Vertically Stacked Low

[John Hudson]

I am probably over-simplifying things, but it seems wherever I have read about systems that are "vertically stacked", IE: surface and upper low vertically aligned, that both systems fill rapidly.

If this is in fact the case, what about the two big lows that were in play yesterday?


John
VE4 JTH

 

[David Wolfson]

I'll give this a shot with what I'd call an accessible partial explanation and real scientists should fill in the details. While there are multiple mechanisms and complicated forces at work, it comes down to what's called baroclinicity. A baroclinic zone is where equal pressure lines (isobars) and equal potential temperature density lines (isentropes) cross. The natural tendency of the atmosphere is to diminish baroclinicity if it can.

In the exit zone of a trough above the surface the result of the pressure gradient (wind) tends to pull the higher temperature air toward higher latitudes, up and over colder air. The net result tends to generate and deepen surface cyclones.

Closed upper systems tend to lose their baroclinicity, so a stacked system lacks the main dynamic mechanism that keeps a surface cyclone going. That doesn't mean, however, that you necessarily have rapid filling either at the surface or above. Not only is there a lot of momentum to disperse, but there are other sources of surface heating and upper cooling that slow the filling. Large warm water surfaces and insolation can be significant at synoptic scales -- or thermal radiation to space working the other direction.

Well I hope that makes some sense and gets the discussion started....

 

[Darrin Rasberry]

As another newbie question to add, would this situation also tend to entail more linear hodographs (with height) rather than clockwise curvature?

 

[David Wolfson]

Well again venturing a bit beyond my pay grade.... That would be true I think except near the surface where friction backs the wind in toward the low. The ultimate barotropic low is, of course, a tropical cyclone. The only directional shear is near the surface but that's enough for tornadoes. The same principle I think applies a bit more diffused and in a deeper section with cold-core systems.

 

[cdcollura]

Good day all,

There are many kinds of "vertically stacked" lows. The cold-core types (extratropical cyclones) are stacked when the upper-level low (500 MB or above) is above the surface low, which usually occludes and is slow moving.

Often this occurs when a highly-amplified trough of the jet stream "pinches" off and becomes a completely closed upper-level-low at 500 to 300 MB with the remainder of the jet stream free to meander as the (now cutoff) low sits there. With the jet stream no longer a major streering factor, these lows can sit in one area for a long time. The surface frontal system under it will usually occlude, and gradually fill-in, but not necessarily related to it being "stacked" (under the upper low) or not.

Many other factors determine filling (or even deepenning) of the surface component, especially lapse rates (as with a cold pool with the 500 MB low aloft) and surface air about the (occluded) low-level low.

Hurricanes / tropical cyclones are also stacked cyclones, but are surface-based and warm core, with DECREASING vorticity with height (NVa) opposed to cold-stacked lows, which usually have INCREASING vorticity with height (PVa).

For example, an upper low over California can cross the rockies and its PVa (reflection) can induce a lee through - and lee cyclogenesis - Causing a stacked low in the central USA - Deepening, not filling, due to the low pressure and lapse rates associated with the 500 MB vorticity.

Another example of a stacked low is the dreaded Hudson Bay low, with it's responsive ridge over the central USA and a "locked" upper air pattern. The same can be said for a "Rex Block", a stacked low sitting off the US west coast, also "locking" the pattern into a west-central-ridge / trough-east pattern.

I believe for a sounding, the hodograph will be more linear closer to the low center. Storm helicity is also least at the center of the surface-upper-low "stack". Rotating cold-core storms developing at the centers of these lows (if unstable enough) feed on the vorticity from the larger low-pressure "storm" itself, kind of like the "hub" of a wheel (highest spin / lowest wind speeds and shear) opposed to the "edges" of it (highest wind speeds, low spin, and high shear). The latter (edges) will be due to friction / boundary layer shear.

 

[Tim Vasquez]

I better clear up some confusion since in the Winter Weather thread last night I referred to the low being "stacked south of the OKC area". The low is by no means vertically stacked. It's tilted, from the DFW area at the sfc right up to NE NM in the upper troposphere. That was a poor choice of words and I'm gonna fix that post right now.

Here's the 10-cent explanation:
Tilted vertical axis: baroclinic, associated with fronts and extratropical lows
Non-tilted vertical axis: stacked, barotropic, common with occlusions, decaying systems, cutoff lows, hurricanes, etc

A low will not fill because it's stacked, but rather become stacked while it's filling (the diminishing vertical tilt is the result of the air mass temperatures homogenizing around the low, or being already homogenous).

Tim

 

[Jeff Duda]

What I learned in my dynamic meteorology class is that when a low becomes vertically stacked (i.e., barotropic) it fills from bottom to top by means of a thing called Ekman pumping, which is basically upward motion of air parcels caused by convergence at the low. This convergence causes the upper level disturbance to flatten out against the tropopause (or just in the upper levels in general), and the associated upper level divergence causes vorticity to decrease. In fact, using Ekman pumping, the equation for vorticity becomes (zeta)g = (zeta-naught)*exp(-t/tau) for some constant tau that depends on the Coriolis parameter, the eddy viscosity coefficient, and the depth of the troposphere.