Clear‐sky infrared aerosol radiative forcing at the surface and the top of the atmosphere

Abstract

AbstractWe study the aerosol radiative forcing at infrared (IR) wavelengths using data from the Aerosol Characterization Experiment, ACE‐Asia, cruise of the National Oceanic and Atmospheric Administration research vessel Ronald H. Brown. The analyses apply to the daytime periods of clear‐sky conditions for the area within the immediate vicinity of the ship. An optical model is derived from chemical measurements, lidar profiles, and visible‐extinction measurements, which are used to estimate the IR aerosol optical thickness and the single‐scattering albedo. The IR model results are compared to detailed Fourier transform interferometer‐based IR aerosol forcing estimates, pyrgeometer‐based IR downward fluxes, and to observations of the direct aerosol solar forcing. This combined approach attests to the self‐consistency of the optical model, and allows us to derive quantities such as the IR forcing at the top of the atmosphere (TOA) and the IR optical thickness. The mean IR aerosol optical thickness at 10 µm is 0.08 and the single‐scattering albedo is 0.55. The modelled IR aerosol surface forcing reaches 10 W m−2 during the cruise, which is a significant contribution compared to the total direct aerosol forcing. The surface IR aerosol radiative forcing is between 10 and 25% of the short‐wave aerosol forcing. The IR aerosol forcing at the TOA can be up to 19% of the solar aerosol forcing. We show good agreement between TOA aerosol IR forcing derived from the model and from the CERES (Clouds and the Earth's Radiant Energy System) satellite data. Over the Sea of Japan, the average IR aerosol radiative forcing is 4.6 W m−2 at the surface, and 1.5 W m−2 at the TOA. The IR forcing efficiency at the TOA is a strong function of aerosol temperature (which is coupled to vertical structure) and changes between 10 and 18 W m−2 (per IR optical depth unit), while the surface IR forcing efficiency varies between 37 and 55 W m−2 (per IR optical depth unit). Copyright © 2003 Royal Meteorological Society