IMPLEMENTATION OF A SEMIPHYSICAL MODEL FOR EXAMINING SOLAR-RADIATION IN THE MIDWEST

A semiphysical solar radiation (SR) model is implemented to generate a new historical daily SR database for 53 locations in nine Midwestern and six adjacent states (available from the Midwestern Climate Center) . This model estimates daily SR using standard hourly meteorological o bservations (surface atmospheric pressure and dewpoint temperature; cl oud height and fractional sky cover by layer) as well as time of day, day of year, latitude/longitude, and the daily presence/absence of sno w cover as input. Because of an extensive effort to interpolate for mi ssing input (especially cloud) data, the daily SR dataset generated is 92% complete for all 53 stations for 1948-91, and 99% complete for th e 43 stations with continuous hourly meteorological observations that commenced during 1945-50 and extended through 1991. Consistent with pr evious work, the model validates favorably against sets of daily SR me asurements from (three) contrasting parts of the study region, and so its output is used here without adjustment. Analyses of the dataset do cument the basic Midwestern spatial and temporal SR variability since the mid-to late 1940s. The spatial variation of calendar monthly mean SR is dominated by a near-meridional (northeastward) decrease in fall and winter. This fundamental pattern is substantially perturbed from m idspring through Summer by subregional-to-mesoscale variability around and across the Great Lakes. Time series of individual monthly station mean SR values exhibit a pronounced, regionwide 1945-91 downtrend for August November. This decline is strongest (similar to 12%) and most statistically significant (>99% level) for October in a belt extending east-southeastward from west-central Wisconsin across southern Lake M ichigan and western Lake Erie to western Pennsylvania. The SR trends f or December-July are largely positive but of lesser spatial coherence, temporal consistency, and statistical Significance.