Assessment of Snow Atmosphere Forcing During Central Idaho Atmospheric Rivers

Abstract

Atmospheric Rivers (AR) are globally occuring weather features and the primary mechanism through which water vapor moves from the tropics and subtropics towards the mid-latitudes, doing so at rates comparable to the world's largest terrestrial rivers. AR that encounter mountains often cause extreme precipitation in the form of rain and snow, high winds, and flooding in many watersheds. They account for as much as 20-30% of cool season precipitation in the central Idaho Mountains. In the Northern Hemisphere, seasonal snow cover during Winter and Spring months is the most variable land surface component in space and time, and acts on the fluxes of energy and mass into the atmospheric system. To date, there has been little effort to understand how the land surface snow cover states prior to and during the arrival of ARs, acting on the surface mass and energy balance, impact the onset, extent, and evolution of precipitation accumulation during AR events. Using a high resolution coupled land-atmosphere model, I examine the sensitivity of the precipitation regime and atmospheric energy balance to an ensemble of realistic snowcover states during a March 1998 AR case study in central Idaho. The results indicate that snow cover forcing 1) causes reductions of shortwave radiation and sensible heating that are balanced by atmpospheric energy transport, 2) increases atmospheric static stability, and 3) modifies the distributions of total accumulated precipitation by as much as 10mm. v