Dual-Pol Applications

Dual-pol installations are now being pushed in full force across the country. As of this post, eight radars are equipped with dual-pol, including the prototype in Norman, OK; Vance, OK; Pittsburgh, PA; Wichita, KS; Phoenix, AZ; Morehead City, NC; Portland, OR; and Langley Hill, WA. Within the coming months and years, many dual-pol installations are scheduled, and it is expected that all radars will be upgraded by May 2013. To see when the radar nearest to you will have dual-pol capabilities, check out the deployment schedule. To learn basic information about dual-pol, check out NWS Amarillo's page. For more thorough dual-pol training for non-NWS users and/or non-meteorologists, visit the Warning Decision Training Branch page. Without the training or prior knowledge of dual-pol, it will be difficult to understand this post.

Over the course of several blog posts, I'll highlight applications that will show how dual-pol base data can supplement and even enhance polarimetric radar base data. I'm not a fan of derived radar products, so you won't see any dual-pol applications on the hydrometeor classification (HC) product or the melting layer (ML) product. If you want to see those, check out the WDTB's training link above. This post will look at how dual-pol products can be applied to a tornadic supercell.

KOUN 0.5 deg base reflectivity 2114 UTC 24 May 2011.

Cross section of supercell above.

The supercell we'll focus on is in Canadian County, OK, about 35 miles northwest of Oklahoma City. This supercell was part of a prolific tornado outbreak across Central Oklahoma on 24 May 2011 and produced an EF-5 tornado. Analyzing a cross section of this supercell, there's virtually no doubt this was a severe thunderstorm given the presence of a bounded weak echo region, weak echo overhang, significant reflectivities aloft, and a tight, strong and persistent mesocyclone. Given these characteristics and a very favorable ambient environment for tornadoes, the likelihood of this supercell producing a tornado is quite high. The question is, how can dual-pol data add value to what we already know?

KOUN 0.5 deg base reflectivity 2114 UTC 24 May 2011. Z (top left), ZDR (top right), CC (bottom left), and KDP (bottom right).

Let's first examine the core of this supercell and look at base reflectivity (Z) along with differential reflectivity (ZDR), correlation coefficient (CC), and specific differential phase (KDP). Z (top left) shows fairly high reflectivities (>56 dBZ) within the core at ~3,100 feet above radar level. This indicates the possibility of heavy rain and also hail. Several ZDR (top right) values within the white circle are between -1 and 1 dB, indicating the presence of large hail (blue and grey pixels) and melting hail. CC (bottom left) values between 0.82 and 0.95 also indicate the presence of large (possibly up to 1.75 in) and small hail within the core. Where CC values are near or below 0.80 (blue and lime green pixels) and ZDR values are -1 to 0 dB (grey pixels), there is a high likelihood of giant hail (2.00 in or larger). KDP (bottom left) values greater than 1 deg/km (light pink pixels) indicate this supercell has very large rain drops (orange pixels) and is capable of producing very high rain rates.

KOUN 0.5 deg base reflectivity 2114 UTC 24 May 2011. Z (top left), CC (top right), SRM (bottom left), and SW (bottom right).

We can also use dual-pol data to identify non-meteorological scatterers, including tornado debris. Dual-pol data will NOT increase tornado lead time, but it can give a warning forecaster high confidence that a damaging tornado is occurring. As a caveat, in order to detect tornado debris, the lofted debris must be within 70 miles from the radar and must fall within the beam during a volume scan. ZDR can be used to identify debris, but correlation coefficient is the best dual-pol product to use for this. CC values between 0-0.8 are pretty good indicators of tornado debris, if collocated with other tornado signatures. In the image above Z (top left) shows a high, blocky looking reflectivity signature southeast of Calumet within the hook echo. Within this high reflectivity signature, there is a rapidly rotating cyclonic couplet (SRM in bottom left) along with high spectrum width (bottom right) values. Correlation coefficient values within this same area range from 0.2 to 0.78. Therefore, the collocation of all these signatures gives extremely high confidence that a damaging tornado is occurring.