5 March 2018

Fig. 12. As in Fig. 6, but for 2230 UTC on 5 March 2018, near the beginning of IOP2.

A cycloidally tracking low pivoted over Minnesota as it began to occlude (Fig. 12). The occluded front, trailing southeast from the surface pressure minimum, pushed northeastward through Indiana during the evening. The NAM and GFS forecast soundings for Lafayette, Indiana (not shown) both indicated the potential for mixed winter precipitation. A refreezing signature IOP was planned for that evening beginning at 2300 UTC.

As the precipitation bands along the occluded front approached Lafayette during the afternoon, some were observed to contain widely varying values of correlation coefficient (Fig. 13). The IOP start time was moved back to 2200 UTC in light of this development. The DOW deployed at ACRE with sleet falling on it, and it collected a nine-minute sequence of PPIs, RHIs, and birdbath scans through 0200 UTC. Unfortunately, no soundings could be launched during IOP2 (or any subsequent IOPs) owing to failure of the helium regulator during IOP1.

Fig. 13. Correlation coefficient at 2111 UTC over western Indiana, measured by the KIND WSR-88D radar at an elevation angle of 0.5°. The bands of precipitation depicted here moved toward the north-northeast.

EAPS 52300 student J. Chen was assigned to analyze IOP 2 DOW data. She found that her QVPs did not conclusively show a refreezing signature at a time one was noted in the log (2327 UTC). She hypothesized that the observed horizontal heterogeneity in the precipitation bands was masking the refreezing signature in the QVPs. Indeed, as the precipitation bands passed over the DOW at 2354 UTC (Fig. 14a), a localized signature partially consistent with refreezing (Kumjian et al. 2013), consisting of enhanced ZDR and slightly decreased correlation coefficient, was observed at low elevation angles to the DOW’s immediate southwest (Fig. 15). The expected enhancement in KDP (Kumjian et al. 2013) was not observed (Fig. 15d). In summary, although sleet was observed at the DOW deployment site, inconclusive evidence for refreezing was found in the polarimetric measurements.

Fig. 14. QVPs of DOW7 observations of (a) reflectivity (in dBZ), (b) ZDR (in dB), (c) correlation coefficient (unitless), and (d) specific differential phase for a portion of IOP2. Image courtesy of J. Chen.


Fig. 15: PPI displays of DOW7 (a) reflectivity (in dBZ), (b) ZDR (in dB), (c) correlation coefficient (unitless), and (d) specific differential phase (in deg km-1) at 2354 UTC on 5 March 2018 at an elevation angle of 4.0°. The view is confined to a 5 km x 5 km box centered on DOW7 (whose location is marked by a black cross). A possible refreezing signature is outlined in white.