The Role of Windward-Side Diabatic Heating in Sierra Nevada Spillover Precipitation

Abstract

his study focuses on the meso-α- and meso-?-scale manifestations of the latent-heat-induced reduction of windward-side blocking to two flood-producing precipitation events on the leeside of the Sierra Nevada. Two simulations were performed?one employing full microphysics [control (CTRL)] and a second in which the latent heating terms are turned off in the microphysics [no latent heating (NLH)]. The differences between the CTRL and NLH are consistent with upstream latent heating?the moist, divergent, and ascending flow dominates the leeside of the mountain range in the CTRL producing copious spillover precipitation while dry high-momentum/downslope-descending flow dominates the NLH simulation on the leeside. A comprehensive sequence of events for spillover precipitation is formulated as follows: 1) Ascent within the exit region of a polar jet streak develops in response to velocity divergence aloft. 2) This ascent phases with ascent from the windward-side flow to create a mesoscale region of heavy upslope precipitation. 3) The latent heat release from the upslope precipitation reduces the upstream static stability and blocking. 4) A mesoscale ridge in the thickness field builds in the upper troposphere and induces subgeostrophic flow in the jet?s exit region above the mountain range. 5) Adjustments to this ridge result in a cross-mountain midlevel jet that facilitates a river of midlevel moisture advected over to the leeside. 6) Stretching of moist isentropic surfaces in proximity to the plume of moisture fluxes causes destabilization on the leeside and formation of a leeside mesolow. 7) Boundary layer air accelerates into the leeside mesolow to form a mountain-parallel low-level flow.