Influence of dust on precipitation during landfalling atmospheric rivers
in an idealized framework
Abstract
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.