Marine stratocumuli make a major contribution to Earth’s radiation budget. Drizzle in such clouds can greatly affect their albedo, lifetime and fractional coverage, so drizzle rate prediction is important. Here we examine a question: does a drizzle rate (R) depend on cloud depth (H) and/or drop number concentration n in a simple way? This question was raised empirically in several recent publications and an approximate H3/n dependence was observed. Here we suggest a simple explanation for H3 scaling from viewing the drizzle rate as a sedimenting volume fraction ( f ) of water drops (radius r) in air, i.e. R = f u(r ), where u is the fall speed of droplets at the cloud base. Both R and u have units of speed. In our picture, drizzle drops begin from condensation growth on the way up and continue with accretion on the way down. The ascent contributes H ( f ∝ H) and the descent H2 (u ∝ r ∝ f H) to the drizzle rate. A more precise scaling formula is also derived and may serve as a guide for parameterization in global climate models. The number concentration dependence is also discussed and a plausibility argument is given for the observed n−1 dependence of the drizzle rate. Our results suggest that deeper stratocumuli have shorter washout times.
Environmental Research Letters
Kostinski, A. B.
Drizzle rates versus cloud depths for marine stratocumuli.
Environmental Research Letters,
Retrieved from: https://digitalcommons.mtu.edu/physics-fp/210