Effect of wind direction on sea surface reflectance

The slope distribution of the sea surface varies with the wind speed as well as directions. 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 (ρ_s) where the geometry of the sensor setup is θ_sensor = 40˚ and φ_sensor = 45˚ counterclockwise to the sun direction. For sun at zenith, the sun direction is assumed at 45˚ clockwise from the sensor direction in a right-hand coordinate system. The results for separated sun glint and sky glint show ignoring the wind direction mostly affects the sun glint (more than 35%). The impact is less than 5% for ρ_sky and for lower wind speed, a lower impact for ρ_sky and higher impact for ρ_sun is projected_. The developed model shows that when the sun is at the zenith, the wind blowing at the sensor direction have the highest impact and maximum difference between isotropic and anisotropic models are projected. For the sun away from zenith, the wind direction somewhere between the sun and the sensor shows the maximum difference between the two models, i.e. with solar zenith angle 30˚, the wind blowing at 25˚ has the maximum impact. The impact of not considering the wind direction for a highly roughed sea surface (U=15m/s) is increased by increasing the solar zenith angle to the point that the separated sun light reflectance (ρ_sun) is higher than the separated sky light reflectance (ρ_sky). For lower wind speed (U=7m/s) or wherever the ρ_sky is higher than ρ_sun, the impact of ignoring the wind direction is decreased by increasing the solar zenith angle.