Distribution of Droplet Effective Radius in Marine Boundary-Layer Clouds During ASTEX

A new aircraft sensor was used on the U. of Washington's C-131A aircraft during the Atlantic Stratocumulus Transition Experiment (ASTEX) to give a continuous 10-Hz record of the liquid water content (LWC) and the effective droplet radius r_e in marine bondary-layer clouds. We show evidence that new sensor produced a data set with superior accuracy: ~5% for LWC and ~10% for r_e. Comparisons with other microphysics probes on the aircraft showed an insufficient time response for the hot-wire LWC probes, and a large underestimate of r_e measured by a FSSP. Retrieval of accurate FSSP droplet spectra by combining outputs of FSSP and new sensor is described.

The distribution of r_e in the clouds is important for parameterizing cloud radiation effects, and for ground-truthing remote sensing of r_e and LWC done by other participants of ASTEX. Vertical and horizontal distributions, and statistics of r_e are presented; and anthropogenic effects are estimated. The distribution of r_e and LWC leads to new insights on cloud-top entrainment: We describe the behavior of r_e and LWC near cloud top for a changing radiation environment; and conditional sampling of r_e and LWC in entrainment features shows the expected effects of inhomogeneous mixing only part of the time; some features show enhanced droplet growth.