UC San Diego

Atmospheric rivers bring warm, moist, and windy conditions to the Sierra Nevada Mountains of California. With warm air and an ample supply of water vapor flowing over an existing snowpack, condensation may occur directly on the snow possibly releasing large amounts of latent heat into the snowpack. Heat supplied to the snowpack can warm the snow and, if isothermal at 0° C, may lead to substantial snowmelt and the possibility of downstream flooding.Full energy balance measurements at a single location are not readily available on the windward side of the Sierra where most large flooding events have occurred in the past. Therefore, an energy balance study site, known as CUES, located on Mammoth Mountain Ski Area, California was utilized. The bulk-aerodynamic method was used to estimate turbulent heat fluxes over snow and identify times of large downward latent heat flux. When snow was present at CUES, daily mean downward latent heat flux occurred 25% of the time with a daily mean value of 3.0 W m-2. An hourly composite plot of integrated water vapor transport when daily mean values of latent heat flux were greater than the 95th percentile show that the largest downward latent heat flux values occur when integrated water vapor transport is well below the typical atmospheric river threshold of 250 kg m-1 s-1. This signifies that downward latent heat flux is not a significant source of energy into the snowpack during atmospheric rivers at CUES.