Characteristics of eastern Australian-western Tasman Sea enhanced-Vs and their connection to severe weather

Previous research, including that by Brunner et al. 2006, and Brunner et al. 2007 - "B06" and "B07", has found a strong relationship between U.S. severe weather occurrences and the enhanced-V infrared satellite imagery signature. The aim of this study is to examine the characteristics of these features in the eastern Australian region, including an analysis of the upper level environments associated with the more intense severe weather outbreaks and enhanced-V signatures.

Color-enhanced 10.8 µm infrared NOAA-AVHRR satellite images for 222 eastern Australian severe thunderstorm event days between 1996 and 2007 were examined. In order to maximize the prospects of enhanced-V detection, event days examined were strongly biased to those associated with (a) major severe weather outbreaks, (b) reports of large hail and/or (far rarer) tornadoes, and (c) warm season occurrences. This revealed 146 enhanced-Vs over the subtropical eastern Australian mainland and adjacent Tasman Sea waters. The signatures were concentrated within a 300-400 km wide longitudinal band, which was centered near the central New South Wales coast, in the south, and 100-200 km inland from the southern Queensland coast, in the north.

Using a similar ±3 hour window detection methodology to that adopted by B07, this study found that 60 (48%) of the 125 enhanced-Vs characterized by a predominantly overland trajectory were associated with severe weather reports (i.e. hail ≥ 2 cm diameter, wind gusts ≥ 48 knots (89 km h^-1), wind damage, and/or tornado sightings). This is considerably below the 80% value obtained by B07. However, satellite and radar data suggest that the relatively low correlation between eastern Australian enhanced-Vs and severe weather episodes could primarily be attributed to the fact that the majority of the “non-severe” signatures occurred in sparsely populated and remote areas, from which severe weather reports were unlikely to be received. Large hail was associated with 84% of the severe weather producing cells, and was by a very wide margin the main severe weather type observed. The vast majority of enhanced-V event days and signatures were detected in the October-January (warm season) period.

Approximately 80% of the enhanced-Vs were characterized by overshooting cloud tops > 5 K lower than the environmental tropopause temperature – i.e. a “Storm Top Temperature Anomaly” (STTA) < -5 K, with an extreme STTA value of –21 K observed in 3 cases. A strong relationship was also observed between the magnitude of the STTA, and threshold values of a revised set of the enhanced-V types found by B06 to be closely linked to U.S. severe weather episodes. Additionally, after allowing for the limitations of the storm spotter network, the STTA and the revised B06 threshold criteria appeared to be stronger discriminants between severe weather and non-severe weather producing eastern Australian enhanced-Vs than the modified B07 threshold criteria.

Case studies are presented that demonstrate that the most intense eastern Australian severe weather outbreaks coincided with the presence of strong upper level forcing. Furthermore, the most intense enhanced-V cold storm top-warm wake couplets (- defined according to the magnitude of the temperature difference and the distance between these two features) were invariably found to occur in environments associated with strong or amplifying sub-synoptic scale upper tropospheric divergence/diffluence.

The Bureau of Meteorology's operational National Thunderstorm Forecast Guidance System (NTFGS) uses algorithms based essentially on characteristics of the low to mid-tropospheric meteorology to diagnose environments favorable for a variety of convective phenomena. The results of this study, consistent with those of many U.S. studies, suggest that NTFGS should be modified to include diagnostics of upper level forcing, in order to enhance its ability to provide quality guidance of (severe) thunderstorm threat areas to forecasters.