Author Topic: Backing vs veering  (Read 20366 times)

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Offline Michael Thomas

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Backing vs veering
« on: 24 July 2011, 06:04:08 AM »
Hello,

There is a lot of information out there with regards to backing and veering winds. I have no problems with these terms when applied in the US (or anywhere in the Northern hemisphere for that matter). I do get confused about what term should be used in Australia (Southern hemisphere). Take this sounding for example-

http://soundings.bsch.au.com/skew-t.html?source=wyoming&lat=-35.1333&lon=147.3667&gribdate=&month=01&day=20&year=2005&hour=00&window=on&hodo=on

Do the winds veer or back with height? Take the weatherzone definition-

"Veering winds - Winds which shift in a clockwise direction with time at a given location (e.g., from southerly to westerly), or which change direction in a clockwise sense with height (e.g., southeasterly at the surface turning to southwesterly aloft). The latter example is a form of directional shear which is important for tornado formation. Compare with backing winds. "

This obviously just taken straight from a Northern hemisphere definition (probably US). "Southeasterly at the surface turning to southwesterly aloft......important for tornado formation." !!! Not in Australia!!!

Now the American Meteorological Society definition-

"Veering - 1. According to general international usage, a change in wind direction in a clockwise sense (e.g., south to southwest to west) in either hemisphere of the earth; the opposite of backing"

So the winds in the sounding above should back with height based on that definition. Personally, I don't like that definition since I believe the process is what is important, not the direction of turning. Definition 2. makes more sense to me-

"Veering - 2. According to widespread usage among U.S. meteorologists, a change in wind direction in a clockwise sense in the Northern Hemisphere, counterclockwise in the Southern Hemisphere; the opposite of backing."

Based on that definition the winds would be described as veering with height in the sounding above! That sounds right to me. What term should we use?

Regards,
Michael

« Last Edit: 24 July 2011, 06:22:55 AM by Michael Thomas »

Offline Michael Bath

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Re: Backing vs veering
« Reply #1 on: 24 July 2011, 07:34:53 AM »
I don't like the definitions as they still cause confusion between NH and SH. Why is there a need for terms veering and backing when clockwise and anticlockwise are accurate descriptions of what is going on.

Something like "winds veered clockwise with height" or "winds veered anticlockwise with height" leave no room for misinterpretation when describing the shear profile.


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Offline Jimmy Deguara

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Re: Backing vs veering
« Reply #2 on: 26 July 2011, 05:39:51 AM »
Hi Michael Thomas and Michael Bath,

Michael T, I am glad you visited these definitions because I hope I have not being using them loosely and perhaps incorrectly over the years rather than the true definitions! I have not been corrected by anyone on this issue to this point.

Just for your interest, an explanation is listed quite well here:

http://www.theweatherprediction.com/habyhints/48/

Jeff Snyder, am meteorologist in the US has an excellent and perhaps more detailed and complex explanation here on storm track (2nd post - though he does ):

http://www.stormtrack.org/forum/showthread.php?7318-Veering-vs.-Backing-winds

Quote
Veering and backing are TRENDS which must be defined either at constant height (or quasi-horizontal surface like a pressure surface, e.g. 850mb) or at a constant location (2D location...e.g. a city) and constant time. In other words, we can either vary time or vary height. Let's say we're going to discuss this in terms of a quasi-horizontal surface (e.g. the surface/ground), which means that we are varying time. Backing winds, then, refer to winds, at a particular location (say, Oklahoma City), which are changing in a counterclockwise direction. For example, winds at 11am are from the SSW, but at 1pm they are from the SSE.

We can also talk about veering and backing winds in terms of a particular/constant time and location but varying height. In this case, backing winds mean that the wind direction is changing in a counterclockwise direction WITH HEIGHT. To help avoid confusion, we typically say that winds are "veering with height" or "backing with height". This is the veering or backing wind profile (profile indicating constant time, varying height) that you hear about.

So, in summary:
+ Veering/backing winds (w/o reference to "with height" or "profile") usually refers to winds that are changing IN TIME at a fixed location and height.
+ Veering/backing of winds WITH HEIGHT implies winds that change in a clockwise/counterclockwise manner at increasing heights at a fixed time and ground location.

As a reminder, veering winds with height implies warm-air advection, while backing winds with height (or a backing wind profile) implies cold air advection. Typically, in the US, we want to see a veering low-level wind PROFILE (so, veering with height) with a backing surface wind tendency. It's also pretty common to see backing winds with height in the mid and upper-levels, which can be favorable since it implies cold-air advection in the mid and upper levels, which can increase instability. Jeff Snyder - KC0HJX
University of Oklahoma Graduate Student
http://www.tornadocentral.com

Note that Jeff also mentions a changing time scale (evolving) as we all as "fixed location". Remember sometimes, a sounding has the veering or backing profiles in place. An evolving event may change a profile to veering/backing or non-veering/non-backing.

The main focus though is the advection of warm air and cold air - again Jeff covers this well.

Regardless of approach, we should be careful in specifically mentioning both hemispheres.

Regards,

Jimmy Deguara
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Offline Michael Thomas

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Re: Backing vs veering
« Reply #3 on: 26 July 2011, 01:01:23 PM »
Thanks for your comments. I agree with Michael B, what is the point of having these definitions if all they mean is clockwise or anticlockwise (as per definition no.1 from the American Meteorological Society)? We don't call low pressure systems anticylones in the southern hemisphere because they rotate clockwise. 

On a totally unrelated note, I have been doing some reading up on hodographs and storm relative helicity, 0-6 km shear etc. Really interesting stuff. I would like to start using hodographs to start pulling out more useful info. The problem is I can't find any programs to plot hodographs. I know there is a program called SHARP but wouldn't have the slightest idea how to get my hands on it.

From what I understand, 0-6 km shear is important for supercells but low-level (say 0-1 or 0-2 km) storm relative helicity (SRH) is important for tornadoes. 0-6 km shear is relatively easy to measure (or estimate), SRH is not. Might be interesting to do some rough calculations of SRH from some Australian events. Using model data (or actual soundings) and modifying low-level winds based on surface observations should be somewhat accurate. Storm speed and direction could be calculated from radar loops.

Offline Michael Thomas

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Re: Backing vs veering
« Reply #4 on: 28 July 2011, 11:48:18 AM »
Regarding software, I have found the Digital atmosphere and RAOB programs. For sounding analysis, the ROAB program seems especially powerful. Both of these programs are not free but I don't mind spending money on something that does a good job. Does anyone here regularly use these programs?

The 2nd Feb, 2005 supercell would be an interesting one to analyse if possible. There was an afternoon sounding that day. Though, whether the data can be imported into the RAOB program is another question. I guess the storm-relative helicity that day was rather high. Would be nice to put a number to it though.

Offline Jimmy Deguara

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Re: Backing vs veering
« Reply #5 on: 28 July 2011, 04:51:20 PM »
Hi Michael Thomas,

I used digital atmosphere many years ago but never got much going with it at that time but it is a pretty good program. I have never really plotted much else, I tend to use models myself.

Regards,

Jimmy Deguara
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