Chasing Storms: Analysis of mid-tropospheric circulation

We know meteorologists spend a great deal of time chasing storms and systems across maps.

Remember Katrina 2005?

Many of those maps portray the state of the atmosphere at standardized levels in the upper atmosphere.

Here, the mid-latitude prevailing westerly winds come into view as everything moves generally in an eastward direction. We have calm, clear areas chasing storms along streamlines. Changes we hear about in our weather forecasts show up as bumps or waves on these continuous curves.

There exists a range of altitudes where nearly all horizontal motion in the air consists of systems following storms and sliding across the map (transformation). They have a minimum of size change over time and distance.

Technically we call this the level of non-divergence, LND. And it happens about halfway up the troposphere (the bottom layer of the atmosphere earth has), nominally at 500 mb, about 20,000 feet above sea level. This is because air moving up or down can continue moving up or down easily without needing to change directions. For example after hitting a strong boundary such as the earth itself or the stratosphere.

Wouldn't it be cool to get your pilot's license and chase storms in a plane. Better use a rented one.

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How many different ways can air flow?

The four flow variables are:

1) Translation – chasing storms across the map,
2) Deformation – change in shape, like squeezing a ball of dough without changing its size,
3) Divergence – spreading out and increasing/decreasing flow area, or apparent size, and
4) Vorticity – rotation about a vertical axis, which may also be moving.

A good film about weather people experimenting and chasing storms is Twister with Helen Hunt and Bill Paxton. See it sometime.

Often all four are happening to the same air parcel at the same time,

moving with weather systems and more or less chasing storms.

Pressure related systems are defined at higher levels by the altitudes above sea level one must go to reach the specified millibar level. Generally the heights decrease as you move toward the nearest pole.

For example, on a North American map usa weather contours at 500 millibars can show heights differing by 500 metres or more in different parts of the country. From a thermodynamic point of view, that makes sense. The colder surface based air following or chasing storms in mid latitudes is more dense (in kilograms per cubic metre) than the warmer, more humid air found elsewhere.

Therefore, one should expect a sharper decrease in pressure with increasing altitude in the cold regions. Considering that the corresponding sea-level pressures do not differ too greatly.

It makes little difference whether we’re chasing storms by attending to a low spot on a height contour map like this one, or a minimum pressure area on a constant altitude map (such as sea-level pressure). The lows and highs shown behave in similar fashions.

Things to keep in mind. The flow goes counter-clockwise around a low and clockwise around a high. Lows are associated with increased cloud, while highs are often located near clearer skies. Here are some other criteria.

Radiosondes and Upper Air Analysis

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Rough weather convection currents
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How different types of clouds form
Different air motions result in different types of clouds. Air moving up and down give the clouds their shapes.

Explain humidity and its consequences
Forecasters spend great effort to explain humidity and its implications. Why? It easily converts into heat energy which drives severe storms.

Figure out Thunderstorm Cumulonimbus Clouds.
When you hear a thunderstorm cumulonimbus clouds can not be too far away. Can we forecast them?

Causes of lightning thunder and storms
What are the causes of lightning thunder and severe storms? Do instability and convection appear out of nowhere? Possibly.

Stable air, weather inversions
What conditions do we get with weather inversions? What causes them? Get the brief rundown.

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