Using airborne vertical-plane dual-Doppler radar to analyze hydrometeor streamline patterns in orographic snow storms

Data from a multi-antenna Doppler cloud radar aboard the University of Wyoming King Air research aircraft, collected on along-wind flight legs over a mountain range in Wyoming, are used to analyze the streamlines of snow crystals as they grow and settle on the upwind side of the mountain crest. With one antenna pointing to nadir, and one 30° forward, the two-dimensional (2D) hydrometeor motion in the vertical along-track plane below flight level can be derived. The sampled orographic clouds are all relatively shallow, with cloud base temperature below freezing, and cloud base height below mountain crest level. The upstream flow below mountain top level is characterized by a bulk Froude number exceeding unity (Fr>1) on all 12 flight days, implying no blocking of the flow by the mountain barrier. The dual-Doppler hydrometeor streamlines, captured at a resolution of ~30 m, are analyzed in terms of wind speed, atmospheric stability, topographic characteristics of the barrier, upstream cloud condensation nuclei (CCN) concentration, and flight-level hydrometeor fall speed.