A study of the aerosol direct effect using ESSP/CALIPSO observation and GCM simulation

Anthropogenic and natural aerosols affect earth's radiation budget directly and indirectly. There are large uncertainties in the aerosol radiative forcing calculated by various global aerosol models to estimate the effect of the climate by aerosols [Schulz et al., 2006]. One of the uncertainties in the evaluated radiative forcing for the aerosol direct effect is the effect of the vertical stratifications of aerosol and cloud. In April 2006, however, the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) satellite was launched with the space-borne lidar CALIOP (the Cloud-Aerosol Lidar with Orthogonal Polarization) as one of the NASA Earth System Science Pathfinder (ESSP) program. CALIOP, for the first time, provides us with global data of aerosols and cloud vertical profiles. CALIOP can detect aerosols above low-level optically thick clouds. These aerosols are not able to be detected from ground-based and ship-born lidar measurements. The optical thickness of aerosol layers overlying optically thick clouds are quantified using retrieval methods [Hu et al., 2007, Chand et al., 2008]. Chand et al., (2009) evaluated aerosol direct effect over the Atlantic Ocean off southwest Africa using these methods, and indicate aerosol direct effect largely depends on underlying cloud. We compare aerosol and cloud vertical distributions between the CALIOP observation and the global aerosol model, Spectral Radiation-Transport Model for Aerosol Species (SPRINTARS) [Takemura et al., 2000, 2002, 2005], and direct radiative forcings of total (natural+anthropogenic) aerosols calculated in their aerosol and cloud distributions in 2007. In this study, shortwave aerosol direct radiative forcing is evaluated using the CALIOP observations and simulated results by SPRINTARS at the top of atmosphere (TOA) for clear-sky, cloudy-sky, and all-sky. The radiative transfer code RSTAR6.2b [Nakajima and Tanaka, 1986, 1988] was used for computation of radiative field.