HR: 0800h
AN: H21A-0182    [Abstracts]
TI: A new method to separate rain and snow
AU: * Kienzle, S W
EM: stefan.kienzle@uleth.ca
AF: University of Lethbridge, 4401 University Drive, Lethbridge, AB T1K 3M4, Canada
AB: Water balance calculations in cold climates require the separation of precipitation into snow and rain. This separation is critical to determine whether water is available for runoff and soil infiltration, or if it is stored as snow. There are currently several approaches available using air temperature to separate precipitation into rain and snow. Based on analyses of daily precipitation data from 113 climate stations in south-western Alberta and south-eastern British Columbia, a curvilinear method is proposed that can be calibrated based on snow observation at local stations, with the two key variables being the threshold mean daily temperature (TT), where 50% snowfall occurs, and the range of temperatures within which mixed precipitation occurs (TR). A sensitivity and verification analysis of the two input variables was carried out for 15 climate stations in southern Alberta, and evaluated using three objective functions. Results reveal that the optimum TT ranges between 1.1 and 4.5°C, with a TR between 10 and 17°C. Verification analyses, using almost 700 years of daily precipitation observation, reveals that the new method significantly improves the estimation of daily snowfall when compared with the static threshold method, and methods developed by Leavesley et al. (1983) and Pipes and Quick (1977). On average, mean annual snowfall (MAS) was estimated to within 0.2% when detailed climate records where available, and within 7.1% when mean estimates of TT (2.6°C) and TR (13.3°C) were used. Respective r² values were 0.789 and 0.802. The investigation of monthly variations of TT and TR revealed that both have strong seasonal variations. These variations generally follow a sin curve, with TT following a sin curve with one annual cycle, while TR follows a sin curve with two annual cycles. Varying monthly TT and TR values based on observations, MAS was estimated on average to within 3.5%, and the r² was 0.828. By varying the mean TT (2.6°C) and TR (13.3°C) using an appropriate sin cycle, MAS was estimated on average to within 2.2%, and the r² was 0.821. While all other tested precipitation separation methods generally work quite well, the newly proposed method appears to offer a significant improvement in the estimation of daily and annual snowfall from precipitation records.
DE: 1816 Estimation and forecasting
DE: 1840 Hydrometeorology
DE: 1843 Land/atmosphere interactions (1218, 1631, 3322)
DE: 1854 Precipitation (3354)
DE: 1863 Snow and ice (0736, 0738, 0776, 1827)
SC: Hydrology [H]
MN: 2007 Fall Meeting