Airborne Spectral Measurements of Ocean Directional Reflectance

Abstract

The Cloud Absorption Radiometer (CAR) was flown aboard the University of Washington Convair 580 (CV-580) research aircraft during the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) field campaign and obtained measurements of bidirectional reflectance distribution function (BRDF) of the ocean in July and August 2001 under different illumination conditions with solar zenith angles ranging from 15° to 46°. The BRDF measurements were accompanied by concurrent measurements of atmospheric aerosol optical thickness and column water vapor above the airplane. The method of spherical harmonics with Cox–Munk wave-slope distribution is used in a new algorithm developed for this study to solve the atmosphere–ocean radiative transfer problem and to remove the effects of the atmosphere from airborne measurements. The algorithm retrieves simultaneously the wind speed and full ocean BRDF (sun’s glitter and water-leaving radiance) from CAR measurements and evaluates total albedo and equivalent albedo for the water-leaving radiance outside the glitter. Results show good overall agreement with other measurements and theoretical simulations, with the anisotropy of the water-leaving radiance clearly seen. However, the water-leaving radiance does not show a strong dependence on solar zenith angle as suggested by some theoretical studies. The spectral albedo was found to vary from 4.1%–5.1% at λ = 0.472 μm to 2.4%–3.5% for λ ≥ 0.682 μm. The equivalent water-leaving albedo ranges from 1.0%–2.4% at λ = 0.472 μm to 0.1%–0.6% for λ = 0.682 μm and 0.1%–0.3% for λ = 0.870 μm. Results of the validation of the Cox–Munk model under the conditions measured show that although the model reproduces the shape of sun’s glitter on average with an accuracy of better than 30%, it underestimates the center of the sun’s glitter reflectance by about 30% for low wind speeds (<2–3 m s−1). In cases of high wind speed, the model with Gram–Charlier expansion seems to provide the best fit.

Sun Glint Correction of High and Low Spatial Resolution Images of Aquatic Scenes: a Review of Methods for Visible and Near-Infrared Wavelengths

“…This difference in response at lower wind speed is reminiscent of Wu's reanalysis of the Cox and Munk data (Equation 18). The study of Gatebe et al [51], using data from airborne instruments for coastal and deep waters in the western Atlantic Ocean, found that the Cox and Munk model fitted their data well for most conditions, but underestimated the maximum glint for wind speeds below 3 m/s.…”

New spectral functions of the near-ground albedo derived from aircraft diffraction spectrometer observations

“…For instance, the NASA airborne data will additionally be presented, which have been obtained below a cloud layer by aircraft flights at an altitude around 350 m at South Africa (20.0-21.7 • S, 13.0-13.7 • E) (Gatebe et al 2003;Genya et al, 2011). These observations were made in eight spectral channels, 340, 381, 472, 682, 870, 1.035, 1.219, and 1.273 nm, with the Cloud Absorption Radiometer (CAR) (King, 1987;King et al, 1990King et al, , 1996Gatebe et al 2003Gatebe et al , 2005.…”

EOS Terra Aerosol and Radiative Flux Validation: An Overview of the Chesapeake Lighthouse and Aircraft Measurements for Satellites (CLAMS) Experiment

“…The observed reflectance patterns show peak values of sun glint near the specular direction. Gatebe et al (2005) compare these observations, under a variety of wind conditions, to traditional model simulations. Spectral and broadband radiative fluxes measured from COVE are shown in Figs.…”

“…The CV-580 also deployed several radiometric instruments to measure spectral and broadband shortwave radiation, including the 14-channel NASA AMES King et al 1986). The AATS-14 provided continuous measurements of aerosol optical depth, while the CAR provided the important lower boundary condition (BRDF) (Gatebe et al 2005), both critical for radiative closure studies and validation of AOD derived from Terra and ER-2 airborne data. Although the CAR measurements are limited to the wind and sea conditions and solar geometries encountered during CLAMS, they will be useful for checking the validity of the long-term measure- (Su et al 2002).…”