Movement of Jet Streams

Peter Wharton

Peter Wharton

EF2
Joined
Mar 16, 2004
Messages
123
Location
New Zealand
Saw an article the other day that suggests that the jet streams are moving closer to the poles, allowing the tropics to expand, and potentially reinforcing drier conditions in the American Southwest.

Here's a link to a representative article - http://www.usatoday.com/weather/climate/20...htm?POE=NEWISVA

Is this having an impact on the setups across the chasing areas that normally would have severe weather? i.e. lack of moisture, capping, changes in supporting winds/shear all seem to be a part of the "busts" this year. My own chasing in May was singularly unimpressive. :(

Does anyone have a perspective on the long-term implications for chasing in the Plains?
 
Thanks alot for puting this to my attention now I am depressed :( :angry: :unsure:

Does this also mean that instead of Oklahoma being nuber 1 for tornadoes it may shift to Kansas or Nebraska?
 
Well I hope global warming (if it is really on going that much) won't effect tornado activity in such way, that we will lost most of April-June activity. Hopefully we just had a second bad year in a row this time. If it moves all from OK to KS or NE it doesnt bother me, I still have to fly 10000km+ :) And definatelly it'd suck if the main action moves further e-ne into the jungle, that would hurt.
 
Thanks alot for puting this to my attention now I am depressed :( :angry: :unsure:

Does this also mean that instead of Oklahoma being nuber 1 for tornadoes it may shift to Kansas or Nebraska?
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Or does it just mean the season would be shifted a bit earlier in all areas, firing a bit earlier than the historical average in the southern plains and accordingly shifted earlier northward ...


Saw an article the other day that suggests that the jet streams are moving closer to the poles, allowing the tropics to expand, and potentially reinforcing drier conditions in the American Southwest.

Here's a link to a representative article - http://www.usatoday.com/weather/climate/20...htm?POE=NEWISVA

Is this having an impact on the setups across the chasing areas that normally would have severe weather? i.e. lack of moisture, capping, changes in supporting winds/shear all seem to be a part of the "busts" this year. My own chasing in May was singularly unimpressive. :(

Does anyone have a perspective on the long-term implications for chasing in the Plains?
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I don't think 1 or 2 bust seasons can be positively correlated to this ... 1987-89 ultra-bust years weren't a sign of things changing in the 1990s-2000s... chase season was gangbusters in the Plains 3-5 years ago... a trend over 10 to 20 years where what we've seen becomes common would be a different story
 
Or does it just mean the season would be shifted a bit earlier in all areas, firing a bit earlier than the historical average in the southern plains and accordingly shifted earlier northward ...
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yeah, I think that makes alot of sense.
 
What I found interesting about the previous thread referenced by Jeff Snyder above, was the inference that the jet stream is driven by temperature gradients, weather systems etc - whereas my impression through years (perhaps driven by the popular press) is that the jet stream is the "driver" of weather systems, and therefore the distribution of warm/cold bodies of air in the atmosphere.

Reference this link where my reading of it is that the waves (and thus the jet streams) in the atmosphere drive weather patterns. http://www.cdc.noaa.gov/MJO/Predictions/wb2006.pdf

My weather "education" is dated, and I would appreciate any thoughts on whether this role of the jet stream has been re-evaluated in more recent weather research/modelling etc :)
 
Dan explained it a bit in the referenced thread. The thermal wind relationship indicates that the vertical geostrophic wind shear (change in geostrophic wind with height) is directly proportional to the horizontal temperature gradient. There are several different ways to write this forumula, but one of the most common is:

thermalwind.png


Let's say temperature on a constant-p surface (say, 1000mb) decreases to the north, resulting in a southerly temperature gradient (gradients "point" from low to high). Both R and p are positive and greater than zero, so "k" (which is the "up" unit vector) crossed with a southerly vector yields an easterly vector. If we take the negative of this (from the - sign), we have a westerly vector. Now, pressure decreases with height, so, since dV/dP is a vector that point to the west, the wind becomes more westerly with height (wind vectors point more to the east as one goes "up" in the atmosphere). Well, this explains why there's a mean westerly current of air in the middle latitudes in the northern hemisphere!

Low-level flow is, generally, pretty weak. If we assume this, then we can see that where the temperature gradient is the strongest we should expect the strongest flow aloft (though, remember, we must consider that isentropic surfaces are sloped, so the upper-level jet tends to be displaced north of the low-level thermal gradient IN THE MEAN). In the case of the strongest temperature gradient, the magnitude of dV/dP is the greatest (since k x grad(t) is greatest there), which means that, in our example above, the winds become more westerly (stronger) with height. Above the tropopause, the temperature gradient reverses, which explains why winds decrease as you near the tropopause.

OK, done with thermal wind relationship. As Dan indicated in the other thread, it seems that, in the long-term, where differential solar heating drives the entire atmospheric circulation, the surface heating tendencies drive the upper-air circulations. On a shorter time scale, you are correct -- the upper-level dynamics and kinematics "drive" low-level pressure and thermal patterns. When the flow is highly curved and/or has a strong ageostrophic component, the strongest flow aloft certainly can occur across a strong low-level temperature gradient. Quasigeostrophic and semigeostrophic vertical motion and height tendency equations can be used to approx the effects of dynamic and kinematic changes on pressure and temperature patterns at the surface and aloft.
 
thermalwind.png


Let's say temperature on a constant-p surface (say, 1000mb) decreases to the north, resulting in a southerly temperature gradient (gradients "point" from low to high). Both R and p are positive and greater than zero, so "k" (which is the "up" unit vector) crossed with a southerly vector yields an easterly vector. If we take the negative of this (from the - sign), we have a westerly vector. Now, pressure decreases with height, so, since dV/dP is a vector that point to the west, the wind becomes more westerly with height (wind vectors point more to the east as one goes "up" in the atmosphere). Well, this explains why there's a mean westerly current of air in the middle latitudes in the northern hemisphere!

Low-level flow is, generally, pretty weak. If we assume this, then we can see that where the temperature gradient is the strongest we should expect the strongest flow aloft (though, remember, we must consider that isentropic surfaces are sloped, so the upper-level jet tends to be displaced north of the low-level thermal gradient IN THE MEAN). In the case of the strongest temperature gradient, the magnitude of dV/dP is the greatest (since k x grad(t) is greatest there), which means that, in our example above, the winds become more westerly (stronger) with height. Above the tropopause, the temperature gradient reverses, which explains why winds decrease as you near the tropopause.


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the scary thing is....i understood most of that. Just dont ask me to derive it mathematically.
 
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