Does orographic snow result from glaciogenic seeding or surface interaction? A downlooking airborne cloud radar view.

Bart Geerts, University of Wyoming

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 and the impact of ground-based glaciogenic seeding. 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. We first report on a field campaign conducted over the Snowy range in Wyoming in early 2006. 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. 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 m/s (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. In early 2008 we will conduct similar flight legs over the mountain, this time over ground generators of AgI. This work will address both the significance of surface-induced nucleation and the efficacy of glaciogenic cloud seeding.

Fig. 1: Transect of WCR Doppler vertical velocity (top) and radar reflectivity (bottom) for a snowstorm over the Snowy Range in Wyoming on 18 Jan 2006. The transect is from west (left) to east, and the wind is from the west. The terrain can be seen as the bold black line in the lower image. The transect was collected between 215552-220402 UTC.