Aerosol and Climate | Jianglong Zhang's Research Group

Shortwave Aerosol Radiative Forcing over Cloud-Free Oceans from Terra: 2. Seasonal and Global Distributions

Using 10 months of collocated Clouds and the Earth's Radiant Energy System (CERES) scanner and Moderate Resolution Imaging Spectroradiometer (MODIS) aerosol and cloud data from Terra, we provide estimates of the shortwave aerosol direct radiative forcing (SWARF) and its uncertainties over the cloud-free global oceans. Newly developed aerosol angular distribution models (ADMs) (Zhang et al., 2005), specifically for different sea surface conditions and aerosol types, are used for inverting the CERES observed radiances to shortwave fluxes while accounting for the effect of aerosol optical properties on the anisotropy of the top of atmosphere (TOA) shortwave radiation fields. The spatial and seasonal distributions of SWARF are presented and the MODIS retrieved aerosol optical depth (τ) and the independently derived SWARF show a high degree of correlation and can be estimated using the equation SWARF = 0.05 - 74.6τ + 18.2τ^{2 Wm^-2} (τ < 0.8). The instantaneous TOA SWARF from Terra overpass time is -6.4 ± 2.6 Wm² for cloud-free oceans. Accounting for sample biases and diurnal averaging, we estimate the SWARF over cloud-free oceans to be -5.3 ± 1.7 Wm², consistent with previous studies. Our study is an independent measurement-based assessment of cloud-free aerosol radiative forcing that could be used as a validation tool for numerical modeling studies.

Longwave Radiation Forcing of Saharan Dust Aerosols Estimated from MODIS, MISR, and CERES Observations on Terra

Using observations from the Multi-angle Imaging Spectroradiometer (MISR), the Moderate Resolution Imaging Spectroradiometer (MODIS), and the Clouds and the Earth's Radiant Energy System (CERES) instruments onboard the Terra satellite, we present a new technique for studying longwave (LW) radiative forcing of dust aerosols over the Saharan desert for cloud-free conditions. The monthly mean LW forcing for September 2000 is 7 Wm^-2 and the LW forcing efficienty (LWeff) is 15 Wm^-2 per unit aerosol optical depth. Using radiative transfer calculations, we show that simultaneous measurements of the vertical distribution of aerosols, surface temperature, and water vapor are critical to the understanding of dust LW aerosol forcing and must come from other sources. Using well-calibrated, spatially and temporally collocated data sets, we have combined the strengths of three sensors from the same satellite to quantify the LW forcing and show that dust aerosols have a "warming" effect over the Saharan desert that will counteract the shortwave "cooling" effect of other aerosols.