A method to obtain the lidar ratio and aerosol size distribution from CALIPSO

Abstract:

The vertical distribution of aerosols critically affects their radiative effect. The CALIOP (Cloud-Aerosol Lidar with Orthogonal Polarization) on board the CALIPSO satellite is a powerful tool in monitoring global vertical distribution of aerosols by measuring the backscattered signal intensity and two backscattered orthogonal polarization components. However, the many aerosol parameters currently retrieved with the Fernald method may have considerable uncertainties due to the lidar ratio assumptions. That said, the current CALIPSO product still needs improvements. In this study, we present an iterative method to obtain the lidar ratio and aerosol size distribution from CALIPSO simultaneously, by approximating the aerosol distribution as a Junge distribution or a Log-normal distribution. With different assumptions of aerosol complex index, the Fernald method is used to calculate the extinction coefficients at 532nm and 1064nm respectively with a starting lidar ratio. Aerosol size distribution can then be estimated by the extinction coefficients at different wavelengths. A new lidar ratio, calculated by the aerosol size distribution with Mie theory, is then used to obtain a set of new extinction coefficients, and so on. Iterate until the lidar ratio reach a stable value. This stable lidar ratio is thus considered closer to truth and the aerosol extinction profile derived using this lidar ratio is more reliable. Our results have potential usage in satellite retrievals of aerosol optical parameters and climate model validation.