Part 2 of a guest blog by Greg Schoor NWS, Baltimore Washington
What are the new products available with Dual-Pol?
There are three base products, each of them go into creating the Hydrometeor Classification (HC) product, previously mentioned. One of the best ways examples to show each of the new Dual-Pol products and what purpose they can serve is to go through case with hail in a thunderstorm. Starting off with what we are more used to seeing, the lowest tilt Reflectivity product. A white oval surrounds the area just north of the storm’s updraft where large hail is likely falling. Reflectivity, however, only shows how “reflective” the surfaces of raindrops, hail, ice and snowflakes, not giving much more information about the echo.
However, when hail is present and mixed in with rain, different characteristics can be seen with Dual-Pol products that cannot be detected by conventional radar. Looking at Correlation Coefficient (CC) product gives us a few key pieces of information:
1). Which echoes are precipitation and non-precipitation (ground clutter).
2). Where echoes are the same type of precipitation and where there is a mix of different precipitation types. The magenta and dark reds are where precipitation types are the same, in this case, all rain. Once you see a mix of lighter oranges and yellows, there is a mix of other precipitation types, in this case, hail.
3). The white circle denotes the same region in the Reflectivity image that is the core of hail.
The other two products show the same hail core feature, in different ways. The Differential Reflectivity (ZDR) product shows the height vs. width difference for each echo. Rain drops are wider (as they are falling) than they are tall, so their ZDR will be a positive value. Conversely, ice crystals and some graupel appear taller than they are wide, so they will return negative values of ZDR. Hail, since it is mostly round will have values closer to zero, since the height vs. width difference is about zero. In the image to the right, the white circle shows the same hail core region, where the values are much closer to zero than the pixels surrounding it.
Lastly, for the hail example, is Specific Differential Phase Shift (KDP). Hail, raindrops, snowflakes and other precipitation types will fall all out of a cloud in different ways. Most raindrops will normally fall straight down, with not much randomness involved. Snowflakes, however, have completely random motions as they fall toward the surface. Even hail will “tumble” as it falls. The more random the falling motion, the higher the value for KDP. The white circle showing the same hail core region shows high values of KDP, which reflects the highly random motion from the medium-to-large sized hail that is falling out of the thunderstorm.
(All images courtesy WDTB: http://www.wdtb.noaa.gov/courses/dualpol/trainingaid/index.htm)
Now that we have Dual-Pol, how can I learn more about it?
Training Courses: There are a number of web sites that offer information on the new suite of Dual-Pol products and what each of these products provides. There are different levels of training for the Dual-Pol products and applications for meteorologists and non-meteorologists:
Training Aides: Here, you can view short instructor-led web modules with full explanations, accompanying graphics and real world examples. Also provided are helpful aides to better understand each of the individual Dual-Pol products:
How can I access Dual-Pol data?
Dual-Pol data is flowing into our local NWS office to help measure our atmosphere better and improve weather warnings. At the moment, NWS public radar pages do not include Dual-Pol products. Many of the WSR-88Ds around the country have yet to be upgraded, so there will be some time before it is readily available to the public. However, radar data is available for download and can be viewed from online and downloadable java viewers provided by the National Climatic Data Center (NCDC), here:
There are also a number of commercial vendors that offer specialized viewing software applications for conventional and Dual-Pol radar data that our local media can utilize.
Written and submitted by:
Radar Program Leader
National Weather Service, Baltimore-Washington (Sterling, VA)