A SOIL-MOISTURE RAINFALL FEEDBACK MECHANISM 1 - THEORY AND OBSERVATIONS

This paper presents a hypothesis regarding the fundamental role of soi l moisture conditions in land-atmosphere interactions. We propose that wet soil moisture conditions over any large region should be associat ed with relatively large boundary layer moist static energy, which fav ors the occurrence of more rainfall. Since soil moisture conditions th emselves reflect past occurrence of rainfall, the proposed hypothesis implies a positive feedback mechanism between soil moisture and rainfa ll. This mechanism is based on considerations of the energy balance at the land-atmosphere boundary, in contrast to similar mechanisms that were proposed in the past and that were based on the concepts of water balance and precipitation recycling. The control of soil moisture on surface albedo and Bowen ratio is the fundamental basis of the propose d soil moisture-rainfall feedback mechanism. The water content in the upper soil layer affects these two important properties of the land su rface such that both variables decrease with any increase in the water content of the top soil layer. The direct effect of soil moisture on surface albedo implies that wet soil moisture conditions enhance net s olar radiation. The direct effect of soil moisture on Bowen ratio dict ates that wet soil moisture conditions would tend to enhance net terre strial radiation at the surface through cooling of surface temperature , reduction of upwards emissions of terrestrial radiation, and simulta neous increase in atmospheric water vapor content and downwards flux o f terrestrial radiation. Thus, under wet soil moisture conditions, bot h components of net radiation are enhanced, resulting in a larger tota l flux of heat from the surface into the boundary layer. This total fl ux represents the sum of the corresponding sensible and latent heat fl uxes. Simultaneously, cooling of surface temperature should be associa ted with a smaller sensible heat flux and a smaller depth of the bound ary layer. Whenever these processes occur over a large enough area, th e enhanced flux of heat from the surface into the smaller reservoir of boundary layer air should favor a relatively large magnitude of moist static energy per unit mass of the boundary layer air. The dynamics o f localized convective storms as well as the dynamics of large-scale a tmospheric circulations have been shown to be sensitive to the distrib ution of boundary layer moist static energy by several previous studie s. These theoretical concepts are tested using field observations from Kansas and explored further in a companion paper [Zheng and Eltahir, this issue] using a simple numerical model.