Thunderstorm outflow and rainfall near the surface over Ft. Collins: 1 August 2011


Vector representation of the divergent radial velocity pattern at low levels in a thunderstorm over Fort Collins, Colorado. Plots depicting selected aspects of this storm's outflow and rainfall characteristics have been assembled.


On 1 August 2011 an isolated afternoon thunderstorm moved across the southern Fort Collins area. To obtain high time resolution observations of the near-surface portion of the storm, a series of four low elevation angle PPI sweeps were conducted through a narrow azimuthal sector. The resultant volume scans repeated on a 74 second cycle time, improving the sampling of the storm's low-level radial velocity and precipitation fields.

Divergent outflow

The downdrafts that characteristically occur during the mature stage of a thunderstorm spread out horizontally as they approach the surface. To visualize this flow pattern, vectors have been used to represent the radial velocity data collected in a 0.5 degree elevation angle PPI sweep. (The vectors are all aligned along radials from the radar; their length is proportional to the magnitude of the radial velocities. The X and Y axes are distances from the CSU-CHILL radar in km.) The diverging nature of the airflow was strong enough to induce outbound velocities on the far-range side of the echo core (X= -36, Y= +9 km). The most intense inbound velocities were occurring on the eastern (leading edge) portion of the storm.

Intense rainfall rates

To monitor rainfall over several urban drainage areas, the city of Fort Collins operates a network of tipping bucket rain gauges. The occurrence of bucket tips is telemetered to a central monitoring site. The 1 August storm most directly affected gauge 6480 (pictured below):

The highest reflectivities were observed over gauge 6480 at 2105:07 UTC. (Due to partial beam blockage at 0.5 degrees, 1.0 degree elevation angle data is shown in the following plot):

The specific differential propagation phase (Kdp) also maximized over the gauge at this same time. At the CSU-CHILL radar's 3 GHz operating frequency, Kdp values of 2.5 - 3 degrees per km imply rainfall rates of approximately 75 mm per hour (~3 inches per hour).

Radar - rain gauge comparison

Rain rate values at gauge 6480 were computed from the polarimetric data in each 1.0 degree elevation angle PPI sweep following the hydrometeor identification-based procedure in Cifelli et al (J. Atmos. and Oceanic Tech, 2011, p 352-364.) These rates were multiplied by the inter-volume times to obtain the radar-estimated rainfall accumulation as a function of time. Ground verification of the rainfall accumulation history was obtained from the times of the individual bucket tips recorded at gauge 6480. As summarized in the following plot, the radar-estimated and observed rainfall accumulation histories were in reasonably good agreement. Rain rate increases were observed slightly earlier in time in the radar data. This may be due to time required for the rain to fall through the several hundred meters between the radar pulse volume height and the surface. By the end of the plotted time period, the radar-estimated accumulation was ~0.1 inches less than the gauge-reported value (0.45 inches vs. 0.55 inches). The occurrence of ~0.5 inches of rain within a 10 minute time period (further compounded by strong outflow winds) can have significant local impacts on transportation and storm water drainage paths. High resolution monitoring of convective rainfall by polarimetric radar and specialized rain gauge networks can be critically important in flood-prone urban areas.



Mr. Chris Lochra, Flood Warning Engineer with the of the City of Fort Collins utilities department made the ALERT network rain gauge data available.