Abstract
Abstract. The ability of a coupled meteorology–chemistry model, i.e., Weather Research and Forecast and Community Multiscale Air Quality (WRF-CMAQ), to reproduce the historical trend in aerosol optical depth (AOD) and clear-sky shortwave radiation (SWR) over the Northern Hemisphere has been evaluated through a comparison of 21-year simulated results with observation-derived records from 1990 to 2010. Six satellite-retrieved AOD products including AVHRR, TOMS, SeaWiFS, MISR, MODIS-Terra and MODIS-Aqua as well as long-term historical records from 11 AERONET sites were used for the comparison of AOD trends. Clear-sky SWR products derived by CERES at both the top of atmosphere (TOA) and surface as well as surface SWR data derived from seven SURFRAD sites were used for the comparison of trends in SWR. The model successfully captured increasing AOD trends along with the corresponding increased TOA SWR (upwelling) and decreased surface SWR (downwelling) in both eastern China and the northern Pacific. The model also captured declining AOD trends along with the corresponding decreased TOA SWR (upwelling) and increased surface SWR (downwelling) in the eastern US, Europe and the northern Atlantic for the period of 2000–2010. However, the model underestimated the AOD over regions with substantial natural dust aerosol contributions, such as the Sahara Desert, Arabian Desert, central Atlantic and northern Indian Ocean. Estimates of the aerosol direct radiative effect (DRE) at TOA are comparable with those derived by measurements. Compared to global climate models (GCMs), the model exhibits better estimates of surface-aerosol direct radiative efficiency (Eτ). However, surface-DRE tends to be underestimated due to the underestimated AOD in land and dust regions. Further investigation of TOA-Eτ estimations as well as the dust module used for estimates of windblown-dust emissions is needed.
Highlights
Solar radiation, the Earth’s primary energy source, plays a crucial role in the climate system
Inconsistencies among satellite retrievals are noticeable in the North Pacific Ocean as well as in the southwestern United States, where higher aerosol optical depth (AOD) is shown in Moderate Resolution Imaging and Spectroradiometer (MODIS)-Terra and MODISAqua but low or no value in other satellite retrievals
Successful estimates of direct radiative effect (DRE) trends depend on accurate estimates of AOD as well as the aerosol direct radiative efficiency (Eτ ), which is defined as the DRE per unit aerosol optical depth and has been used for comparisons between different methods (Yu et al, 2006)
Summary
The Earth’s primary energy source, plays a crucial role in the climate system. The decadal variations in surface shortwave radiation (SWR) starting from the 1950s and related climate impacts have been well documented (Ohmura and Wild, 2002; Mercado et al, 2009; Ohmura, 2009; Wild et al, 2005, 2007; Wild, 2009). The global aerosol effective radiative forcing at the top of atmosphere (TOA) is estimated to have cooling effects from both aerosol–radiation interactions (−0.95 to +0.05 W m−2) and aerosol–cloud interactions (−1.2 to 0.0 W m−2), which are comparable in magnitude to the warming effects by anthropogenic greenhouse gases (2.83 W m−2) (IPCC, 2014). Large discrepancies in aerosol radiative forcing exist amongst the results estimated by different approaches, at a regional scale.
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