Large-eddy simulations of surface influences on planetary boundary layer development in southwest Western Australia

Observations from near-simultaneous atmospheric soundings released over contrasting land surfaces in the southwest of Western Australia during December 2005 (austral summer) and August 2007 (late austral winter or early spring) have shown higher planetary boundary layer (PBL) heights over native vegetation as compared to agricultural land. The large-eddy simulation technique is used to investigate the drivers behind these observed differences in PBL, and sensitivity tests are carried out with modified soil moisture and vegetation cover. It is shown that the differences in PBL for the December case are mainly driven by the change in vegetation cover, while a soil moisture gradient also played a role for the August case. The mixing diagram approach is used to further quantify the relative contributions of surface and entrainment fluxes on the growth of the PBL and it is shown that, while dry-air entrainment plays an important role in PBL development, it is the higher surface Bowen ratio which drives the more vigorous PBL development over the native vegetation. It is also shown that the enhanced PBL development over the native vegetation leads to the preferential formation of shallow convective clouds for the August case. Copyright (c) 2012 Royal Meteorological Society