AN EXAMPLE OF SUPERCOOLED DRIZZLE DROPS FORMED THROUGH A COLLISION-COALESCENCE PROCESS

The microphysics associated with observations of supercooled drizzle d rops, which formed through a condensation and collision-coalescence pr ocess, are reported and discussed, The growth environment was an 1100- m-thick stratiform cloud with cloud-base and cloud-top temperatures of -7.5 degrees and -12 degrees C, respectively. The cloud was character ized by a low droplet concentration of 21 cm(-3) and a large droplet m edian volume diameter of 29 mu m, with a concentration of interstitial aerosol particles of less than 15 cm(-3) (larger than 0.13 mu m in di ameter). The evolution of drizzle drops was traced downward from cloud top, with a maximum diameter of 500 mu m observed at cloud base. The air mass was sufficiently clean to ensure only a small number of activ e cloud condensation nuclei. Consequently, small concentrations of clo ud droplets led to concentrations of over 300 L(-1) for droplets large r than 40 mu m, which set up strong conditions for continued growth by collision-coalescence. Ice crystals in concentrations of 0.08 L(-1) w ere measured simultaneously with the drizzle drops and were not effect ive in glaciating the cloud, even though the drizzle drops were estima ted to have taken at least 1-2 h to form. While the growth of precipit ation-sized drops through collision-coalescence has been well document ed, there are few measurements of this phenomena at temperatures less than 0 degrees C. This study provides a well-documented example of suc h an event at subfreezing temperatures. The applicability of this meas urement in terms of hazardous aircraft icing is discussed.