INVESTIGATION OF UPPER-AIR CONDITIONS OCCURRING WITH HEAVY SUMMER RAIN IN UTAH

Proximity soundings and upper-air fields are analyzed for 25-44 heavy summer rain events in Utah, using the period 1950-1993. Means and stan dard deviations for 44 and 62 variables are calculated. The emphasis i s placed on sample statistics and event/climate differences. A goal of the study is to determine if large-scale, non-terrain effects contrib ute, on average, to the occurrence of heavy summer rain. Results indic ate that the heavy rain event environment tends to be cooler, moister, and more stable than normal in lower portions of the troposphere. Bas ed on examination of composite and individual height maps as well as t he statistical results it is concluded that the air mass has been made more favorable for heavy rainfall through identifiable circulation ch anges. The low to middle level tropospheric flow is backed about 25-35 degrees from normal and lift is enhanced through significantly increa sed low level convergence and upper level divergence. These indication s are consistent with the finding of upstream troughing on the composi te maps. It also appears that environmental conditions for heavy summe r rain are different in some respects from those for lighter events an d those associated with convective downbursts, particularly with respe ct to amount and depth of moisture inflow and degree of low-level conv ergence/upper level divergence. (C) 1998 Royal Meteorological Society.