Important New Research On The Role Of Aerosols On Precipitation By Professor Chidong Zhang

Professor Chidong Zhang of the University of Miami presented an important talk on June 19 2008 at NOAA’s Earth System Research Laboratory entitled “Climatic Effect of Aerosol on Tropical Rainfall: Evidence from Satellite Observations.” The abstract reads

“Many efforts have been made to investigate whether and how aerosol may affect precipitation. These efforts have yielded inconsistent and therefore controversial results. Especially, in the absence of observed aerosol effects on precipitation on climate scales, the null hypothesis “There is no climatic effect of aerosol on precipitation” has not been rejected. This has led the IPCC 2007 report to state “the sign of the global change in precipitation due to aerosols is not yet known”. In this study, we use long-term satellite observations of aerosol and rainfall to document large-scale co-variability of the two variables over the tropical Atlantic and West Africa. When influences due to known climate phenomena (e.g., ENSO, NAO, TAV) and meteorological factors (e.g., water vapor) are ruled out and analysis domains are carefully designed, such co-variability, especially its spatial patterns, suggests possible aerosol effects on rainfall. Large reductions in rainfall are found over the western tropical Atlantic Ocean and over the Gulf of Guinea in months of anomalously high aerosol concentration. Such reductions are statistically different from random and overall interannual variability. We propose that these reductions signify the climatic effect of aerosol on precipitation distribution and variability. Statistical results based on long-term satellite data are confirmed by consistent results based on more recent and higher quality satellite observations.”

This very important talk is based on the research of Professor Zhang in the following two papers

Huang, J., C. Zhang, and J. M. Prospero, 2008: Large-Scale Effects of African Aerosol on Precipitation of the West African Monsoon. Quart. J. Royal Meteor. Soc., in press.

The abstract reads

“We used multiyear satellite observations to study aerosol effects on the large-scale variability in precipitation of the West African Monsoon (WAM). We found a significant precipitation reduction associated with high aerosol concentration near the Guinea coast from late boreal autumn to winter. The largest aerosol-induced reduction (~ 1.5 mm d-1) is about 50% of the mean in the region and is mainly in the rain rate range of 2-17 mm day-1) under maritime environment off the northern coast of the Gulf of Guinea. This reduction cannot be attributed to known climate and weather factors such as El Niño-Southern Oscillation, North Atlantic Oscillation, Atlantic sea surface temperature, and water vapour. The fractional precipitation variance related to aerosol is about 13%, a value comparable to those attributed to the known climate factors. Aerosol responsible for the observed precipitation reduction can be traced back to various African sources where aerosol emissions have varied considerably over the past several decades, in part attributable to human activities.”

and

Huang, J., C. Zhang, and J. M. Prospero, 2008: Aerosol-Induced Large-Scale Variability in Precipitation over the Tropical Atlantic. J. Climate, submitted.

The abstract reads

“We used multiyear satellite observations to document a relationship between the large-scale variability in precipitation over the tropical Atlantic and aerosol traced to African sources. During boreal winter and spring, there is a significant reduction in precipitation south of the Atlantic marine intertropical convergence zone during months when aerosol concentrations are anomalously high over a large domain of the tropical Atlantic Ocean. This reduction cannot be attributed to known climate factors such as El Niño-Southern Oscillation, North Atlantic Oscillation, and zonal and meridional modes of tropical Atlantic sea surface temperature, or to meteorological factors such as water vapor. The fractional variance in precipitation related to aerosol is about 12% of the total interannual variance, which is of the same order of magnitude as that related to each of the known climate and weather factors. A backward trajectory analysis confirms the African origin of aerosols that directly affect the changes in precipitation. The reduction in mean precipitation mainly comes from decreases in moderate rain rates (10 – 20 mm/day), while light rain (<10 mm/day) can actually be enhanced by aerosol. Our results suggest aerosols have a clearly identifiable effect on climate variability in precipitation in the Pan-Atlantic region.”

The role of aerosols on precipitation (from industrial activity but also fires and blowing dust) was reported in my recent House subcommittee testimony (see) on overlooked and/or under reported issues in the IPCC and CCSP reports. Professor Zhang’s research provides further documentation of this critically important topic with respect to the human role on the climate system.

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