An analysis of the relationship between soil moisture, rainfall, and boundary layer conditions, based on direct observations from Illinois

This study of the soil moisture-rainfall feedback uses a dataset beginning in 1981 of biweekly neutron probe measurements of soil moisture at up to 19 stations in Illinois to show that soil moisture can play a significant role in maintaining drought or flood conditions during the summer. Results of a linear correlation analysis between initial soil saturation and rainfall in the subsequent three weeks showed that a positive correlation between these two variables is present from early June through mid-August. This correlation is more significant than the serial correlation within precipitation time series, suggesting the likelihood of a physical mechanism linking soil moisture to subsequent rainfall. This result prompted further investigation into the nature of such a physical pathway linking soil moisture to subsequent rainfall. Near-surface hourly observations of pressure, P, temperature, T, wet-bulb temperature, T., and relative humidity,f, from 13 stations in and close to Illinois were used in these analyses. From each hourly set of direct observations of P computed. Time series of the spatial average of each of these quantities were then calculated by averaging data from the 13 stations at each hour. An analysis of the connections between an average soil saturation time series for the whole state of Illinois with these statewide average boundary layer conditions did not yield the anticipated positive correlation between soil moisture and moist static energy, as quantified by Tw, 0., or OE. It is not clear if this is due to limitations of the data or of the theory. There was evidence, however, that moisture availability (or lack thereof) at the surface has a very strong impact on the wet-bulb depression of near-surface air, particularly from mid-May through the end of August, showing good correspondence to the period of significant soil moisture-rainfall association. The final set of analyses performed included an investigation of hourly boundary layer and rainfall data. Data from the 82 hourly rainfall stations were averaged to compare statewide hourly rainfall to statewide hourly boundary layer conditions. A link between high MSE and high rainfall was noted for much of the range of MSE during the summer months, and a link between low Tdpr and high rainfall was evident for all of the months analyzed (April through September). These analyses, then, suggest that the significant but weak correlation between soil moisture and rainfall during Illinois summers is due not to soil moisture controls on the boundary layer entropy, …