Written by cuweathernerd    Sunday, 15 January 2012 20:51   
Chasing 101: Section 1:5 Forecasting: Helicity and Bulk Shear

This is part of the continuing series called chasing 101, a course to help people who are new to chasing learn the fundamental skills to chase productively and safely. They are meant as both information and as a forum for discussion.


This is a continuation of the previous posts on wind shear and the hodograph, and presumes that you have a comprehension of all the material posted there.

While hodographs and a theoretical understanding of wind shear gives us a nice operative framework from which to work, neither is exactly quantitative, and so comparing a given set up to another can be a challenging task. It would be nice if there was a quantitative way to compare the shear, especially on a map.

The way we do this is a measure called storm relative helicity, or SRH. This is a term you will hear thrown around a lot by chasers, but most people don't know what the numbers represent. That's ok, but it is helpful to understand where these numbers come from if we want to use them to their full potential.

The word helicity seems odd -- but think of the word 'helix'. This, then, is a measure of the potential for air to follow a helical flow, or flow that looks like a corkscrew.

Helicity is proportional to the strength of the wind, the amount of speed shear with height, and the amount of turning in the flow (vorticity).

The actual "stuff" of helicity is very math intensive, but that's why we have computers. The way the math works out, a veering (clockwise turning with height) produces positive helicity, and a backing (counter-clockwise turning with height) produces negative helicity. We also tend to calculate helicity with respect to storm motion, hence the phrase storm relative helicity. It is almost certain that you'll only encounter helicity in this form.

You'll also find that helicity is calculated with respect to layers of the atmosphere, normally 0-1km above ground level (agl) and 0-3km agl. This is the region of the atmosphere we are most likely to care about, because that's where our tornadoes and wall clouds and some of our mesocyclones form.

It turns out that helicity is related to the area under a hodograph curve -- while the mathematical relation is interesting, its application is more so. Because of this relationship, helicity helps us to identify regions where the most beneficial hodographs exist.

All this leads to some empirical studies which found the following critical values for 0-3km agl helicity:

SRH = 150-299 ... supercells possible with weak tornadoes

SRH = 300-499 ... very favorable to supercells development and strong tornadoes

SRH > 450 ... violent tornadoes

Note there is a 'Goldilocks' zone -- too strong, and the storms will shear apart. Be skeptical of tornado potential of helicity much above 500 unless there is also extreme instability as well.

For 0-1km, you'll want values over 100.

It's important to know that helicity has some shortcomings; SRH helicity varies a lot in the actual environment, and a good forecast is dependent on an accurate forecast of storm movement. It is hard to model these local changes, and storm movement is one of the more difficult things to forecast. So use this value with some caveats.

Nonetheless, helicity shows up in several parameters which are quite important, namely energy helicity index (EHI) and updraft helicity, both of which are very useful in producing a chase day forecast. We'll cover both towards the end of chasing 101.


Also important is bulk shear, which is just a measure of the wind speed at one height minus the speed at a lower height. This, obviously enough, is just a measure of the speed shear independent of other factors, but like helicity, it is a measure across a layer, normally 0-1, 0-3, and 0-6km.

You'll tend to use the 0-6km parameter the most, 0-6 km bulk wind shear of at least 30-35 kts, preferably 40-50 kts. Any more than that, and you run the risk of shearing storms apart.

This measure breaks down for elevated base storms, but you don't tend to chase those looking for tornadoes anyway (we'll cover that later).

This pretty much covers then end of the sections on wind shear.

We'll be going through the rest of forecasting using "ingredient based science", where each of the ingredients for tornadoes are examined. We've already gone through the first, and probably most complicated, wind shear. We'll also take a look at instability, forcing, mesoscale considerations, and moisture. We'll also look at advanced modeling (HRRR) and a chasing flowchart.


Glossary

EHI (Energy Helicity Index) - Measure of the instability and helicity that serves as an important forecasting tool in quickly identifying optimal regions for tornado and severe weather development.

EmpiricalInformation gained by means of observation. Relationships are understood through patterns in data, and values come out of these patterns instead of theory.

Updraft HelicityA measure of the likelihood of a thunderstorm updraft to rotate. Mainly a forecast tool produced by the HRRR.

VorticityA measure of the rotation of air in a horizontal plane. We'll cover it in part when we discuss shortwaves, but this is one of the more complex topics in atmospheric sciences.


 

The above is an article written by Reddit user cuweathernerd and has been modified by Ryan Lehms. The original article and discussion can be found here:

http://www.reddit.com/r/stormchasing/comments/oicjj/chasing_101_section_15_forecasting_helicity_and/

Last Updated ( Tuesday, 06 August 2013 18:52 )
 

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