ATMOS 301: Troughs, Ridges, Waves, and Fronts

Wednesday, October 30, 2013
10:40 p.m.

It almost sounds like it could be a Dr. Seuss poem. Troughs, ridges, waves and fronts. Ditches, mountains, surfboards, and stunts.

Anyway, this will be the last of my “light duty” reviews/online lectures/blogs. The next section will be on gas laws and atmospheric thermodynamics. I’ll try and write them in a manner that I can understand, and let me tell you, it won’t be easy. But before I write one blog, two blog, red blog, blue blog (and possibly more), let’s do some more qualitative analyses of atmospheric phenomena.

Ridges and troughs are pretty easy to conceptualize. They represent the shape of the jet stream in the upper atmosphere. In the northern hemisphere, a ridge represents a northern shift in the jet stream caused by a high pressure “pushing” it northward, whereas a trough represents a southern sagging of the jet stream caused by an area of low pressure.

The general flow of the atmosphere in the mid-latitudes in the northern hemisphere is eastward. Because these ridges and troughs travel eastward, they are often called waves. Waves come in all sorts of shapes and sizes, but for simplicity’s sake, we divide them into two main categories: short waves and long waves. Generally, the shorter the wave, the faster it moves. I haven’t taken any ocean physics classes, but I would like to find out if this same phenomena is true with water waves. I can’t remember if/how the length of light or sound waves affects their speed when they are not in a vacuum. The picture below is another one I got from Professor Houze’s presentation, and it shows the ridges and troughs quite nicely. The far right hand portion got cut off because of a formatting problem with the PDF.

Retrieved from http://www.atmos.washington.edu/~houze/301/protected/Notes/CompObsMaps.pdf

Here’s an example of a 500mb chart from our latest WRF-GFS model here at the UW. Can you distinguish the shortwaves from the longwaves?

Add captValid 07:00 am PST, Tue 05 Nov 2013 – 135hr Fcst: 500mb Heights, Absolute Vorticity. UW WRF-GFS 36km Resolution: Initialized 00z 31 Oct 2013. Retrieved from the UW Pacific Northwest Environmental Forecasts and Observations Website. Model URL: http://www.atmos.washington.edu/~ovens/wxloop.cgi?mm5d1_x_500vor+///3

Remember, pressure is the vertical coordinate here, not height. Using pressure as the vertical coordinate is useful because it helps us get rid of some variable in some equation that I didn’t write down. But trust me, it does in fact help get rid of some variable in some equation.

Fronts:

Many people don’t know what troughs or ridges or waves are. Even I didn’t have the firmest grasp of them for a long time. And I still don’t think I do… I plan to study these and their effect on lower atmospheric dynamics at some point in the future over this quarter or over winter break. Whilst working with Steve Pool, we don’t usually put fronts on our weather maps because he claims that much of the populace that watches the news doesn’t understand what fronts are and just wants to know whether their suit should be of the swimming or survival variety. Most of my friends and family have a general idea of what they are, though.

Fronts mark the warm edge of a zone of strong temperature contrast; they are not simply the boundary between two different air masses. This horizontal temperature gradient is conducive to the formation of a low pressure system. Hopefully you all know what a low pressure system is.

Retrieved from http://www.atmos.washington.edu/~houze/301/protected/Notes/CompObsMaps.pdf

The formation of a low pressure system, known as cyclogenesis, is really complicated, and the above diagram is greatly simplified. Still, it offers a fantastic idea of the general life cycle of a cyclone.

Below is a diagram of the vertical structure of a midlatitude cyclone. The occluded front results because the cold front moves faster than the warm front, and when it does this, the warm air goes away. When I was young and studying this stuff on my own, I learned that there were two types of occlusions: warm occlusions and cold occlusions. It turns out there is only one – the cold occlusion – with cold air on both sides.

Retrieved from http://www.atmos.washington.edu/~houze/301/protected/Notes/CompObsMaps.pdf

Fronts are responsible for clouds and precipitation, but explaining that and a lot of the other intricacies of fronts wouldn’t be the best use of my time. If you are interested in that stuff, I recommend clicking on the links above where I am getting the pictures from.

One last thing that I think is important, however, is the relationship between ridges and troughs in the upper atmosphere and fronts at sea level. Fronts tend to form on the right edge of the trough as it transitions into the ridge. The trough and low/front move at the same speed, but remember, not all troughs themselves move at the same speed. Those with shorter wavelengths move faster than those with longer wavelengths.

Shortwave troughs are also commonly superimposed on longwave troughs. Imagine a worm wiggling, and then these wiggles having their own individual wiggles associated with them. That’s what the upper atmosphere is like. A worm wiggling its own wiggles.

Now, onto thermodynamics!

Charlie

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