Direct aerosol radiative forcing based on combined A-Train observations and comparisons to IPCC-2007 results

We describe a technique for combining CALIOP aerosol backscatter, MODIS spectral AOD (aerosol optical depth), and OMI AAOD (absorption aerosol optical depth) measurements for the purpose of estimating full spectral sets of aerosol radiative properties, and ultimately for calculating the 3-D distribution of direct aerosol radiative forcing. As preparatory work, we carried out sensitivity studies, tested our methodology using suborbital observations, and investigated the consistency between various combinations of satellite input data sets. These studies were carried out largely to investigate the propagation of uncertainties in the multi-sensor input data into the aerosol radiative property estimates. We present results using one year of collocated CALIOP V3, MODIS and OMI data collected in 2007 and show comparisons of the aerosol radiative property estimates to collocated AERONET retrievals. An apparent bias in the input aerosol absorption optical depth and a related bias in the input aerosol single scattering albedo (SSA) are removed after application of the multi-sensor aerosol retrieval. We surmise that the removal of the SSA bias from the input data is a consequence of requiring the multi-sensor retrievals to be consistent with all input data and that the multi-sensor retrievals may be better constrained than retrievals from individual A-Train sensors. Initial calculations of seasonal clear-sky aerosol radiative forcing based on our multi-sensor aerosol retrievals compare well with over-ocean and top of the atmosphere IPCC-2007 model-based results, and with more recent assessments in the "Climate Change Science Program Report: Atmospheric Aerosol Properties and Climate Impacts" (hereafter referred to as CCSP-2009). This is noteworthy, since the observationally based estimates in CCSP-2009 did not compare well with the model-based estimates. Our multi-sensor based forcing calculations compare well with observationally based CCSP-2009 estimates at the surface, indicating that differences in column aerosol absorption properties between our assessment and the CCSP-2009 observationally based estimates need to be investigated. We emphasize that our forcing estimates are purely observational, without use of aerosol transport model assumptions. As such, our results should be well suited for comparisons to initiatives such as AeroCom (Aerosol Comparisons between Observations and Models), that ultimately aim to understand the differences in the 3-D distribution of aerosol radiative properties and effects in global aerosol models.