An LES study on the CBLs over heterogeneous land surfaces

The convective boundary layer (CBL), which plays a significant role in the initiation and development of moist convection, has been described by mixed-layer similarity. The similarity has been constructed based on the assumption of local homogeneity even though land surfaces are continuously heterogeneous. Thus, for a realistic description of CBLs a mixed-layer similarity theory applicable over heterogeneous surfaces needs to be developed. In this study, a recently-developed large eddy simulation (LES) is run with prescribed sinusoidal surface heat flux variations. Based on the mean surface heat flux variation, and the amplitude and wavelength of the surface heat flux variability, the simulated CBLs can be divided into cases with well-organized and less-organized circulations. The criteria for separation into these two categories are found by understanding the mechanisms governing the growth and decay of surface-heterogeneity-induced mesoscale flows. The CBLs having well-organized circulations have mesoscale forcing strong enough to produce mean horizontal winds that within a limited time overcome the turbulent flow and penetrate into adjacent areas with contrasting surface flux. These CBLs, however, have a self-decay mechanism that stabilizes the CBL with cold advection at lower levels and warm advection at upper levels that is associated with the well-developed circulation. In the CBLs with less-organized circulations, on the contrary, the mesoscale forcing is not strong enough to completely overcome the turbulent forcing and the surface-heterogeneity-induced mesoscale flows are broken up by turbulence. The applicability of mixed-layer similarity to CBLs over heterogeneous surfaces is discussed in light of these two mesoscale flow regimes.