A beginner’s guide to atmospheric shortwaves

2010 February 17
by Colin

My note: This was originally written on Sons of Sam Horn on December 29th. I have slightly modified it, but it still is rather raw. It is not meant to be an exhaustive or textbook-level discussion of atmospheric shortwaves, but is intended for non-scientists who want a little more than what their TV weatherman tells them. That said, it’s always nice to have my work published on my domain so it’s now here for posterity.

What’s an S/W? Is that a shortwave? What does a shortwave look like on one of the maps that get posted?

We’ll start with the first question. Yes, S/W (sometimes written SW– which is obviously confusing to those of us who also care about directions– which is, well, all mets) means “shortwave.” A shortwave is essentially a “kink” in the large-scale upper level trough/ridge system.

For examples, I’ve pulled some output from the last 84-hr NAM run. Below is the 300 mb map at 30-36-42 hours from model spin-up (18Z Wed, 00Z Thu, 06Z Thu) which is Wednesday afternoon through about 1 AM Thursday AM. Color contours are wind speed, arrows are wind direction (length corresponds to magnitude) and the line contours are height contours (you can think of higher heights being higher pressure and lower heights being lower pressure– they are the height in geopotential meters that you find the 300 mb pressure level for a given X/Y (lat/lon) location).

NAM 300mb animation

There are a couple prominent shortwaves in the trough. One begins in ID and makes its way down to the NE/CO/WY border by the end of the run. The one we’ll focus on begins in KS/OK and makes its way up through IL into IN by 42hr.

Here’s a better look; from now on we’ll freeze at 36hr (00Z Thursday).

NAM 300mb

You can see the counterclockwise kink in the white (height) contours very well in this picture. You can see hints of it as far north as Minnesota and it almost parallels the Mississippi River all the way down to the gulf coast. Shortwaves (like this one) are usually formed by cool pools aloft and upper level fronts.

There are phenomenon associated with shortwaves that are evident at all layers in the atmosphere. Next is the 500mb map, which shows color contours of “vorticity.” Vorticity is essentially the amount of rotation an infinitesimal parcel of fluid has.

NAM 500 mb

We can see a brightly colored area right in the region of the most “kinkage.” This is conceptually expected. The smaller radius of curvature at the bottom of the kink implies more vorticity since the an air parcel following the curved contours would need to rotate more than one following non-curved contours. Also, from now on I will refer to positive and negative vorticity. In this graph the “warmer” the colors (reds, oranges, and yellows) the more positive the vorticity. The way I tell most people to keep it straight is (in the NH) positive vorticity = good for rising motion = exciting weather (storms) = cyclonic = counterclockwise. On the downwind (eastern) side of the kink we get what is called “positive vorticity advection.” For those not familiar with advection, it is (essentially) the transport of a fluid property from location X to location Y. Here, we see the wind is blowing from Kansas through Arkansas and up the Ohio River. Positive vorticity advection is occurring around the Mississippi River (east of the big yellow bulls-eye) because the highly positive vorticity is being transported (via wind) eastward. In simplistic terms, advection can be thought of as the derivative (d/dx) or gradient of a field. If you have no gradient, you have no advection because you’ll just replace your parcel of air with another parcel of air with the same properties (vorticity, temperature, moisture, whatever property you want to advect). However, if the gradient is steep, then you will notice rapid changes as air travels to and then through your location because you are transporting air that is very different than what was previously in the region (high magnitude of advection).

This kink also causes another type of advection. Temperature advection. Meteorologists typically like to look at the 850 mb map to see this.

NAM 850 mb

Here we same the same thing as vorticity. The color contours are temperature (warm colors are higher) and the white arrows are wind. In the black circle we see the wind “blowing” from warmer air to cold. Therefore, in regions like Arkansas, warm air advection (WAA) is occurring due to the shortwave– it’s causing warm air to be transported into a region where cooler air previously existed. Over time we expect to see temperatures rise. You are also seeing cold air being advected BEHIND the kink (KS/OK). This is called cold air advection (CAA). The WAA and CAA are what cause the thermal gradients more commonly interpreted as “warm fronts” and “cold fronts.”

So why is this all important? Well, in very simple forecasting terms, both WAA and positive vorticity advection (PVA) are associated with rising motion in the atmosphere (there are a lot of very complicated dynamical reasons this is the case; I have spent 6+ years learning all of this so it’s not something I can really delve into the “why” of on a blog). Not surprisingly, our shortwave is associated with both of these lifting mechanisms– lifting implies latent heat release through condensation and we typically see precipitation. Well do we? I’ll show one last chart. This is the surface map with QPF (precipitation) in color contours.

NAM 1000 mb

Voila. We have precipitation in the area of our shortwave! Now there’s a couple small issues with this. One, this precip is actually the accumulated rainfall over the past 6 hours (30hr-36hr). For it to “line up better” with our other charts above we would have to average the “before” and “after” (36hr-42hr) QPF to get one centered on 36hr. Two, the shortwave is part of a complicated system; it may the main driver, but it isn’t working alone and is always interacting with the atmosphere. For instance; we see the heaviest precipitation a bit further south than we’d expect given the vorticity plots. Well from just a quick glance at the maps (don’t worry– I didn’t point this out earlier so as not to confuse the topic at hand) there is some upper level jet divergence near the Gulf Coast which is enhancing the vertical motion (and hence rainfall). We also expect QPF to be highest near the warm Gulf waters because there is a greater moisture flux from the surface to the atmosphere there (which means higher vapor pressures, more rapid condensation and more precipitation).

If all these features persist as the shortwave moves and they continue to work the way they need to; we increase baroclinic instability: instability driven by temperature gradients in the atmosphere (which is a result of the WAA/CAA as well as the rising motion induced by factors such as vorticity advection). This creates an energy source that can intensify tiny shortwaves into powerful mid-latitude cyclones. Now, this is an extremely simplified conceptual model; and isn’t EXACTLY how big nor’easters form (which is obviously more complicated; otherwise you could get a met degree in 6 weeks ;) ); but it should give everyone a broad understanding of the dynamics at play and how to spot a few of these features on either weather maps or model output.

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One Response leave one →
  1. 2010 February 23

    Good crisp explanation.The level at which the advection takes place is also important and the impact of cold air advection.Coild these be explained in some detail.

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