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

item Marks, Daniel
item Winstral, Adam
item Reba, Michele
item POMEROY, JOHN - University Of Saskatchewan
item KUMAR, MUKESH - Duke University

Submitted to: Advances in Water Resources
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/4/2012
Publication Date: 5/1/2013
Citation: Marks, D.G., Winstral, A.H., Reba, M.L., Pomeroy, J., Kumar, M. 2013. An evaluation of methods for determining during-storm precipitation phase and the rain/snow transition elevation at the surface in a mountain basin. Advances in Water Resources. 55:98-110. DOI: 10.1016/j.advwatres.2012.11.012.

Interpretive Summary: Using data from the Reynolds Creek Experimental Watershed, the authors discuss the difficulty associated with determining precipitation phase during a storm, show that the air temperature is not an effective determinate of phase and that humidity, either dew point or wet bulb temperature can be used to accurately characterize phase. Twelve years of precipitation, snow depth and SWE, temperature and humidity data from the Reynolds Mtn. East catchment are used to validate the method. Data from a transect along 800 m of elevation are used to illustrate the dynamic nature of the rain-snow transition during a major storm. This information will be used to improve our understanding of precipitation in mountain regions and will result in better hydrologic models that more accurately predict streamflow and water supplies.

Technical Abstract: 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 (Ta) at 0°C, 2) dual Ta threshold, -1 to 3°C, 3) dewpoint temperature (Td) at 0°C, and 4) wet bulb temperature (Tw) at 0°C. The comparison shows that at the RME Grove site, the dual threshold approach predicts too much snow, while Ta, Td and Tw 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 hours transitioned to rain at the lowest through highest elevations. Using 15-minute measurements of precipitation, changes in snow depth (zs), Ta, Td and Tw, 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 reliably estimate precipitation phase and effectively track the elevation of the rain/snow transition during the event.