A grad student at Arizona State University, Bohumil Svoma, yesterday gave a fascinating talk at the American Meteorological Society convention on new research showing that tiny particles of air pollution, mostly from car tailpipes, work to reduce the amount of winter rainfall in the Phoenix area.
It's a very clever study. Svoma and his advisor, Robert Balling, knew from much past research that this sort of pollution tends to suppress rainfall. The cleverness comes in finding a way to demonstrate that with data, which they did by correlating the amount of rainfall in Phoenix over the last twenty-five years against days of the week. (As Svoma said in the talk, there is nothing in nature comparable to the build-up of particulate matter in a city caused by increased commuter traffic.)
If the theory held true, then rainfall would be suppressed midweek, and more likely to fall on Sunday or Monday. [Ed. note — technical difficulties are hindering insertion of the graph, which is not protected, but the signal is clearly visible if you look at the study, as rainfall is dramatically lower midweek than on the weekend or on Monday.]
From the abstract (An Anthropogenic Signal in Phoenix, Arizona Winter Precipitation:
varying concentrations of fine atmospheric
aerosols (PM2.5) impact precipitation processes;
generally, higher concentrations of these
aerosols tend to depress winter precipitation
especially in short-lived, shallow, and orographic
clouds. Phoenix, Arizona has a large population
relying heavily on motor vehicles as the primary
means of transportation. This results in a strong
weekly cycle of PM2.5 concentrations with a
maximum on Wednesday and Thursday and a
distinctive minimum on the weekend. To
determine any influence on rainfall, we analyze
daily precipitation records from 116 stations in
the Phoenix area and find a strong weekly cycle
in winter precipitation frequencies with maximum
values on Sunday and minimum values on
Thursday.
Svoma noted that although midweek winter rainfall was suppressed, total precipitation was not…but he added that he expected to see this signal in Los Angeles and other cities. Intriguing.
Kit Stolz 
If you get a chance could you check out Peter Webster’s new dynamic warm pool hypothesis (predicting stronger but not more TCs with continued warming)?
I’m very interested in any details, in particular 1) whether he expects that the size of the DWP won’t increase even with warming of 3C or more, 2) the relationship of the hypothesis to Kerry Emanuel’s proposal that enhanced TC activity explains equable climates, 3) the degree of storm enhancement that can be expected, and 4) the meaning for the hypothesis of the recent results finding enhanced comvective cloud activity in the tropics. Finally, is there any reason to think SoCal might get more TC activity with continued warming?
Thanks.
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Hi Steve:
Apparently that paper has been withdrawn:
http://ams.confex.com/ams/htsearch.cgi?words=Peter+Webster&action=search&formaction=http%3A%2F%2Fams.confex.com%2Fams%2Fhtsearch.cgi&meetingid=120&dir=techprogram&override=&exclude=&config=&method=and&format=builtin-long&sort=score
That session happened yesterday…but thanks for the suggestion. If you have anything else you’d like me to check out, I’m listening.
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Was that both of them? Neither seemed to be marked withdrawn.
Anyway, I can’t think of anything else, but thanks for the offer. If Peter or Carlos are still around, maybe you could ask them about the DWP.
Oh, a photo of Jim and Peter clinking Rossbys would be nice. 🙂
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Hmmmm…well, I know I saw that note about Webster’s paper somewhere…I thought it was at that URL.
“Clinking Rossbys”…now there’s a thought. Is that why people become climatologists? So they can think that big?
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Don’t all young scientists dream of Nobels etc., just as bloggers dream of getting paid? 🙂
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