Friday, May 17, 2013

Radar Images from the May 15, 2013, EF4 Hood Co. Texas Tornado

The following are radar images from the NEXRAD Doppler in Dallas-Fort Worth. At the time the data was collected the maximum estimated wind (doppler + forward motion) was 182 mph.

 One of the most wound up hook echoes I have seen.with a distinct debris ball and a rear flank downdraft (RFD) gust front. The RFD gust front reminds me of a hurricane feeder band.
 Spherical targets have a correlation coefficient (from dual polarization technology) between the vertical and horizontal axes of 1.0 or close to that. Low correlation is expected where you find targets that are not spherical, like debris.  So the debris ball shows up well above. Note this confirms what we see on the reflectivity image at the top as being the debris ball

 Differential Reflectivity ( ZDR) is another dual pol product. It is the difference (Horizontal - Vertical) reflectivity. Spherical targets will have a near zero ZDR because the horizontal and vertical dimensions are equal. Large rain drops flatten out as the fall so will have a positive ZDR. But debris looks nothing like a sphere so why does the debris ball have near zero ZDR. Because there are many objects, randomly oriented in space and moving and tumbling through space.  The net result is that the energy returned to the radar is about equal in the vertical and horizontal.

 Radial Velocity is movement directly to or away from the radar. In the image above green is air moving to the radar (inbound) and red is air moving away from the radar (outbound). HOLD ON THERE IS AN EXCEPTION. The green kidney shaped area surrounded by red and the red areato the right of it are exceptions.  The winds are so strong that the radar sees those spots as the opposite of what  they are.This is velocity folding, the winds exceed the maximum the radar can measure so the colors "fold over" to other part of the color table. The velocity can be mathematically unfolded. See the next image.

 Tornado circulation at the lowest level the radar can see.  When the software unfolded the velocity the maximum inbound was 159.7 mph, and the maximum outbound was 115 mph. If the highest velocity was alligned at some point in time with the forward motion of the storm which was moving about 22 mph then the maximum wind speed would be 182 mph, right in the middle of the EF4 category.