Atmospheric radiative forcing at the surface derived from aircraft irradiance and spectral optical depth measurements

In this paper we report atmospheric radiative forcing, radiative forcing efficiency, and fractional forcing efficiency by aerosols determined from airborne measurements performed during the Indian Ocean Experiment (INDOEX) and the Asian Pacific Regional Characterization Experiment (ACE‐Asia). The atmospheric radiative forcing at the surface is calculated in the visible spectral region (400–700 nm) using near‐surface observations from the National Center for Atmospheric Research (NCAR) C‐130 aircraft. A total‐direct‐diffuse radiometer (TDDR) with seven spectral bands spanning 400 to 700 nm is used to determine irradiances at the surface as well as spectral (in all seven channels including a narrowband channel at 500 nm, τ500) and broadband (400–700 nm) aerosol optical depths. For INDOEX, using observations in the Arabian Sea, the visible surface forcing efficiencies range from about −74 to −120 W m−2 τ500−1, and the corresponding fractional forcing efficiencies range from about −14 to −30% τ500−1. These forcing values in the Arabian Sea are higher than those derived in a previous study using measurements at a surface site located at the Kaashidhoo Climate Observatory (KCO). During ACE‐Asia, the aerosol forcing and forcing efficiency at the surface are slightly lower than those during INDOEX and range from about −54 to −110 W m−2 τ500−1 and −11 to −23% τ500−1, respectively. These values are consistent with similar measurements made at the surface site at Gosan, Jeju, Republic of Korea. Diurnal forcing efficiencies are also estimated for the aircraft measurements giving a range of −23 to −54 W m−2 τ500−1 for INDOEX and −19 to −37 W m−2 τ500−1 for ACE‐Asia. Forcing efficiencies are determined using broadband aerosol optical depths and reflect small differences (related to changing aerosol characteristics) relative to those values obtained using narrowband quantities.