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Attribution of the United States “warming hole”: Aerosol indirect effect andprecipitable water vapor
Citation:
Yu, S., Kiran Alapaty, R. Mathur, Jon Pleim, Y. Zhang, Chris Nolte, B. Eder, K. Foley, AND T. Nagashima. Attribution of the United States “warming hole”: Aerosol indirect effect andprecipitable water vapor. Scientific Reports. Nature Publishing Group, London, Uk, 4:6929, (2014).
Impact/Purpose:
The National Exposure Research Laboratory′s (NERL′s)Atmospheric Modeling Division (AMAD) conducts research in support of EPA′s mission to protect human health and the environment. AMAD′s research program is engaged in developing and evaluating predictive atmospheric models on all spatial and temporal scales for forecasting the Nation′s air quality and for assessing changes in air quality and air pollutant exposures, as affected by changes in ecosystem management and regulatory decisions. AMAD is responsible for providing a sound scientific and technical basis for regulatory policies based on air quality models to improve ambient air quality. The models developed by AMAD are being used by EPA, NOAA, and the air pollution community in understanding and forecasting not only the magnitude of the air pollution problem, but also in developing emission control policies and regulations for air quality improvements.
Description:
Aerosols can influence the climate indirectly by acting as cloud condensation nuclei and /or ice nuclei, thereby modifying cloud optical properties. Observations show a striking cooling trend in summertime daily maximum temperature (T<SUB>max</SUB>) in the central and south central United States during the 20th century (termed the U.S. “warming hole”), but the attribution of this trend is unclear. We quantitatively studied this by analyzing the relationship between observations of temperature, shortwave and longwave cloud forcing (SWCF, LWCF), aerosol optical depth and precipitable water vapor, and global coupled climate models. We find that the observed cooling trend in summer T<SUB>max</SUB> can be attributed mainly to SWCF due to aerosols with offset from greenhouse effect of precipitable water vapor. These results provide compelling evidence of the role of the aerosol indirect effect in cooling regional climate on the Earth. Our results reaffirm that LWCF can warm both winter T<SUB>max</SUB> and minimum temperatures.
