Written by cuweathernerd    Wednesday, 11 January 2012 20:51   
Chasing 101: Section 1:4 Forecasting: The Hodograph

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 post on wind shear, and presumes that you have a comprehension of all the material posted there. If you have not read it yet, it would be beneficial to do so.

Most of the discussion amateur chasers will do preceding a severe weather set up will center on analyzing the SPC's convective outlook and talking about how a certain model is treating the CAPE. This isn't the optimal way to actually chase effectively -- if our goal is to be able to predict severe convection more accurately, it is far more important to learn some of the more 'gritty' forecasting --which is why we are starting with wind shear and tools we can use to evaluate that shear.

You'll recall shear effectively is comprised of horizontal winds that change speed and/or direction with respect to height above the ground.

We get data on the wind speed and direction above the ground through different instruments, including radar, satellites, and most importantly, weather balloon launches.

Reading the raw data can tell you a lot about the wind profile. They are normally included on what is known as a skew T sounding. You can see what this raw data looks like here:

If you don't know how to read wind barbs, the symbols the used to display the wind profile, that is a critical skill to learn. Here is a great little guide if you need it.

What would be nice is if there were a tool that helped us to diagnose changes in wind speed and direction easily. Luckily, such a thing exists, and is called a hodograph.

At first glance they are pretty imposing, but you will find they are pretty much a cornerstone of actually predicting how a given chase day will play out.

It is instructive to see how a hodograph is actually made to learn how to use them.

I made this little diagram that should help:


 

Of course, in the real world, most hodographs aren't that nice and clear. Luckily, on chase days, there tends to be a few prevalent patterns and the hodographs tend to be easier to understand.

We want higher winds aloft than at the surface to generate the kind of shear we need, so we'll want the hodograph to be further from the origin with greater heights.

Also, we'll want cyclonic, or counterclockwise (in the Northern Hemisphere) rotating storms, so we want to see our hodograph turn to the right with height. This is a little counter-intuitive. (How do clockwise turnings with height lead to counter clockwise rotating storms?)

The answer is that speed shear tends to create a pair of rotations. Adding directional shear will determine which of this pair will be favored; it turns out that a right-turning hodograph favors right-moving, cyclonic storms. So we want to see right turning hodographs (sometimes called veered hodographs). If this is confusing, just start looking for patterns first.

Add to that the fact that southerly surface winds in tornado alley bring warm, moist gulf air often, and it makes the most sense for our tornado-favorable hodographs to occur in the top right quadrant.

Also of note is the sickle shape of the hodograph -- that particular shape is associated with violent tornadoes and tornado outbreaks. While it doesn't necessitate a good chase day, that kind of shear should catch your attention, and is normally what leads to a high or moderate risk by the SPC.

Another thing you may come across is something known as the wind shear vector. If you draw a line tangent to the curve, this is the wind shear vector, and it represents the change in wind between two points. You will see this come up in some storm discussions, and it plays an important role in determining storm mode. We'll cover that later.

Hodographs are integrally related to a measure of shear known as helicity. We will cover that and bulk shear in the next post.

In summary, hodographs are a way of quickly diagnosing the directional and speed shear present in a given setup. Right turning, widely looping hodographs in the top right quadrant are most favorable for tornadoes.


Glossary

CAPE (Convective Available Potential Energy) - Measure of potential energy that can be liberated if a parcel of air is raised above its level of free convection. It s a measure of instability and will be covered in depth later.

Skew T - one way of plotting the raw data from a balloon launching, showing how winds, temperature, and dew point vary with height. Many calculations, including CAPE, finding the height of the cloud base, finding convective temperatures, analyzing the cap, and more can be performed using this diagram. In the end, the skew-t will become the anchor of your forecasting, and will be covered soon.


 

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/odl4m/chasing_101_section_14_forecasting_the_hodograph/

Last Updated ( Sunday, 13 March 2016 09:15 )
 

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