Precipitation properties of a cool-season tornadic storm inferred from C-band dual-polarimetric radar and 2D-video disdrometer observations

A cool-season severe storm produced an EF-2 tornado and isolated small (< 2.5 cm) hail reports in and around downtown Huntsville, Alabama during the early evening hours of 21 January 2010. Just prior to tornado touchdown, the storm passed directly over a bermed research area heavily instrumented to measure precipitation properties, including two 2D-Video Disdrometers (2DVDs). Storm evolution was well observed by the Advanced Radar for Meteorological and Operational Research (ARMOR). ARMOR is a C-band dual-polarimetric Doppler radar located at the Huntsville International Airport, which is about 14 km southwest of the disdrometers. ARMOR sampled low-to-mid levels (0-4 km) of the storm at close range (5-25 km) every 1-3 minutes leading up to and including tornado touchdown.

Precipitation properties, including particle size distribution (PSD), shape and type, are inferred from the 2DVD, ARMOR and storm report observations of the tornadic event. The key result confirmed by all observations is the predominance of low concentrations of very large raindrops, likely resulting from the melting of small hail, throughout most of the low-to-mid level echo. Unlike other tornadic supercells documented in the literature, the differential reflectivity (Z_dr) was routinely in the range of 3-5 dB and 5-8 dB was not uncommon in the high reflectivity echo (Z_h > 45 dBZ) of the entire forward flank downdraft (FFD). A Z_dr-arc, or enhanced (5-8 dB) region of Z_dr in a cyclonically curved pattern, was consistently present along the southern flank of the FFD prior to and during tornado touchdown. The Z_dr-arc has been associated with size sorting in the presence of wind shear in prior studies. Differential phase measurements in the Zdr-arc appear to be dominated by backscatter differential phase of 20 degrees or more, which is consistent with resonant scattering of low-concentrations of large drops. 2DVD observations confirm that the FFD and Z_dr-arc regions were likely composed of mostly large raindrops with mass weighted mean diameters (D_m) of 3 mm and maximum rain drop sizes of over 5-6 mm. Drop concentrations (50-1100 m^-3) and rain rates (2-24 mm h^-1) from the 2DVD were low throughout the entire FFD and Z_dr-arc regions. Gamma fits to the 2DVD PSDs clearly indicate negative shape parameters, which has also been associated with size sorting in the presence of shear. 2DVD drop shapes are consistent with large oblate raindrops. Only two hail reports (1.3 cm and 2.2 cm) came from storm spotters for the entire event. Z_dr in the area of melting hail was also from 3-7 dB near the surface, making it difficult to differentiate from large raindrops.

A precipitation streamer (in 2D) or sheet (in 3D) was present at low-to-mid levels on the rear flank of the tornadic storm prior to tornadogenesis. The narrow streamer was a rearward lateral extension of the larger and broader Zdr-arc region. The streamer was characterized by very large Z_dr (3-8 dB) and typically low Z_h (< 30 dBZ), suggesting extremely low concentrations of large raindrops or melting hail at low levels. Its presence was most evident in Zdr (and was not always clear in Z_h). This “Z_dr-streamer” wrapped up cyclonically in the mesocyclonic flow and later became part of the tornado appendage or hook echo that was apparent at low levels prior to and during tornado touchdown.