Wednesday, May 18, 2011

A reminder of what we're really looking at...

This will be a short post today.  I recently had to install Unidata's Integrated Data Viewer program on a computer and thought I'd use some of the program's capabilities to remind  us what we're actually looking at when we look at upper-level height charts.

Take the 300mb chart for instance.  I often will show charts like this:
Fig 1 -- NAM 6 hour forecast of 300mb height (contours) and winds (colors) at 18Z, May 18, 2011.
Those black contours are the height of the 300mb surface above ground, and we use them to diagnose where there are troughs (generally low heights) and ridges (generally high heights).  We could color-shade the contours of height (instead of wind) to get a better sense of the topography of the 300mb surface:
Fig 2 -- Color-shaded version of figure 1 in IDV.
This is the exact same map as I showed above, only I removed the winds and color-shaded the height contours instead.  Cool colors indicate lower heights and warmer colors indicate higher heights.  You can see that the deep trough over the Pacific northwest and the shortwave trough over the middle Atlantic states clearly stand out as locally low heights (intrusions of blue colors into areas of warmer colors).

But remember--these are heights above the ground.  When we look at height contours of a particular pressure surface, we're actually looking at a topographical map of the elevation of that surface above the ground.  As such, if we were to consider a three-dimensional map of the 300mb surface, it would look something like this:
Fig 3 -- Same as figure 2, only tilted.
I've used the 3-d rendering capabilities of the IDV program to change our three-dimensional angle of viewing the 300mb surface.  Now you can really start to see how these contours show the "topography" of the surface.  Where we saw troughs indicated above, the height of the surface is lower.  In fact, if we were to look at this surface from the side (looking straight north) at a cross section, it would look like this:
Fig 4 -- Side view of figures 2/3 looking north.
Those 300mb troughs really are "troughs" in that they dig down beneath the mean height of the 300mb surface.  That Pacific northwest trough is the downward bulge on in the middle-left side of the image while the relatively shallower mid-Atlantic trough is the smaller bulge just to the east of it.  Note in the background you see what looks like extended troughing across the back.  That's just an artifact of this perspective--the 300mb height surface naturally decreases in elevation as you head toward the poles.  The anomalies in the 300mb height are the bulges in the foreground--the troughs that make our weather happen.

Now, the 300mb surface doesn't actually vary as much as these figures imply.  I've greatly exaggerated the vertical scale on these to show the features.  If I decrease the exaggeration, the horizontal cross section in figure 4 quickly turns to this:
Fig 5 -- Figure 4 at a much smaller vertical exaggeration.
The cross-section quickly reduces to looking like a horizontal line.  The troughs and ridges are still there--believe me.  They're just so small over such large horizontal scales.  The actual height of the 300mb surface maybe varies by 500-800m from its mean height, at most.  That's less than a kilometer.  The US is several thousand kilometers across.  That means the scale of the vertical perturbations is actually very small compared to their horizontal extent.  But, incredibly, even these tiny perturbations in height are enough to cause all the powerful weather we experience.  Pretty amazing.

Also, the troposphere isn't nearly as thick as that outline box implies. The tropopause is usually at 10-15 km above the ground.  Once again, compared to horizontal scales of thousands of kilometers, the troposphere is a very, very thin layer.  It would be hard to even see it if we looked at a cross section over the US at actual scale.  So, we exaggerate it vertically quite a bit to show these small contrasts.

Anyhow, I just wanted to use this kind of 3-d visualization to remind people that all these flat, horizontal maps we look at are representations of a three-dimensional atmosphere.  The troughs and ridges on an upper-air map really are troughs and ridges in the height of that surface.  It's a great topography that's constantly changing.  And, even though these changes may seem small in scale, they make all the difference when it comes to our weather.

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