Modifications of the atmospheric moisture field as a result of cold-pool dynamics

Abstract:

This study investigates the origin of moisture that feeds deep convective clouds in the absence of large-scale forcing. Thereby the focus lies on the interplay between atmospheric moisture and deep convective clouds via cold-pool dynamics. Redistribution processes such as advection are contrasted with local moistening processes in a series of cloud-resolving modelling studies. More specifically the contribution of moisture advection, evaporation of rain and surface fluxes to the moisture budget over particular regions of the domain is investigated. This is done both for a continental case and an oceanic case although both cases show a very similar behaviour. The accumulation of moisture in confined regions of the sub-cloud layer that constitute preferred locations for future cloud development mainly results from the advection of moisture. The latter contributes to ~86 %, minor evaporation of precipitation contributes to ~4%, whereas surface moisture fluxes yield ~11% in the continental case. In the oceanic case advection contributes to ~125%, surface moisture fluxes to ~-32% and evaporation of precipitation to ~7%.

To further identify the origin of the advected moisture additional scalars marking moisture originating from the surface and from the evaporation of rain are introduced into the model. It is seen that the surface moisture and the evaporated rain water that were released within the last two hours only make 55% of the moisture accumulated in the moist patches in the land case, the rest stemming from older moisture. In the ocean case this share increases to 72%. The contribution of recently released moisture drops to 28% at cloud base in the continental case and to 56% in the ocean case. The contribution at cloud base is dominated by surface fluxes, the evaporation of rain is negligible.