Overview

The Polarimetric Cloud Analysis and Seeding Test (POLCAST) is an ongoing field and research program located in Eastern North Dakota, centered on Grand Forks, ND. The field programs are conducted every other summer. The last field program was conducted in the summer of 2010 (21 June 2010 through 23 July 2010). The previous field experiment was conducted from 09 June through 11 July 2008. The original field program took place in August–September 2006. The experiment utilizes the University of North Dakota (UNDs) polarimetric C–Band Doppler weather radar, Weather Modification Incorporated (WMIs) instrumented Cessna 340 aircraft, and the University of North Dakota Citation II research aircraft. The POLCAST studies are supported through the North Dakota Atmospheric Resource Board (NDARB) and are collaborative efforts between the National Center for Atmospheric Research (NCAR), NDARB, University of North Dakota (UND), and Weather Modification Incorporated (WMI).

Figure 1. PPI image of randomized case observed on 09 July 2008

The main objective of the POLCAST studies is to better understand the effects of hygroscopic seeding at cloud base on convective clouds in Eastern North Dakota. Specifically, the focus of the POLCAST is to determine identifiable signatures of hygroscopic seeding in polarimetric observables or derived fields; characterize the effects of hygroscopic seeding at base in cloud base which was stratified by ambient distributions of aerosol and cloud condensation nuclei (CCN) in the atmosphere. All the cases are randomized to characterize cloud droplet distributions above cloud base for seeded and non–seeded clouds to confirm inferences observed in the polarimetric Doppler weather radar fields.

Figure 2. Reflectivity (top panels) and hydrometeor retrieval
(bottom panels) PPI images for elevation tilt of 2.4° (left panels)
and 8.0° (right panels) for storm observed around 2040 UTC 09
July 2008 (click to enlarge)

For the second POLCAST field campaign in 2008 (POLCAST2), the seeding aircraft flew 13 randomized events. During 2010 (POLCAST3), a total of 14 randomized events were collected. The plan is to conduct the fourth field campaign in summer 2012 in an effort to increase the sample size of randomized cases.

Figure 3. Distribution of hydrometeors observed at 2040 UTC 09
July 2008 (click to enlarge)

An example of results obtained from POLCAST is shown in Figures 1–3. Figure 1 shows a radar snapshot of an event that was targeted on 09 July 2008 between 2013 and 2025 UTC. The aircraft targeted feeder cells on the western side of the storm for this case. Figure 2 shows an example of the spatial distribution of hydrometeors retrieved using the NCAR polarimetric hydrometeor identification (HID) analysis. Figure 3 shows the distribution of hydrometeors from the HID retrieval of the storm shown in Figure 2.