Hodograph vs. Directional Shear

[BBauer]

I understand that clockwise hodographs are good for warm air advection, the "ideal" being a veered setup with southeasterly winds at the surface then shifting to westerly etc. as you go higher up. I understand this can contribute to explosive updrafts. But what I am not understanding is how can clockwise hodographs contribute to rotating updrafts that rotate counterclockwise. It just seems counter-intuitive. Am I associating too closely clockwise hodos and low level directional shear?

 

[Richard Dickson]

I'm not a meteorologist, but I'll take a shot at this one.

If you consider the hodograph as being the graphical representation for 'layers of air' at different levels, it might be a little easier to see. Given: Surface wind SE@5, 850mb S@10, 700mb SW@15, 500mb WSW@20. If each of those values were attached 'head to tail' bottom layer to top layer, this would create a clockwise hodograph in ascending height like you spoke of.

Now, picture each of these layers individually from the bottom up and it should become apparent that the forces involved would rotate a parcel of air in a counter-clockwise manner under those criteria.

I may not be correct, but at least it makes sense that way to me.

 

[Tim Supinie]

There are two important things to remember here.

The first is that observed soundings have sampling issues in that they're only sampling a small part of the atmosphere at a given time. Given the sparse radiosonde network relative to the surface network, we often assume that conditions in the general area (~200 km radius) of the radiosonde are the same as at the radiosonde, which is probably not a good assumption when you're close to the storm and the radiosonde is not. Thus, the sounding may not be a true representation of what the storm is experiencing.

The radiosonde also is not a true vertical profile of the winds because it's being blown by the wind. This means that you could be sampling a point in the atmosphere that is 50 km away from the radiosonde launch site and assuming it's over the launch site. This is usually only a problem in the upper levels of the atmosphere on days when the winds are really strong, but it's something to keep in mind.

The second thing is that without a thunderstorm updraft, there is no vertical vorticity*. In that case, all the vorticity is in the horizontal. The updraft acts to tilt that vorticity into the vertical, which produces a pair of "vortexes" rotating in opposite directions. The image below gives a good illustration of that.

(Source: http://bit.ly/kftw1y, adapted from Bluestein 1999)

Which one of those vortexes gets accentuated into the mesocyclone depends on the storm motion and the exact shape of the hodograph. If you have a clockwise-curved hodograph, you probably have some storm-relative component of the wind that is "pushing" the counter-clockwise vortex into the updraft, where it gets stretched and accentuated. I can't find the picture right now, but on the wind profile on the image above, think of a southerly component to the wind (a north-pointing arrow) in the mid-levels of the atmosphere, and you can kind of get the picture.

Does that help?

*If you're not familiar with the term "vorticity," you can think of it as a measure of how much the air is rotating. Really, this rotation is on the microscopic scale, but for visualization, it helps to think of it on the scale of something you can see.

 

[Zach Young]

The curve ofthe hodograph influences which of those vortices become dominant when the cell splits. In the diagram above, the hodograph would be very linear, and it's likely that both the left and the right split will last for a while. Clockwise curved hodographs favor right splitting cells, which contain the counterclockwise vorticity in their updrafts. When a right storm split becomes dominant, it's usually referred to as a "right-mover" or that the cell has made a right turn. This usually orients the cell in such a way that it is receiving the best possible storm relative inflow (best moisture, best temperature, best CAPE, and best inflow direction), which can cause the storm to rapidly intensify and makes it more likely to produce a tornado. On days where the hodographs favor right splits (and tornadoes) the left split of the cell weakens and dies off so fast that you don't even realize it was there at all. If you find yourself behind a supercell, you may be able to notice an area of clockwise spin drifting off the back side of the storm.

A counterclockwise hodograph would tend to favor the left split which have the clockwise vorticity in their updrafts, but tornadoes from left splits are rare.

 

[BBauer]

Yes, all your responses help a ton, thanks!

 

[Andrew Burnett]

great thread guys. Thanks.. helped me out too.

 

[Jeff Duda]

Although Tim and Zach covered it pretty well in their posts, the image in Tim's post comes from Joe Klemp's 1987 write up on the dynamics of tornadic thunderstorms. It's quite mathematical, but if you check out the images, you'll see how a clockwise turning wind profile results in favored counter-clockwise turning tornadoes.

 

[Robert Dewey]

Also note that just because there is a curved hodograph does not mean there is excellent low-level speed / directional shear (top chart; blue hodograph would be most desirable), just as a straight-line hodograph isn't always unidirectional (bottom chart) [as you can see, it would result in poor / zero speed shear in the lowest layer and thus wouldn't be suitable for organized severe storms and/or tornadoes IMO].

 

[Jeff Duda]

Also note that just because there is a curved hodograph does not mean there is excellent low-level speed / directional shear (top chart; blue hodograph would be most desirable), just as a straight-line hodograph isn't always unidirectional (bottom chart) [as you can see, it would result in poor / zero speed shear in the lowest layer and thus wouldn't be suitable for organized severe storms and/or tornadoes IMO].

I don't know if I would agree with that, Robert. Assuming the levels in both hodographs in the top plot are the same, then those have the same amount of shear and would have the same amount of ESRH if the same pattern was used to find the motion vector. Thus those two hodographs are, for most situations, identical in terms of their ability to create tornadic supercells. The hodograph in the lower plot, despite being straight, could still foster the development of tornadic supercells, too, despite not having much speed shear in the lowest levels. I'm not exactly sure at which point speed shear and directional shear become different enough for one to matter way more than the other, but there is certainly plenty of low level shear in that sounding as well (again depending on where the different height levels are on it).

 

[Jeff Snyder]

I agree with Jeff D. above. SRH can be calculated from a hodograph as twice the area swept out by the hodograph bounded by the storm-relative wind vectors at the top and bottom bound of the layer for which SRH is being calculated. As Jeff D. noted, the shear both the red and blue hodographs in top chart is the same. The storm motion would be different, no doubt, but the SRH would likely be the same.

As Robert also correctly noted, a nice veering wind profile is not necessarily any better than a unidirectional wind profile. There are times, such as those that are represented in the bottom hodograph, when the winds veer with height (from southeast at the surface to west-southwest aloft), yet the result is still a unidirectional shear profile. The straightline hodograph typically favors splitting supercells (see Tim's embedded image), and each member of the split may end up with a storm motion that is off of the hodograph. As soon as the storm motion moves off the hodograph, SRH is "produced" (either anticyclonic or cyclonic depending upon the split/storm motion), which may then lead to updraft intensification (via the nonlinear term in the diagnostic pressure perturbation equation) and storm motion that may deviate further from the hodograph (via the linear term in the diagnostic pressure perturbation equation). In general, a more curved hodograph (e.g. the top hodograph) allows for greater streamwise vorticity / SRH ingestion, which then can result in a more significant supercell (all other things being equal). To maximize positive SRH, you typically want to see a highly-curved hodograph, and evidence exists to suggest that a near right-angle turn in the hodograph in the low-levels (often giving yield to the sickle-shape hodograph) is favorable for stronger tornadoes.

 

[Robert Dewey]

Assuming the levels in both hodographs in the top plot are the same, then those have the same amount of shear and would have the same amount of ESRH if the same pattern was used to find the motion vector.

Ah, you are right. Thanks for the additional input.