Investigation of upper-air conditions occurring with heavy summer rain in Utah

Abstract

Proximity soundings and upper-air fields are analyzed for 25–44 heavy summer rain events in Utah, using the period 1950–1993. Means and standard deviations for 44 and 62 variables are calculated. The emphasis is placed on sample statistics and event/climate differences. A goal of the study is to determine if large-scale, non-terrain effects contribute, on average, to the occurrence of heavy summer rain. Results indicate that the heavy rain event environment tends to be cooler, moister, and more stable than normal in lower portions of the troposphere. Based on examination of composite and individual height maps as well as the statistical results it is concluded that the air mass has been made more favorable for heavy rainfall through identifiable circulation changes. The low to middle level tropospheric flow is backed about 25–35° from normal and lift is enhanced through significantly increased low level convergence and upper level divergence. These indications are consistent with the finding of upstream troughing on the composite maps. It also appears that environmental conditions for heavy summer rain are different in some respects from those for lighter events and those associated with convective downbursts, particularly with respect to amount and depth of moisture inflow and degree of low-level convergence/upper level divergence. © 1998 Royal Meteorological Society