On the Relationship between Diurnal Mesoscale Circulations and Precipitation in a Mountain Valley

Abstract

Diurnal signatures of precipitation at mountain stations in Utah have been studied and precipitation events in the Salt Lake Valley compared with local wind circulations. Results of harmonic analyses of hourly precipitation data show small-amplitude diurnal cycles in winter and large-amplitude cycles in summer. Frequencies of diurnal precipitation events in summer were dependent on elevation, with maximum frequencies peaking around 1430 MST at elevations above 2100 m and nearly three hours later below 1500 m. High rainfall intensities exhibited the largest diurnal modulations with maximum amplitudes occurring in the higher terrain. These results were based on over 25 years of data and have important implications for flash flood forecasting and mesoscale modeling of precipitation in complex terrain. A detailed objective analysis of hourly resultant winds for a 45?75 km area with a 5 km grid showed the influence of mountain-valley winds on the diurnal variation of precipitation in the Salt Lake Valley. Low-level divergence had well defined diurnal signatures and displayed marked differences between summer and winter. Influences of these local circulations on precipitation were evident when the timing of wind convergence was compared to the relatively high frequencies of precipitation events at the Salt Lake City Airport in the early morning in winter and late afternoon in summer.