Radar Operations Summary (2006-2007)



The in house designed digital receiver / signal processing systems used in the CSU-CHILL and Pawnee radars continued to be tested and refined during 2007. (They were initially fielded on both radars during the summer of 2006). CSU-CHILL operations were suspended in July 2007 to prepare the Greeley radar site for the installation of the new dual offset feed antenna.

(An overview of the new antenna installation can be found here.)

Winter Season

During the later months of the 2006 – 2007 winter season, CSU-CHILL operations were conducted when significant snow accumulations were expected to occur. The best organized snowstorm case was observed on 24 February 2007 when blizzard conditions were forecast for the region immediately east of the greater Denver metropolitan area. In the pre-dawn hours on this date, a well-defined snow band had developed to the southeast of the CSU-CHILL radar. This snow band was located within the northwest quadrant of the associated surface low pressure center. Elevated frontogenetic flow patterns in this zone can promote the organization of banded precipitation. The horizontal flow fields in the snow band were synthesized by combining the CSU-CHILL radial velocity data with those observed by the KFTG (Denver) WSR-88D radar. Figure 1 shows the resultant streamline pattern at the 3.5 km MSL height level at 1336 UTC. (In this plot, the average U and V wind components on the CAPPI surface have been subtracted.) A well-defined frontogenetic streamline pattern was collocated with the snow band reflectivity axis. Analyses of both the air motion and polarimetric data fields associated with this snow band are continuing.

Figure 1: Horizontal wind field streamlines at the 3.5 km MSL CAPPI level based on a dual Doppler synthesis using radial velocities from the CSU-CHILL and KFTG WSR-88D radars at 1336 UTC on 24 Feb 2007. The CAPPI plan average U and V wind components have been subtracted. CSU-CHILL reflectivity levels (ZH) in dBZ are shown with color-filled contours. The origin of analysis grid is the CSU-CHILL radar location near Greeley, Colorado. (Processing of the input radar data files was done using NCAR’s SPRINT and CEDRIC software packages.)

REU 2007 Project

During the summer months of 2007, the CSU-CHILL facility supported an NSF-sponsored Research Experience for Undergraduates (REU) program directed by Prof. V. Chandrasekar of the CSU Electrical and Computer Engineering Department. This summer’s REU class consisted of seven students from seven educational institutions. One of the initial activities for the class was a visit to the CHILL radar site on 29 May to receive introductory lectures on various hardware aspects of the CSU-CHILL radar and meteorological applications of polarimetric data that the system collects. Following these conference room presentations, the students visited the user van and observed real time RHI scans through a bright band (Fig. 2).

Figure 2: RHI scan through a bright band observed during the visit made to the CSU-CHIL radar by the REU 2007 class on 29 May. Tick marks are in km relative to the radar. (The basic radar data image was generated using NCAR’s SOLO software.)

ICE-L Field Project

In the late October through mid-December 2007 period, the CSU-Pawnee radar was operated on a 20hr project basis to support the Ice in Clouds (ICE)-Layer (L) cloud field experiment. (The CHILL radar was out of service during this time period due to the antenna replacement activities.) ICE-L was conducted under the general direction of Dr. A. Heymsfield of NCAR. The primary objective was collecting in-situ observations of the life cycle of ice particles in various types of cloud systems. (For additional project information see: http://www.eol.ucar.edu/deployment/field-deployments/field-projects/ice-l/ice-lweb). Pawnee radar operations were conducted when low level upslope flow conditions were expected to generate precipitation-producing clouds in the northern Colorado / southeastern Wyoming area.

Figure 3 shows the Pawnee reflectivity field observed in a 1.6 degree PPI scan on 11 December 2007. At this time, a broad area of light snow was centered in the northwest azimuth quadrant. Further south, a much narrower, slightly more intense linear echo was present near azimuth 225 degrees / range 35 km. As indicated by the overlaid track points, the NSF C-130 research aircraft operated by NCAR transited the narrow echo feature between 1923 and 1924 UTC.

Figure 3: CSU-Pawnee radar reflectivity 1.6 degree PPI image at 1922 UTC on 11 December 2007. Range rings are at 30 km intervals.

The University of Wyoming’s 94.92 GHz (W-Band) cloud radar (http://flights.uwyo.edu/wcr/) was mounted on the C-130 for the ICE-L project. Figure 4 shows the reflectivity data collected by the downward-looking beam of the Wyoming cloud radar as the C-130 passed through the narrow echo band. (This “first look” Wyoming cloud radar data, and technical assistance on reading it, were supplied by Dr. Samuel Haimov of the University of Wyoming.) The data collected by the C-130 instrumentation will be used to examine the composition of the thin echo band; the overall evolution of this echo feature can be obtained from analyses of the Pawnee radar data.

Figure 4: Time – height plot of reflectivity data collected by the downward-looking beam of the University of Wyoming cloud radar between 1923 and 1924 UTC on 11 December 2007. (Data provided by Dr. Samuel Haimov of the University of Wyoming.)

22 May 2008 Tornado

Main article: Tornado Observation from May 22, 2008

During the early months of 2008 the CSU-CHILL radar was out of service while the new dual offset feed antenna was being installed. Test scanning operations with the new antenna began in mid-May 2008. During this initial experimental period, antenna movement in the elevation plane was mechanically locked while the motion control system software was being adapted to the new antenna’s inertial characteristics. The radar was in this limited operational state when a significant tornado developed in the area in the late morning hours of 22 May. (The planned date for the start of CSU-CHILL data collection was 1 June.)

Radar data collection efforts were started as the rapidly-developing severe weather situation became apparent; these efforts were hampered by commercial power fluctuations that developed as surface wind gusts began to exceed 40 knots at the Greeley airport. Recording of single elevation (approximately 1.7 degrees) PPI scans began near 1823 UTC when the tornadic storm was centered about 12 km north-northeast of Fort Collins. The reflectivity data contained a local minimum that was essentially coincident with the tornado vortex signature (TVS) in the unfolded radial velocity data (Figures 5a and b). One prominent feature in the differential reflectivity field was the band of relatively large positive Zdr values that curved towards the TVS from the right flank of the storm (Fig. 5c). (The storm was moving towards the northwest.) This pattern appears to be similar to the Zdr arc identified in Kumjian and Ryzhkov’s summary of polarimetric radar signatures observed in supercell storms (July 2008 JAMC.)

Figure 5a: Co-polar H reflectivity (dBZ). Black dot marks TVS location.

Figure 5b: Manually unfolded radial velocities (mps)

Figure 5c: Differential reflectivity (dB). Curved arrow denotes axis of positive Zdr values.