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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. D23, 8661, doi:10.1029/2002JD003260, 2003

A model for the radiative forcing during ACE-Asia derived from CIRPAS Twin Otter and R/V Ronald H. Brown data and comparison with observations

William C. Conant

Department of Environmental Sciences and Engineering, California Institute of Technology, Pasadena, California, USA

John H. Seinfeld

Department of Environmental Sciences and Engineering, California Institute of Technology, Pasadena, California, USA

Jian Wang

Department of Environmental Sciences and Engineering, California Institute of Technology, Pasadena, California, USA

Gregory R. Carmichael

Center of Global and Regional Environmental Research, University of Iowa, Iowa City, Iowa, USA

Youhua Tang

Center of Global and Regional Environmental Research, University of Iowa, Iowa City, Iowa, USA

Itsushi Uno

Research Institute for Applied Mechanics, Kyushu University, Fukuoka, Japan

Piotr J. Flatau

UCAR Senior Visiting Scientist at Naval Research Laboratory, Monterey, California, USA
Scripps Institution of Oceanography, University of California, San Diego, California, USA

Krzysztof M. Markowicz

Institute of Geophysics, University of Warsaw, Warsaw, Poland

Patricia K. Quinn

Pacific Marine Environmental Laboratory, NOAA, Seattle, Washington, USA

Abstract

Vertical profiles of aerosol size, composition, and hygroscopic behavior from Center for Interdisciplinary Remotely Piloted Aircraft Studies (CIRPAS) Twin Otter and National Oceanic and Atmospheric Administration R/V Ronald H. Brown observations are used to construct a generic optical model of the Asian Pacific Regional Aerosol Characterization Experiment (ACE-Asia) aerosol. The model accounts for sulfate, black carbon, organic carbon, sea salt, and mineral dust. The effects of relative humidity and mixing assumptions (internal versus external, coating of dust by pollutants) are explicitly accounted for. The aerosol model is integrated with a Monte Carlo radiative transfer model to compute direct radiative forcing in the solar spectrum. The predicted regional average surface aerosol forcing efficiency (change in clear-sky radiative flux per unit aerosol optical depth at 500 nm) during the ACE-Asia intensive period is −65 Wm⁻² for pure dust and −60 Wm⁻² for pure pollution aerosol (clear skies). A three-dimensional atmospheric chemical transport model (Chemical Weather Forecast System (CFORS)) is used with the radiative transfer model to derive regional radiative forcing during ACE-Asia in clear and cloudy skies. Net regional solar direct radiative forcing during the 5–15 April 2001 dust storm period is −3 Wm⁻² at the top of the atmosphere and −17 W m⁻² at the surface for the region from 20°N to 50°N and 100°E to 150°E when the effects of clouds on the direct forcing are included. The model fluxes and forcing efficiencies are found to be in good agreement with surface radiometric observations made aboard the R.H. Brown. Mean cloud conditions are found to moderate the top of atmosphere (TOA) radiative forcing by a factor of ∼3 compared to clear-sky calculations, but atmospheric absorption by aerosol is not strongly affected by clouds in this study. The regional aerosol effect at the TOA (“climate forcing”) of −3 Wm⁻² is comparable in magnitude, but of opposite sign, to present-day anthropogenic greenhouse gas forcing. The forcing observed during ACE-Asia is similar in character to that seen during other major field experiments downwind of industrial and biomass black carbon sources (e.g., the Indian Ocean Experiment (INDOEX)), insofar as the primary effect of aerosol is to redistribute solar heating from the surface to the atmosphere.

Received 30 November 2002; accepted 24 March 2003; published 20 August 2003.

Index Terms: 0305 Atmospheric Composition and Structure: Aerosols and particles (0345, 4801); 1620 Global Change: Climate dynamics (3309); 3359 Meteorology and Atmospheric Dynamics: Radiative processes.

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Citation: Conant, W. C., J. H. Seinfeld, J. Wang, G. R. Carmichael, Y. Tang, I. Uno, P. J. Flatau, K. M. Markowicz, and P. K. Quinn (2003), A model for the radiative forcing during ACE-Asia derived from CIRPAS Twin Otter and R/V Ronald H. Brown data and comparison with observations, J. Geophys. Res., 108(D23), 8661, doi:10.1029/2002JD003260.

Copyright 2003 by the American Geophysical Union.