17.3 Snow growth and transport patterns in orographic storms as estimated from airborne dual-Doppler radar data in along-wind transects

Friday, 22 August 2014: 8:30 AM
Kon Tiki Ballroom (Catamaran Resort Hotel)
Bart Geerts, Univ. of Wyoming, Laramie, WY; and Y. Yang

Airborne vertical-plane dual-Doppler cloud radar data, collected on wind-parallel flight legs over the Medicine Bow Range in Wyoming in 15 winter storms, are used to analyze orographic snow growth, transport, and deposition. The sampled clouds are all mixed-phase, quite shallow (typical depth 1-3 km), have cloud base heights below mountain crest level, and generally produce snowfall over the mountain only. With one radar antenna pointing to nadir, and one 30° forward of nadir, the 2D hydrometeor motion in the vertical along-track plane below flight level can be synthesized. This yields cross-mountain hydrometeor streamlines, including the streamline that terminates at the mountain crest and thus separates hydrometeors falling on the windward side from those falling on the lee side. The cross-mountain flow and radar reflectivity fields are averaged over several transects during a single flight, to obtain flow patterns and precipitation distributions that characterize the sampled storms. Two flow patterns over the mountain are distinguished: shallow convection, with relatively undisturbed cross-mountain flow; and vertically propagating mountain waves, often with strong, plunging flow in the lee. Precipitation falls mostly in the lee in the former flow pattern, and mostly upwind of the crest in the latter. The sensitivity of snowfall distribution to several ambient and cloud parameters is considered, including wind speed, Froude number, droplet number concentration, and flight-level particle fall speed. Precipitation tends to spill over more on the lee side with the onset of stronger flow and higher Froude number. Spillover tends to be enhanced also when droplet concentration is high (suppressing snow growth by riming) and particle fall speed low, consistent with expectation, although the best predictor of transport across the mountain crest is wind speed
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