momentum fluxes due to subgrid-scale eddies exist in the subcloud layer but small upward fluxes exist in the cloud layer.A diagnostic cloud model is applied to the heat and moisture budgets to determine spectral distributions of cloud mass fluxes. Sallower clouds have larger mass fluxes than deeper clouds. Large net heat sources are resulted from condensation heating in the lower cloud layer, but net cooling occurs in the inversion layer due to evaporation cooling effects. Vertical flux convergence of the water vapor due to clouds is important for the moisture balance as well as condensation and evaporation effects.Difference in momentum between inside and outside clouds is evaluated by combining the results of budget and cloud model computations. Momentum inside clouds is very close to that in the environment indicating that the clouds over the AMTEX region quickly adjust their momentum to that in the environment.Vertical eddy fluxes of heat, moisture and momentum obtained by the budget computations are compared with those measured by the NCAR Electra. The fluxes computed by the budgets agree quite well with those directly measured by the aircraft.

[学科分类] 大气科学