National Weather Service Weather Forecast Office

Originally posted: Friday, October 28, 2011

The upgrade to the Chicago National Weather Service's WSR-88D has been completed. The radar (KLOT) is officially back up and providing all of the data that has normally been provided. Over the past 24 hours we have done multiple tests to ensure the dual-polarization data feed is working correctly, and everything has tested well.  We thank you for your patience over the past couple of weeks of missing radar data as we performed the upgrade.

In addition, we should mention that one of the advantages of the new radar data will be better discrimination of precipitation type (rain, snow, sleet, hail, etc.).  Note that we said "better" and not "perfect."  There will still be uncertainty, so especially in the early weeks and months after receiving this new radar, we will be looking for as much ground-truth information as we can get.  To submit a report of mixed precipitation you can email our webmaster or put a note on our Facebook page.

And speaking of mixed precipitation, on Thursday afternoon (Octrober 27), within the first couple hours of turning on our newly upgraded radar, we were delighted to see the immediate benefits of this new technology.  Three of the new dual-polarization radar products are Differential Reflectivity (ZDR), Correlation Coefficient (CC), and Specific Differential Phase (KDP).  Using the information derived from these three products, in addition to the legacy Base Reflectivity data (Z), the radar is able to provide a best guess of precipitation type at a given location.  On Thursday afternoon (October 27), around 5:00 PM, the radar was indicating the presence of rain showers and graupel (frozen and/or slushy pellets) in the area around Rolling Meadows, near I-90 and Hwy 53 (outlined in the yellow box below).  Multiple observers in the area did in fact report graupel with these showers around that time.

(Click on the image to enlarge)

 

An early dual-polarization image from the newly upgraded KLOT radar, valid at 5:05 PM CDT on Thursday, October 27, 2011.  The images shown are Base Reflectivity (Z, upper left); Differential Reflectivity (ZDR, upper right); Correlation Coefficient (CC, lower right); and Specific Differential Phase (KDP, lower left).

The explanation of Z, ZDR, CC, and KDP is somewhat beyond the scope of this brief web story, but for those who are interested, here are the approximate observed values for those four parameters at that time:

Z:  40 to 48 dBZ
ZDR: 0.3 to 0.86 dB
CC:  > 0.995
KDP:  -0.4 to 0.5 deg/km

If you are interested in more of the specifics behind the variables, please visit the NSSL dual-pol information page by clicking here.

As shown in the reference sheet below, these values fit nicely within the expected ranges for each parameter when graupel is likely present.