The inluence of dry air entrainment on a growing convective boundary layer through land surface feedbacks

The influence of dry air entrainment on surface heat fluxes and the convective boundary layer (CBL) properties is investigated. To address the influence on the surface fluxes, a simple expression for the Priestley-Taylor parameter is derived, based on the definition of equilibrium evaporation and the mixed layer equations. To assess the CBL modifications through the land-atmosphere feedbacks, a mixed layer CBL model coupled to the Penman-Monteith equation is used. The essential physics and feedbacks in the coupled land atmosphere system are retained in this model. It is shown that the moisture content of the free atmosphere largely influences the surface evaporation. The Priestley - Taylor parameter indicates that especially at low temperatures, entrainment of dry air can significantly enhance the evaporation and consequently reduce the sensible heat flux. Under these conditions, the CBL growth is suppressed significantly. Therefore, the free atmosphere moisture conditions can be considered as a sensitive parameter for the CBL height.