|HOW IS THE REFLECTIVITY GRADIENT|
IMPORTANT TO SEVERE WEATHER?
METEOROLOGIST JEFF HABY
The reflectivity gradient is defined as how much the value of radar reflectivity changes over distance. If
the reflectivity changes significantly over a small distance then that would be a strong reflectivity
gradient. If the reflectivity changes only slightly over a significant distance then that would be a weak
reflectivity gradient. Determining what is a strong gradient, weak gradient or a gradient that is in-between
takes practice. With a few examples we will show how each look on radar.
First we will look at a commonly used scale for reflectivity values. The scale is shown below:
The green colors represent light reflectivity (light rain aloft), the yellow colors are more of a moderate
reflectivity (moderate rain aloft) and the red colors represent heavy reflectivity (heavy rain and possible
An example of a strong reflectivity gradient is a red color next to no reflectivity while an example of weak
reflectivity would be a gradual transition from green to yellow colors. Below is an example of a weak, moderate
and strong reflectivity gradient.
WEAK REFLECTIVITY GRADIENT
MODERATE REFLECTIVITY GRADIENT
STRONG REFLECTIVITY GRADIENT
The reflectivity gradient is important because it can give clues to if
severe convective wind gusts are occurring.
Severe convective wind gusts are more likely to occur when there is a strong reflectivity gradient. The leading
edge of severe thunderstorms often have an abrupt transition zone of
heavy precipitation and wind. In this
region the outflow from the storm is progressing into the environmental air ahead of the storm. Strong convective
winds will force the reflectivity into an abrupt zone where the reflectivity changes rapidly
over a small distance. Look for severe convective wind gusts when red reflectivity is next to very little
or no reflectivity. This is especially true if the storms are in a line segment that is bowing.