CIMSS wind shear analysis--controversy over its accuracy

[miamicanes177]

There has been great debate as to just how accurate the shear maps are at the CIMMS university of wisconsin website. I've seen numerous professional mets (even those with Doctoral degrees) reference the site and swear by its accuracy. However, some have reservations about it. This thread is the place to discuss just how reliable (or unreliable) of a source it is.
http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/winds.html

[windstorm99]

The maps are perfectly fine thet just need to be read the correct way.

I use the CIRA page for wind shear analysis sometimes.

[Aquawind]

I think it's an interesting tool, however we can barely get real time data that is reliable, consitant, or complete in coverage. To think we can forecast the winds at varied altitudes when we can barely do a good analysis doesn't sound to promising. The forecast can't be any better than the data put into it. Entirely computer generated maps always seem suspect to me. Just another tool for interpretation and certainly not to be taken as absolute or even accurate.

[cpdaman]

i think the direction of the wind shear combined with the direction the storm is moving supercedes the general raw windshear numbers that 90% of the focus seems to be about (relative windshear more imortant) unless of course the storm is standing still! then remember that the wind shear in the upper levels could be low, but mid level wind shear is a killer as well and may not be depicted on the standard wind shear maps

[WindRunner]

The CIMSS map is only as good as the data it's given. Sometimes it's accurate with a couple of knots, sometimes it's just a decent ballpark estimate. Occasionally it won't be even close, though it usually is in the ballpark range.

As for the what has been mentioned as "relative shear," it might be useful to know that shear is relative. Upper level wind shear is calculated by subtracting the wind vector at that upper level from the wind vector at the lower level (i.e. the 850-700mb level). The important thing to realize is that these lower level wind vectors are also the storm motion vector for the weak disturbances . . . i.e., as long as the storm's motion is relatively close to the 850-700mb steering layer winds, then the shear maps have already accounted for the storm's motion and no further work needs to be done.

[wxmann_91]

I almost never look at the scalar component of the shear vector (i.e. how many kts of shear there is). I only look at the directional component. (The orange lines.) I have found that the closer the lines are to one another, the stronger the shear. And of course since it's a directional component, the storm relative shear can be better assessed.

[benny]

There has been great debate as to just how accurate the shear maps are at the CIMMS university of wisconsin website. I've seen numerous professional mets (even those with Doctoral degrees) reference the site and swear by its accuracy. However, some have reservations about it. This thread is the place to discuss just how reliable (or unreliable) of a source it is.
http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/winds.html

What data do you have that suggest the data is inaccurate?

As far as I know.. they use layer averages to calculate the shear. I believe it is based off of NOGAPS initial fields as a first guess then manipulated using the cloud-drift winds. It isn't perfect but I have used it effectively for a long time.

[miamicanes177]

What data do you have that suggest the data is inaccurate?

As far as I know.. they use layer averages to calculate the shear. I believe it is based off of NOGAPS initial fields as a first guess then manipulated using the cloud-drift winds. It isn't perfect but I have used it effectively for a long time.

I believe it is accurate. Some are questioning its reliability so I created a thread to get to the bottom of the issue.

[btangy]

i think the direction of the wind shear combined with the direction the storm is moving supercedes the general raw windshear numbers that 90% of the focus seems to be about (relative windshear more imortant) unless of course the storm is standing still! then remember that the wind shear in the upper levels could be low, but mid level wind shear is a killer as well and may not be depicted on the standard wind shear maps

This is a common misconception. The wind shear is the difference of two vectors (wind at 200mb and wind at 850mb, usually). Adding the storm motion onto these two vectors and then subtracting them gives you the exact same result. You are right about shear in the mid-levels though not being represented, though this is only important for a small subset of storms, mostly in the E Pacific and in the Cape Verde region where mid-level jets are more common.

The CIMSS product is mostly based on cloud track winds and give much higher resolution analysis compared to model initializations (GFS, NOGAPS, and ECMWF), especially in the area around tropical cyclones where there are clouds to track. In case you're wondering, the tropical cyclone is filtered from the data before the analysis is done so that you get the environmental value of the vertical shear and not the shear due to the storm itself.

[Derecho]

It always seemed fine to me.

What I find baffling is people persistently using and posting the CIMSS shear TENDENCY maps rather than the actual shear maps themselves (the tendency maps are more coloruful, I hope that isn't the reason.)

I don't really care that much that 40 kt shear dropped to 30 kt in the last 24 hours - I just want to know the actual shear.

[skysummit]

I also care to use the current shear maps in conjunction with water vapor imagery for shear analysis, however, those who rather the tendancy map can still use the color codes on the right for current shear.

[Aric Dunn]

I also care to use the current shear maps in conjunction with water vapor imagery for shear analysis, however, those who rather the tendancy map can still use the color codes on the right for current shear.

i personally use.. both .. becasue just the shear map itself does not tell the whole story.. the tendency help shows the propagation of and past trends of the shear .. so if a system is currenlty getting shear and you compare the last 24 hours of shear tendency maps you can see which way the shear is heading and how much can be expected to increase or decrease .. which is much more helpful then just the current shear.. becasue its only current now.. no wait now ... no wait ...now its current ... wait ... now its current.. you see what im saying.. the shear is always changing that why its important to use both ..

[caneflyer]

i think the direction of the wind shear combined with the direction the storm is moving supercedes the general raw windshear numbers that 90% of the focus seems to be about (relative windshear more imortant) unless of course the storm is standing still! then remember that the wind shear in the upper levels could be low, but mid level wind shear is a killer as well and may not be depicted on the standard wind shear maps

This is a common misconception. The wind shear is the difference of two vectors (wind at 200mb and wind at 850mb, usually). Adding the storm motion onto these two vectors and then subtracting them gives you the exact same result. You are right about shear in the mid-levels though not being represented, though this is only important for a small subset of storms, mostly in the E Pacific and in the Cape Verde region where mid-level jets are more common.

You are exactly correct! It astounds me how many otherwise smart people, including professionals, have deluded themselves with this concept of storm-relative shear. It does not even make sense dimensionally - the proper units for shear is 1/[time] - the difference between two wind vectors divided by the distance between the two levels. The latter division is commonly left off, but is implicit when you say, for example, the 200-850 mb shear. You can't properly take a shear value and subtract a wind vector from it - they are not dimensionally consistent quantities.

What trips people up is substituting storm motion for the actual flow at some presumed steering level. It is proper to talk about "storm-relative flow", which is simply the difference between the wind at some level and the storm motion. But this is NOT shear. Wind shear is wind shear, regardless of the motion of the storm.

When people talk about storm-relative shear, what they usually really mean, even if they don't realize it, is UPPER-LEVEL shear. In other words, the difference between the upper-level flow and mid-level flow (the latter of which is generally close to the storm motion, hence the confusion).

[wjs3]

There has been great debate as to just how accurate the shear maps are at the CIMMS university of wisconsin website. I've seen numerous professional mets (even those with Doctoral degrees) reference the site and swear by its accuracy. However, some have reservations about it. This thread is the place to discuss just how reliable (or unreliable) of a source it is.
http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/winds.html

OK, I think some more context is useful here for those who may have missed it.

This came up in the context of the latest invest. Several people on the thread (inclouding me) thought that the invest presneted as "sheared" at some point in its existence. Others pointed out that the CIMSS shear anaylis showed only small amounts of shear over the system. So several made comments that CIMSS was perhaps not estimating the shear correctly.

I was (am) in the "it was sheared" camp. Though not a great deal, the convection was off to the south (SSW? I can't remember) of the Low level center. I argued (and argue) that the CIMSS analysis is good, but not perfect. It is in fact based on cloud drift winds (with the cyclone removed) as far as I know and as others in this thread have said. However, it is based on cloud drift winds....If there are no clouds, then it's talking a guess (some in this thread have suggested it's based on NOGAPS...I don't know).

The bottom line is that like any other remote sensing, it can miss things. I d think it was generally correct and shear was modest over the system. However, I thought it was a pretty clear and easy analysis by checking out visible satellite that SOME shear was helping to weaken the system. (Dry air was a HUGE cuplrit too). So, I would argure that the CIMSS shear analysis may have slightly underestimated the shear. It's done that before. I've seen it miss ULLs until A TC gets close enough to get some clouds from it wrapped in.

FWIW, I think the maps are very accurate, but, if a closeup visible analysis of a TC is showing one thing, and the shear maps disagree, then I have to guess that the shear maps are misestimating. I treat them like any other tool for measuring the atmosphere (especially in a low resolution environment like the open tropics/ocean). They are GREAT directional tools for assessing the big picture, but can have issues in their fine details.

WJS3