Observations of the spectral clear‐sky aerosol forcing over the tropical Indian Ocean

Meywerk, J.; Ramanathan, V.

doi:10.1029/1999jd900502

Cited by 75 publications

(65 citation statements)

References 7 publications

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“…The results are not sensitive to the absolute values of the emission because we are scaling the emission to its value for 1995-1999. We could have scaled it to some fraction of SO 2 and BC emissions, but the scaling factors for SO 2 and BC are nearly the same because in Fig. 1 they are both due to fossil fuel combustion.…”

Section: Methodsmentioning

confidence: 99%

“…The absorption of solar radiation by ABCs contributes to atmospheric solar heating. The absorption together with the scattering leads to a large reduction of UV and visible wavelength solar radiation at the surface (2), alternately referred to as dimming (3). In addition, aerosols nucleate more cloud drops that enhance scattering of solar radiation and contribute to additional dimming (4).…”

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confidence: 99%

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Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle

Ramanathan

Chung

Kim

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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South Asian emissions of fossil fuel SO2 and black carbon increased Ϸ6-fold since 1930, resulting in large atmospheric concentrations of black carbon and other aerosols. This period also witnessed strong negative trends of surface solar radiation, surface evaporation, and summer monsoon rainfall. These changes over India were accompanied by an increase in atmospheric stability and a decrease in sea surface temperature gradients in the Northern Indian Ocean. We conducted an ensemble of coupled oceanatmosphere simulations from 1930 to 2000 to understand the role of atmospheric brown clouds in the observed trends. The simulations adopt the aerosol radiative forcing from the Indian Ocean experiment observations and also account for global increases in greenhouse gases and sulfate aerosols. The simulated decreases in surface solar radiation, changes in surface and atmospheric temperatures over land and sea, and decreases in monsoon rainfall are similar to the observed trends. We also show that greenhouse gases and sulfates, by themselves, do not account for the magnitude or even the sign in many instances, of the observed trends. Thus, our simulations suggest that absorbing aerosols in atmospheric brown clouds may have played a major role in the observed regional climate and hydrological cycle changes and have masked as much as 50% of the surface warming due to the global increase in greenhouse gases. The simulations also raise the possibility that, if current trends in emissions continue, the subcontinent may experience a doubling of the drought frequency in the coming decades.aerosols ͉ black carbon ͉ regional climate change

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Section: Methodsmentioning

confidence: 99%

mentioning

confidence: 99%

Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle

Ramanathan

Chung

Kim

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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1,317

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“…The shipborne measurements during 1997 are based on a hand-held Sun photometer onboard R/V Sagar Kanya [Jayaraman, 1999] cruising between latitudes of 13øN and 13øS during January 1-31, 1997 (interpolated to 630 nm using observations at 497 and 667 nm). The shipborne measurements during 1998 were done using the spectroradiometer [Meywerk and Ramanathan, 1999] The satellite-derived AOD is sensitive to the aerosol phase function; the single scattering albedo, the surface reflectance, and the AVHRR calibration constants used in the retrieval.…”

Section: Introductionmentioning

confidence: 99%

Direct observations of clear‐sky aerosol radiative forcing from space during the Indian Ocean Experiment

Rajeev¹,

Ramanathan²

2001

J. Geophys. Res.

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Abstract. This study presents the regional estimates of the seasonal and diurnal mean broadband (0.3-5.0 •m) clear-sky aerosol radiative forcing at the top of atmosphere (TOA) due to both the natural and the anthropogenic aerosols over the tropical Indian Ocean from 25øN to 25øS. We propose two new methods, the slope method and the differencing method, to obtain clear sky aerosol forcing from solely satellite measurements. The focus of the study is January to March 1997, 1998, and 1999. The TOA clear-sky aerosol forcing was obtained by integrating satellite data for aerosol optical depth (AOD) and the broadband radiation budget. Over 30,000 pixels were collocated to

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“…The diurnally averaged broadband forcing efficiency at the bottom of the aerosol layer for the CalNex case is À58.6 AE 13.8 W/m 2 , whereas for the ARCTAS case it is À48.7 AE 11.5 W/m 2 . Other radiometrically determined estimates of diurnally averaged forcing efficiency show À45.8 AE 13.1 W/m 2 for INTEX-NA, À48 W/m 2 from broadband (400-700 nm) irradiance measured during an Indian ocean experiment by Meywerk and Ramanathan [1999], 38.5 AE 4.0 W/m 2 and 42.2 AE 4.8 W/m 2 from ground-based radiometer measurements of broadband (400-700 nm) irradiance taken during the Indian ocean experiment and another in Asia by Valero [2002, 2003]. Although CalNex represents the highest below-layer diurnally averaged forcing efficiency presented here, its uncertainty falls within the reported values from ARCTAS, INTEX-NA, and the Indian Ocean experiment.…”

Section: Corresponding Diurnal Average Of Forcing Efficiencymentioning

confidence: 99%

“…Aerosol direct radiative forcing depends on aerosol optical thickness and incident irradiance at the top of the layer. Forcing efficiency, the radiative forcing normalized by aerosol optical thickness at 500 nm, was introduced by Meywerk and Ramanathan [1999] to reduce large variability in aerosol loading which affects the comparison of forcing for different cases. Relative forcing efficiency ( f e ) [Redemann et al, 2006]:…”

Section: Relative Forcing Efficiencymentioning

confidence: 99%

Spectral aerosol direct radiative forcing from airborne radiative measurements during CalNex and ARCTAS

et al. 2012

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This study presents the aerosol radiative forcing derived from airborne measurements of shortwave spectral irradiance during the 2010 Research at the Nexus of Air Quality and Climate Change (CalNex). Relative forcing efficiency, the radiative forcing normalized by aerosol optical thickness and incident irradiance, is a means of comparing the aerosol radiative forcing for different conditions. In this study, it is used to put the aerosol radiative effects of an air mass in the Los Angeles basin in context with case studies from three field missions that targeted other regions and aerosol types, including a case study from the Arctic Research of the Composition of the Troposphere from Aircraft and Satellites (ARCTAS). For CalNex, we relied on irradiance measurements onboard the NOAA P‐3 aircraft during a flight on 19 May 2010 over a ground station. CalNex presented a difficulty for determining forcing efficiency since one of the input parameters, optical thickness, was not available from the same aircraft. However, extinction profiles were available from a nearby aircraft. An existing retrieval algorithm was modified to use those measurements as initial estimate for the missing optical thickness. In addition, single scattering albedo and asymmetry parameter (secondary products of the method), were compared with CalNex in situ measurements. The CalNex relative forcing efficiency spectra agreed with earlier studies that found this parameter to be constrained at each wavelength within 20% per unit of aerosol optical thickness at 500 nm regardless of aerosol type and experiment, except for highly absorbing aerosols sampled near Mexico City. The diurnally averaged below‐layer forcing efficiency integrated over the wavelength range of 350–700 nm for CalNex is estimated to be −58.6 ± 13.8 W/m2, whereas for the ARCTAS case it is −48.7 ± 11.5 W/m2.

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Observations of the spectral clear‐sky aerosol forcing over the tropical Indian Ocean

Cited by 75 publications

References 7 publications

Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle

Atmospheric brown clouds: Impacts on South Asian climate and hydrological cycle

Direct observations of clear‐sky aerosol radiative forcing from space during the Indian Ocean Experiment

Spectral aerosol direct radiative forcing from airborne radiative measurements during CalNex and ARCTAS

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