Finding the rain/snow transition elevation during storm events in mountain basins

Authors: Marks, Daniel; Winstral, Adam

Submitted to: European Geosciences Union General Assembly Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 11/1/2007
Publication Date: 12/8/2007
Citation: Marks, D.G., Winstral, A.H. 2007. Finding the rain/snow transition elevation during storm events in mountain basins. Presented in Joint symposium JHW001: INteractions between snow, vegetation and the atmosphere, The 24th General Assembly of the IUGG, Perugia, Italy, July 2-13, 2007.

Interpretive Summary:

Technical Abstract: In mountainous, forested environments, the interaction between vegetation and precipitation during winter storms is strongly dependant on whether the precipitation falls as snow or rain. In remote mountain regions, the elevation of rain/snow transition is very transient, and difficult to determine without onsite observation. In mid-latitude locations, at elevations within the rain/snow transition zone (1000-2000m), snow commonly falls when air temperatures are above C. In general hydrologists and watershed modelers use a calibrated air temperature relationship to determine precipitation phase, though such relationships are site specific, and highly variable, depending on season and storm track. Further, as the climate continues to warm, these relationships will need to be re-calibrated each year. In an effort to develop a more stable indicator of precipitation phase in mountain basins, we present data from a transect of 8 sites at 50m elevation intervals in a small mountain catchment in the northwestern US. The 1.8 km2 catchment in the Owhyee Mountains of Idaho, USA, ranges in elevation from 1488-1868m. In this region, it is typical for precipitation events to occur as rain in the lower and snow in the upper elevations. Air temperature, humidity and snow depth are measured at all eight transect sites, while full meteorology and precipitation are measured at three of the eight sites. Two years of data from the transect show a weak and highly variable relationship between air temperature and precipitation phase, while dew point temperature shows a strong, nearly 1:1 relationship to precipitation phase. When the dew point temperature is C or less, precipitation is snow; if the dew point temperature is above C, it is raining. The method was also satisfactorily tested over a mid-sized basin in Idaho (~240 km2), and a large river basin in southern Oregon (~5,000 km2). In all cases the elevation of the rain/snow transition was very dynamic, changing rapidly and continuously during the storm event. Because dew point temperature during a storm is a property of the air mass, it is less likely to be influenced by site or local conditions, and will be a more stable predictor of precipitation phase for modeling snow properties and processes in mountain basins.