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Why we should observe an increment on the mean intensity in rainfalls and an increment on mean dry days with global warming?

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closed as off topic by Sklivvz, David Z Dec 30 '12 at 23:27

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Because the statement isn't true while it is more convenient for the popular idea that there is some climate change to worry about.

In reality, irregularities and extremes in precipitation, temperatures etc. are driven primarily by temperature gradients. Because the cold poles are predicted to warm faster than the warm equatorial areas because of various feedbacks, the pole-equator temperature difference is predicted to drop. If true, this will reduce temperature gradients and make many location-dependent quantities more spatially uniform. A higher temporal uniformity is expected as well.

So if there were (significant) global warming, precipition and other processes would probably get more uniform and less extreme. The frequency of rains that are stronger than a given threshold would drop. However, the relative difference in the intensity of rain etc. would be very small because one or few Celsius degrees is a tiny change of the absolute temperature while the natural noise and fluctuations in quantities describing precipitation are of order 100 percent.

This is a textbook material. I was taught those things mainly by Richard Lindzen but e.g. his textbook on atmospheric physics is recommended:

http://www.amazon.com/Dynamics-Atmospheric-Physics-Richard-Lindzen/dp/0521018218/ref=sr_1_2?ie=UTF8&qid=1354290340&sr=8-2&keywords=Richard+Lindzen

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You should mention that the work by Lindzen is highly controversial. Without such a mention, the claim This is textbook material is misleading, because it implies the exact opposite. For example, tropical cyclones are driven by sea surface temperatures, not by equator-pole gradients. Many models, such as the ones by Trenberth, do show an increase in intensity of precipitation events, and warmer climates do tend to have more intense precipitation. –  gerrit Feb 12 '13 at 22:37
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This site by climate scientist Frank Wentz discusses the rainfall possibilities of increased temperatures. Although warmer atmosphere contains more water thereby increasing rainfall, changes in circulation could minimize rainfall.

It's also possible that currently wet areas will get wetter and currently dry areas will get drier.

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Dear Michael, Wentz only observes what the actual water content is but he doesn't have solid attribution of the changes or justified predictions for the future. After all, this experimenter isn't really qualified to do such things. A higher temperature of course means a higher absolute humidity for the same relative humidity but absolute humidity, and therefore temperature, is irrelevant because rain is sparked when relative humidity reaches 100 percent due to water/pressure/temperature conditions. –  Luboš Motl Nov 30 '12 at 15:54
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All the guesses about changed circulation or increased non-uniformities are pure speculations and/or numerical artifacts of overfitted untested models. None of these claims actually follows from any proper physics analysis of the atmosphere and physics actually implies that the opposite sign is to be expected. Global warming, if any, would reduce all the non-uniformities in precipitation and other things. –  Luboš Motl Nov 30 '12 at 15:56
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Incidentally, the fluctuations are so significant and the record is so short so far that we can't even observationally verify the proposition that the average relative humidity of the atmosphere should be more constant than the absolute humidity - because the former is between 0 and 100 percent while the latter may grow larger because of warming. The 1 deg C warming only represents a few percent change in the ratio of the two humidities but the precision with which the humidity trends may be extracted from the observations has an error that's larger. So no changes like that are obs. seen. –  Luboš Motl Nov 30 '12 at 16:29
    
I believe that forecasting climate effects like rainfall is still a work in progress. I'm not inclined to dismiss Dr. Wentz,s views on the subject. –  Michael Luciuk Nov 30 '12 at 16:45
    
You may not be inclined but it's still true that you don't have any physical evidence for your claims, e.g. that "it's possible that wet will get wetter" etc. You (and Mr Wentz and dozens of others) just use the uncertainties in order to promote one particular kind of answer that is as unjustified as any other answer (and actually less justifiable than the opposite answer) because you find this answer more convenient, for certain reasons. It's called scientific dishonesty. –  Luboš Motl Dec 1 '12 at 11:56
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