DPWX/Cold frontal passage at the CSU campus weather station: 7 December 2016
Author: Patrick C. Kennedy
CSU-CHILL X-band reflectivity data collected in a 1.4 deg elevation PPI scan through a snowband associated with a cold front. Plots showing various indications of this cold front, seen both in the surface data collected at the CSU campus in Fort Collins as well as in the CSU-CHILL radar observations, have been prepared.
During the predawn hours on 7 December 2016, light snow fell in the greater Fort Collins Colorado area. The snow intensity briefly increased around 1000 UTC (0300 MST) when an arctic cold front overspread the region from the north. Surface indications of this frontal passage were documented at the weather station maintained by the state climate office on the main CSU campus. The CSU-CHILL radar also observed this frontal passage during an overnight period of unattended operations. The arctic frontal passage was associated with a developing snow band. The radial velocity patterns recorded in RHI scans done essentially over the campus weather station site were consistent with the surface wind shift that occurred during the frontal passage.
The Colorado State Climate weather station is located at azimuth 291 degrees / range 40.63 km from the CSU-CHILL radar. Several basic meteorological parameters (wind, temperature, barometric pressure, etc.) measured at this site are archived as 10 minute averages. Additionally, ultrasonic snow depth sensors provide remotely-sensed information on snow depth at 5 minute time intervals. These snow depth measurements are made by directing beam of ultrasonic acoustical pulses towards the ground surface beneath the instrument (see following photograph). The time delay in the reception of the signal reflected back from the underlying surface decreases as the snow depth increases.
The red dots in the following plot show the locations of the campus weather station and the CSU-CHILL radar as seen in a 1.4 degree elevation angle PPI scan presentation. The radar data is reflectivity as measured by the CSU-CHILL X-band channel at 0958 UTC.
X-band 1.4 degree elevation angle reflectivity loop
The evolution of the snow band echo associated with the arctic frontal passage is shown on the following loop of X-band 1.4 degree elevation angle angle scans during the 0958 - 1045 UTC period. Peak reflectivity values of ~20 dBZ occurred at the campus site around 1022 UTC.
290 deg azimuth RHI data at 0958 and 1033 UTC
The pre-programmed scan sequence that was being done by the CSU-CHILL radar included RHI scans on an azimuth of 290 degrees at time intervals of 5 minutes and 40 seconds. In the following plots, the RHI scan range gate data was interpolated to a Cartesian grid in which the positive X axis was rotated to coincide with the RHI azimuth. The vertical levels in the interpolation grid are referenced to sea level and have a spacing of 200 m. The first pair of RHI images shows the X-band reflectivity and radial velocity patterns at 0958 UTC. Reflectivity levels in the shallow, surface-based layer were mostly under 15 dBZ. (The reduced reflectivities just above the ground are due to beam blockage effects.) Radial velocity values in the vicinity of the campus weather station were positive, indicating a motion component away from the radar. (i.e., a general southeasterly flow that was upslope with respect to the local terrain contours).
The second pair of RHI scans were taken ~30 minutes later at 1033 UTC. At this time, the higher reflectivities associated with the frontal snow band snow band had entered the RHI plane from the north. The more northerly wind directions associated with the cold airmass were nearly perpendicular to the 290 degree azimuth of the RHI plane. This reduced the radial velocities to near zero values. When the northward-shifting wind directions passed through 020 degrees (perpendicular to the RHI plane), the low-level flow at the range of the campus site began to develop an inbound to the radar (negative sign) radial velocity component.
Surface observations from the campus WX station
The next plot shows the time histories of the 10 minute averaged surface temperature, pressure and wind direction data at the campus weather station for the 07 - 13 UTC time period. The two gray reference lines indicate the times of the two RHI scans shown in the previous section. The temperature trace contains the sharp drop associated with the arrival of the arctic airmass that began shortly after 10 UTC. Pressure changes due to the frontal passage were less dramatic, but the overall pressure rise signal became stronger after ~10 UTC. During the early portion of the plotted time interval, the recorded wind directions were consistent with the southeasterly flow that was implied by the positive radial velocities seen in the first RHI scan at 0958 UTC. By the time of the second RHI scan, the observed surface wind directions were shifting through the perpendicular to the RHI plane direction (shown by the red 020 degree reference line).
The final plot shows the precipitation time history at the campus weather station. The upper "rain gauge" trace shows the liquid equivalent of the snow based on weight of the accumulating precipitation. While the accumulation rate remained modest, precipitation maximized during the 09 - 12 UTC period that contained the frontal passage. The lower plot panel shows a smoothed trace of the snow depth data collected by one of the ultrasonic sensors. The snow depth increased most rapidly in the ~1030 - 1100 UTC time period as the frontal passage was occurring. It is possible that the snow tended to become more powdery with a higher snow - water equivalent ratio as the cold air moved in. This could account for the rapidly increasing snow depth while the liquid mass accumulation rate (presented in the upper panel rain gauge trace) did not change appreciably.
While the early morning cold frontal passage on 7 December 2016 was not a major event, the data recorded by the campus weather station and the CSU-CHILL radar documented several organized aspects of the precipitation system. The combination of the ~ 4 inch snow accumulation and temperatures declining into single digit Fahrenheit levels was sufficient to cause various transportation complications.
The data records from the CSU campus weather station were provided by Nolan Doesken, Zach Schwalbe and Noah Newman of the Colorado Climate Center.