An evaluation of methods for determining during-storm precipitation phase and the rain/snow transition elevation at the surface in a mountain basin

Determining surface precipitation phase is required to properly correct precipitation gage data for wind effects, to determine the hydrologic response to a precipitation event, and for hydrologic modeling when rain will be treated differently from snow. In this paper we present a comparison of several methods for determining precipitation phase using 12 years of hourly precipitation, weather and snow data from a long-term measurement site at Reynolds Mountain East (RME), a headwater catchment within the Rey-nolds Creek Experimental Watershed (RCEW), in the Owyhee Mountains of Idaho, USA. Methods are based on thresholds of (1) air temperature (T a) at 0 °C, (2) dual T a threshold, À1 to 3 °C, (3) dewpoint temperature (T d) at 0 °C, and (4) wet bulb temperature (T w) at 0 °C. The comparison shows that at the RME Grove site, the dual threshold approach predicts too much snow, while T a , T d and T w are generally similar predicting equivalent snow volumes over the 12 year-period indicating that during storms the cloud level is at or close to the surface at this location. To scale up the evaluation of these methods we evaluate them across a 380 m elevation range in RCEW during a large mixed-phase storm event. The event began as snow at all elevations and over the course of 4 h transitioned to rain at the lowest through highest elevations. Using 15-minute measurements of precipitation, changes in snow depth (z s), T a , T d and T w , at seven sites through this elevation range, we found precipitation phase linked to the during-storm surface humidity. By measuring humidity along an elevation gradient during the storm we are able to track changes in T d to reliably estimate precipitation phase and effectively track the elevation of the rain/snow transition during the event. To understand and model the hydrologic response of a mountain basin to precipitation we must know the phase of precipitation (rainfall or snowfall) and the spatial distribution of phase over the basin. Phase is required to properly correct precipitation gauge data for wind undercatch, to simulate snowcover development and ablation, and for hydrologic modeling. Mountain precipitation is often measured using weighing gauges that do not distinguish between liquid and solid input. Where tipping bucket rain gauges are collocated with weighing gauges allowing differentiation between liquid and solid precipitation, the spatial density of collo-cated measurements is rarely …