Radar Documentation of a Cyclic Supercell in the San Joaquin Valley, California

This study is a documentation of the evolution and structure of a right-moving cyclic supercell thunderstorm on the basis of WSR-88D radar information. The study is somewhat unique because the supercell did not occur in the Great Plains but near the Lemoore Naval Air Station in the San Joaquin Valley of California on November 22, 1996. The storm produced a mesocyclone-induced F0 and a subsequent F1 tornado. This storm was the first California supercell tornado event to occur near a WSR-88D radar [that at Hanford (KHNX)]. Furthermore, because of the flat expanse of the San Joaquin Valley, the Doppler radar had an unobstructed view of the tornadic storm that resulted in unprecedented quality of the low (0.5°) elevation radar scans for this storm. Hence, this case study is the first observation and documentation in California of a tornado cyclone signature (TCS) from WSR-88D radial velocity data.

The severe storm was the southernmost cell in a line of strong thunderstorms that developed in the cold sector of a wave cyclone and upstream of a progressive subsynoptic trough associated with a broad upward vertical motion field. The storm matured in an environment characterized by strong vertical wind shear and near a localized region of maximum CAPE. Radar reflectivity and radial wind velocity signatures showed well-defined supercell structure was present during the nearly 1.5 hour lifespan of the storm. The supercell was also cyclic with numerous updraft redevelopments during the maturation stage before becoming tornadic. The storm evolved on base-reflectivity images from a classic supercell with a hook echo and a highly reflective (>68-dBZ) updraft core (Fig. 1; top image, solid arrow) into a storm that resembled a high-precipitation (HP) supercell with strong returns across a wide swath of a more pronounced and reflective (68-dBZ) hook appendage (Fig. 1; top image, double headed arrow) during the F1 tornado event. During the tornado phase, the (VIL) product showed that large hail was likely associated with these intense updrafts and especially with an updraft in the hook echo region where base-reflectivity scans detected returns of 71 dBZ.

SRV volume scans during the storm’s lifespan showed the presence of mid-level mesocyclone indicated by numerous detections of a deep circulation by the WSR-88D mesocyclone algorithm. SRV images at 0.5° elevation showed that the development of a low-level mesocyclone (Fig. 1; bottom image, dashed circle) occurred simultaneously with the initiation of a RFD and just prior to the formation of the first tornado. Inbound radial velocity signatures associated well with the location of the hook echo (Fig. 1; bottom image, dashed arrow) and continued to increase in speed due to downdraft accelerations within the RFD. Furthermore, just before the formation of the first tornado, reflectivity cross-section and 0.5° tilt base reflectivity images indicated a BWER (Fig. 1; top image, dashed arrow) adjacent to a tilted updraft vault containing strong mid-level echo overhang.

Two distinct circulations were evident on the SRV data embedded within the storm during both tornado episodes. The larger (~1.75 nm/3.5 km), weaker (rotational shear 14.6 X ) circulation (Fig. 1; bottom image, dashed circle) was the low-level mesocyclone and the smaller (0.5 nm/1 km), intense (rotational shear 51.4 X ) one was a TCS (Fig. 1; solid arrow). The TCS evolved from small-scale downdrafts in the RFD that descended to near the surface and developed into an area of intense and increasingly rotational convergence. In this region of strong vertical shear is where the tornadoes occurred. The case study of this cyclic tornadic supercell highlights the usefulness of Doppler radar for analyzing the evolution and structure of such storms in Central Valley of California.

Fig. 1. 0.5° KHNX WSR-88D base reflectivity (top) with storm magnified in bottom left corner and storm-relative velocity (bottom) with dashed box indicating magnified area in top right at 22:52 UTC 22 November 1996. Arrows are discussed in text. Note: Top and bottom images not to scale.