HR: 1340h
AN: A13A-0885    [Abstracts]
TI: Collision-Coalescence Nuclei Measurements During the Rain In Cumulus over the Ocean Experiment
AU: * Small, J D
EM: jsmall@es.ucsc.edu
AF: Earth Science Department University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 United States
AU: Chuang, P Y
EM: pchuang@es.ucsc.edu
AF: Earth Science Department University of California Santa Cruz, 1156 High Street, Santa Cruz, CA 95064 United States
AB: Precipitation is a key process in both weather and climate prediction, but one that is not yet well understood. In warm clouds, 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 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. There are several mechanisms used to explain the production of C-C nuclei. These mechanisms are broadly described as: (1) the existence of ultra-giant aerosol (UGA), (2) enhancements in drop collision rates due to small scale turbulence and (3) entrainment based mixing. In order to investigate each of these mechanisms in- and out of cloud measurements of C-C nuclei are necessary. Previous studies have identified difficulties in observing and measuring C-C nuclei with radar and in-situ optical probes. Past attempts have been unsatisfactory for several reasons, including inadequate instrument sample volumes, difficulties making measurements in the necessary size range, measurement quality concerns, and instrument artifacts. The Phase Doppler Interferometery (PDI) technique allows for the accurate measurement of C-C nuclei size, concentration and velocity. Aircraft PDI measurements 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. Shallow, warm trade wind cumuli are the dominant cloud type in this region. In this talk, the PDI technique will be introduced briefly, focusing on its capabilities for measuring large drops. Measurements in clear air are analyzed to constrain UGA concentrations from sea spray. In-cloud measurements of C-C nuclei are analyzed to understand the role of small scale turbulence and entrainment mixing processes in their formation.
DE: 3311 Clouds and aerosols
DE: 3314 Convective processes
DE: 3354 Precipitation (1854)
DE: 3379 Turbulence (4490)
DE: 3394 Instruments and techniques
SC: Atmospheric Sciences [A]
MN: Fall Meeting 2005