Sources of Updrafts in Orographic Cloud Systems over the Payette Mountains of Idaho—Results from the SNOWIE Project

The overarching goal of SNOWIE, the Seeded and Natural Orographic Wintertime clouds—the Idaho Experiment, was to understand the natural dynamical and microphysical processes by which precipitation forms and evolves within orographic winter storms and to determine the physical processes by which cloud seeding with silver iodide impacts the amount and spatial distribution of snow falling across a river basin. One goal of SNOWIE was to document and understand the processes by which updrafts form within winter cloud systems over the Payette Mountains of Idaho where the experiment was conducted. To document the spatial and temporal distribution of updrafts within the cloud systems, the University of Wyoming cloud Doppler radar (WCR) was flown aboard the University of Wyoming king air aircraft and pointed vertically both upward and downward to measure the vertical radial velocity of particles along flight tracks parallel to the flow in the middle atmosphere as the aircraft crossed the Payette Mountain range. These measurements were first used to estimate the average terminal velocity of ice particles as a function of altitude by averaging the radial velocity over sufficiently long distances that the median value could represent the approximate terminal velocity of the particles. This value was then subtracted from the radial velocity to obtain an estimate of the vertical motions within the storms. Data were then analyzed using contoured frequency by altitude diagrams to quantify the vertical distribution and magnitude of updrafts and downdrafts. In this paper, we will present the results and relate the updrafts to potential forcing mechanisms that created them.