Initial soil moisture as a predictor of subsequent summer severe weather in the cropped grassland of the Canadian Prairie Provinces

Abstract

Soil moisture, along with the type and stage of the vegetation, influences the thermodynamic structure of the atmosphere by regulating heat and moisture fluxes to the planetary boundary layer. This study examined whether modeled areal-average root-zone soil moisture (RzSm) in the "wet" and in the "dry" regions of the cropped grassland of the Canadian Prairie Provinces had predictive value in determining whether these areas would subsequently have above or below average number-occurences and event-days of summer severe convective weather (i.e., tornadoes, large hail, heavy rains and/or damaging winds). RzSm, simulated by the Prairie Agro-climate Model, for the 1997 to 2003 growing-seasons was analyzed three times per season. Dry areas, with RzSm <_ 50% of available water holding capacity (AWHC), and wet areas, with RzSm > 50% of AWHC, were delineated post-snowmelt, on June 15th, and on July 15th. The areal-average RzSm levels in the *dry'and in the "wet" areas were calculated, and plotted against the relative number-of-occurrences and number-of-event-days which were recorded during the remainder of the growing season for each type of summer severe convective weather. In each case; the best-fit linear regression line, and the variance that it explained (r2 value) were computed. The hypothesis that the slope of each regression line was significantly different than zero was then tested. A relationship with r2 near or greater than 0.25, and with a regression line slope that was significantly different than zero, was selected as one which could have potential value in the climatological forecasting of summer severe convective weather. For most of the severe weather types, the relative number-of-occurrences and the relative number-of-event-days, which were recorded subsequent to the three dates on which the areal-average RzSm was determined, were greater over the "wet" areas than over the "dry" areas. This thesis represent an advancement in the development of our understanding of the linkage between RzSm and severe weather associated with moist deep convection in the cropped grassland of the Canadian Prairies. It demonstrated that modeling RzSm may improve climatological forecasts of severe convective weather.