Atmospheric rivers can provide as much as 50% of the total annual rainfall to the U.S. West Coast via orographic precipitation. Dust is thought to enhance orographic precipitation via the “seeder-feeder”; mechanism, in which ice particles from a high cloud fall through a lower orographic cloud, seeding precipitation in the low cloud. Using the Weather Research and Forecasting model, we vary dust concentrations in simulations of two-dimensional flow over a mountain. This idealized framework allows us to test the sensitivity of the precipitation-dust response to a variety of different dust concentrations and initial conditions. The model is run using an ensemble of 60 radiosondes collected from Bodega Bay, CA in 2017-2018, clustered based on their vertical moisture profile into “deep moist”, “shallow moist”, and “subsaturated” clusters. The principle impact on precipitation is to increase the ratio of precipitation falling as snow. This produces a “spillover” effect, decreasing precipitation upwind of the peak and increasing precipitation downwind of the peak. The largest impacts on the snow/rain ratio occur at the end of the event, during cold front passage. The ensemble mean does not produce a significant seeder-feeder response, however in individual cases with favorable initial conditions there is a significant increase in precipitation throughout the domain due to dust effects on the seeder-feeder mechanism. These findings afford an opportunity to build a more comprehensive understanding for the conditions under which dust aerosol can have a significant impact on precipitation during atmospheric rivers, with implications for future developments in forecasting.

The Salton basin is an arid, sub-sea level basin located in southeastern California. Mountain ranges border the basin directly to the west and east, to the north extends a narrow valley that terminates with a transverse mountain range, and to the south spans heavily irrigated, agricultural lands that gradually rise in surface elevation. Within the basin lies the Salton Sea, and surrounding the Sea is diverse terrain that includes dry playa, rocky and vegetated surfaces, and highly emissive alluvial fans and dry washes, among others. Although large dust outbreaks due to orographically forced high wind speed events are frequent in the area, measurements from a new AERONET site stationed within the basin shows that dust is a standard component of the region's atmosphere, even on days with otherwise low wind speeds. We find a diurnal cycle of background dustiness that peaks in the early afternoon. An analysis of boundary layer structure using a Vaisala CL51 suggests that deepening of the boundary layer due to dry convection results in downward mixing of momentum, increased surface wind speeds, and an accumulation of dust in the mixed layer. Further analysis of the CL51 observations at nighttime suggest that high AOD concentrations persist in the shallow nocturnal boundary layer, and can become elevated throughout the night due to westerly katabatic flows. An analysis of surface meteorological station data suggests possible pathways of suspended dust due to mountain and valley flows within the basin.