Geoscience Reference
In-Depth Information
3.2.
Wind and wind-wave effects on TOA reflectance
The analytical algorithm mentioned in Sec. 2.2 can now be applied to model
specular reflection from a rough liquid surface, such as the ocean, as seen
from a remote sensing satellite. However, in order to evaluate the top of the
atmosphere radiances or reflectances, we need to include atmospheric pro-
cesses such as aerosol scattering and absorption processes by the underlying
atmosphere. In our model atmosphere, we have included Rayleigh and mar-
itime aerosol scattering, each of them being of importance especially in the
optical and near-IR region and ozone gas absorption. The model atmosphere
is divided into fifty atmospheric layers resolving the aerosols, gas molecules
and ozone vertical distribution (from 0 to 120 km altitude with aerosols in
the lower 10 km). Molecular scattering is described by a Rayleigh phase
function. The aerosol scattering phase function is modeled using refractive
indexes provided by Shettle
16
as a function of relative humidity (Maritime
aerosols with 98% rel. hum.). All calculations were done for an atmosphere
at a wavelength of 0
.
551
µ
m and an optical depth of approximately 0
.
14.
For the surface term part, refractive indexes of pure water at a given wave-
length were taken from Ref. 17, a correction was applied to account for
water salinity as in Ref. 18.
Plots in Fig. 2 show the Sun glint pattern simulation using both, Cox
and Munk
1
,
2
formulation and the new MSS (at several wave ages) computed
using the new wind-wave formulation. The solar angle is about 24
◦
and the
calculations were done for wind speeds of 2
.
0 and 8.0 m/s. These values
were chosen as representative examples for low and intermediate winds.
From both panels of Fig. 2, we can qualify the effect of surface wind
speed on the Sun glint pattern. As expected, at low wind speed, a smoother
surface is encountered by the down-welling radiation field, therefore a well
defined glint pattern would dominate. In both cases, the glint pattern is
predominantly driven by the direct solar beam. However, if wind speed
increases, the liquid surface becomes rougher and the developing wave pat-
tern drives the amount of scattered photons away from the specular direc-
tion of the Sun. Hence, at higher wind speed, the sea surface increases
the amount of diffuse radiation as the surface becomes rougher. This effect
that can be easily observed over the ocean when wind conditions are rapidly
changing, the surface roughness will increase and the glint pattern will be
less dominant at higher speeds.
Comparing each individual plot from Fig. 2, we see that the Cox and
Munk
1
,
2
formulation shows stronger glint reflectance as compared with
