Huneeus, N, Boucher, O (2007). One-dimensional variational retrieval of aerosol extinction coefficient from synthetic LIDAR and radiometric measurements. JOURNAL OF GEOPHYSICAL RESEARCH-ATMOSPHERES, 112(D14), D14303.
The Cloud and Aerosol Lidar with Orthogonal Polarization (CALIOP) [onboard the Cloud-Aerosol Lidar and Infrared Pathfinder Satellite Observations (CALIPSO) platform] and the Moderate Resolution Imaging Spectroradiometer ( MODIS) instrument ( onboard the AQUA platform) will provide simultaneous measurements as part of the AQUA- train,'' thus offering a unique opportunity to improve our knowledge on aerosol properties and their spatial distribution. Here we investigate to which extent both the vertical distribution of the aerosol extinction coefficient and the aerosol bimodal size distribution can be retrieved from a synergetic use of the vertically-resolved lidar signal and the spectral radiance measurements. To this effect, a variational retrieval scheme based on a simplified radiative transfer model was developed. The extinction-coefficient profile for fine and coarse-mode aerosols was retrieved from synthetic observations of the profile of the attenuated backscatter lidar signal at two wavelengths and radiances at six wavelengths. Our method aims at minimizing a cost function which measures the departure of the solution to the observations. The adjoint method was applied to find the gradient of the cost function with respect to the input parameters. The retrieval scheme was tested under a realistic noise level and different microphysical perturbations. The retrieval of extinction-coefficient profiles, for fine and coarse particles, is successful if there is a predominance of fine particles. If coarse particles dominate over fine ones, the scheme retrieves the profile of the total extinction coefficient with a higher confidence than that of the fine mode. When perturbations on the aerosol microphysical properties are introduced, thus simulating a more challenging case with incomplete information of the aerosol model present in the atmosphere, the scheme shows a very good performance in terms of total extinction- coefficient retrieval but less success for individual modes. It retrieves the modal radii for both modes simultaneously but can not retrieve at the same time the refractive index and true-mode radii for both modes. Results also reveal that there is some prospect for improvement in the quality of the retrieval by either increasing the size of the predefined set of aerosol models or by including other sources of independent information such as Polarization and Directionality of Earth Reflectances ( POLDER)- like measurements.