Measures of the Potential for Rotating Updrafts

Do meteorologists have a direct way of estimating the potential for horizontal flow to be associated with the development of rotation (mesocyclone) in an updraft? Yes, and this can be assessed from the hodograph.

Vertical Shear and Mesocyclone Development

Mesocyclones first appear in the midlevels (say, around 10000 feet to 18000 feet) of thunderstorms growing in an environment of sufficient vertical shear (i.e., in the presence of favorable jet stream winds). Meteorologists estimate this favorable vertical shear by subtracting the surface wind direction and speed (combined, called the surface wind vector) from the wind vector at 6 km (~18000 feet, approximately the 500 mb level). This is sometimes called the "deep layer shear".

When the magnitude of this difference is around 35 knots or greater in an environment (and, usually, in the lower level winds are nearly at right angles to the winds at 18000 feet or so but this is not neccesary), the shear is thought to be favorable for midlevel mesocyclone development. (Note: the 500 mb wind can be used as a rough estimate of the deep layer shear.) Such vertical shear also ensures that the storm will be long-lived enough to give the rotation time to develop because the stronger upper winds prevent precipitation from falling through and suppressing the updraft.)

The magnitude of the deep layer vertical shear can be calculated directly from the hodograph. It is simply the magnitude of the shear vector from the surface wind to the the wind at 6 km with the value read directly from the hodograph rings.

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Vertical Shear and Low Level Shear/Helicity

Since the jet stream is almost always present over the United States (except in mid summer, usually), it might be assumed that the deep layer shear is almost always favorable for storms to develop rotating updrafts. However, only in areas in which the surface layer wind is at a substantial angle relative to the 500 mb wind will the inflow air to developing thunderstorms possess substantial spin due the vertical shear. A measure of the degree to which the surface wind streams possess this spin is the STORM RELATIVE HELICITY (SRH), which is a measure of the "low level shear." This is normally calculated for various layers from the ground to about 1 km or up to 3 km. Helicity (from the word helix) is a measure of how "helical" the updraft of a growing thunderstorm will be.

For example, the helicity in the 0-3 km layer is proportional to the sum of the magnitudes of the shear vectors from the surface wind to 3 km and not just a function of the wind veering with height. In the hodographs below, both have a veering wind with height, but the curved hodograph has much greater helicity (because the length of the hodograph is greater). We are visualizing the helicity here with respect to the ground.


In actuality, the the helicity that is important is that which "the storm" experiences. The storm motion can be plotted on the hodograph as a vector. The initial storm motion can be estimated by drawing a line for the 0-6km shear magnitude and plotting a point halfway along it. That would be the estimate for the initial storm motion.

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Supercell Thunderstorms

When the updraft develops rotation through a deep portion of the mid section of the storm, and if that rotation persists for periods on the order of 15 minutes or more, the storm is classified as a "supercell." Despite popular myth, the definition of the supercell contains nothing about the storm size or depth, or whether the storm produces a tornado. In fact, studies have shown that only 10% or so of supercells produce tornadoes. However, supercells account for a disproportionate fraction of the severe weather reports received by SPC (meaning, most thunderstorms are not supercells, yet most severe weather reports are associated with supercells). Clearly, if supercells have rotating updrafts (updrafts that have helicity), then thunderstorms that develop in an environment with a curved hodograph will have the most "rotation."

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The difference between the two hodographs shown above is that the winds at levels between 0 and 6 km are stronger for the curved hodograph case. This occurs particularly when the winds between 900 and 800 mb are particularly strong. This creates a stronger "paddle wheel" effect (helicity generation) in the 0-3 km layer, nominally the inflow layer for most Great Plains' thunderstorms.

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The "storm relative helicity" is directly proportional to the area of the figure obtained by drawing a line from the storm motion to the lowest level hodograph point, and the highest level hodograph point. The interesting thing is that once the initial storm forms in a deep layer shear environment favorable for supercells, the storm splits into two portions, one with a storm motion to the left of the hodograph and one with a storm motion to the right of the hodograph. The storm relative helicities calculated for these various motions determine the strength (and nature) of the updrafts with the storms.

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Tornadic Supercell Thunderstorms

All supercells that directly produce tornadoes have mesocyclones that appear to extend to the ground from the midlevels of the storm; this is due to the vertical shear in the lowest layers of the atmosphere. (There are several ways in which to assess this, but these are beyond the level of Metr 302).

Studies have shown that supercells developing in environments in which 0-3 km SREH values of 150 or greater are observbed often associated with weak tornadoes (F0). Values of 250 or greater suggest to meteorologists that if tornadoes occur they are liable to be associated with moderate (F1) to considerable (F2) damage.

These are just general limits---some studies have shown that SREH values do not need to be this great if the buoyancy (as measured by CAPE) is great, while situations of relatively low buoyancy (low CAPE) can be mitigated by very strong vertical shear. (Recent research suggests that low level positive shear (0-1 km positive shear) may be a more direct measure of the potential of the low level rotation to develop.) In order for helicity to be important, however, there must be some buoyancy. So meteorologists often look at the combination of helicity/deep layer shear and CAPE in a given area. Keep in mind that there are other factors that relate to whether or not a supercell will produce a tornado in the so-called "supercell cascade." But these other factors do not become important unless the deep layer and low level shear develops in a proper manner, as outlined above.

The most recent research indicates that there is a favorable range of CAPE values and 0-3 km SREH values for strong tornadic supercells. An index called the "Energy Helicity Index" is the product of CAPE and the 0-1 SREH (and a constant).

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