William P. Hooper

Scanning lidar measurements of surf-zone aerosol generation

William P. HooperNaval Research LaboratoryRemote Sensing DivisionCode 72214555 Overlook AvenueWashington, DC 20375-5000E-mail: hooper@ccf.nrl.navy.milLee U. MartinNaval Research LaboratoryRadar Division4555 Overlook AvenueWashington, DC 20375-5000Abstract. A scanning lidar was used at Wallops Island, Virginia, to lo-cate regions where aerosols are generated by breaking waves and tocharacterize the resultant aerosol plumes. False-color lidar images showviews of both the horizontal and the vertical extent of the plumes abovethe surf zone. A 30-min-average location of aerosol generation is shownas a contour overlay on the lidar images. The strength of the backscatteris separated into three regions: the land side of the surf zone, the surfzone, and the ocean side of the surf zone.

Lidar observations of turbulent vortex shedding by an isolated topographic feature

Scanning measurements by a single wavelength lidar (1.06 Μm) were made downwind of the Pt. Sur rock, an isolated hill (height 110 m) along the California coast. Turbulent eddies (approximate diameter 50 m) were observed detaching from a stationary aerosol feature above the rock and moving downwind. Under conditions with a high Froude number [1.8], the Strouhal number [0.22] of vortex shedding was close to that observed in tank experiments with a Reynolds number of 200.

Lidar Observations of Ship Spray Plumes

As part of the Monterey Area Ship Track experiment, which was designed to study ship-generated cloud tracks, ship-based measurements were made by a gyroscopically stabilized scanning lidar system. This paper focuses on the spray plume observed by lidar behind the USS Truxton, a nuclear-powered surface ship. Measurements are included from five passes at different speeds. Observed parameters include the speed of the plume meander, maximum speed of vertical mixing, and dispersion time.

Down looking lidar inversion constrained by ocean reflection and forward scatter of laser light

The laser aureole at 1.06 microm resulting from the redirection of light by sea surface reflection and forward scatter through maritime boundary layer aerosols to a sensor high above the ocean surface is modeled for profiles with typical North Atlantic aerosol size distributions. The magnitude of this laser aureole is highly correlated with the optical depth for these profiles. This optical depth, estimated from the laser aureole, is used to adjust the power of the extinction-backscatter relationship in a Bernoulli-Riccati lidar inversion. Using a lognormal marine aerosol model, 150 profiles of aerosol size distributions are selected by their probability of occurrence in the North Atlantic boundary layer. For these profiles, the lidar inversion using the estimated optical depth predicted the surface extinction 5 times better than the lidar inversions using a climatological backscatter-extinction relationship.

Using backward Raman scattering from coupled deuterium cells for wavelength shifting

Measurements and simulations of a dual gas-cell system, which uses Raman scattering to convert infrared laser radiation from 1064 to 1560 nm, are presented. The cells contain deuterium at a pres- sure of 25 atm. A beamsplitter is used to distribute the energy from the YAG laser between the seed and pump cells. The laser light is focused into the seed cell, which generates a lower-power, backward- propagating, pulse. This seed pulse and a laser pulse counterpropagate in the pump cell to generate a stronger 1560-nm pulse. The goal is to create a single pulse without any shorter pulses or oscillations. Simula- tion results are presented, which indicate that a beamsplitter directing 25% of the laser energy into the seed cell and the rest into the pump cell is optimal for 1560-nm generation. Laboratory testing of a dual-cell sys- tem using a 32% beamsplitter shows the generation of a 1560-nm pulse with 250 mJ/pulse with an efficiency of 35%.

Lidar detected spike returns

Lidar measurements of spike returns from clear air are presented. These spikes occur infrequently (approximately one in hundred returns) but provide returns that are significantly stronger (occasionally an order of magnitude larger) than the average aerosol backscatter signal. The spike density is 5.7e-3 spikes m -3 for backscattering cross sections estimates to be between 0.003 and 0.080 mm 2 sr -1. A modified form of the lidar equation which includes returns from large particulates is presented and the probability distribution for the spike magnitudes is derived from five million measurements.

Retrieval of near-surface temperature profiles from passive and active optical measurements

Two measurement techniques are used to measure the range and apparent elevation of surface targets. In one set of experiments, a survey station measured the apparent elevation angle, and Global Positioning System data were used to determine the range. In the other experiments, a lidar measured both elevation and range. Two different analysis techniques (one analytical and one involving an optimal estimation model) are used to characterize the shape of the temperature profiles. The six case studies discussed are equally split between stable and unstable cases. In four cases, data from a survey station are analyzed and include measurements of boats, buoys, and a tower. In two cases, lidar measurements of the sea surface were made. Elevation and range data are derived from a nonlinear model of the surface return. For the analytical model, the errors between the derived and the measured range are minimized; typically the range errors are less than 100 m. For the optimal estimation technique, the errors in derived and measured heights are minimized; the height errors are typically less than 1 m.

Simulation of Laser-Induced Light Emissions from Water and Extraction of Raman Signal

Abstract A simulation of the laser-induced Raman and fluorescence spectra produced by laser irradiation of the water column is described as well as a method of extracting the Raman signal from the fluorescence spectra using multiple laser excitation wavelengths. The simulation includes Gelbstoff fluorescence at 440 and 580 nm, chlorophyll-a fluorescence at 685 nm, Raman signal, and noise sources. Water temperature, relative intensities of the fluorescence signals, sky noise, water clarity, and depth of the return can be varied as well as system optical parameters such as collector size or sea surface range to test system design specifications. A simple method to extract the Raman signal from the fluorescent background signal is developed that uses five laser excitation wavelengths within the range of 510–570 nm. An average fluorescence signal is calculated and subtracted from the raw signal at one of the laser excitation wavelengths to obtain the Raman signal. A two-component Gaussian fit is made to the e...

Aureole lidar: instrument design, data analysis, and comparison with aircraft spectrometer measurements

A lidar system is developed to map extinction under the flight path of a P-3 aircraft. With a modified Cassegrainian telescope, signals from both wide and narrow fields of view are detected. The wide field-of-view detector senses the aureole signal generated by sea surface reflection and aerosol forward scattering. The narrow field-of-view detector senses the backscattering profile and the direct reflection off the sea surface. Optical depth and extinction profiles are derived from these signals. In comparisons made beween in situ aerosol-size spectrometer and lidar measurements, lidar profiles are smaller in magnitude but similar in shape to the spectrometer profiles.

Monte Carlo simulations of laser-generated sea surface aureole.

Monte Carlo simulations are used to study the signals generated by the scatter from aerosol in the marine boundary layer and reflection off a rough sea surface when a laser pulse at 1.06microm passes down through the atmosphere. The model estimates the probability of photons being returned to a receiver collocated with the laser which has two detectors: one with a narrow field of view (lidar detector) and another with a wide field of view where the directly reflected photons are blocked (aureole detector). The simulations are done for nine different aerosol size distributions, three different boundary layer depths, and three different wave conditions. A comparison of the boundary layer optical depth and normalized aureole signal is presented. In addition, a comparison is made between the normalized aureole signal at 1.06 microm (when the detector field of view is reduced) and boundary layer optical depths at 3.75 microm.