VIL Density as a Hail Indicator

In current severe thunderstorm warning operations, forecasters frequently use the vertically integrated liquid water content (Vn) 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 alway 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 VV, by the echo tap 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 (greater than or equal to 19 mm, 3/4 in.) reports as VIL density increased above 3.5 gm(-3) At values greater than 4.0 gm(-3), virtually every thunderstorm produced severe-criteria hail, regardless of the actual VIL or the thunderstorm height. At values below 3.5 gm(-3), very few thunderstorms produced severe-criteria hail.