DPWX/Evolving bright band: 28 March 2018
Author: Patrick C. Kennedy
CSU-CHILL S-band RHI scan along the axis of an area of light rain showers located just southeast of the radar on 28 March 2018. Time lapse loops of similar RHI scans showed that the reflectivity of the main, horizontally-oriented echo layer increased as it descended through the melting level.
The CSU-CHILL radar was conducting a series of system test operations during the mid-day hours of 28 March 2018. Continuous two sweep RHI scans were done on azimuths of 160 and 161 degrees as a part of these tests. These azimuths were selected due to their alignment with the narrow band of light showers that was passing the CHILL site. Surface temperatures at the radar were a few degrees above freezing during this period; the precipitation at the surface was observed to be light rain. This precipitation developed several hours after the passage of a surface cold front. Cold air advection was present in the NWS 500 hPa analyses valid at 12 UTC on the morning of 28 March. The sounding data from Denver (located ~80 km to the south of CSU-CHILL) showed that the environmental freezing level fell from ~1100 m AGL ahead of the cold front at 12 UTC to ~270 m at 00 UTC on the 29th. (These heights are reference to the CHILL radar elevation.) The following low level (0.5 degree elevation angle) PPI scan done by the NWS KFTG radar at 1909 UTC shows the general echo distribution:
S-band reflectivity loop from the 160 degree RHI sweeps
The following loop was assembled from the 160 degree RHI sweeps that were done between 1910 and 1935 UTC. (By the time of the image captures, the azimuth shown in the annotation block had shifted from actual 160 degree data collection angle.) The time interval between the sweeps is 2:07. The primary feature of interest is the general descent of a quasi-horizontal layer with reflectivity levels reaching ~20 dBZ. These reflectivities increased to ~30 dBZ as the descending echo layer reached the 0 C level. By the final loop frames, the narrow echo band was apparently moving out of the RHI scan plane.
Reflectivity data from the 7.58 km range gate
A more detailed view of the reflectivity evolution in the RHI scans was developed by extracting the reflectivity data in the 7.58 km range gate in each sweep. It should be noted that this selection of a single range gate means that the plots are not vertical profiles. (i.e. The horizontal projection of the sample volume approaches the radar as the elevation angle increases. The white cursor cross in the preceding RHI loop frames marks the 7.58 km range point at the surface.) Due to the quasi-horizontal echo layering in the RHI scans, this simple, single range gate extraction is capable of capturing the general evolution pattern. A blue arrow included in each each image frame marks the descending reflectivity maximum. To highlight the peak reflectivity values that are attained in the melting layer, the trace color is changed to red when the reflectivity is 27 dBZ or greater.
The increase in the dielectric factor when the hydrometeor phase changes from from solid ice to water should produce a reflectivity increase of ~6 dB (Battan 1973 p. 193). The ~34 dBZ peak reflectivities seen at 1926:56 UTC represent a reflectivity increase of ~10 dB. This suggests that the hydrometeors were probably also aggregating into larger sizes as they descended through the melting layer.
RhoHV and LDR data at 1924 UTC
The final plot provides a snapshot of two dual-polarization variables (Co-polar correlation coefficient (rhoHV), and Linear Depolarization Ratio (LDR)) at 1924 UTC. To maximize resolution, the RHI plots have been zoomed into the first few km from the radar. (The height scale marking is located at the 3 km range point; less than half of the 7.58 km range data shown in the earlier reflectivity profile plots.) In both rows of the RHI plots, S-band reflectivity is shown on the left. Correlation coefficient (field name RH) and LDR (field name LH) are on the right. The highest reflectivities are found just above the 0.2 km AGL height mark while the rhoHV minimum and LDR maximum occur just below the 0.2 km height level. The reflectivity maximum is associated with ice particles that are still fairly early in the melting process. Below this maximum reflectivity height, continuing melting causes the smaller particles to completely liquefy, while more massive particles are still partially frozen. This diverse particle population reduces rhoHV near the base of the bright band (Zrnic et al, 1994). The irregularly-shaped (non-spherical), melting aggregates that survive to the greatest fall distance below the 0C level probably contribute to the LDR enhancement (Zrnic et al, 1993).
Battan, L. J., 1973: Radar Observation of the Atmosphere. University of Chicago Press.
Zrnic, D. S., N. Balakrishnan, C. L. Ziegler, V. N. Bringi, K. Aydin, and T. Matejka 1993: Polarimetric signatures in the stratiform region of a mesoscale convective system. J. Appl. Meteor., 32, p. 678 - 693.
Zrnic, D. S., R. Raghavan, and V. Chandrasekar 1994: Observations of copolar correlation coefficient through a bright band at vertical incidence. J. Appl. meteor., 33, p. 45 - 52.