Realtime snow ratio forecast concept

A review of the microphysical literature reveals that many factors may contribute to snow density, including in-cloud (crystal habit and size, the degree of riming and aggregation of the snowflake), sub-cloud (melting and sublimation) and surface processes (compaction and snowpack metamorphism). Despite this complexity, surface and radiosonde data are sufficient for useful accuracy in the classification of snowfall density (Roebber, Bruening, Schultz and Cortinas 2003, Weather and Forecasting). A principal component analysis isolated seven factors that influence the snow ratio: solar radiation (month), low- to mid-level temperature, mid- to upper-level temperature, low- to mid-level relative humidity, mid-level relative humidity, upper-level relative humidity, and external compaction (surface wind speed and liquid equivalent). The most critical inputs are related to the month, temperature, and external compaction. One key aspect of snowfall density is the role of in-cloud vertical motions. The maximum growth rate is expected to occur near the level of maximum upward air motion within the cloud, where the greatest water vapor delivery occurs (Auer and White 1982). Through natural variations in ice crystal size, some crystals will grow relative to their neighbors within the cloud and begin to fall, thus promoting the sweepout of smaller particles. If this sweepout occurs in a cloud of ice crystals, then aggregation leads to the formation of snowflakes and relatively low snow density. An ice crystal falling through a cloud of supercooled water droplets, on the other hand, will lead to rimed crystals (graupel) through accretional growth, and very high snow densities (Power et al. 1964). Empirical evidence for the importance of vertical motion is provided at: http://www.nws.noaa.gov/er/hq/ssd/snowmicro. Data on in-cloud vertical motion was unavailable for this study, hence the neural network implemented on these web pages does not include this effect. Under circumstances in which the neglect of this factor is deemed significant (as described above), it should be possible to empirically account for its absence and correct the network forecasts accordingly. Some additional physical effects have also been neglected, including early and late season ground-temperature effects (which may lead to melting and compaction of new snowfalls) and strong low-level winds (which can lead to snow crystal fragmentation).

Comments and/or suggestions? Send them to Prof. Paul Roebber at roebber@uwm.edu