We have used two-dimensional correlation on two-dimensional extinction cross-sections measured by a scanning lidar to determine the velocity structure of the salt-spray aerosols. The lidar scans were collected over a reef at Bellows Beach, on the Northeast side of Oahu, Hawaii. The resulting velocity streamlines suggest that lifting of sea-spray aerosols as high as 200 m occurs in the vicinity of opposing horizontal roll vortices. The velocities vary rapidly over distances of less than 500 m and show a complex pattern which is inadequately represented by conventional anemometer measurements.
We performed a calibration experiment on a Spectralon Target using a wavelength of 0.532 μm and a 60 cm lidar system coupled to a photon counter via a Triax 190 monochrometer. The complete system optics, including the telescope and monchromator, were modeled using a commercial ray-tracing program. Energy efficiency was calculated by generating large numbers of equal-area rays on the telescope input pupil from points in the plane of the target. The number of these rays received by the PMT cathode was then used to estimate the optical efficiency. Comparison of the calculated and observed signals gave agreement to within 30%. We discuss the possible sources of disagreement between the calculated and observed signals.
We present the results of our multi-wavelength scanning lidar investigations of time dependent 3-D marine aerosol fields as a function of meteorological parameters at a coastal site in Hawaii. We describe our measurements of salt-aerosol plumes generated at a reef >1.3 km from the lidar and their effect on the aerosol extinction coefficient. At typical trade wind speeds of ~7 m/s, plumes of salt spray have been observed to rise to heights of about 50 m above the reef. A time sequence of vertical scans at three wavelengths (355, 532, 1064 nm) was taken under light (1.9 m/s) wind conditions over the same reef. Large salt plumes more than 600 m high were found to develop under these conditions. The much greater height of light-wind plumes suggests that they are being dispersed less rapidly, allowing them to rise to greater heights because of the presence of thermals. Earlier data collected at Bellows showed reef plumes rising to 120 m in winds of 5 m/s, indicating a consistent trend of increasing plume height with decreasing wind speed.
Due to the complexity of atmospheric aerosol, validation efforts are required to test satellite retrievals. Here we give an overview of our aircraft and ship validation measurements near Hawaii. Some examples of the measurements are shown which illustrate some of the variability we have encountered. This effort is ongoing and can provide important background measurements for satellite validation as well as radiation studies.
Lidars are ideal for mapping the spatial distribution ofaerosol concentrations, however efforts to convertthe lidar measurements into estimates of the aerosol extinction or scattering coefficient are usually complicated. The difficulties arise from the uncertainty in the aerosol backscatter-to-extinction ratio and the lidar calibration. In marine conditions with little absorption, the aerosol backscatter-to-extinction ratio is identical to the aerosol phase function/4? at 1 80 degrees (backscatter) . Uncertainty in the lidar calibration is another source ofuncertainty, which can change with time depending on the state ofthe optics (clean or dirty). Here we investigate several techniques to obtain calibrated aerosol extinction coefficient values. The first approach uses horizontal lidar measurements over the open ocean where the atmosphere is horizontally homogeneous. The lidar calibration or aerosol phase function is adjusted until the derived aerosol extinction coefficients are flat with distance. Modeling shows that this provides correct aerosol extinction values. A second approach uses a target to reflect the lidar beam at different distances. The aerosol extinction is derived from the differential transmission measurements. As an independent measurement, the aerosol phase function and scattering coefficients can be measured with a polar nephelometer.
On the windward side of Oahu, a multi-wavelength Mie-Rayleigh (MR) scanning lidar is used on a regular basis for measuring aerosol attenuation in the marine boundaiy layer. The lidar data are being used for investigathg dynamic effects of marine aerosol fields on electro-optical (EO) properties. The lidar has been operated mostly at 532 and 1064 nm, and recently at 355 nm. We have observed that the vertical aerosol distribution can be very non-uniform. Under certain atmospheric conditions, ascending and descending streaks of aerosols with high extinction (2x 1O-4 per meter) have been observed, indicating that both the surface and cloud drizzle effects are important. Horizontal lidar scans at 6 meters above the sea surface indicate that aerosol is fairly uniform on a large scale but can exhibit significant variability on small scales particularly close to protruding reefs and shorelines. Above the reefthe enhanced aerosol fields have been observed to rise as high as 100 meters. As expected there is a strong correlation between wind speed and sea salt extinction values. The temporal and spatial distribution ofthese aerosol fields and their dependence meteorological parameters and wave height are discussed.
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