Towards an analysis of the liquid-water fraction for orographic precipitation

Knowing the liquid-water fraction (LWF) in the rain/snow transition zone has useful applications in hydrology, road transportation, and aviation. Several control parameters that dictate the LWF are identified through a series of idealized, two-dimensional experiments using a spectral-bin microphysical model. These are the stability, the drop-size distribution (DSD), and the degree of riming (RF). The most important of these is stability. Though DSD and RF have secondary impacts, they are still nonnegligible. In real case studies, stability can be determined from numerical-model output, but DSD and RF are unknown. Therefore, several sets of experiments are conducted to gauge the effects of DSD and RF variations on the rain/snow transition zone for actual case studies of orographic precipitation in the western United States. Each experiment uses a unique combination of DSD and RF, which are assumed to be constant across the entire domain. These experiments show that DSD and RF have nontrivial effects on the LWF and depth of the rain/snow transition zone. In some case studies, the depth of the transition zone varies by as much as 450 m. This result suggests that more precision is needed to account for DSD and RF variations. Given limited observations, these fields must be derived from radar or satellite measurements or assumed based on model-derived stability. Efforts to this end are currently underway.