This is a nice discussion to get started - but keeping in mind the audience I want to avoid getting too technical. I think Jeff may have implied that you only get a negatively tilted system with an occluded surface cyclone, but this is misleading as occusion generally follows from negative tilt troughs, not the other way around.
More often, a negatively tilted trough results from a smaller scale intense disturbance riding around a larger scale trough. Below is a long-winded description of this process.
We can reduce this down to considering jet stream level dynamic features only. I found this image on Steve Ackerman's web site:
[Broken External Image]:
http://mapmaker.meteor.wisc.edu/~jbrunner/ackerman/upperair/upair5.gif
The lines here could represent a few of the height contours typically seen on a 300 mb surface map. Geostrophic wind speed is the air speed that you would expect based on the how close together the contours are - so the closer the line contours - the faster the wind speed. If you were to look at the temperatures at 500 mb - you'd find a relative cold pocket of air in the trough and a relative warm pocket of air in the ridge. Now, because of the effect of the Earth's rotation on air motions, air flowing through a trough (the "dip") is slowed (subgeostrophic), whereas air flowing through the ridge is accelerated (supergeostrophic) relative to the geostrophic flow - and this leads areas of divergence and convergence aloft. The surface low is below the region of divergence aloft (where are is being removed from the air column) and the surface high pressure is below the convergence region (where air is piling up). The tilt of the trough shown above is neutral - as in if you were to draw a line down the axis of lowest heights, it would be up/down as viewed on your screen. The "normal" orientation of upper troughs is positive tilt - meaning the line drawn down the axis of lowest heights would lean from SW-NE as viewed on a weather map in the northern hemisphere. With positive tilt troughs, the amount of curvature leading to divergence is less - so weaker forcing for a surface system. If we could get a more negative tilt (SE-NW orientation), the amount of curvature is increased and subsequently so is the amount of upper level divergence and forcing for rapid intensification of a surface cyclone.
So, how can we get the "normal" positive tilt trough to become negatively tilted? We need help from a smaller scale disturbance riding around the upper trough. The larger the scale of a disturbance - the slower it moves, so the trough/ridge pattern we talked about above moves very slowly (it actually it is quickly retrograding against the mean flow). Shortwaves - which we often look for as vortcity maxima on 500 mb charts - are much smaller in size and therefore retrograde much slower, and so move through the upper level trough ridge pattern acting to distort the upper ridge trough. This shortwave can often be observed as a jet streak embedded in the ridge/trough pattern. So, to get our negatively tilted trough - let's start with a jet streak (shortwave) on the west side of the upper trough (where convergence is occuring owing to the curvature effect). Shortwaves/jet streaks cause imbalances in the forces that leads to areas of rising/sinking motion. I like the jet streak model - so let's look at that one:
[Broken External Image]:
http://www.ems.psu.edu/Courses/Meteo200/lesson6-2/graphics/jetstrk.gif
The above image is from a Penn State web site. It's busy - but let's just focus on what's important. The upper panel shows an elongated bullseye of strong winds - with the maxima in the center. the top right corner is labeled "left exit" and there air is divergergent at upper levels forcing upward air motion (see bottom panel). Back to the top left is the entrance region where air converges at upper levels causing sinking air and pressure rises (again see bottom panel). When air rises - it cools due to expansion as it finds itself in progressively lower pressure (recall pressure decreases with increasing height). Conversely, air sinking warms due to compression. Recall that the upper trough is collocated with a mass of relatively cold air aloft - so the localized lifting caused by the jet streak in effect adds a smaller "bulge" of cold air as it slips around the larger scale trough.
Ok, now comes the difficult part. Again, as we started above, let's begin with our shortwave on the west side of the upper trough. As the left front edge jet streak approaches the base of the trough (bottom right edge if we are viewing it on our screen, rotate the jet streak model 90 degrees clockwise), it is lifting air in the base of the upper trough - and this leads to further cooling (height falls) in the base of the trough and the trough subsequently expands southward. Since the jet streak is smaller in scale, it continues to move around the larger scale upper trough until the left exit region of our jet streak is colocated with the upper level divergence owing to curvature. Here, things get interesting as the bulge from cooling has slipped around to the east side of the upper trough, and with the phased together upper level divergence - the low-level cyclone intensifies rapidly owing to potent height falls which in turn leads to a marked increase in warm air advection (often called the warm conveyor belt). Warm air advection leads to height rises - and in effect leads to ridge building ahead of the region of upper divergence - causing an indentation in the upper height contours back to the NW. So - adding together our cool lump on the southeast edge of the upper trough with our warm indentation to the north of the upper divergence core we have created a negatively tilted upper trough.
Glen
! Sorry, by the time I posted my last message I guess you had already posted yours. Thanks again for all the great info & explanations! Now we have quite a bit to munch on for the impending season, before mother nature throws us her first real curveball! :wink:
