|Here's a quick look at some of the new data we've been using at the National Weather Service in La Crosse after our radar underwent an upgrade to Dual Polarization.
This image shows three of the new graphics that we are able to view:
- The top left image is Reflectivity, which we had before this upgrade. The pink to purple colors suggest hail is within the storm (at this elevation about 15000 feet above the ground).
- The top right image is Differential Reflectivity (ZDR), which provides a ratio of how the shape of the precipitation compares vertically and horizontally (round or flat). If it is round, like hail, the ratio is closer to zero (large rain drops fall more hamburger-shaped and have a high ZDR) . Most of the grey-blue color is close to zero (thus likely hail). Meanwhile in the white circle the ZDR is very high. This indicates a region of strong updrafts. A storm needs a strong updraft to produce large hail.
- The bottom right image shows the Correlation Coefficient (CC), which gives an idea if the precipitation particles are all the same (e.g., rain) or a mix (e.g., rain and ice). The dark blue in the white circle highlights a region of CC less than 0.70 which indicates large hail. Further, note the spike of dark blue (in the light blue box) down radial (southwest) of the hail indication. This blue spike of CC is also a large hail indicator.
- The bottom left image shows the Specific Differential Phase (KDP), which is mainly used for determining rainfall rates.
This radar snapshot came from 4:19 pm on Saturday, May 5th, 2012 near the Charles City, Iowa area just before it produced golf ball sized hail at the Floyd County fairgrounds.