An Examination of VIL and Echo Top Associated with Large Hail in Middle Tennessee

Timothy W. Troutman and Mark A. Rose
NWSO Nashville, TN

1. Introduction

Numerous large hail events (hail diameter 3/4 in or more) were studied across the Nashville County Warning Area (CWA) in middle Tennessee during 1995-1997 in order to determine the relationship between Vertically Integrated Liquid (VIL) and storm echo top. VIL Density was also computed. The methodology used in deriving these comparisons is provided to assist other NWS offices in developing their own VIL/echo top and hail size/VIL Density correlations.

2. VIL and VIL Density

VIL is a function of reflectivity, and converts reflectivity data into an equivalent liquid water content value based on studies of drop-size distribution and empirical studies of reflectivity factor and liquid water content (Amburn and Wolf 1996). The general equation for VIL as used with the WSR-88D is:

VIL = SUM 3.44 x 10^-6 [(Z_i + Z_i+1)/2]^4/7 dh

and has units of kg m^-2. Z_i and Z_i+1 are radar reflectivity at the bottom and top of the layer dh, whose thickness (dh) is in meters.

VIL Density is simply the VIL divided by the echo top (m) and multiplied by 1000 in order to express the result as g m^-3. The importance of the VIL Density is its use in quickly identifying thunderstorms with high reflectivities relative to their height. Such thunderstorms often contain hail cores, and as the VIL Density increases, the hail core tends to be deeper, more intense, and the resulting hail sizes tend to be larger (Amburn and Wolf 1996).

3. Methodology

During the years 1995-1997, 151 severe hail events occurring within the Nashville CWA were analyzed. VILs, echo tops, and hail sizes were gathered for each of these events so that correlations could be determined.

In order to select the echo top, the WSR-88D function was used. The echo top selected was always located on the same pixel as the maximum VIL. The echo top and VIL values used were in increments of 5 units. (A VIL in the range of 45-49 kg m^-2 was reported as 45 kg m^-2. An echo top in the range of 35-39 kft was reported as 35 kft.) Only storms for which severe hail was reported were used. Thus, in this study, it is unknown whether large values of VIL or VIL Density may be associated with hail of less than 3/4 in diameter.

Severe hail events from all seasons of the year were utilized in this study. Severe hail was observed in thunderstorms with VILs as low as 15 kg m^-2 and echo tops as low as 15 kft, and in thunderstorms with VILs as high as 80 kg m^-2 and echo tops as high as 50 kft. A comparison was also made between hail size and VIL Density. Linear regression was used to fit a straight line to these two variables to assist in interpreting results.

Most of the cases used in this study were gathered from WSR-88D data in which Volume Coverage Pattern 21 (not VCP 11) was used. VCP 21 samples fewer elevation angles than VCP 11. Because of the gaps in the scan strategy, VCP 21 performs less thoroughly than VCP 11 in the estimation of VILs and echo tops (Federal Meteorological Handbook No. 1, Part C 1991).

Also, all hail sizes used in this study were obtained using storm verification data. These data are collected to assist with warning verification and are not necessarily complete as far as supporting scientific studies. It is not known whether the hail sizes reported with each event represent the largest hail which occurred.

4. Results

It was hypothesized that a relationship existed between VIL and echo top, and that such a relationship might be useful in determining the potential of a thunderstorm in producing severe hail. The results of this study show that not only is there a strong relationship between VIL and echo top, but a relationship between hail size and VIL Density also exists. These results confirm earlier studies which indicate that, in cases of large hail, there is a strong positive correlation between echo top and VIL.

VIL Density, especially when compared with hail size, does provide useful information. For the 151 large hail cases, 81% had a VIL Density of at least 3.5 g m^-3. Interestingly, Amburn and Wolf (1996) identified the VIL Density of 3.5 g m^-3 as correctly identifying over 90% of the severe hail cases in their Oklahoma study. Also, the average VIL Density associated with 3/4 inch hail (the smallest of the severe categories) was 4.1 g m^-3. The VIL Density of 1.6 g m^-3 correctly identified all 151 severe hail events, 3.3 g m^-3 identified all hail occurrences of 1.75 in and larger, and 3.6 g m^-3 identified all hail occurrences of 2.75 in and larger.

A regression line describing this relationship was developed as well (Figure 1). This figure may be used in actual severe weather events as an indicator of a thunderstorm's potential for severe hail. The VIL Density should, however, be used with a reliable minimum VIL threshold technique in such operational situations.

5. Conclusion

The data gathered for this study show a strong correlation between VIL and echo top in storms which produce large hail. It has also been shown that a relationship between hail size and VIL Density exists. This relationship can be utilized operationally to assist in issuing severe thunderstorm warnings when used with the "VIL-of-the-day" or other minimum VIL threshold techniques.

Acknowledgements

The authors thank Scott Sharp and Darrell Massie (meteorologists) and Henry Steigerwaldt (SOO) NWSO Nashville, for their reviews of this paper. Special thanks to Steve Amburn (SOO, NWSO Tulsa) for his review and numerous suggestions.

REFERENCES

Amburn, S. and P. Wolf, 1996: VIL density as a hail indicator. Preprints, 18^th Conf. on Severe Local Storms. Amer. Meteor. Soc., San Francisco, CA.

Federal Meteorological Handbook 11, 1991: Doppler Radar Meteorological Observations, Part C, Office of the Federal Coordinator for Meteorological Services and Supporting Research, U. S. Department of Commerce/NOAA, Washington, D. C.