tommy
Registered User
Reged: Thu
Posts: 6
|
|
http://wombat.met.psu.edu/cgi-bin/GFStc2...n&hem=NWHEM can someone look at this link and tell me what the colors stand for and the # below it also . the # go from 2/40 one way and 2/40 another way . ty for your time.
tommy
|
Ed Dunham
Former Meteorologist & CFHC Forum Moderator (Ed Passed Away on May 14, 2017)
Reged: Sun
Posts: 2565
Loc: Melbourne, FL
|
|
Guess that I need some help with this one. Looks like the link is time-sensitive and I can't find it on the Penn State site??
Thanks,
ED
|
Ed Dunham
Former Meteorologist & CFHC Forum Moderator (Ed Passed Away on May 14, 2017)
Reged: Sun
Posts: 2565
Loc: Melbourne, FL
|
|
This has got to be one of the best questions that anyone has ever asked on the site - and one of the toughest ones to answer.
Here is a simplified explanation:
The colors to the right are associated with areas of positive vorticity (rising air - bad weather - lower pressure) and the colors to the left represent negative vorticity (sinking air - good weather - higher pressure). The larger the number, the stronger the vorticity (positive or negative).
So the next question is, okay, what is vorticity? Well here goes, but this one is going to 'hurt your head' a little.
Courtesy of: theweatherprediction.com
and
METEOROLOGIST JEFF HABY
"The 500-millibar prog represents the level where about one half of the earth's atmospheric mass is below it and half the earth's atmospheric mass is above it. The number 1 item a forecaster looks for on this prog is advection vorticity. Vorticity is a spinning motion (or eddy) created by directional and/or speed changes in the wind field. This model prog is available on UNISYS weather at:
http://weather.unisys.com/eta/4panel/eta_500_4panel.html
This prog shows colorized 500-millibar absolute vorticity, height contours, and wind vectors. Absolute vorticity is a combination of curvature, shear and Coriolis vorticity. Coriolis vorticity is always positive because the earth rotates counterclockwise when viewed from the North Pole. Curvature or Shear vorticity can be positive OR negative. Since the model shows absolute vorticity, the values of vorticity will be skewed toward positive values (because Coriolis vorticity is always positive). The model determines the value of vorticity for numerous locations across the model panel. These vorticity values are then colored using the scale below the panel.
The normal range of vorticity values are from near zero (or single digit negative) to sometimes more than +30 units. A unit of vorticity is equal to 1 times 10 to the negative 5th with units of seconds to the negative 1. The units of vorticity are derived from the change is wind speed over a horizontal distance. Since wind speed has units of m/s and distance has units of m, the units cancel to seconds to the negative 1. The highest vorticity regions on the panel (caused by the greatest combination of positive shear and positive curvature) are termed vorticity maximums. Positive curvature is a counterclockwise windflow (such as in a trough) and positive shear is a horizontal speed change of wind with distance that causes a counterclockwise rotation.
An eddy of vorticity spins like a low pressure in that it spins counterclockwise. This counterclockwise spin-up causes the air to diverge and rise downwind from the vorticity maximum when there is a wind flow through the vorticity maximum region. A rough guide to the intensity of a vort max is: less than 14: small vorticity; 14 to 20: moderate; 22+ large. The value of the vorticity maximum does not tell the whole story. For strong upward motion to result with a vort max, there must also be a strong wind flow (long wind vectors) flowing through the vort max. A strong gradient in vorticity values is also helpful in strengthening vorticity advection. Strong wind flowing through the vort max will create regions of NVA and PVA, where NVA stands for negative vorticity advection (wind flow region toward the vort max; advection of smaller values of vorticity) and PVA stands for positive vorticity advection (wind flow region away from the vort max; advection of higher values of vorticity). It is the downstream PVA region of a vort max that has upper level divergence and rising air (also called the downwind region of the vort max)."
In your example, you were looking at the 950 millibar level (less than a thousand feet high) rather than the 500MB level that was noted in the description above (about 18 thousand feet high).
Vorticity is one subject that can cause an instant migraine for many mets. Its easier to know what it represents rather than understand how its derived. Personally I like my simple version - hope that this has been of some help. Since I'm an 'old-school' met, maybe someone else can chip in with a clearer explanation (I hope) 
Credits to CoconutCandy for providing the corrected link.
Penn State 950MB Vorticity Chart
Cheers,
ED
|
CoconutCandy
User
Reged: Fri
Posts: 245
Loc: Beautiful Honolulu Hawaii
|
|
OK. I'll take a stab at shedding some light on this one. But please keep in mind that I'm *not* a trained meteorologist!
Although I've taken a couple meteorology classes at U of Hawaii back in the 80's, including a 300-level course on Tropical Meteorology, my main interest (hence the style of my posts) is with Mesoscale Convective Systems (areas of persistent thunderstorms) as they relate to Tropical Cyclone formation, intensification and, when it happens, regeneration of remnant systems.
And while the study of vorticity can become quite complicated, with advanced mathematics involved, and the fact that there are various 'types' of vorticity, at least the *concept* of vorticity is not difficult to understand.
So, with due apologies to the Mets that contribute and 'lurk' on these forums, here goes.
Think of a figure skater, as she is spinning. She starts off spinning slowly, and as she pulls her arms in closer to her body, she spins faster and faster. Finally, as she suddenly extends her arms out fully, she stops spinning. Although used more often as the classic 'textbook' example of the "conservation of angular momentum", you can also think of it as a simplified example of one type of vorticity. Vorticity = Spinning. That's essentially the take-away of this whole discussion.
Now getting back to the chart with the cool color shadings you were asking about. This is a global computer *forecast* model of *where* vorticity will occur in the atmosphere, and how *strong* that vorticity (spinning) is forecast to be.
The stronger the 'Positive' vorticity, the faster the spin (*counter-clockwise* in the northern hemisphere) and is illustrated with shades of yellow, orange and then red, with values of 2 to 40.
Conversely, 'negative' vorticity (the spin is *Clockwise* in the northern hemisphere) is depicted in shades of green, light blue and then dark blue, with values ranging from -2 to -40 (think below zero).
And, as Ed has so succinctly pointed out ...
Quote:
The colors to the right are associated with areas of positive vorticity (rising air - bad weather - lower pressure) and the colors to the left represent negative vorticity (sinking air - good weather - higher pressure). The larger the number, the stronger the vorticity (positive or negative).
In areas where the atmosphere is NOT spinning clockwise OR counter-clockwise, but where winds are blowing in more-or-less *straight* lines from some direction on the synoptic (large) scale, the vorticity is nearly ZERO, and shown as WHITE on the color scale at the bottom of the chart.
Important to note in the following graphic that the atmosphere has nearly ZERO vorticity ... 2 units or less ... over most areas of the earth at any given time, at least at the 950mb (1000 foot) level shown in the chart.
Interestingly, you can see from the graphic that the small positive vorticity area near 10N and 145W (small elongated tan area) is associated with a weak tropical low that has now developed into Tropical Storm 'Kika'. It's expected to pass harmlessly, far to the south of the Hawaiian Islands. See the 'Other Basins' forum for the latest info on TS Kika.
And you can also clearly see the remnant circulation of TS 'Edouard' over Texas and the newly christened Eastern Pacific Tropical Storm 'Hernan'. Also of note is the Bahamas wave and the next wave coming off the west coast of Africa. And a handful of extra-tropical storms at high northern latitudes.
It's worth mentioning that atmospheric vorticity is best (most often) analyzed and modeled at the 500mb height in the atmosphere, whereas the chart you linked to is the 950mb vorticity analysis and forecast, which is much closer to the earth's surface.
Here are a few links to learn more about vorticity, but be aware that this subject can quickly become rather complicated and, frankly, over my head too !! But the following links are not too hard to wrap your mind around, and the Flash Tutorial below is especially user-friendly and highly recommended !! What's the word? Edu-tainment?
http://www.theweatherprediction.com/habyhints/56/
Here's the nice, short (two and a half minutes) Flash Tutorial that discusses vorticity as it relates to tropical cyclone development and movement.
As synchronicity would have it, it actually uses, as its' example, the same chart, with the exact same color shading, as the link you provided! The only difference is that it depicts vorticity at the 850mb height, instead of the 950mb chart you linked to.
This is a MUST SEE if you want to see how vorticity can affect the tracks of tropical cyclones.
http://stormjunkie.com/training/models/fsu_models/fsu_850vort/fsu_850vort.html
Finally, there are (or can be) a number of misconceptions about vorticity, even among meteorologists. Credit goes to Meteorologist Jeff Haby for delineating these common misconceptions regarding vorticity. 'Misconceptions' 3, 4 and 5 are most relevant to this discussion and are the most easily comprehensible for most of us.
http://www.theweatherprediction.com/charts/500/vorticity/
Keep up your interests and keep learning and you will go far in your understanding! 
--------------------
"Don't Get Stuck on Stupid" - General Honore, following Hurricane Katrina
|
Ed Dunham
Former Meteorologist & CFHC Forum Moderator (Ed Passed Away on May 14, 2017)
Reged: Sun
Posts: 2565
Loc: Melbourne, FL
|
|
This question and its responses were reconstructed as a separate thread to eliminate a duplicate post.
ED
|
|
Previous
Index
Next
Threaded
Edit
Reply
Quote
