Development of algorithms to retrieve aerosol optical properties from dual wavelength polarization lidar and application to the lidar data measured over the tropical Pacific Ocean

We developed two-type of algorithms, i.e., forward and backward. The algorithms determine aerosol types and retrieve the vertical profiles of the extinction coefficient at a wavelength of 532 nm for each aerosol type, from three channel lidar measurements: co-polarization and cross-polarization components at the wavelength of 532 nm and a co-polarization component at a wavelength of 1064 nm. The algorithms have the following main features: (1) they first determine the aerosol type, e.g., water-soluble, sea-salt, or dust, for each layer. (2) The vertical profiles of the value of extinction-to-backscattering ratio (S) are retrieved, in distinction from the widely used Fernald method. (3) The microphysical and radiative properties of aerosols are estimated. (4) The backward-type algorithm needs the data measured under the clear-sky condition, and it can retrieve calibration constant of the lidar. (5) The forward-type algorithm can estimate the aerosol optical properties under a cloud bottom layer though the calibrated lidar-signals are needed.

We applied the algorithms to cruise data measured with a dual-wavelength polarization lidar of NIES (National Institute for Environmental Studies) installed on the research vessel MIRAI of JAMSTEC (the Japanese Maritime Science and Technology Center). The cruise was performed on the coast of Indonesia and Philippine islands from September 21 to December 17, 2001. We first removed the data contaminated by clouds using the lidar and also cloud radar data. In the cruise, 95GHz cloud profiling radar of NICT (National Institute of Information and Communications Technology) was also installed. Next, we applied the backward-type algorithm to the data measured under clear-sky condition, and then calibrated the lidar data using the aerosol optical properties retrieved. Finally, the forward-type algorithm was applied to the calibrated lidar-data, which were measured under cloud bottom layers. The result shows that few dust aerosols were found in all the observation period. Most of water-soluble and sea-salt aerosols existed in the lower layers below 1 km. The aerosol optical thickness at a wavelength of 532 nm averaged in all the observation period was 0.14. The value of 0.14 is relatively larger than that reported from sunphotometry measurements over the open sea in the Pacific region. This might imply advection of aerosols originated from the Indonesia and Philippine islands.