Vertically Integrated Liquid (VIL) can be used to identify some features in storms. These features include hail, multicell thunderstorms and downbursts.
VIL is the total amount of water vapor in a column of air. It was originally used to estimate potential hail size; higher values of VIL were initially thought to correlate to larger hail size. However, it was later found that the relationship between hail size and VIL was much more complicated than originally thought. Some of the other variables that influence hail size include the time of year, location and weather patterns. Below are images of reflectivity and VIL from a hail producing storm in South Dakota back on July 19th.
Multicell thunderstorms can be identified with VIL. A multicell thunderstorm is one in which a parent thunderstorm collapses and then a new thunderstorm develops. New cells tend to develop to the west or southwest of the main cell. When looking at VIL, a multicell will typically have VILs that fluctuate. When a cell weakens, the VILs will decrease. When a new cell intensifies, VILs will increase once again.
Downbursts have a similar VIL pattern to multicells, but instead of increasing and decreasing, VILs will decrease rapidly in a downburst. The rapidly decreasing VIL is a sign that the storm’s updraft is weakening. As a result of the storm weakening, the air in the top of the storm is forced to the ground. Once the air reaches the ground, the air spreads out, potentially causing damage.
A few other points about VIL to keep in mind:
• VIL will be underestimated for storms close to the radar because of the storms being in the cone of silence. It will also be underestimated for storms far away from the radar because the entire storm is not being sampled (the radar beam curves upward as the distance from the radar increases).
• When a storm is highly tilted, VIL will be underestimated because it is computed assuming a vertical column.
• Non-meteorological echoes can contaminate VIL calculations.