Storm dynamics orographic kinematics and naturally emitted aerosols conspire to create a natural cloud seeding environment over California

Abstract:

The precipitation from a US West Coast winter storm grows in a complex system. To attempt to quantify the effect of changing aerosol properties on precipitation from such storms, we have conducted a model sensitivity experiment using two case studies from February 2011 over Northern and Central California. The storm from each case was simulated using the Weather Research and Forecast model with Spectral Bin Microphysics which has been modified to allow for aerosol chemistry and surface area sensitive ice nucleation. Each storm contains a strong baroclinic zone which marks a discontinuity in upper-layer long range transported aerosol source. We will present evidence herein which suggests that the baroclinic zone discontinuity in aerosol source is common in West Coast winter storms and may have profound impacts on the growth of precipitation in orographic post-frontal clouds. To isolate the change in aerosol source from the meteorology, we have simulated the time period before and after a cold front transits our model domain in two configurations. In the first, the temporal evolution of aerosol chemistry, size, concentration and vertical location follows that observed by aircraft during an intensive field phase, during which elevated layers containing ice-nuclei active dust and biological aerosols were observed after a cold front. In the second configuration, the aerosol properties approximate those observed during the pre-frontal period and remain constant throughout the simulation. Simulated cloud and precipitation properties are validated using data from a continuous flow diffusion chamber, the Moderate Resolution Imaging Spectroradiometer, and cooperative raingauge stations. We find that introducing an elevated dust and biological aerosol layer immediately after cold front transit increases post-frontal hourly precipitation rates by 23 percent. Greater increases are found when near surface winds are westerly and temperatures are cold. The frequency of clouds with top pressure less than 400 hPa increases dramatically. Analyses of water phase segregation and cloud particle size distributions suggest that cloud microphysical processes other than ice nucleation respond to the proliferation of ice-nuclei active aerosol particles, and are primarily responsible for the precipitation rate increase.