Plume Fluxes in Clear and Cloudy Convective Boundary Layers

Kurzfassung

From results of large-eddy simulations of the clear convective boundary layer and of a stratus-topped boundary layer, mean properties of “plumes” that consist of “updrafts” and “downdrafts” are determined. The plumes are defined locally by the sign of the vertical velocity or of moisture fluctuation or by a combination of both. As a further alternative, updraft and downdrafts in which the vertical velocity magnitude exceeds certain threshold values are considered. The first two variants divide the motion field into two streams, whereas in the other variants “environmental” air forms a separate stream. The computed mean properties are in general agreement with existing measurements. From the results we compute mean vertical fluxes assuming “top-hat profiles” and compare these with the actual fluxes. It is shown that the most uniform flux approximation is obtained if the plume structure is classified in terms of vertical velocity w. For such “w plumes,” the top-hat profiles approximate about 60% of the actual fluxes if updrafts and downdrafts are distinguished with zero threshold values just according to the sign of the vertical velocity. A higher percentage is obtained with nonzero threshold values.