Investigation of surf-zone plumes with a two-dimensional bistatic lidar and the polarization ratio method

A vertically fanned argon-ion laser beam revealed tall plumes suspended over surf zones during Electro-Optical Propagation Assessment in Coastal Environments (EOPACE) experiments. A simple (albeit versatile) non-iterative estimator using the polarization-ratio method was derived and then applied to images taken by two distant CCD cameras to estimate the aerosol concentrations over the surf zone near the Pier of the Scripps Institution of Oceanography (La Jolla, CA). The use of this estimator bypassed the need for both calibration and a first guess, but required the postulation of an aerosol model such as the Hybrid Coastal Aerosol Model (HCAM). HCAM modified the Navy Oceanic Vertical Aerosol Model (NOVAM—a multimodal aerosol model designed for open-ocean use) for optical use and for an additional suspected mode radius at 15 micrometers. HCAM fulfilled three goals: (1) delivery of a baseline of concentrations expected over the open ocean, (2) estimation of concentrations over the surf zone by applying a best fit of HCAM onto rotorod data, and (3) determination of the reliability of the concentrations estimated by the polarization-ratio method. Once the two-dimensional concentrations estimated via the polarization-ratio method were reduced to a most-likely value, the estimated concentrations (on average) revealed an increase in concentrations by 57% (mode 2) and by a factor of 6 (mode 3), as compared to the baseline concentrations, while falling below those from the HCAM fits by 38% (mode 2) and by a factor of 46 (mode 3). Similarly, extinction estimations from the polarization-ratio method slightly exceeded those from the baseline by 27% (on average). Those results confirm both the success of the polarization-ratio method at statistically estimating concentrations within the self-consistent framework of HCAM and the role of the surf zone as a source of aerosols. The two-dimensional estimated concentrations described inconclusive shapes. This result is attributed to the inhomogeneity of the concentrations over the surf zone, to atmospheric motion, and to errors associated with applying HCAM over the surf zone.