VIL (Vertically Integrated Liquid) is a function of reflectivity, and converts reflectivity data into an equivalent liquid water content value based on drop-size distribution and a reflectivity factor. This factor is proportional to the total number of targets within a measured volume and to the target diameters taken to the sixth power. Thus, target diameter has a much greater effect on reflectivity than does the number of targets. Reflectivity increases exponentially as target diameter increases. Thus, VIL increases exponentially with reflectivity, so high VIL values require high reflectivity values, usually implying the presence of large targets (hail) aloft. As a result, VIL is used to identify thunderstorms that likely contain large hail and/or a deep layer of large drop sizes.
However, there are several potential problems with using VIL by itself:
To eliminate some of the inherent problems with using VIL alone, the use of "VIL Density" as a severe hail indicator was studied at the weather office in Tulsa, OK in 1994 and 1995. Their study included 221 thunderstorms, the majority of which produced severe hail.
VIL Density = VIL ÷ Echo Top (units are g/m3 when multiplied by 1000)
VIL Density is a parameter that "normalizes" the VIL using the height/depth (echo top) of a thunderstorm, i.e., a cell's VIL is compared to the cell's echo top, which basically eliminates the air mass dependency of VIL alone and the problematic assumptions of VIL-of-the-day. Therefore, VIL Density can be used to identify thunderstorms with high reflectivities relative to their height. Unlike VIL which usually increases as storms increase in height, VIL Density increases primarily due to increases in target size. Thus, as VIL Density increases, hail cores tend to be deeper, more intense, and reported hail sizes tend to be larger. Thunderstorms with larger VIL Density values generally produce larger hailstones at the surface.
VIL Density Results:
|SEVERE THUNDERSTORM / HAIL THRESHOLDS|
|VIL Density > 3.28||Identified 97% of severe hail cases, but gave a 25% false alarm rate.|
|VIL Density > 3.50||Identified 91%of severe hail cases, with only a 5.5% false alarm rate.|
|VIL Density = 3.0-3.5||Small hail and marginal SVR Hail|
|VIL Density = 3.5-3.9||Mostly SVR Hail|
|VIL Density > 4.0||SVR Hail|
|HAIL SIZE VERSUS VIL DENSITY THRESHOLDS|
|VIL Density < 3.0||Mostly non-severe/small hail but a few dime to nickel size severe cases.|
|VIL Density = 3.0-3.4||About half the cases non-severe hail, and half of cases severe with dime to nickel size.|
|VIL Density = 3.5-3.9||Nearly all severe cases, with most dime to nickel size, with a couple one inch size.|
|VIL Density = 4.0-4.4||All severe hail cases, with most dime to nickel size but several an inch or more and a few golfball size.|
|VIL Density = 4.5-4.9||All severe hail cases, with half of cases dime to nickel size and half of cases an inch in diameter or more, including several golfball size or larger.|
|VIL Density > 4.9||All severe hail cases, with most over an inch in diameter and a number of events larger than golfball size.|
|Hail Size > 1 inch||VIL Density > 4.0 in 91% of the cases.|
|Hail Size > Golfball||VIL Density > 4.3 in 88% of the cases.|
VIL Density limitations/considerations:
Procedure to use VIL Density graph operationally:
The VIL Density graph below reveals VIL values versus echo top (in kft). To use the chart, determine from the same volume scan on the WSR-88D a storm’s VIL value and echo top (>18 dBZ) using either the echo tops product or a vertical cross section. (Remember that the echo tops product may only give coarse top values, especially if using VCP 21 and for storms close to the radar). After obtaining this information, employ the chart to find your point of intersection and compare it to the VIL Density lines. If your point is to the right of the appropriate critical VIL Density line, then consider issuing a severe thunderstorm warning. A value to the right of the 3.5 line has a Moderate potential for severe hail. A value to the right of the 4.0 line has a High potential. However, remember that this method has limitations and certainly is not fool-proof. For example, it is possible that a storm may (may not) produce large hail even with a VIL Density value below (above) a threshold. Also, be aware of the VIL Density limitations cited above. Use this method as an additional tool for assessing hail potential. Always consider the overall storm structure and evolution before issuing a warning.