High Rain Rates at Denver International Airport Observed with Indexed Beams

On 10 September 2006, the CSU-CHILL radar made continuous 0.5 deg elevation PPI scans in alternating VH polarization mode. In these scans, the beam revisited a given location at essentially one minute intervals. While this scanning procedure was in use, heavy thunderstorm precipitation developed around Denver International Airport. To provide geographic references for the radar data, the FAA airport taxi diagram for KDEN is shown below. A red dot has been added to indicate the approximate location of the ASOS sensors where the basic elements of the surface weather reports (METARS)are measured. A horizontal size scale can be obtained from the runway length depictions; runway 35L / 17R (immediately to the west of the ASOS dot) is 3.66 km (12,000 ft) long.

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For each PPI scan, the one-way specific propagation differential phase (Kdp in degrees per km) was calculated using the methods of Cifelli et. al (J. Geophys. Res.; 2002). Kdp-based rain rates were then provided by R = (40.5 * (Kdp**.85)) from Eq 8.14 in the text of Bringi and Chandrasekar (2001). In the time lapse sequence shown below, these rain rates have been color coded as follows:

(make this a table)

light blue= 25 mm/hr

light red= 50mm/hr

green= 75 mm/hr

blue= 100 mm/hr

yellow=125 mm/hr

red=150 mm/hr

grey=175 mm/hr

(Note: PPI times (UTC) appear in the lower left quadrant of each image). The patterns evident in the time lapse sequence demonstrate the small time and space scales on which thunderstorm precipitation evolves. The METAR prevailing visibility decreased to 0.25 statute miles in heavy rain at 2342UTC. The CSU-CHILL data indicate that even higher rain rates existed over runway 35R - 17L, while rainfall rates never reached 25 mm per hour over the approach end of runway 25 in the western portion of the airport. Since the radar data were collected in indexed beam mode, the range gate azimuths remained constant. These indexed beams improve the frame-to-frame stability in the time lapse sequence.