Wind Shear Questions

[Jon Miller]

Are there any good web sites that explain the different types of wind shear used in storm prediction ? ie vertical shear, horizontal shear, speed shear, bulk shear , brn shear, SRH, effective shear etc. - I get confused occasionally by the terminology as a non Met person .

Is SRH the same as horizontal or speed shear ?

Am I correct in assuming that vertical wind shear is the same as the 0-1, 0-3, 0-6, 0-8 values given in forecast models and on the SPC Meso web site etc ?

How about BRN shear, bulk shear and effective shear - are they important in tornado genisis ?

Thanks
Jon Miller
KT8NDO

 

[Shane Adams]

I can't really answer this question the way you asked, because scientifically I'm fairly retarded. What I can say is, looking at a wind profiler is probably the best thing you can do to familiarize yourself on what a supercellular windfield looks like. Try not to get lost in the specific labels and "types" of shear; what you need to understand are the basic parameters that are required at each level (at least the ones that are more or less universally agreed-on by chasers) for supercell/tornado development.

Years ago when I was still a dumba**, I happened to meet Gene Moore at some dive motel in southwest Iowa. He told me something that day that I have lived by since: 30kts @ 500mb is sufficient for sustained supercell structures. This is but one example. Higher up at 250mb (or 'anvil-level'), you'll want something like 50-60kts at least, to ensure that your precipitation is adequately vented downstream, preventing your updraft from becoming "seeded" or rained out. Down closer to the ground, at 700mb, you'll want something close to the 500mb level. And your 850mb level, which is where the LLJ lives, is usually decent at 20-25kts, to support a legitimate tornado threat.

These numbers are of course highly subjective depending on the individual chaser and his/her own tastes, but they give you a starting point. Personally, I like around 40-45kts at 500mb best, and of course all tornado chasers love high octane LLJs, on the order of 40-50kts. Stronger winds in the lowest levels are greatly responsible (at least a lot of us think so) for producing the stronger, longer-lived tornadoes.

Someone more book-schooled can jump in here and explain this better.

 

[Chris Nuttall]

Are there any good web sites that explain the different types of wind shear used in storm prediction ? ie vertical shear, horizontal shear

I'll see if I can take a stab at this because this is quite a bit to answer. First, I highly recommend signing up for COMET's MetEd website and taking some of their modules. Anyone can do it. Most of the material on there is geared towards mets, so it can be highly scientific in nature. However, even if you don't fully understand the material, you'll still get exposed to the terminology and its applications. Now to the answers....

Wind shear is simply the change in wind speed and direction over a short distance. Vertical wind shear can be seen on a sounding where you have winds changing direction/speed with height. The best example I can think of for viewing horizontal wind shear would probably be viewing a velocity couplet or downburst signature on radar. As far as supercell forecasting goes, we're mainly concerned with the vertical wind shear, which creates horizontal vorticity, which gets titled and stretched upward by an updraft, rotating that updraft.

Think of horizontal vorticity like this: Imagine of a horizontal tube of air. You have a weak wind moving along the bottom of the tube and a stronger wind moving at the top. The stronger wind aloft will "push" the top of that tube along faster than the bottom, causing it to rotate. For a funny example, think of the log roll where you stand on a log, run on it, and try to knock the other person off into the water. When that log spins, think of that as horizontal voriticity.

Speed and directional shear simply refer to whether we are looking at the speed changes or direction changes.

bulk shear , brn shear, SRH, effective shear etc.
Is SRH the same as horizontal or speed shear ?

Instead of trying to explain what all of these parameters are and how they differ, I'm going to refer you back to SPC Mesoanalysis page. Select a region to view. Click on the parameter you want in the menu on the left. Above the map, there is a link that says "Description" that will pop-up and explain what it is you are looking at.

Am I correct in assuming that vertical wind shear is the same as the 0-1, 0-3, 0-6, 0-8 values given in forecast models and on the SPC Meso web site etc ?

0-1, 0-3, etc. are not values of vertical wind shear. These refer to layers of the atmosphere. 0-1km AGL, 0-3km AGL, etc.

How about BRN shear, bulk shear and effective shear - are they important in tornado genisis ?

I sort of already answered this by referring you back to the SPC Meso page. These are simply different parameters that have been developed to help us quantify and measure how much shear is out there. Sort of like how CAPE and the lifted index help us determine how much instability there is. It's important to note that they have their limitations and just because you see values above a certain threshold, it does not mean supercells are a given. For example, CAPE values can get pretty high during the summer under a death ridge.

Are those shear measures important in tornadogenesis? Sort of. They are measures taking the whole 0-6km or 0-8km layer of the atmosphere into account, which is really all about supercell development. For tornadoes, we need to be concerned about the lowest layers, for example the 0-1km Storm-Relative Helicity (SRH) is one to watch in regards to tornadoes. Again, it has limitations and there must be a lot of other things in place for tornadoes to develop.

All these parameters are more about the potential for development and how the storms will evolve and behave, assuming they form.