Question On Shear

[Bailey1777]

Please answer this only if you are sure on this. Does the angle/direction that shear impacts a system from maybe have as much impact on a system as the shear speed? I've seen some systems do relatively well in higher shear and some systems get ripped in lower shear.

Thanks in advance to those who answer.

[Evil Jeremy]

There is an additional factor that plays a big role in weather or not shear affects a storm: What levels in the atmosphere the shear is in. Upper level shear will effect systems differently than low level shear in certain situations. But yes, the direction of the shear also matters.

[Bailey1777]

So then when looking at a shear tendency map are you being given upper or lower shear probabilities?

[Evil Jeremy]

There are different shear tendency maps for the different shear levels I believe, though I might be wrong. I might be confusing it with regular wind direction charts.

[Bailey1777]

Okay,

You are correct I went and looked and there are upper and lower shear tendency charts. So not only do you have direction and speed but you could have 2 different speeds pulling at the top and bottom.

[wxman57]

Please answer this only if you are sure on this. Does the angle/direction that shear impacts a system from maybe have as much impact on a system as the shear speed? I've seen some systems do relatively well in higher shear and some systems get ripped in lower shear.

Thanks in advance to those who answer.

I think I detect a general misunderstanding of what shear is. There are two types of wind shear - speed and directional. Speed shear occurs when winds horizontally or vertically blow from the same direction but at different speeds. The difference between the two speeds across that distance determines the speed shear. Similarly, there is directional speed shear. The wind speed may be the same (horizontally or vertically) but the direction from which the wind is blowing may be different. So there is really no directional component, it's just shear. The arrows on the wind shear charts at CIMSS (http://cimss.ssec.wisc.edu/tropic/real- ... g8shr.html) represent the predominant direction of upper level or mid level winds.

What I think you're asking may be "does the direction of the mid to upper level winds have an impact on potential development?" The answer to that question would be yes. For example, let's consider the interaction of a disturbance with an upper-level low or trof. Upward vertical velocities are enhanced around certain parts of the upper low, like on the south and southeast side of the low (the "entrance region of the jet"). That's why the interaction of an upper low and a tropical disturbance can cause a temporary flare-up in squalls as the disturbance passes to the south of the upper low. And if this disturbance then moves into a lower-shear environment with the increased squalls, it could have a better chance of development. Often, this increased southwesterly flow becomes hostile wind shear, preventing development near the upper low. But if the low is weak enough, the flow can enhance convection without resulting in significant wind shear.

I could type more, but I have a 2pm-4pm meeting to go to.

[Ed Mahmoud]

I found this years ago on the interweb


3.2 Vertical Wind Shear


Read the whole thing, not just the sub-section

As previously stated, latent heating is one of the primary ingredients in tropical cyclone formation. The latent heat released from cumulus convection can lower the surface pressure and begin a cyclonic circulation and low-level inflow. But in order for this event to occur, the heat generated by the convection must not be removed. For example, if the cloud cluster disturbance travels at nearly the same speed as the environmental flow in which it is embedded, its heating will not be removed with respect to the disturbance center. However, if it is too slow, the heating in the upper troposphere will be carried away by the mean flow (Gray, 1975). Thus, there must be nearly zero vertical wind shear in the area of TC formation (Fig. 3.9).

In addition to small values of vertical wind shear near the disturbance center, Gray (1979) has shown that the orientation of the horizontal gradient of zonal vertical wind shear is also important. Figure 3.10 shows that in those cloud clusters which develop, the magnitude of zonal vertical wind shear determines the extent to which the cloud cluster is ventilated. If the cloud cluster ventilation is small, as in the third diagram from the left, heat and moisture can accumulate and development can occur. In the Northern Hemisphere, this implies westerly shear (westerlies aloft, easterlies below) to the north and easterly shear to the south. This pattern is satisfied if a low-level cyclonic rotation is overlaid by an upper-level anticyclone. Initially it was thought that the upper-level anticyclone was a result of the heating from the incipient TC. But recent studies have shown that a small amount of anticyclonic rotation must exist prior to development of a tropical disturbance.

By rearranging the terms in the vertical wind shear equation, Gray and Frank (1978) summarized this parameter as the difference of the low-level (e.g., 900 hPa) and upper-level (e.g. 200 hPa) relative vorticity average over an appropriate radius (2-6 degrees). They found this value to be three times larger for developing systems than for those which did not develop.

[Bailey1777]

Thankyou Ed. That's good reading and very helpful.

[Bailey1777]

Here is some info I dug up on shear that I thought others might find intresting.
1. In general shear refers to any change in wind speed or direction.
2. Wind shear is primarily implicated in the vertical direction; surface to top of troposphere, about 40,000ft.(200 mb)
3. Storms are normaly steered by the avg. wind thru this layer.
4. Shear is typically the wind speed difference between 200 mb 40,000ft. and a layer where 850 mb is found about 5,000ft. above the surface.
5. Wind shear is computed for very large areas.
6. These computations are a crude measure of actual shear a storm may experience since changes in wind speed not direction are considered.
7. Computations do not take in smaller scale changes for the large area earlier disscussed such as it is common at 600mb to find a jet of wind along the SAL, and this jet can create signifigant shear that won't show up.
8. Hurricane Wilma Had 125mph winds and a pressure of 955mb while encountering 40 kts. of shear.

Pro's if I misinterpited anything please add or take as I am learning and trying to help other's do the same.