Thursday, 10 January 2013: 11:45 AM
Room 5ABC (Austin Convention Center)
This presentation will include an overview of aerosol, cloud, and precipitation properties associated with small marine cumulus clouds observed during the Barbados Aerosol Cloud Experiment (BACEX, March-April 2010) and a discussion of their interactions. The principal observing platform for the experiment was the Cooperative Institute for Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter (TO) research aircraft that was equipped with aerosol, cloud, and precipitation probes, standard meteorological instrumentation and upward-looking FMCW Doppler 95 GHz cloud radar to observe the mean and turbulent thermodynamic and wind structures. The relationship between aerosol, cloud droplets, and precipitation rates will be addressed. The TO was able to sample many clouds in various phases of growth during BACEX. The maximum cloud depth h observed is about 3 km and in most of the clouds h is less than 1 km; Two types of precipitation features were observed for the shallow marine cumulus clouds. In one, precipitation shafts are observed to emanate from the cloud base with evaporation in the sub-cloud layer. In the other, precipitation shafts emanating near the cloud top on the down-shear side of the cloud evaporate in the cloud layer. Only 35 % of clouds sampled were purely non-precipitating throughout the clouds; the remainder of the clouds showed precipitation somewhere in the cloud and dominantly closer to the cloud top. The principal variability in the background aerosols observed during these flights was associated with African dust above and within the boundary layer. An average CCN (at super-saturation 0.6 %, for example) varied from 50 c m-3 to 800 c m-3. The precipitation susceptibility SR, the relationship between precipitation rate (R) and cloud droplet number concentrations (Nd) at cloud base, in marine shallow cumulus clouds is investigated for a fixed cloud thickness h. This study shows that as cloud droplets Nd increase, precipitation R increases to a certain threshold of Nd, but: beyond the threshold, R decreases with Nd increases. Similarly, Nd increases to a certain threshold of h, as h increases. However, Nd decreases for the thicker clouds (larger h), as h increases. The results also show that wet scavenging of aerosols and cloud droplets by precipitation have large effects on susceptibility estimates.
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