A numerical study of daily transitions in the convective boundary layer

摘要

We examine daily (morning-afternoon) transitions in the atmospheric boundary layer based on large-eddy simulations. Under consideration are the effects of the stratification at the top of the mixed layer and of the wind shear. The results describe thetransitory behaviour of temperature and wind velocity, their second moments, the boundary-layer height Z_m (defined by the maximum of the potential temperature gradient) and its standard deviation a_m, the mixed-layer height zi (defined by the minimum of the potential temperature flux), entrainment velocity W_e, and the entrainment flux Hi. The entrainment flux and the entrainment velocity are found to lag slightly in time with respect to the surface temperature flux. The simulations imply that the atmospheric values of velocity variances, measured at various instants during the daytime, and normalized in terms of the actual convective scale w_*, are not expected to collapse to a single curve, but to produce a significant scatter of observational points. The measured values of the temperature variance, normalized in terms of the actual convective scale THETA_*, are expected to form a single curve in the mixed layer, and to exhibit a considerable scatter in the interfacial layer.