Boundary-layer turbulence and orographic precipitation growth in cold clouds: evidence from profiling airborne radar data

Airborne vertically-pointing Doppler radar data collected in 10 winter storms over a mountain range in Wyoming are used to examine the importance of boundary-layer (BL) turbulence for orographic precipitation growth. In all 10 cases, the cloud base temperature was below 0°C and bulk Froude number more than 1.0, implying little or no blocking of the flow by the mountain barrier. Seven of the 10 storms sampled were post-frontal, with weak static stability and relatively shallow cloud tops. Doppler vertical velocity transects depict a ~1 km deep turbulent layer draped over the terrain, sometimes clearly distinct from the stratified flow in the free troposphere aloft, where vertical motion is largely controlled by gravity wave dynamics. Spectral analysis of near-surface Doppler vertical velocity data in terrain-following coordinates reveals an inertial subrange with decreasing power with height towards the BL top. The composite of radar data profiles from the 10 flights is analyzed in frequency-by-altitude diagrams, with altitude expressed above ground level. These diagrams indicate a wide range of vertical velocities in the BL, and rapid snow growth within the BL as air rises through the cloud base, especially when BL turbulence is more intense. This snow growth is concentrated on the windward side of mountains, above the terrain – cloud base intersection. The dominant snow growth mechanism in the BL, i.e. by accretion or vapor deposition, cannot be established because of restrictions in aircraft flight level over complex terrain. Snow aggregation also may have contributed to the observed rapid increase in reflectivity in the BL along the windward slope. The snow growth mechanisms will be examined in a follow-up NSF-funded campaign (ASCII, AgI Seeding of Clouds Impact Investigation), to be conducted in early 2012 in the context of ongoing glaciogenic seeding operations over the target mountain range.