Retired senior lecturer in the Department of Meteorology at Penn State, where he was lead faculty for PSU's online certificate in forecasting.
By: Lee Grenci , 8:16 PM GMT on December 29, 2012
The secondary low-pressure system that developed east of the Appalachians and dumped heavy snow on northern New England on December 27 (snowfall map) occluded and deepened early on Thursday. For confirmation, compare the central pressure of the occluding low at 06Z surface analysis on December 27 with the barometric reading at 09Z. Note that the pressure fell from 989 mb to 987 mb in three hours.
The 09Z surface analysis on December 27, 2012. Note the central pressure of the low along the New Jersey Coast was 987 mb, 2 mb lower than it was 06Z (despite the fact that the low had entered the early stages of occlusion). Courtesy of HPC.
So the low-pressure system intensified during the early stages of occlusion (not unusual). Aloft, the closed 500-mb low associated with the negatively tilted short-wave trough was moving toward a vertical alignment with the surface low (compare the 12Z NAM model analysis at 500 mb with the 12Z surface analysis).
The movement toward a vertical "stacking" is characteristic of the occlusion stage (note the vertical alignment of the surface low and the 500-mb low at 12Z). Even after the vertical stacking takes place, the central pressure of the occluded low is still pretty close to its minimum central pressure. Indeed, check out the 18Z surface analysis and note that the minimum barometric reading was 988 mb.
When the 500-mb low and surface low are vertically stacked, the upper-level divergence that had supported the surface low during its mature state and the stages of early occlusion has all but vanished. Keep in mind here that the upper-level divergence that sustains the surface low typically occurs to the east of the 500-mb short-wave trough. So, when the system is vertically stacked, the surface low is flat out of luck when it comes to upper-level divergence.
How does the occluding surface low respond as it starts to run out of upper-level divergence? Simple answer: It starts to move back into the cold air. At first glance, such a move seems self-defeating. Why's that? Well, above the center of a low-pressure system, the air column that extends from the earth's surface to the top of the atmosphere weighs the least compared to all the neighboring air columns. Keep in mind that, when the atmosphere is hydrostatic, surface pressure corresponds to the weight of the overlying air column (for convenience, we assume the column has unit cross-sectional area).
At any rate, the column of air with the lowest weight (above the center of a low) has the highest average air temperature (more precisely, the highest virtual temperature) compared to neighboring air columns. In turn, the column with the highest average temperature also has the lowest average air density (which is consistent with the lowest weight). The bottom line here is that an occluding low moving back into the cold air, which has a higher density than warm air (at the same pressure), appears, at first glance, to be counterproductive for a low's well being.
An idealized cross section showing the movement of an occluding low back into the cold air and the tropopause undulation (TUL) associated with a lower tropopause in the cold air mass behind the low and a higher tropopause in the warm air mass ahead of the low. Larger image. Courtesy of the American Meteorological Society.
So how can an occluding low move back into the cold air and still have its central pressure decrease (like the low along the Northeast Coast on December 27)? To answer this question, check out the idealized cross section of an occluding low above (larger image) (from Tropopause Undulations and the Development of Extratropical Cyclones by Paul A. Hirschberg and J. Michael Fritsch). The rightmost "L" along the bottom represents a surface low about to enter the occlusion stage at some time, t sub 0. The two "L"s to the left indicate the position of the occluding low at two later times. Back deep in the cold air mass, the tropopause, indicated by the thin dark curves (solid and dashed) roughly between 200 mb and 500 mb, lies at a relatively low pressure altitude. I say "relatively" because the tropopause is higher to the east of the low (in the warm air mass). Meteorologists call these variations in the heights of the tropopause a tropopause undulation.
To understand why the tropopause lies at a lower altitude in the cold air mass, recall that pressure decreases relatively rapidly with height.
In the cold air mass, sinking motion associated with collapsing heights prompts air to sink from the lower stratosphere, dramatically warming on descent. This dramatic warming of sinking stratospheric air creates a warm pocket at roughly 200 mb (and higher up). In turn, winds at 200 mb blow warm air at 200 mb eastward. This warm advection at 200 mb transports relatively warm air into the column of air over the center of the occluding low. The arrival of warm air at 200 mb over the center of the low increases the average mean temperature in the air column over the low. Thus, the average air density in the column decreases. In other words, the air column over the center of the low loses some weight, and, as a result, the surface pressures decreases (even as upper-level divergence vanishes).
So the low cuts back into the cold air, making it seem like an ill-advised move. But the low is actually moving back toward the warm pocket of air at 200 mb, banking on warm advection at this altitude to increase the mean column temperature and, thus, lower the mean column air density and weight. Eventually, the low moves far enough back into the cold air where warm advection at 200 mb ceases. Now the stage is set for the low to rapidly weaken and eventually dissipate.
The 12Z GFS model analysis of 200-mb isotherms (dashed contours in degrees Celsius) and 200-mb streamlines (thin blue contours with arrows). Larger image. Courtesy of Penn State.
Let's get back to the coastal low on December 27. Revisit the 12Z surface analysis. Okay, let's look (above) at the 12Z GFS analysis of 200-mb isotherms (dashed, green contours in degrees Celsius) and 200-mb streamlines (thin, blue contours with arrows). Larger image. Note the pocket of relatively warm air off the DelMarva Peninsula (inside the closed contour of minus 48 degrees Celsius). Note how 200-mb winds blew warm air toward the surface low (warm advection), helping the surface low to deepen (or to maintain its strength) as it moved back into the cold air during the early stages of occlusion.
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