RADAR EVIDENCE OF A GENERATING LEVEL FOR SNOW
1954; American Meteorological Society; Volume: 11; Issue: 1 Linguagem: Inglês
10.1175/1520-0469(1954)011 2.0.co;2
ISSN2163-5374
AutoresK. L. S. Gunn, M. P. Langleben, A. S. Dennis, B. A. Power,
Tópico(s)Fire effects on ecosystems
ResumoVertical-section radar observations of precipitation in the winter of 1951–1952 have been related to upper-air data. On three of the 22 days, there was very little signal and practically no pattern observed. It is possible that these are cases, such as described recently by Wexler and Austin (1953), in which the snow crystals grow gradually while descending through cloud-free air, at a saturation intermediate between that of ice and water. On the remaining 19 days, trail patterns were observed. On 13 of them, well-defined snow trails, usually with generating elements visible, were detected. Upper-air analyses showed the air aloft to be stable on these days of good pattern. On six days, the records contained only parts of trails, the whole pattern being less well-defined; the air aloft was unstable on these days. Since the well-defined patterns occur on stable days, instability is not the initiating mechanism; rather, the presence of instability confuses the pattern. For the 13 stable days, the top of the trail echo was, on the average, near the top of a main middle cloud, the top of this cloud being defined as the height above which radiosonde data show the relative humidity to drop sharply. The average cloud-top height for these 13 days was some 1300 ft above a frontal surface, the average frontal-surface height over Montreal being about 11,000 ft. The results of this study indicate the presence of a relatively shallow “active layer” of stratiform cloud, straddling a frontal surface, in which snow-generating elements of considerable lifetime exist.
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