The images are all from 1.7 degree elevation angle PPI scans; UTC time, date, etc. appear in the annotation block in the lower right corner. The range ring interval is 20 km. Geographic references are provided by the orange base map. The plus symbol at an azimuth of 291 degrees and a range of 40.6 km is located immediately north of the CSU Student Center Building, which suffered extensive water damage.
Four different radar measurements are presented in the various images; the data field identifications are shown by the abbreviations above the color scale. The basic reflectivity (labeled "10CM DBZ") shows the intensity of the signal received by the radar. This signal strength increases in regions where large numbers of large raindrops exist. Differential reflectivity ("ZDR") shows the degree to which the received signal strength changes when the polarization of the pulse transmitted by the radar is alternated between horizontal and vertical. This mesurement is sensitive to the mean shape of the particles being observed by the radar. Particles whose cross sectional shapes are flattened, like large raindrops, produce positive ZDR values. Differential propagation phase ("DP") is a mesurement of the amount by which the horizontally polarized return signal lags the vertically polarized return signal when the two signals traverse a common path. This lag increases as the quanity of large, flattened raindrops present in the radar beam path increases. Thus, regions in which the DP values are changing rapidly with distance from the radar indicate heavy rain is in progress. Finally, the motions towards or away from the radar in meters per second are shown by the Doppler velocity field ("VEL"). Positive values represent motion away from the radar; negative values signify motion radially towards the radar.
On the synoptic scale, deep "monsoon" moisture was present over NE Colorado on the day of this event. Upslope flow existed at low levels, while winds aloft were generally light. Rain fell steadily over the western half of Ft. Collins starting around 00 UTC (1800 MDT). The radar data indicate that this portion of the city continued to receive rain at least through 04 UTC (2200 MDT). An apparent rainfall maximum took place between 03 and 04 UTC. The ZDR's are distinctly positive during this period, indicating that large raindrops were predominate. The DP values were changing by as much as 4 degrees per kilometer at 0345 UTC. This amount of phase change implies rain rates of approximately 5 inches per hour. This first look at the radial velocity pattern shows that the both the general low level upslope, and local convergence beneath the Ft. Collins echo system, were enhanced at 0330 UTC.
Storm rainfall totals of 6 - 11 inches were reported from the western portions of the city. (At the author's home in eastern Ft. Collins a total of only 1.1 inches fell, and no flooding occurred). Newspaper accounts suggest that the initial reports of serious flooding conditions began shortly after 03 UTC (2100 MDT). City officials issued a disaster declaration at 0725 UTC (0125 MDT).
Pat Kennedy
CSU-CHILL Radar
1 August 1997
