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. Houze and Medina and (2005) 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, not in a stable shear layer intersecting the terrain (Houze and Medina 2005), but rather in the boundary-layer, roughly in the lowest 1 km above the ground, especially on the windier days. This turbulence enhances the hydrometeor growth rate by riming, and thus more precipitation reaching the ground before the crest. Also, ice nucleation may occur along rimed surfaces on the ground, such as trees, or by snow on the ground lofted by turbulent wind gusts. The ice crystals mix readily in the BL, and thus the Bergeron process too may enhance the growth rate of snow. The BL turbulence may bring supercooled liquid water and near-surface ice crystals up to higher level, which may interact with overlaying shear layer. This work addresses the significance of BL turbulence, surface-induced ice nucleation, and the interaction between the turbulence and upper-level processes.