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Trade wind cloud evolution observed by polarization radar: relationship to aerosol characteristics

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Title: Trade wind cloud evolution observed by polarization radar: relationship to aerosol characteristics
Author(s): Minor, Hilary A.
Advisor(s): Rauber, Robert M.
Department / Program: Atmospheric Sciences
Discipline: Atmospheric Sciences
Degree Granting Institution: University of Illinois at Urbana-Champaign
Degree: M.S.
Genre: Thesis
Subject(s): Rain In Cumulus over the Ocean (RICO) Trade Wind Cumuli Radar cloud physics Warm Rain aerosol cloud organization
Abstract: One of the classical unsolved problems in cloud physics is the explanation of the observed short time between initial cloud formation and the onset of precipitation in warm clouds. Although a time interval of about 20 minutes is often quoted for initiation of warm rainfall, few field observations have carefully defined the starting or ending times of trade-wind cumuli development. The goals of this research are to: 1) investigate the characteristic time and height of warm rain formation in trade-wind cumulus using radar and 2) determine the sensitivity of precipitation development in shallow maritime cumuli to variations in giant (sea salt) condensation nuclei (GCN) and other factors such as cloud organization. The microphysical evolution of trade wind cumuli, focusing on the potential role of giant nuclei in influencing the ZDR signal, is characterized using data collected by the National Center for Atmospheric Research S-Pol radar during the Rain In Cumulus over the Ocean (RICO) field campaign. The analyzed data set consists of 76 trade wind cumuli, observed over six days of the field project, that have been tracked from early echo development through rainout. Analysis days were chosen based on the results of Colόn-Robles et al. (2006) which show GCN concentrations are a strong function of near-surface wind speed. Days were chosen to insure a wind range of low-level wind speeds. Each cloud was analyzed with a time-height section approach that displays the spatial and temporal evolution of the maximum radar reflectivity factor (Zm) and differential reflectivity (ZDRm) within each constant-elevation angle sweepfile, and the correlation between these two fields. Other measures used for the time-height analysis include ‘averaged' Z (AZ) and ZDR (AZDR), values which are calculated for entire constant-elevation angle sweeps through the cloud. A statistical analysis of the radar observables for the ensemble of trade wind clouds is performed in order to determine if there is a statistically significant difference in the precipitation evolution that can be related to the initial giant aerosol concentration. Our results show a great deal of temporal variability in warm rain development. Warm rain was observed over a wide time range, from within 30 minutes of cloud formation to as long as 170 minutes after cloud formation. The data showed a large amount of spread in both the characteristic time and height for warm rain formation, indicating giant nuclei have a minimal influence on warm rain development. Rather, our results and the statistics imply that precipitation formation in warm clouds is heavily influenced by cloud organization and dynamics. It was found that giant nuclei do influence the rain drop size distributions as larger drops were present above cloud base when the giant nuclei concentrations below cloud base were greater. The Z and ZDR fields show better temporal and spatial correlation when the giant nuclei concentrations were greater as well.
Issue Date: 2010-08-20
URI: http://hdl.handle.net/2142/16916
Rights Information: Copyright 2010 Hilary A. Minor
Date Available in IDEALS: 2010-08-20
Date Deposited: 2010-08
 

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