Analysis of the pathways relating soil moisture and subsequent rainfall in Illinois

摘要

This study is a continuation of an earlier work [Findell and Eltahir, 1997] on the soil moisture-rainfall feedback using a data set of biweekly neutron probe measurements of soil moisture at up to 19 stations throughout Illinois. Analyses in this earlier work showed a positive correlation between initial soil saturation and subsequent rainfall from early June to mid-August. This correlation was more significant than the serial correlation within precipitation. suggesting the likelihood of a physical mechanism linking soil moisture to subsequent rainfall. This paper probes the nature of such a physical pathway linking soil moisture to subsequent rainfall. The pathway is divided into two stages: soil moisture and near-surface air, and nearsurface air and rainfall. An analysis of the connections between an average daily soil saturation for the whole state of Illinois with statewide average near-surface air conditions did not yield the anticipated positive correlation between soil moisture and moist static energy (MSE). It is not clear if this is due to limitations of the data or of the theory. Other factors, such as clouds, could potentially be masking the impacts of soil moisture on the energy of the nearsurface ari. There was evidence, however, that moisture availability at the surface has a very strong impact on the wet-bulb depression of near-surface air, particularly from mid-May to the end of August, showing good correspondence to the period of significant soil moisture-rainfall association. The final set of analyses performed used hourly boundary layer and rainfall data. A link between high MSE and high rainfall was noted during some summer months, and a link between low wet-bulb depression and high rainfall was evident for all of the months analyzed (April through September). These analyses suggest that the significant but weak correlation between soil moisture and rainfall during Illinois summers is at least partially due to soil moisture controls on the wet-bulb depression of near-surface air.