The Second Polarimetric Cloud Analysis and Seeding Test
Keywords:Hygroscopic Seeding, Weather Modification Research
AbstractThe Polarimetric Cloud Analysis and Seeding Test 2 (POLCAST2) program was an effort to evaluate the effectiveness of hygroscopic flares to increase precipitation in the North Dakota region. As part of the POLCAST2 program, a field project was conducted between 9 June 2008 and 11 July 2008. Thirteen randomized seeding candidates were found during twelve flights of the Cessna 340 seeding aircraft. The aircraft carried instrumentation to conduct in situ measurements of properties important for precipitation development, which enables seeding candidate stratification by aerosol amount. The University of North Dakota's NorthPol radar was used to continuously monitor cloud structure, measure precipitation rates, and investigate polarimetric radar observation variability between seeded and nonseeded candidates. Cloud candidates were randomly selected to be either treated with the release of hygroscopic material at cloud base by burning fou sets of two flaress, or were instead not treated and only cloud base measurements conducted. NorthPol<92>s radar data were ingested into the Thunderstorm Intensity Tracking Analysis and Nowcasting (TITAN) software to analyze canditate cases. TITAN analysis of six cases (an insufficient number to be statistically significant) indicated that the methodology of using polarimetric radar data to analyze "areas of influence" is promising for evaluation of possible seeding effects. Airborne measurements show that the cloud base aerosol and droplet concentrations are generally relatively high in summer time North Dakota with Passive Cavity Aerosol Spectrometer Probe (PCASP) aerosol concentration of 890 cm-3, Cloud Condensation Nuclei (CCN) concentrations of 1,030 cm-3, and cloud droplet concentrations of 360 cm-3. The cloud base CCN concentration is higher in North Dakota than in other areas (Mali and Saudi Arabia) where similar measurements have been made. The cloud base aerosol concentration varies from day to day, and hence it is an important field to quantify when evaluating the effectiveness of hygroscopic seeding. Additional field projects are planned that will add to the number of randomized cases obtained so far, which will provide greater confidence in the POLCAST2 case study results and provide additional measurements for understanding the physical processes of precipitation development in the North Dakota region.
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