Microphysics of Marine Stratocumulus Clouds with Two Drizzle Modes

This observational study looks at the distribution of some cloud microphysical properties measured from the University of Washington's aircraft in mostly unbroken stratocumulus (Sc) clouds in the vicinity of the Azores Islands during ASTEX. The average behavior of the Sc includes the presence of two drizzle modes. In 85% of the Sc the amount of drizzle LWC (liquid water content in droplets with radius r > 20 µm) is less than 0.01 g m−3, while in the rest, drizzle LWC is much larger than 0.01 g m−3. The microphysics of light-drizzle Sc approach classical conditions, because measured and adiabatic LWC profiles are similar, and droplet spectral dispersions decrease with height. The lognormal function fits approximately measured droplet spectra of light-drizzle Sc. Standard deviations of these spectra remain about constant with height, vary over a small range, and average 1.74-µm radius. Drizzle LWC > 0.01 g m−3 is found on the average in Sc with spectra that have effective radii re &ge 16 µm. This heavy-drizzle threshold coincides approximately with the experimental coalescence threshold of 19-µm radius when the largest droplets in the average droplet spectrum are considered. This leads to the conclusion that droplet growth by condensation plays an important role in the formation of heavy-drizzle Sc. Measurements near Sc cloud top show narrow regions of depleted LWC with unchanging values of re, suggesting the presence of entrainment and inhomogeneous mixing.