Originally posted by B Ozanne
This is a great question. One I had to answer on a test several years ago.
In deepening cyclones (not tropical cyclones) the surface low precedes the upper level low. You can get into a which came first argument here, but simply put its because the cold air at the surface comes before the upper level cold pocket. They are not vertically stacked.
Mature cyclones are vertically stacked. The upper level cold pocket and low sits directly over the surface low.
Ah, QG theory comes into play here...
Let's start with the upper-air pattern. When flow is straight, upper-level flow is nearly always in geostrophic equalibrium -- meaning that the two force balances are the pressure gradient force and coriolis force. The real wind will follow isobars / isohypses when this is the case. For curved flow, however, there is another force at play, and that one is courtesy of centripetal acceleration (giving rise to the fictitous force that is the centrifugal force_. When air "rounds" the base of a trough, the wind speed is a bit less than what geostrophic balance would suggest; when air rounds the top of a ridge, the wind is stronger than geo balance would suggest. We call these subgeostrophic flow (at the base of the trough) and supergeostrophic flow (at the top of the ridge). Now, let the ageostrophic wind be the difference between the real wind and the geostrophic wind. If we plot the ageostrophic wind vectors, we'd see that, for the northern hemisphere, the ageo wind vector points max westward at the base of the trough and max eastward at the top of the ridge. So, what about between the trough and the ridge? The ageo wind is southwesterly upsteam (near the base of the trough) and northeasterly downstream (near the top of the ridge) -- we have divergence! More specifically, the area downstream of a trough but upstream of a ridge is characterized by ageostrophic curvature divergence aloft. This creates upward motion, surface convergence, pressure falls at the surface, and an environment generally favorable for cyclogenesis.
Let's look at the other side of the trough -- the area downstream (east in the northern hemisphere) of a ridge and upstream (west) of a trough. Here, we have the opposite situation as that described above; this is an area of ageostrophic curvature CONvergence aloft. This creates large-scale subsidence, pressure rises, and an environment generally favorable for anticyclogenesis.
Of course, there are other sources of ageostrophic motion that can create synoptic-scale upward and downward motions. For example, for a straight upper-level jet streak, there is divergence in the right-entrance and left-exit regions, and convergence in the left-entrance and right-exit region of the jet streak. This creates something called the transverse circulation, where these is USUALLY upward motion in the left-exit and right-entrance regions, and downward motion in the left-entrance and right-exit regions. These motions result in a thermally direct circulation in the entrance region of the jet streak, since there is upward motion on the warm side of the jet and downward motion on the cold side of the jet (with a mean southerly flow between the two in the low-levels). In the exit region, there is a thermally indirect circulation -- upward motion on the cold side of the jet and downward motion on the warm side (with a mean southerly flow between the two). This is a very idealized explanation, but serves for a basic explanation.
Oh yes, can't forget about vorticity. In a trough, there can be significant cyclonic vorticity courtesy of the strong curvature. Throw in a strong jet (which can add significant shear vorticity) and you can see why you often see vort maxes along the trough axis, and vort mins in ridge axes. In the low-levels, wind speeds aren't usually very strong, so the shear-component of vorticity is usually relatively small. However, there is often strong curvature in the low-level flow field, yielding relatively strong vorticity from the curvature-component. In the upper-levels, we usually have the opposite situation -- large shear component and small curvature component. But what about in the mid-levels, like at 500mb? Here, we can still have significant shear vorticity (since we can have 60-90kt winds at 500mb), and we can still have significant curvature vorticity. This means that, ideally, vorticity is maximized in the mid-levels.
So, again, imagine that we are downstream of a trough (say a trough in the western US and ridge in the eastern US -- we're in the plains). As the vort max approaches, there is local positive vorticity advection across the plains. QG theory tells us that differential positive vorticity advection (or positive vorticity advection increasing with height) results in upward motion, which in turn tends to yield surface pressure falls. Note again that on the other side of the trough-ridge pattern, we have the opposite -- the vort max is moving away, so we can have different negative vorticity advection. Again, this is an idealized explanation, as it's not only positive vorticity advection increasing with height that causes upward motion, but there's also upward motion when there's negative vorticity advection decreasing with height.
Of course, we shouldn't forget thermal advection patterns. In terms of temperature/thermal advection, we often see the strongest advection occuring in the low-levels, with lesser advection in the upper-levels. Above the level of maximum warm air advection, there will be height rises; below the level of maximum warm air advection, there will be height / pressure falls. Warm-air advection that is maximized around 850mb yields surface pressure falls and height rises aloft (assuming the surface pressure isn't more than 850mb).
OK, so putting all of this together. Let's set up a west-coast trough and east-coast ridge situation, and let's place a jet streak at the base of the trough. Let's say that the jet streak is ejecting out of the trough axis, with the trough moving eastward. So, over the Plains, we can have several sources for surface pressure falls -- (1) ageostrophic curvature divergence aloft, (2) differential positive vorticity advection, and (3) left-exit region upper-level divergence. This makes the area between the upstream trough and the downstream ridge a mighty fine area of cyclogenesis, as we have numerous sources for surface pressure falls.
Perhaps the Plains example isn't the best, since the Plains has a very significant factor not really found in other places of the US (except for more local effects in the lee of the Appalachians and other mountain ranges)-- downsloping! As winds subside along the lee of the rockies, the air warms and expands, leading to surface pressure falls. Assuming a ridge across the eastern US, this lee trough creates an easterly pressure gradient and resultant southerly flow. This southerly flow tends to result in warm-air advection, further lowering the pressure across parts of the plains.
I've used a lot of simplifications, but this should describe, in a basic sense, why the area between a trough to the west and a ridge to the east is often favorable for cyclogenesis.
EDIT: One important thing to remember is that, when not saturated, air follows isentropic surfaces! This means that when air rises, for example in a warm-air advection regime, it follows a sloped ascent path. So, when you look at upward motion in the right-entrance region, remember that the air that is rising in the low-levels is NOT located immediately under the right-entrance region of the jet streak. If we take a jet streak in the westerlies, there may upward motion from the right-entrance region well south of that region of the jet streak. So, if there's a jet streak over OK, there may be low-level ascent associated with the right-entrance region that's well into southern TX.
Also realize that, when saturated, air follows moist isentropes (or theta-e surfaces), which are usually sloped more steeply than dry isentropes. This means that, in a saturated environment, the path of air parcels from the low-levels to the upper-levels is more erect. This also helps explain why precip in the warm-air advection zone can help deepen surface lows and amplify the upper-level pattern. With more focused and significant ascent along the more steeply-sloped theta-e surface (relative to theta, or dry isentropic, surface), stronger pressure falls may occur.
EDIT II: Fixed several grammatical and spelling errors.
