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Peter L. Wolf

Abstract

During a severe thunderstorm outbreak on 17 April 1995, a bowing line segment of severe thunderstorms intercepted an isolated supercell over eastern Oklahoma. The result of the supercell–bow echo interaction was unexpected. Instead of showing a weakening supercell, with diminished severe weather potential, as the bow echo’s cold pool undercut, and spread in advance of, the supercell’s updraft, WSR-88D imagery showed a different outcome. The bow echo rapidly weakened, while the supercell maintained its identity and severity for over an hour after the interaction took place. WSR-88D imagery showed the evolution of a large high-precipitation supercell with a “comma-shaped” echo appearance.

The archive II dataset from the WSR-88D radar at Inola, Oklahoma, was retrieved, and the reflectivity and velocity images for this event were reproduced and examined. The images showed the supercell–bow echo interaction and the changes to supercell structure and resultant weather that appeared to result from this interaction. This paper documents the remarkable evolution of a large, comma-shaped supercell, as shown by WSR-88D imagery, and illustrates an important scenario that can result from the interaction between a supercell and a bow echo.

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Steven A. Amburn
and
Peter L. Wolf

Abstract

In current severe thunderstorm warning operations, forecasters frequently use the vertically integrated liquid water content (VIL) product from the WSR-88D to estimate thunderstorm severity and, particularly, hail size. Since VIL varies greatly based on airmass characteristics, forecasters have typically determined a threshold VIL to be used for each new thunderstorm event. A product that is independent of airmass characteristics, and thus independent of season and geographic location, would be more desirable in an operational warning environment.

It has been observed that high-topped thunderstorms with high VILs do not always produce large hail. It has also been observed that low-topped thunderstorms with low VILs occasionally do produce large hail. However, the maximum reflectivity in both high-topped and low-topped thunderstorms is similar when both produce similar-sized hail. From this, it was hypothesized that dividing the VIL by the echo top would “normalize” the VIL and produce a common value, or range of values, for thunderstorms producing large hail, independent of airmass characteristics. This quotient is defined as VIL density in this study.

To test the hypothesis, thunderstorm VIL and echo tops were recorded over a wide range of airmass characteristics, and VIL density was calculated. The data were correlated to surface-based reports of hail. The results showed a substantial increase in severe hail (≥19 mm, ¾ in.) reports as VIL density increased above 3.5 g m−3. At values greater than 4.0 g m−3, virtually every thunderstorm produced severe-criteria hail, regardless of the actual VIL or the thunderstorm height. At values below 3.5 g m−3, very few thunderstorms produced severe-criteria hail.

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