WSR-88D Radar Imagery

An analysis of Weather Surveillance Radar 1988 Doppler (WSR-88D) radar images from Monterey, California (KMUX) is presented here. To the authors' knowledge, these are the first WSR-88D images of an anticyclonic tornadic supercell ever analyzed and recorded. Reflectivity characteristics were typical of a Northern Hemisphere supercell, except that the E-W axis was reversed. For some volume scans at the lowest elevation scan (0.5^o), a pendant echo, most likely a degraded view of a hook echo, was apparent on the left flank of the anticyclonic supercell, as opposed to the typical right flank on cyclonic storms (). Analysis of radial velocity data shows anticyclonic storm-scale vortices (meso-anticyclones) with vertical and time continuity. The storm motion was to the left (poleward) of the mean flow, opposite to a classic right-moving cyclonic supercell. The supercell, and both tornadoes associated with it, moved from southeast to northwest.

2331 UTC 4 May 1998 reflectivity at 0.5 o tilt from the KMUX (Monterey) WSR-88D, showing cyclonically-curved hook near Mountain View. (Click HERE for animated loop; Click Image for Larger Version)

A version of the NSSL MDA, tuned to detect clockwise-rotating storm-scale vortices, was run on the WSR-88D Level-II data from KMUX. The Level-II archive includes data beginning at 2334 UTC 4 May 1998, which is during the time of the F2 Sunnyvale tornado. Therefore, it is difficult to ascertain from the radar data alone the origins of the vortex that produced this tornado. However, the radar velocity characteristics of this first tornado at 2334 UTC are similar to other non-supercell tornadoes, in that only a very small vortex (albeit anticyclonic) is observed, with no accompanying or surrounding larger mesocyclonic vortex (). In fact, the vortex is too small to be accurately detected by the MDA at this time. The subsequent volume scan, at 2340 UTC (not shown), continues to show the anticyclonic Sunnyvale vortex; this is near the time of the tornado's demise based on visual observations.

2331 UTC 4 May 1998 radial velocity (left) and reflectivity (right) at 2.4 o tilt from the KMUX WSR-88D. Location of anticyclonic vortex associated with the F2 Sunnyvale tornado is shown, along with the direction of motion (arrow). Radar time is during F2 Sunnyvale tornado. (Click image full size version)

Velocity data shows the development of a second, and separate vortex, northwest of the first vortex, which was associated with the second tornado, the F1 Los Altos tornado. This second tornado developed at 2349 UTC, and was also anticyclonic. At 2355 UTC, the tornado was associated with a more-pronounced radar velocity vortex, which was detected by the MDA and classified as a "low-topped meso-anticyclone", a vortex detection which is defined as occupying at least 25% of the storm depth, but is less than 3 km tall. The storm depth was about 9 km, which is in the general range of low-topped storms. shows the WSR-88D data at 2355 UTC, with the locations of the radar-observed storm-scale vortices from both tornadoes overlaid. represents an 8-panel display of WSR-88D velocity data at various horizontal cross-sections through the storm. The MDA detected two-dimensional anticyclonic features with vertical continuity.

2355 UTC 4 May 1998 radial velocity (left) and reflectivity (right) at 0.5 o tilt from the WSR-88D. Location of anticyclonic vortex associated with the F2 Sunnyvale and F1 Los Altos tornadoes are shown, along with the directions of motion (arrow). Radar time is during F1 Los Altos tornado. Red-in-yellow circle is MDA output of a "low-topped meso-anticyclone". (Click image full size version)

This success augers well not only for detecting relatively rare clockwise-rotating vortices, but also for capturing tornadic precursor circulations developing in the low-topped convection which is typical in the Pacific Coast states. The high-siting elevation of many of the WSR-88D sites in the mountainous west will mitigate the ability to detect such vortices, however. For example, in the case considered here, KMUX's elevation of about 1000 m resulted in the radar not being able to "see" the lowest 20-25% of the tornadic supercell, even though the storm was quite close to the radar site.

2355 UTC 4 May 1998 multi-panel radial velocity image, at 1.1, 3.0, 4.9, 6.6, 8.5, 11.8, 19.2, and 28.0 k AGL, from the KMUX (Monterey) WSR-88D. Blue circles indicate the 2D anticyclonic feature output from the MDA (as detected for elevation scan) associated with the F1 Los Altos tornado. The diameter of the circles correspond to the diameter of vortices in the radar velocity data.(Click image full size version)