Concept of Helicity
Helicity is a measure of the tendency for rotation to develop in a thunderstorm's updraft when horizontal vorticity found in the environment is tilted into the vertical (as demonstrated in class). The horizontal vorticity is due to vertical shear. Normally, the layer of air ingested into the bottom of a thunderstorm is between 1 to 3 km deep. Hence, you will often see plots of labeled 0-1 or 0-2 or 0-3 km helicity.
Symbolic Equation for Helicity
Note that the equation is simplified here. The symbolic equation suggests that helicity can be calculated from wind information rather easily. In concept, the equation says that at any given level the tendency for the wind to twist is measured by the product of the wind speed (in natural coordinates, along the "s" axis) with the vertical shear. The summation sign means that the for the inflow layer to a thunderstorm, the helicity must be obtained through the depth of the inflow layer, usually around 3 km.
|Discussion Question: What are the units of helicity?|
Helicity values give you a direct way of estimating the chances for development of rotation (mesoscyclone) in the updraft of a growing thunderstorm. It is also a measure of the vertical shear's contribution to a rotating updraft. Values of 150 m2/s2 or greater are thought to support minimal mesocyclones. Values of 250 m2/s2 or greater suggest to meteorologists that thunderstorm updrafts are liable to have moderate to strong mesocyclones.
In order for helicity to be important, however, there must be some buoyancy. So meteorologists often look at the combination of helicity and CAPE in a given area.
Thunderstorms developing in an environment characterized by rich helicity tend to be supercell thunderstorms. A supercell thunderstorm is a thunderstorm characterized by a persistant (~15 minutes or longer) rotating updraft (mesocyclone or mesoanticyclone) through a great depth of the storm (chiefly about 1/3 of the depth of the radar echo).
Supercell Thunderstorms on Radar
Below: Two Supercell Hooks in Oklahoma on 3 May 1999.
The eastern-most storm has a very strong mesocyclone and, at the time of the image, was producing an F4 tornado in southeastern Oklahoma City. (Please note: the hook is NOT a signature of the tornado, but of the mesocyclone that eventually "gave birth" to the smaller, much more intense vortex).
Doppler Radar Also Allows Visualization of Motion to and from Radar
The radar acually can be used to measure the speed of motion of precipitation (dust and large debris) being carried by the wind towards the radar or away from the radar. Thus, Doppler radar can be used to actually spot the true motions in mesocyclones. In addition, when storms are particularly close to the radar site, the radar can detect the smaller scale motion of the developing tornado: the Tornado Vortex Signature.