Vertical motions forced by small-scale terrain and cloud microphysical response in extratropical precipitation systems

Airborne vertically profiling Doppler radar data and output from a similar to 1-km-grid-resolution numerical simulation are used to examine how relatively small-scale terrain ridges (similar to 10-25 km apart and similar to 0.5-1.0 km above the surrounding valleys) impact cross-mountain flow, cloud processes, and surface precipitation in deep stratiform precipitation systems. The radar data were collected along fixed flight tracks aligned with the wind, about 100 km long between the clouds: the Idaho Experiment (SNOWIE).

Data from repeat flight legs are composited in order to suppress transient features and retain the effect of the underlying terrain. Simulations closely match observed series of terrain-driven deep gravity waves, although the simulated wave amplitude is slightly exaggerated. The deep waves produce pockets of super-cooled liquid water in the otherwise ice-dominated clouds (confirmed by flight-level observations and the model) and distort radar-derived hydrometeor trajectories.