Saturday, September 22, 2007
TD # 10
Tropical Depression Ten never quite earned the status of a named storm. Now it appears as if the remnant Low is taking more of a southern path as it continues to spin down and weaken. We may not see a whole lot of precipitation from this feature. There are some inherent problems with computer models and these can show up in both mid-latitude cyclones and tropical storms.
The behavior of the atmosphere is governed by physical laws which can be expressed as mathematical equations. These equations represent how atmospheric quantities such as temperature, wind speed and direction, humidity, etc., will change from their initial current values (at the present time). If we can solve these equations, we will have a forecast. We can do this by sub-dividing the atmosphere into a 3-D grid of points and solving these equations at each point. Super computers do these equations at ridiculously fast rate and "model" the atmosphere. To put the speed into perspective, it would take eight months to manually do the math just for a tomorrow forecast.
These models have three main sources of error:
1) Initialization. We have an imperfect description of what the atmosphere is doing right now, due to lack of data (particularly over the oceans). When the model starts, is has an incorrect picture of the initial state of the atmosphere, so will always generate a forecast that is imperfect.
2) Resolution. Models are run on 3-D grids that cover the entire globe. Each grid point represents of piece of atmosphere perhaps 40 km on a side. Thus, processes smaller than that (such as thunderstorms) are not handled well, and must be "parameterized". This means we make up parameters (fudge factors) that do a good job giving the right forecast most of the time. Obviously, the fudge factors aren't going to work for all situations.
3) Basic understanding. Our basic understanding of the physics governing the atmosphere is imperfect, so the equations we're using aren't quite right.
Posted by Ted Zarras at 10:50 PM
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