An Example of Supercooled Drizzle Drops Formed through a Collision-Coalescence Process

Abstract

The microphysics associated with observations of supercooled drizzle drops, which formed through a condensation and collision-coalescence process, are reported and discussed. The growth environment was an 1100-m-thick stratiform cloud with cloud-base and cloud-top temperatures of ?7.5° and ?12°C, respectively. The cloud was characterized by a low droplet concentration of 21 cm?3 and a large droplet median volume diameter of 29 µm, with a concentration of interstitial aerosol particles of less than 15 cm?3 (larger than 0. 13 µm in diameter). The evolution of drizzle drops was traced downward from cloud top, with a maximum diameter of 500 µm observed at cloud base. The air mass was sufficiently clean to ensure only a small number of active cloud condensation nuclei. Consequently, small concentrations of cloud droplets led to concentrations of over 300 L?1 for droplets larger than 40 µm, which set up strong conditions for continued growth by collision-coalescence. Ice crystals in concentrations of 0.08 L?1 were measured simultaneously with the drizzle drops and were not effective in glaciating the cloud, even though the drizzle drops were estimated to have taken at least 1?2 h to form. While the growth of precipitation-sized drops through collision-coalescence has been well documented, there are few measurements of this phenomena at temperatures less than 0°C. This study provides a well-documented example of such an event at subfreezing temperatures. The applicability of this measurement in terms of hazardous aircraft icing is discussed.