Written by cuweathernerd    Thursday, 05 January 2012 20:51   
Chasing 101: Section 1:3 Forecasting: Wind 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.


It may seem odd that we are starting with forecasting in a discussion of chasing -- why not talk about storm structure or how to chase first?

The rationale that the underlying themes we will cover over the next few posts are the theoretical framework from which we understand tornadoes and other severe phenomena. Covering how to decide which days will produce severe weather helps us to understand what makes severe storms severe.


With that in mind, we will be talking about wind shear, and it will be covered from a largely theoretical viewpoint, because it is important to understand why a specific set up or storm is behaving as it is.

Meteorologists really like to use the words "wind shear" when talking about severe weather because it dictates, in part, what kind of weather will occur. The quality of shear present can dictate the difference between a day with discrete supercells favorable for tornadoes and a bow echo event. Exactly how to forecast this will be covered in a future post.

Shear, for our purposes, is changes in wind with respect to height above the surface. For now, we will cover the primary types of shear and talk about how they contribute to severe weather.


The first kind of shear is speed shear. This is caused by a difference in wind speed at different heights.

Instead of going into a discussion of the mathematics, I want you to find a pen or pencil, and put it between the palms of your hands. Move the top hand along the pencil. You'll see the pencil rotates.

See this illustration for clarification:

This is effectively how speed shear works -- a layer of air moving faster relative to a layer moving more slowly above or below it will create an area of spin oriented along a horizontal axis (like the pencil).

This is important to forecasting severe weather, because rotations like this play an important role in creating the storms we that interest us.

It may be counter-intuitive how a horizontally oriented vortex tube (think of the pencil as a tube -- any point on the pencil rotates similarly) can produce a vertically oriented rotating phenomena like a tornado.

The answer lies in the idea of vortex tilting, and while it is not complete, it certainly plays an important role in tornadogenesis, the formation of tornadoes.

I made this diagram to illustrate the process:

Speed shear will also tilt a storm's updraft. This allows the updraft and downdraft to occur in separate regions of the storm. As a result, the downdraft will not cut-off the updraft and actually it will even reinforce it. This is critical in the formation of supercells as well as hail.

I made this diagram to illustrate how this works:

 


The second type of shear is directional shear, which is a change in the direction of wind with height.

So if the wind is from the south at the surface, and higher up in the atmosphere, maybe they are from the west, this would represent an area of directional shear.

The main reason we are concerned with this kind of shear is that it helps updrafts to start rotating as they form, which helps to make supercells and mesocyclones, which are the parent systems of tornadoes, and the storms we traditionally chase.

This can be used in identifying warm and cold air advection, which we'll cover later.

Even more importantly, certain kinds of directional shear (called veering winds) are particularly favorable for tornado development. We'll talk about identifying these in the future.

In the field, a rotating updraft might look like this:


In general, shear will show up in three 'parameters' when you are forecasting: hodographs, storm relative helicity, and bulk shear. We'll cover these later, once we finish building up a theoretical framework.


Glossary

Advection - Physical movement of some existing variable (temperature, moisture, momentum, vorticity, etc) by wind. Think of it like leaves on top of a flowing river.

Bow Echo - Multicellular storm system organized into a line that bows (like a bow and arrow). They are associated with very damaging winds near the apex of the bow.

Discrete supercell - A type of thunderstorm which is dominated by a deep, persistently rotating updraft. The thunderstorm exists independently of other cells, and has the potential to be the most severe of any kind of thunderstorm. Most tornadoes form from supercells.


 

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/o4f3s/chasing_101_section_13_forecasting_wind_shear/

Last Updated ( Tuesday, 06 August 2013 18:54 )
 

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