UC Santa Barbara

Two extreme snowfall events associated with extratropical cyclones, one interacting with the western and one with the central Himalaya, are simulated with the Weather Research and Forecasting (WRF) model over 8 days. One event in January 1999 was driven by a longwave trough over west Asia, with the cyclone becoming terrain-locked in the western Himalayan notch. Another event in March 2006 was driven by a trough further south and east, facilitating the passage of two successive cyclones along the entire Himalayan ridge, drawing moisture from warm tropical waters. These flow patterns are typical for extreme winter precipitation in the western and central Himalaya, respectively, but were amplified in these two cases. In the WRF simulations, snowfall is confined to the western Himalaya in the January simulation, while a near-continuous band of accumulated snowfall along the Himalayan ridge forms in the March simulation. Precipitation rate throughout both simulations is largely determined by cross-barrier moisture flux, which is generally greatest wherever the cyclonic winds converge on the mountains at each time. However, the March 2006 simulation evolves in an environment with greater convective instability upwind of and moisture transport toward the mountains than in the January 1999 event. Hence, greater precipitation rates and more solid snowbands are generated in the March than in the January simulation. However, due to the terrain-locking of the cyclone in the January event, individual locations receive more persistent snowfall, so that the greatest 8 day accumulations are similar between the two events, although these accumulations are more widespread in the March event.