Questions on Shear

[kevin]

::Topic for the pros to answer and the rest to ask::

I've noticed that many people are looking at the shear maps and shear forecasts, but I wonder if all of us have any understanding of how they are produced or the mechanisms that create shear. Since I personally know next to nothing on this subject, I'll start the questions with the simplest.

What is shear?

[Weatherboy1]

shear is simply a state where the winds at one level of the atmosphere are blowing the opposite of winds at another level. This situation prevents storms from forming and/or strengthening. If you have a surface high to the North of a storm, for example, there will be easterly (out of the east) winds blowing in the lower levels of the atmosphere. That's because winds flow clockwise around an area of high pressure.

But if you have an upper level low to the NW of that same storm, you'll get upper level winds out of the W or SW (since winds flow counterclockwise around an area of low pressure). This will inhibit storm development because it will rip the tops off the building convection.

Depending on the strenght of the shear, it can be an inhibiting factor to strengthening, or it can outright rip a storm apart. Andrew, for instance, almost got sheared apart in 1992 due to an upper level low N of it. But it survived, got under an area of high pressure to the W of that low, strengthened due to the elimination of shear ... and the rest is history.

Hope this helps.

[rbaker]

shear is an enemy to a hurricane, since if you looked a a profile of a hurricane it is vertically stacked where there is low pressure at the surface spinning counter-clockwise, and since the air is rising it has to be evacuated some how which is out through the center (eye) in a different direction, which is why you need a ridge or high pressure aloft or at 20-30,000 ft range to allow the rising air to lift and spin out of the system.
Shear causes the thundertstorms surronding a center of low pressure to be blown off at the tops of the thunderstorms where the thunderstorms cannot persist or grow and complelety surround the center (eye) and be displaced, and the only way a hurricane can survive is with warm rising air off the ocean, with little or no shearing winds in the upper atmosphere, and someway to get rid of the rising air (high pressure aloft) to survive or maintain itself.
I hope in laymans terms this gives you an answer.

[ColdFront77]

For the record, higher upper level wind speed are needed for severe weather developing ahead of a cold front.

[ncweatherwizard]

Tropically speaking...

If you're just talking about simple horizontal "shear" at a given altitude, then it is measured by the wind vector acting upon the system at that pressure level, etc. In turn, and simply explained, horizontal "shear" is simply wind unassociated on a storm that acts upon the storm.

Vertical shear (at least the value) is a change in horizontal shear values over a change in altitude (say 850mb to 250mb or the avg b/w of upper levels and avg b/w lower levels).

Of course, I've got a problem with what I've just said...horizontal shear is a wind value at one level; but the same term, 'shear' is used to describe the change in those winds at different levels...two completely different ideas. That's why I used "shear" in those quotation marks. But, for example, you can have fairly low vertical shear, but very strong horizontal winds, and the storm's flow will be disrupted.

But if you have an upper level low to the NW of that same storm, you'll get upper level winds out of the W or SW (since winds flow counterclockwise around an area of low pressure). This will inhibit storm development because it will rip the tops off the building convection.

Well, that ULL could help the system to, depending on the proximity of the low to the storm. If the periphreal of that ULL meets a tropical low and the wind direction (particularly at upper levels) matches the wind vectors associated with the storm, then the ULL would actually enhance the flow of the storm.

And of course, speaking in nontropical terms, shear will enhance thunderstorms...particularly the horizontal winds at a range of upper levels.

So I have a question now: is this 'shear' term arbitrary or not?