Impact of soil moisture on boundary-layer cloud development

Michael Ek


We study the daytime land-atmosphere interaction using an atmospheric boundary- layer (ABL) model coupled with a land-surface model (LSM) using observations taken on 31 May 1978 at Cabauw, Netherlands. In a previous study it was found that in coupled model simulations using a simple LSM did not accurately represent surface fluxes and coupled atmospheric boundary-layer development; using a more sophisticated LSM in the study here allows the land-atmosphere system the freedom to respond interactively with the ABL. Results indicate that in coupled land-atmosphere simulations, realistic daytime surface fluxes and atmospheric profiles are produced, even in the presence of ABL clouds.

Subsequently, the role of soil moisture in the development of ABL clouds is explored via model simulations, analytical development, and with observational (Cabauw) data in terms of a relative humidity (RH) tendency equation at the ABL top which involves a number of land-atmosphere interactions. It is shown that the effect of soil moisture is to increase ABL-top RH tendency and thus potential for ABL cloud formation (confirming intuition), but only if the stability above the ABL is not too weak (and given sufficient initial RH in the ABL and air above the ABL that is not too dry). Alternately, for weak stability above the ABL, drier soils yield a greater ABL-top RH tendency and thus potential for ABL cloud formation (somewhat counter-intuitive), where in this case soil moisture acts to limit the increase of ABL-top RH, and that the largest values of ABL-top RH tendency are achieved not over moist soils, but rather over dry soils.