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

[ 1] 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, tau(500)) 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) tau(-1)(500), and the corresponding fractional forcing efficiencies range from about - 14 to - 30% tau(-1)(500). 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) tau(-1)(500) and - 11 to - 23% tau(-1)(500), 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) tau(-1)(500) for INDOEX and - 19 to - 37 W m(-2) tau(-1)(500) 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.