CONDITIONS ASSOCIATED WITH LARGE-DROP REGIONS

In light of the significant icing hazard large drops pose to general a viation, two conditions have been previously associated with large-dro p formation; these being a warm cloud-top temperature and a low drople t concentration. This paper identifies an additional condition associa ted with the development of large-drop regions. Wind shear is hypothes ized as being a necessary but not sufficient condition for the formati on of large drops. Wind shear at cloud top may cause turbulence, Kelvi n-Helmholtz waves, and thus the inhomogeneous mixing leading to large drops. This hypothesis was tested in 29 cases where the Wyoming King A ir aircraft made a climb or descent through the top of stratiform clou ds. The presence of a wind shear layer was defined by the magnitude of the wind shear and the value of the bulk Richardson number across the layer. In 23 of the 29 cases, wind shear was associated with large-dr op regions. A chi(2) statistical test was applied to the data. The nul l hypothesis, where wind shear and large drops were considered indepen dent of each other, was rejected to a significance level of 0.01. From this it can be inferred that large drops and wind shear are related. The depth of the shear layer was usually small, less than 150 m. The v alidity of the condition of low droplet concentration is questioned si nce several cases of large drops were found in the presence of a high droplet concentration. These cases were marked by strong wind shear.