Date of Award

January 2020

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Earth System Science & Policy

First Advisor

Xiaodong Zhang

Abstract

The slope distribution of the sea surface varies with the speed as well as the direction of the wind. However, the dependence on wind direction is frequently ignored in the studies of the sea surface reflectance. In this study, we investigate the effect of wind directions on the sea surface reflectance (ρs). Zhang et al. 2017 sea surface reflectance model is followed where the sea surface in our study is modeled using the Cox and Munk (1954) anisotropic model. The Cox and Munk model has an inherent uncertainty relating to the distribution of capillary wave facets and wind speeds, which affects the estimate of surface reflectance. This leads to an inherent uncertainty in estimating surface reflectance of 5-20%, depending on the Sun-viewing geometry and wind speeds. For a typical setup of sensors measuring the sea surface reflectance, where sensor viewing angle(θsensor) = 40° and sensor azimuth angle (φsensor) = 45° to 90° relative to the Sun direction, we found the wind direction would either enhance or diminish Sun glint by up to a factor of 10, whereas its effect on skylight glint is less than 5%. The effect on total sea surface reflectance, including both Sun and skylight glints, therefore depends on the relative importance of Sun glint and the exact direction of the wind. In general, the effect of wind directions is less than the inherent uncertainty of the Cox and Munk model and hence can be ignored when Sun zenith angle (θSun) is greater than 40°. When θSun < 40°, the effect varies with the exact Sun-viewing geometry and the wind direction. In particular, when θSun < 20° and wind speed > 7.5 m s-1, the maximum effect of ignoring the wind direction could reach up to 35%.

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