WRF simulations of extreme snowfall events associated with extratropical cyclones over the Himalayas.

Abstract:

Two extreme snowfall events associated with extratropical cyclones interacting with the Himalaya Mountains are simulated with the Weather Research and Forecasting (WRF) model. These two events exemplify contrasting large-scale conditions typically observed in the winter and early spring. One event in January 1999 was characterized by a strong zonally oriented subtropical jet flowing directly over the mountains, with the cyclone becoming terrain-locked in the western-Himalayan “notch”. Another event in March 2006 featured a deep longwave trough on the jet, facilitating cyclogenesis further south, with two successive cyclones passing along the entire Himalayan ridge. These features of the events are captured by the simulations, so that 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. Satellite rainfall estimates (TRMM) and interpolated rain-gauge measurements (APHRODITE) largely validate these precipitation distributions, but highlight some possible shortcomings of the model when simulating such events. Snowfall rate throughout both simulations is largely determined by cross-barrier moisture flux, which is generally greatest wherever the cyclonic winds are convergent against the mountains at each time. However, the March 2006 simulation evolves in an environment with greater moisture transport towards 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 accumulations are similar (about 150 mm) between the two events, although these accumulations are much more widespread in the March event.