|Fig 1 -- Base reflectivity radar mosaic for 1946Z, Nov 30, 2010. From the College of DuPage website.|
|Fig 2 -- Sea-level pressure (contoured) and temperature (shaded) from the RUC analysis at 12Z, Nov 30, 2010. From the HOOT website.|
- Since one of the "adages" I mentioned in an earlier post was that pressure tends to fall as a cold front approaches and then rise again in its wake, we can conclude that a cold front tends to lie in a local pressure trough. We can see an elongation on the contours around the low pressure center along a line stretching from near Chicago, though Indiana and down into northern Alabama. We would suspect that some sort of boundary would like in this pressure trough.
- There is a shift in the winds along that same line. To the west, winds are out of the west and become more northwesterly to northerly the further south you go. To the east, winds are generally southerly. This implies an area of convergence in the winds along that line. Convergence like that is typically associated with a front. But, in connection with our first observation above, if there is a low pressure trough along that same boundary, since air tends to flow from areas of high pressure to low pressure, we would expect winds to head toward our trough. Thus, this convergence makes even more sense.
- There is a strong temperature gradient across the front, particularly to the south. Since the technical definition of a front is a strong gradient in potential temperature, this is the surest sign we have a front there.
|Fig 3 -- 300 mb wind and height analysis from 12Z, Nov 30, 2010. From the HOOT website.|
So what can we conclude about this cyclone, then? Considering the weakening temperature gradients around the low over western Lake Superior, stronger temperature gradients further south, and consequently the better upper air support further south, I might suspect that the northern low is going to slowly weaken in favor of a stronger low somewhere further south.
Now this was at 12Z--what has happened since then? Here's a look at the pressure falls as of 20Z this afternoon:
|Fig 4 -- 3-hour pressure falls and wind vectors as of 20Z, Nov 30, 2010. From the College of DuPage website.|
Of course, one thing that this cyclone is definitely doing is bringing a lot of rain. However, it's not just along the cold front as we can see in the radar image back in figure 1. So, what is providing the lift in that broad region? The answer is isentropic lift. When air moves, it tends to want to do so without gaining or losing any energy, or rather it moves isentropically--keeping the same entropy. This means that the air may rise or sink depending on whatever path satisfies this condition. We measure the entropy by using potential temperature which accounts for the energy in both the actual temperature and the pressure. Two parcels of air at the same potential temperature have the same entropy. Therefore, a map of a constant potential temperature surface is an isentropic map. Since parcels want to conserve their entropy, if a parcel starts out at a certain potential temperaure, it wants to stay at that potential temperature as it moves around. Therefore, we can start inferring how air is going to move based on the structure of isentropic surfaces.
Below is the 300 Kelvin Isentropic surface from 12Z this morning.
|Fig 5 -- 300K Isentropic Surface with heights and winds from 12Z, Nov 30, 2010. From the HOOT website.|
Take a look at what's happening in the southeast and on the east coast. Note now the pressure contours decrease as you go further north. Since pressure decreases with height in the atmosphere, this implies that the isentropic surface is higher off the ground to the north and closer to the ground to the south. Also note the winds in this area. They are all blowing from south to north in a region with lots of moisture. We can conclude that there is very moist air on this surface being advected from south to north. But as it moves north, this air must follow the surface. Since the surface is getting higher as we move north, the air must be rising too. This is a phenomenon known as isentropic lift--and there's a lot of it going on in the eastern US. That's what's causing the precipitation to form over such a large area and to be so heavy--lots of isentropic lift.
Of course, once air becomes saturated it doesn't follow potential temperature surfaces anymore, but once the air is saturated--we're getting condensation and rain. So it still gives a good solid reasoning behind all the rain in the eastern US...