SPATIAL AND ELEVATIONAL VARIATIONS OF SUMMER RAINFALL IN THE SOUTHWESTERN UNITED-STATES

This study examines the spatial variability of mean monthly summer rai nfall in the southwestern United States, with special attention given to the effect of elevation. Rain gauge data from a consistent 60-yr pe riod show that mean rainfall increases linearly with elevation within a local area. A simple model( rain = normalized rainfall as a function of latitude and longitude + elevation coefficient X elevation) explai ns a large part of the spatial variability of mean rainfall. The rainf all model (the MSWR model) and digital elevation data were used to pro duce a 1 degrees X 1 degrees gridded rainfall climatology for July, Au gust, and September. Regional rainfall estimated with this model is 9. 3% higher than an estimate based on arithmetic averaging of gauge data over 2 degrees x 2 degrees areas. For individual 2 degrees X 2 degree s cells, the difference between model rainfall and the arithmetic mean of gauge rainfall ranged from -250% to +41%. The MSWR model was used to remove orographic effects from regional rainfall fields. When rainf all is normalized to sea level, two rainfall maximums emerge: one in s outh-central Arizona associated with the Mexican monsoon maximum and o ne in southeastern New Mexico associated with the Gulf of Mexico. Detr ended block kriging (using the MSWR model as an estimate of the long-t erm trend) and monthly rain gauge data were used to produce unbiased a real rainfall estimates that were compared to 1 degrees X 1 degrees sa tellite-based rainfall estimates. On a month-by-month basis, there wer e large differences between the two estimates, although the comparison improved after temporal averaging.