Orographic precipitation enhancement by boundary-layer turbulence: a vertically pointing airborne cloud radar view

The University of Wyoming King Air, with multi-antenna 3 mm Doppler radar (the Wyoming Cloud Radar, or WCR), is used to examine the vertical structure of orographic precipitation. The key question regards how boundary-layer turbulence affects orographic precipitation growth in cold clouds. Medina and Houze (2005, Mon. Wea. Rev.) speculate that BL turbulence is important in snow growth, mainly though riming in turbulent eddies whose updraft speed far exceeds the average ascent rate over the terrain. Flight-level cloud microphysics data and WCR data were collected in flights across the Snowy range in Wyoming. The WCR Doppler velocity transects across the mountain clearly indicate intense turbulence in the lowest ~1000 m above the ground on several flight days, especially on the windier days. An example is shown in Fig. 1. The flight track is roughly along the wind. Note the specks of red and blue in the top panel, just west of the mountain top. These indicate vigorous vertical motions, down & up respectively. Some updrafts exceed the blue upper range of 3 ms-1 (see key above the top panel). In such updrafts snow can be generated rapidly, and in fact the radar reflectivity (lower panel of Fig. 1) appears to be increasing from left to right just above the upwind slope of the Snowy Range. This natural growth process may be enhanced by ice nucleation along rimed surfaces on the ground, such as trees, or by snow on the ground lofted by turbulent wind gusts. This work will address both the significance of surface-induced nucleation and BL turbulence