Steer vs. Sheer

[NativeLiberty]

Hello Weather Gurus

This may not be a very simple question...if weather were simple, we'd know where every cloud moving off Africa will end up 10 days later. Anyway, what characteristics of a weather feature that could potentially interact with a tropical system determine whether it will steer the system or sheer the system?

I ask because, while watching Ernesto in its earlier stages, a weather feature was moving eastward across the GOM. I anticipated our meteorologist to say, "This front will steer Ernesto..." But, he said that it was going to produce some sheer. The weather spot was short, so no explanation was given as to why "sheer" and not "steer."

Thanks in advance!

[wjs3]

Hi, Native Liberty:

I hate to start with a nitpick, but it's "shear", not "sheer".

To try answer your question, I'm going to do a lot of defining. You may already know these. If so, please bear with me.

Wind shear is a change in wind speed or direciton over a slice of the atmosphere. Here, we're talking about vertical wind shear, which means that the wind speed or direction changes with altitude. So, if you have east winds at 5 knots at the surface, and east winds at 10 aloft, that's wind shear. Similarly, if you have east winds at 5 knots at the surface, but west winds at 5 knots aloft...that's wind shear too.

So, a simple first answer to your question might be this...any system or combination of systems that produces a change in wind speed or direction over the altitude of the storm is going to create shear.

Again, you probably are aware of these, but the best place to look for current shear is at CIMSS:

http://cimss.ssec.wisc.edu/tropic/real- ... s-shr.html

OK, so what about steering? It's kind of a related concept in my mind, having to do with winds in a vertical slice of the atmosphere. Tropical Cyclones are steered by the mean wind over their depth. For instance, a weaker storm can be very shallow (not a lot of vertical development) and be steered by a layer that's not very deep, whereas a stronger system is steered by the mean flow over a deeper layer.

Again, I turn to CIMSS where there's a suite of mean layer winds--along with the pressure of a hypothetical tropical cyclone they'd steer. Note that the layer gets deeper as the cyclone gets stronger:

http://cimss.ssec.wisc.edu/tropic/real- ... s-dlm.html

So, to answer your question, a weather system that changes wind speed or direction over the altitude of a TC creates shear. One that changes the mean winds for the TC will change the steering. And, to make things more complicated, shear can also change the steering winds...but I digress.

Bottom line is this...armed with a reasonable understanding of the concepts, the forecast models combined with CIMSS can tell you whether or not a system is being steered, sheared or both.

Anyone else want to try?

[NativeLiberty]

Thanks, wjs3. (Smack the forehead) Of course it's "shear"...hope you had a laugh...Teach me to post after 10:00pm.

This is very helpful, and I get it. I also appreciate your explanation and links for the Coriolis effect in another post.

Fascinated Native

[wjs3]

No problem, Native Liberty--ot was a great question. As I said in that other post, please fire away if there's any other questions you have.