Relationships between Storm Total Supercooled Liquid Water Flux and Precipitation on Four Mountain Barriers
AbstractThere is a commonly held view that large winter orographic storms tend to be efficient in converting supercooled liquid water to snowfall while small storms tend to be inefficient and, therefore, are more susceptible to cloud seeding. To test this conceptual picture, supercooled liquid water flux and precipitation amounts were compared for 4 different mountain regions in Arizona, Colorado, and Utah. Supercooled liquid water flux totals were estimated for entire storm periods from vertically-integrated microwave radiometer measurements and wind speeds within about 1 km of the mountain crest, corresponding to the layer expected to contain most of the supercooled liquid water. Per storm precipitation totals were measured near the radiometer sites. Comparison of storm total supercooled liquid water flux with total precipitation revealed apparent significant relationships between the two variables at all sites. The larger supercooled liquid water flux-producing storms tended to have larger precipitation amounts. When the effect of storm duration was removed, partial correlation coefficients between supercooled liquid water flux and precipitation were significant at 3 of the mountain sites. None of the data sets supported the concept that large precipitation-producing storms are highly efficient in converting supercooled liquid water flux to snowfall. This suggests that large storms are efficient during some phases when abundant snowfall is produced, but inefficient during the other phases when supercoled liquid water flux is abundant. This hypothesis was tested for one well-observed storm. Precipitation efficienty was estimated throughout the storm's duration by comparing snowfall with upwind supercooled liquid water flux plus ice flux. The prefrontal phase was very inefficient and produced most of the total supercooled liquid water flux. The frontal phase was more efficient and about half the flux of liquid and ice was converted to snowfall. The postfrontal phase had little supercooled liquid water flux but significant ice flux. However, it was inefficient in converting cloud ice to snow on the ground. One to a few storms at each site produced most of the seasonal total supercooled liquid water flux. From 50 to 67 percent of all storms at each site produced small amounts of supercooled liquid.
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