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 Reynolds Creek Experimental Watershed (RCEW), in the Owyhee Mountains of Idaho, USA. Methods are based on thresholds of (1) air temperature (T-a) at 0 degrees C, (2) dual T-a threshold, 1 to 3 degrees C, (3) dewpoint temperature (T-d) at 0 degrees C, and (4) wet bulb temperature (T-w) at 0 degrees 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. Published by Elsevier Ltd.