Interactions between dry-air entrainment, surface evaporation and convective boundary-layer development

The influence of dry-air entrainment on Surface heat fluxes and the convective boundary-layer (CBL) properties is studied for vegetated land surfaces, using a mixed-layer CBL model coupled to the Penman-Monteith equation under a wide range of conditions. In order to address the complex behaviour of the system, the feedback mechanisms involved were put into a mathematical framework. Simple expressions for the evaporative fraction and the Priestley-Taylor parameter were derived, based on the concept of equilibrium evaporation. Dry-air entrainment enhances the surface evaporation under all conditions, but the sensitivity of the evaporation rate to the moisture content of the free troposphere falls as temperature rises. Due to the evaporation enhancement, shallower CBLs develop beneath dry atmospheres. In all cases, dry-air entrainment reduces the relative humidity at the land surface and at the top of the CBL. However, because of dry-air entrainment-induced land-atmosphere feedback mechanisms, relative humidity at the top of the CBL responds nonlinearly to temperature rise; it decreases as temperature rises beneath a moist free troposphere, whereas it increases beneath a dry free troposphere. Finally, it was found that in certain conditions the evolution of the surface fluxes, relative humidity and CBL height can be as sensitive to the free tropospheric moisture conditions as to the land-surface properties. Therefore, studies of the land surface and of convective clouds have to take into account the influence of dry-air entrainment through land-atmosphere feedback mechanisms. Copyright (C) 2009 Royal Meteorological Society