Precipitation Phase Discrimination by Dew Point and Air Temperature

Correctly reported precipitation phases are crucial for estimation of snow storage in hydrological, regional and global climate models. Precipitation phase is especially critical for models simulating processes in tree canopies, since the canopy storage capacity is about one order of magnitude larger for snow than rain. The number of manned meteorological stations is decreasing, making determination of precipitation phase more difficult. Most hydrological models use an air temperature threshold to separate rain from snow, but there are indications that a dew-point temperature threshold might work better. This study utilized forty-five years of three-hour man-made observations for nineteen Swedish station ranging from 55 deg N to 68 deg N consisting of precipitation mass and phase, air and dew point temperatures. Precipitation events were classed as snow or rain, excluding mixed precipitation, were used for the initial analysis. Air temperature was found to be a better indicator of precipitation phase then dew point temperature. On occasion 0 deg C is used as an air temperature threshold, but if the air temperature rain/snow threshold 0 deg C is replaced by 1.0 deg C, the misclassified precipitation would be reduced by almost half in Sweden.