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.