Ryan
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can someone please explain the whole shear factor to me i dont think i understand..thanks
(post moved to a more appropriate Forum)
Edited by Ed Dunham (Tue Aug 09 2005 03:04 AM)
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Clark
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The whole shear factor? Need to be a bit more specific than that...like how is it measured, where does it come from, what does it do? Or something else? Thanks.
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Big Kahuna
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According to NOAA "Shear - Variation in wind speed (see speed shear) and/or direction (see directional shear) over a short distance. Shear usually refers to vertical wind shear, i.e., the change in wind with height, but the term also is used in Doppler radar to describe changes in radial velocity over short horizontal distances. "
For more info:
http://www.srh.noaa.gov/oun/severewx/glossary4.php#Glossary
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Ryan
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well first off, what is shear?
how is it measured?
what affects does it have on tropical cyclones?
any other info i should know about shear?
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Clark
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Shear as it deals with hurricanes (and in the most general sense) describes the change in winds -- both direction and speed -- with height. You can also have horizontal shear, where the winds change in direction and speed over a horizontal area, but we are usually more concerned with vertical shear.
It is measured simply by taking wind observations throughout the depth of the troposphere (from the surface to ~10-15km up). Generally, the upper level and lower level winds are subtracted (using vector subtraction) to come with the shear magnitudes. You can also see mid-level shear calculated, where winds at the high and low end of the mid-levels (generally 300-700mb) are subtracted.
Shear values under 10 m/s or 20kt are necessary for any significant development....even lower is better. You will never see a storm with no wind shear; that describes what we call a barotropic environment, one believed to exist in theory only. But, you can see storms with very low (1-3kt) wind shear values; these are often the most intense storms and what we call "annular hurricanes." Mitch (1998) and Isabel (2003) are two good examples of such a storm.
Shear has a negative effect on a storm by disrupting the vertical structure of a tropical cyclone, affecting the convective pattern associated with the storm, and impacting the outflow at upper-levels requisite for the storm to survive and thrive.
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Lysis
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I have a question Clark… How does the direction and speed of the cyclone itself (not the shear) factor into the shear’s disruptiveness? Such as if the storm was going up stream or downstream, so to speak (I suppose this would only work for horizontal shear). Is it like a vector quantity or something?
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Clark
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Storm motion can help alleviate the influence of shear (or similarly, strengthen its influence). You'll see storms under southwesterly shear ahead of a trough strengthen as they recurve and accelerate because of the lessened impacts of the shear upon the storm. You'll still see asymmetries result from the storm motion, but not as much of a negative impact due to the shearing winds. It's all relative, as they say, so shear does work in a storm-relative sense.
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Lysis
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Makes sense. So is that why storms in the more northern latitudes are more resilient to shear (as noted per many discussions)?
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Clark
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Generally speaking, yes. Part of it likely also deals with the structure of these storms at higher latitudes; most of them are closer to hybrid structures or are undergoing transition. Since shear is a key source of energetics for midlatitude systems, such storms are generally going to be able to handle slightly more shear than would be expected for a purely tropical cyclone. But, a good bit of it has to do with storm motion -- often at those latitudes, storm motion goes with the shear vector, lessening the effect upon the storm.
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Lysis
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Since shear is a key source of energetics for midlatitude systems...
Man... that absolutely fascinates me. I have been asking allot about hurricanes in accordance with the subject matter of this site, but I do remember you saying that your specialty was transition (correct me if I am wrong) (?). I was reading about HF storms in an old issue of Mariners Weather log that my neighbor gave me (he is a great lakes big ship captain… had friends on the Edmund Fitzgerald and everything)… and it was something that really interested me. Unfortunately, my school and local library have little content on the matter of any substance. I was wondering if you have any good links to sites or articles on the matter?
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Clark
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Yeah, my research and specialty is transition. We know that a change in the energetics of the two types of storms must occur during transition -- hurricanes draw energy and moisture from the ocean's surface, while midlatitude systems draw their energy moreso from dynamical processes -- but part of what we don't know about the whole thing is how this all occurs, how the storm itself impacts the environment, and so on. We're just starting to crack the iceberg on all of those topics...I expect we'll know a lot more within 10 years.
There's a new climatology that some forecasters at the Ocean Prediction Center did on HF-extratropical storms; they presented this work in Washington D.C. last week. You can read more about it here, including being able to listen to/watch the presentation as it was given: http://ams.confex.com/ams/WAFNWP34BC/techprogram/paper_94332.htm
Regarding the whole energetics issue and such....there is some research on both types, but unfortunately most of it is in archives that are accessible only to the educational community or for a small fee. You can do a search at http://ams.allenpress.com through most of the meteorological journals to see if there is anything that piques your interest -- maybe the library even has a subscription to the service -- and go from there. Sometimes a search for a particular article will come up with a free article somewhere.
The cool thing is that the midlatitude storms get a lot of their energy from conversions of potential energy into kinetic energy (with a bunch of transfers between small, medium, and large scales complicating matters). This occurs largely because of the vertical shear of the wind. I'd have to refresh my memory about everything, but it is one big cycle...potential to kinetic back to potential energy from birth to maturity to occlusion and death. I'll try to remind myself to look over the stuff again tomorrow and see what else I can add; if you don't hear back from me by Friday here, drop me a PM and we'll go from there.
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Lysis
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Thanks alot buddy. First week of school, so I am a bit busy, but I will get right on that.
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Clark
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With baroclinic (e.g. midlatitude systems) energetics, you get kinetic energy extracted from the potential energy of the mean flow (which itself has two components -- internal potential energy and that from gravity). However, not all potential energy can be converted over to kinetic energy...in fact, the percentage of such is pretty low. Even less of the total potential energy can be converted over to available potential energy.
Getting to the math behind all of this is very complex...it's entry-level graduate dynamics in meteorology. But, skipping that, the conversion term to get between potential and kinetic energy is directly related to the thermal wind, which itself is a measure of the vertical difference in the winds between two levels, and vertical motion processes. In that respect, the thermal wind is not a true wind shear, but is in many respects similar to one. Generation of potential energy that can be converted to kinetic energy comes out of heating processes. Ultimately, the kinetic energy goes back to potential energy as the system evolves and matures, leading to dissipation.
The big thing about weather systems is that they occur to restore stability to an unstable situation. They grow out of instability -- it is required to even get to this whole debate on energetics -- but serve as facilitators to return the atmosphere to a stable state.
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