PATTERN IN THE VERTICAL OF SNOW GENERATION

Abstract

The beam of a moderately sensitive 3-centimeter radar has been kept pointed to the zenith. Height/time records of snow echoes for seven winter weeks have been correlated with analyses of standard upper-air data. The major part of the record in nearly every storm contains trail patterns, formed as the snow falls from generating cells aloft. The majority of the related generating levels occur somewhere between 11,000 and 20,000 feet, between ?12 and ?34 degrees Celsius. Combined data from this and a previous study show that of 24 generating levels, 16 occur in maritime polar air and 16 occur in the lowest fifth of the relevant air-mass. In all but three of the cases of the present study, the snow generation takes place in stable air. Terminal speeds, deduced from the trail patterns, are estimated to range from 1 to 6 feet per second. At the leading edge of a storm, the radar usually records snow overhead for some time before any snow falls at the radar site. Occasionally the lower edge is patterned by pendulous extensions; these ?stalactites? are associated with snow falling into dry air and are probably the pattern of overturning as air, chilled by evaporation, descends. Toward the end of a storm, the records tend to lack pattern, and the height of echo tends to lower. It may be that a different precipitation mechanism is involved; on the other hand, it may be that with decreasing intensity of snow the radar is unable to see to the higher levels where pattern exists. Observations of snow cells, together with terminal-speed evidence of aggregation at low temperatures, suggest a turbulent convective mechanism, even though the cells occur in stable air. The role of growing ice crystals, acting as a thermal source, is examined. In moist stable air, the latent heat of sublimation released by growing ice crystals may result in significant vertical development, comparable to the observed depth of snow-generating cells. Calculated updraft velocities are comparable to the terminal speeds of ice crystals or aggregates. In air containing supercooled water cloud, the ?sublimational? updraft is much lower than in cloud-free air; thus, shear and turbulence could develop across a cloud boundary in the presence of growing ice crystals, and be a significant factor in their aggregation. It is suggested that cloud boundaries, along which aggregation is favored, may serve as the bases for the snow-generating cells.