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Pollution may increase summertime rainfall

March 31st, 2008 by obrien · 1 Comment

In the article “Midweek increase in U.S. summer rain and storm heights suggests air pollution invigorates rainstorms” printed in the Journal of Geophysical Research, Bell et al. make a strong case that air pollution plays a role in changing both precipitation coverage and intensity during summer afternoon thunderstorms. Pollution aerosols act to delay precipitation in a building thunderstorm, which allows the clouds to develop to a higher altitude. When cloud droplets reach these higher, and colder, altitudes they can freeze thereby releasing latent heat and strengthening the updrafts. This can delay the start of precipitation and the development of downdrafts with can allow the cloud to continue to grow. These processes are stronger in a moist, unstable atmosphere so the precipitation changes are best observed over the southeast U.S. Interestingly there is a weakened correlation between pollution and rainfall over the southwest U.S. and some research shows that the increase in convection due to pollution is less in drier conditions and pollution may even inhibit convection in drier climates.

Rainfall estimates over the southern U.S. and adjacent waters were made using the Tropical Rainfall Measuring Mission (TRMM) satellite’s Microwave Imager which, due to its orbit, can see from 40 S to 40 N. The analysis showed that there was a highly significant increase in precipitation estimates during mid-week summertime afternoon thunderstorms and that storm heights were higher. Reanalysis winds also showed that the midweek low-level wind convergence, mid-level vertical wind velocity and upper-level wind divergence all increased as would be expected with stronger thunderstorm development. Data from rain gauges also showed an increase in precipitation, but to a lesser extent than was shown in the satellite estimates. The opposite was found over the adjacent ocean waters where precipitation was suppressed during the midweek period but showed a peak over the weekends. This was attributed to the air at the top of the land-based thunderstorms moving out and descending over the ocean thereby suppressing the development of thunderstorms over the ocean waters, especially the Atlantic.

The reason for the midweek peak in precipitation over the southeast U.S. is due to weekly variations in particulate concentrations. Data from the Environmental Protection Agency was analyzed and it was found that the average particulate concentration smaller than 10 microns peaked from Tuesday to Thursday. There was also a Tuesday to Thursday peak for particulates smaller than 2.5 microns, but it was less pronounced than for the 10 micron size. These measurements were taken at the surface so more research is needed into these particulate concentrations in the vertical.

It is pointed out in the article that both pollution levels and types have changed over the years as technology has developed, populations have increased and moved about, and the government regulated industry. Therefore, pollution of yesteryear may not have had the same effects as that of today, and pollution in the future may affect precipitation differently.

The authors do mention that this weekly variation shows up in the afternoon precipitation in most, but not all summers. I wonder why this is so. Also, I wonder how fast pollutants move about in both the vertical and horizontal. If heavy rain “scrubs” some of the pollution from the atmosphere, how fast do the particulates repopulate the air? And, as also pointed out in the article, which particular pollutants are involved in the processes described in the article that invigorate the convection?

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