Collision-Coalescence Nuclei and Entrainment Mixing During RICO

Precipitation initiation in warm clouds is not yet fully understood. In such clouds, formation of precipitation requires that cloud drops approximately 10 to 20 microns in diameter which form by condensation grow to become precipitation drops on the order of 1000 microns in diameter. This occurs by the process of collision-coalescence, which, as typically described, requires the presence of a small concentration of large drops >55 microns in diameter in order to be effective. The presence of these large drops, or collision-coalescence nuclei (C-C nuclei), cannot be explained by condensation of water vapor on cloud condensation nuclei (CCN) alone when constrained by the typical lifetime of warm rain clouds. Thus, it is required that additional mechanisms occur within warm clouds to account for the existence of drops large enough to begin the collision-coalescence process and thus initiate precipitation. One potential mechanism is the entrainment of unsaturated air into developing clouds, which may accelerate the development of precipitation through the enhanced production of C-C nuclei. If regions of entrainment lead to greater concentrations of C-C nuclei, then one would expect that the local environment surrounding such large drops would also bear the signature of entrainment mixing.

Measurements of C-C nuclei were obtained using the Phase Doppler Interferometer (PDI) which provides accurate measurement of C-C nuclei size, concentration and velocity. Aircraft PDI measurements of shallow, warm trade cumulus clouds were made during the Rain In Cumulus over the Ocean (RICO) project in January 2005 off the Atlantic coasts of Antigua and Barbuda in the Eastern Caribbean. In this talk, the PDI technique will be described, focusing on its capabilities for measuring large drops. C-C nuclei events and the environment surrounding each C-C nucleus are analyzed to evaluate the hypothesis that entrainment mixing plays a key role in the initiation of precipitation in warm clouds. Preliminary results suggest that there is evidence to support this hypothesis for the clouds sampled during RICO.