Day 235: Central United States Cold Front

Posted by pmarsh on 23 Aug 2010 | 4 comments

Well, it’s the start of another academic school year, which means my favorite meteorology class is once again back in session — Synoptic Meteorology. Although I’m not a TA for the course this year, I will once again be helping where and when I can. I also hope to use this blog to supplement the lecture materials by having my nightly post tie into whatever was discussed in class that day. (We’ll see how long this lasts…)

Today we had several brave students get up and do a spot map discussion. (The students did very well for the first day. This bodes well for the rest of the semester!) One of the maps we had the students discuss was today’s water vapor imagery. While the students were discussing the large dry region over much of the eastern US apparent in the water vapor imagery, I noticed something interesting in the high plains near the Rocky Mountains. If one examines a loop of today’s water vapor imagery, it is possible to watch the progression of a surface cold front as it surges south. Although water vapor imagery tends to depict moisture in what meteorologists refer to as the “mid and upper troposphere”, it can actually “see” very near the ground in regions of high elevation (such as mountains and the high plains). This is because in these locations the ground is actually near the bottom of what we consider the mid troposphere (700mb or so).

The cold front shows up in water vapor imagery as a thin, dark, curved line extending southward along the ridge of the Rocky Mountains and then curves east and then northeastward as it extends out over the lower plains. The satellite is detecting the density gradient along the leading edge of the cold front. (Remember, cold air is more dense than warm air!) The satellite is most likely detecting the subsidence portion of the mesoscale circulations within the frontal zone. The reason for the “surge” southward nearer the peaks of the Rocky Mountains can be attributed to the location of the surface high pressure (not shown). As the surface high pressure moves southeastward out of western Canada, the circulation around the high pressure results in a northeast surface wind to the southeast of the center of the high pressure.

This northeast wind advects the “cold” air behind the font into the Rocky Mountains. As the cold air piles up along the Rocky Mountains one of two things has to happen in order to conserve mass: 1) the cold air can flow over the Rocky Mountains, or 2) the air must spread out horizontally along the front range. Since cold air is dense (heavy) it is very hard to lift this air over the Rocky Mountains, which means the air tends to spread out along the eastern slopes of the Rocky Mountains. Because of the north-to-south component of the surface wind, the cold air does not spread out equally to the north and south. it predominantly moves south. Thus, the cold front surges southward faster the closer to the peaks of the Rocky Mountains. (In actuality, the fastest surge occurs just east of the Rocky Mountain peaks due to frictional effects of the mountains…)

Below is a series of water vapor images in sets of two. The top most image has no annotation whereas the bottom image has the hand-drawn location of the surface cold front. Notice in the last few images higher moisture is detected by the satellite. This is because thunderstorms developed in response to the surface convergence along the advancing cold front. Also, in the first few images, a second cold front is detected to the north of the primary cold front. This cold front appears to dissipate as the day progresses, most likely the result of daytime heating weakening the temperature/density gradient along this secondary front.

Water Vapor Imagery Valid 1215 UTC (7:15 AM CDT)

Water Vapor Imagery Valid 1515 UTC (10:15 AM CDT)

Water Vapor Imagery Valid 1815 UTC (1:15 PM CDT)

Water Vapor Imagery Valid 2115 UTC (4:15 PM CDT)

Water Vapor Imagery Valid 0015 UTC (7:15 PM CDT)