Solar radiation budget and radiative forcing due to aerosols and clouds

Abstract

This study integrates global data sets for aerosols, cloud physical properties, and shortwave radiation fluxes with a Monte Carlo Aerosol‐Cloud‐Radiation (MACR) model to estimate both the surface and the top‐of‐atmosphere (TOA) solar radiation budget as well as atmospheric column solar absorption. The study also quantifies the radiative forcing of aerosols and that of clouds. The observational input to MACR includes data from the Multiangle Imaging Spectroradiometer (MISR) for aerosol optical depths, single scattering albedos, and asymmetry factors; satellite retrieved column water vapor amount; the Total Ozone Mapping Spectrometer (TOMS) total ozone amount; and cloud fraction and cloud optical depth from the Cloud and Earth's Radiant Energy System (CERES) cloud data. The present radiation budget estimates account for the diurnal variation in cloud properties. The model was validated against instantaneous, daily and monthly solar fluxes from the ground‐based Baseline Surface Radiation Network (BSRN) network, the Global Energy Balance Archive (GEBA) surface solar flux data, and CERES TOA measurements. The agreement between simulated and observed values are within experimental errors, for all of the cases considered here: instantaneous fluxes and monthly mean fluxes at stations around the world and TOA fluxes and cloud forcing for global annual mean and zonal mean fluxes; in addition the estimated aerosol forcing at TOA also agrees with other observationally derived estimates. Overall, such agreements suggest that global data sets of aerosols and cloud parameters released by recent satellite experiments (MISR, MODIS and CERES) meet the required accuracy to use them as input to simulate the radiative fluxes within instrumental errors. Last, the atmospheric solar absorption derived in this study should be treated as an improved estimate when compared with earlier published studies. The main source of improvement in the present estimate is the use of global distribution of observed parameters for model input such as aerosols and clouds. The agreement between simulated and observed solar fluxes at the surface supports our conclusion that the present estimate is an improvement over previous studies. MACR with the global input data was used to simulate the global and regional solar radiation budget, aerosol radiative forcing and cloud radiative forcing for a 3‐year period from 2000 to 2002. We estimate the planetary albedo for a 3‐year average to be 28.9 ± 1.2% to be compared with CERES estimate of 28.6% and ERBE's estimate of 29.6%. Without clouds (including aerosols) the planetary albedo is only 15.0 ± 0.6%. The global mean TOA shortwave cloud forcing is −47.5 ± 4 W m−2, comparing well with the CERES and ERBE estimates of −46.5 and −48 W m−2, respectively. The clear‐sky atmospheric absorption is 72 ± 3 W m−2, and the surface absorption is 218 ± 4 W m−2. Clouds in all‐sky conditions enhance atmospheric absorption from 72 ± 3 W m−2 to 79 ± 5 W m−2 and decrease surface solar absorption from 218 ± 4 W m−2 to 164 ± 6 W m−2. The present estimate of 79 W m−2 for all‐sky solar absorption is much larger than the Intergovernmental Panel on Climate Change (2001) values of about 67 W m−2. Most of the increased atmospheric solar absorption is due to improved treatment of aerosol absorption (backed by surface based aerosol network and chemical transport models) and water vapor spectroscopic data. The global mean clear‐sky aerosol (both natural and anthropogenic) radiative forcing at the TOA and the surface are −6.0 ± 1 W m−2 and −11.0 ± 2 W m−2, respectively. In the presence of clouds the aerosol radiative forcing is −3.0 ± 1 W m−2 (at TOA) and −7.0 ± 2 W m−2 (at the surface). The study also documents the significant regional variations in the solar radiation budget and radiative forcing of aerosols and clouds.