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Determining the Amount of Precipitation


Falthon

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If 100% of the water in the atmosphere in a given region precipitated out very quickly, how many inches of rain would be produced?

I'm trying to set this up as a math problem, but I don't know enough about meteorology to do so. I realize weather is extremely complex, so I'm just looking for a rough approximation. To keep the problem relatively simple, I'm assuming things like mountains and prevailing winds are not factors.

How would I set up this problem, and what do I need to know to solve it?

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Integrate the total water vapor in the column. This has a specific name: precipitable water. It's the integration of the mass density (water vapor only). Say, for example, you have a box that is one meter squared, extending through the atmosphere. Then the water vapor in each cubic meter is WVdensity*volume (WVdensity*1m3). You add this up for all boxes going up to the top of the troposphere (or entire atmosphere.)

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Operational note on this as well ... PWATs can be used as a rough estimate of the "weight" of the column. This is particularly helpful in determining hail potential. You generally want PWATS (at least in my experience others with more operational knowledge may be able to tweak this a little) under 1.2". As you increase PWAT values above that, you start working against large hail potential.

I can probably count on one hand the number of times I have seen 1.25" hail and larger with PWATS over 1.4" in our region.

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If 100% of the water in the atmosphere in a given region precipitated out very quickly, how many inches of rain would be produced?

I'm trying to set this up as a math problem, but I don't know enough about meteorology to do so. I realize weather is extremely complex, so I'm just looking for a rough approximation. To keep the problem relatively simple, I'm assuming things like mountains and prevailing winds are not factors.

How would I set up this problem, and what do I need to know to solve it?

Not entirely sure on this, but I am pretty sure you would need to know the precipitable water values (PWATs) in the region. Basically, precipitable water is a measure of the amount of water in a given vertical column of air. The value assigned to a particular location is how much rain would fall (in inches) if all of that location's water were to fall as rain. I guess that is a simple way of determining how much rain would fall if 100% of water in the atmosphere were to condense and precipitate.

Here is an example from the GFS showing FORECAST precipitable water values 36 hours from now:

post-532-0-52731400-1352679053_thumb.png

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