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Terminal Doppler Radar Data Proven Beneficial


TCVG Terminal Doppler Radar Supplements KILN/KIND WSR-88Ds

Sunday, March 8, 2009 -- Ripley County Tornado

During a period from April to September in 2008, National Weather Service offices gained access to data from forty five Terminal Doppler Weather Surveillance Radars (TDWRs) at major airports across the country. Reflectivity data has also recently become available to the public in the RIDGE 2 radar display mosaic. The Wilmington, OH (ILN) County Warning Area (CWA) has been fortunate to have a TDWR located at major airports near Dayton, Columbus, and Cincinnati. The TDWRs bring the potential for data sampling at lower levels of a storm, more frequent radar data at the lowest levels, more detailed information given a storm's closer proximity to the radar, and ultimately more information for warning forecasters. On March 8, 2009, the Cincinnati-Northern Kentucky International Airport radar (TCVG) provided data which helped detect rotation associated with a brief tornado touchdown.




Figure 1:
Top panels- TCVG Z/SRM looped minute scans 2115Z-2125Z;
Bottom panels- KIND Z/SRM 4.5 minute scans 2115Z-2125Z


More Frequent Radar Data
TDWRs provide a radar scanning strategy which provides new data at the lowest elevation tilt every minute. In comparison, the WSR-88Ds can provide radar scans at the lowest tilt every four and a half minutes in their fastest scanning strategy. During the Ripley County tornado, the radar provided valuable reflectivity and storm relative motion (SRM) velocity data. In Figure 1, the TCVG SRM shows the broad rotation of the storm tightening into a couplet. The couplet is not visible from KIND, but more of a broad rotation is evident with the storm. The storm touched down briefly at approximately 2122Z. Notice the couplet on TCVG from 2121Z-2124Z. The 0.5 degree elevation scans from KIND provided data of the storm at 2120Z and 2124Z, missing the first three minutes of the couplet. In Figure 2, seen below, KILN shows no evidence of a tightening circulation. The reflectivity data from TCVG indicates an apparent cell merger on a pre-existing storm scale boundary. This suggests that the tornado may have developed from a non-descending mesocyclone. These processes can be difficult to detect using WSR-88D data since they are often short-lived and develop just a few thousand feet above ground level. The addition of one minute data from the 0.1 degree elevation scan of TCVG in this case provided the ability to see short-lived reflectivity and velocity signatures which occurred between scans from both KILN and KIND. TCVG provided more scans by the minute, detecting the evolution of the storm.




Figure 2:
Top panels- TCVG Z/SRM looped minute scans 2115Z-2125Z;
Bottom panels- KILN Z/SRM 4.5 minute scans 2115Z-2125Z


Closer Storm Proximity to the Radar


Figure 3: Google Earth map of the distance from each radar to the storm.

The resolution of radar data decreases the further a storm is away the radar. Range limitations provide challenges for forecasters, as their furthest most counties are not sampled as well by the WSR-88D radar. The resolution decreases with range and the lowest levels are not sampled at all. TDWRs can provide data at a finer resolution than the WSR-88s with a smaller data bin size. In this case, the storm was approximately 65 nm from KILN and 60 nm from KIND. Both the KIND and KILN 0.5 degree elevation SRM data show only weak rotation at this distance from the radar. Thus, the low-level rotation was poorly sampled by the WSR-88Ds, and not sampled at all below 5,000 feet. Fortunately, the TDWR was approximately 33 nm from the storm, providing higher resolution scans of the storm, especially in the lowest tilt where the couplet was more evident. Although the range on the WSR-88Ds is greater, the proximity of TCVG to the storm in this case provided more detailed scans.

Lower Elevation Data Sampling


Figure 4: Top panels- TCVG Z/SRM at 2122Z; Bottom panels- KILN Z/SRM at 2122Z.

Despite the fact that TDWRs have less range in comparison to the WSR-88Ds, their scans provide a lower elevation angle (0.1 or 0.3 degrees). TCVG's lowest elevation angle in this scanning strategy is at a 0.1 degree tilt, where the WSR-88Ds provide a 0.5 degree tilt. With the storm being closer to TCVG, the 0.1 degree elevation scan provided data of the storm at approximately 1,100ft AGL. Meanwhile, the 0.5 degree elevation scan of both KIND and KILN displayed data at 5,400ft and 5,800ft AGL respectively. Notice in Figure 4, the TCVG SRM velocity couplet is well-defined at the 0.1 degree tilt, whereas the KILN SRM shows inconclusive data. The WSR-88Ds could not sample the most valuable data near the surface which is sampled by TCVG.

Conclusions
The radar data provided by TCVG revealed a well-defined rotation with the storm as it moved into northeast Ripley County and approached the Dearborn County line. The lower tilted beam, proximity to the radar, and faster scanning at the lowest tilt of the TDWR provided data which corresponded with a brief touchdown of a weak tornado. This supplemental information was beneficial for forecasters. In this case, WSR-88D data was limited, and could not detect the low-level rotation as adequately as the TDWR data. Although the TDWR provided more detail in this case, there still are limitations which are not covered in this study. The storm's proximity to the radar can vary the quality of the data provided by both the WSR-88Ds and TDWRs. The Ripley County storm is proof that the TDWRs can provide beneficial supplemental data to the WSR-88Ds in certain scenarios.

Case Study Performed by: Charlie Woodrum
Contributions by: Stephen Hrebenach and Dan Hawblitzel
Technology Support: Todd Shobe and Jonathan Blaes

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