Satellite Remote Sensing of Evaporation over Sea
449
on it the polarization by the surface reflection decreases due to different mechanisms (Wentz,
1992):
• Surface waves with wavelengths greater than the wavelength of the radiation interacting
with the surface change the local incidence angle due to tilted surface facets. This effects a
mixture of horizontal and vertical polarizations by which the polarization of the reflected
radiation is decreased (Stogryn, 1967).
• Whitecapping on the sea surface increases the reflectivity at visible wavelengths and
decreases the reflectivity at infrared and microwave frequencies (Koepke, 1986; Stogryn,
1972).
• Surface ripples being small compared to the wavelength of the incident radiation cause
diffraction of the radiation. Considering capillary waves as the shortest surface waves this
effect is only relevant for radiation exceeding millimetre wavelengths (Wentz, 1975).
The three effects have been parameterised in terms of the rms slope of the large undulations, a
fractional whitecap coverage and the correlation length of the small-scale ripples which all can in
turn be described, more or less, as function of wind speed or, better, friction velocity (Cox and
Munk, 1954; Monahan and O'Muircheartaigh, 1986; Mitsuyasu and Honda, 1982). The surface
roughness especially modifies the reflectivity at microwave frequencies where the flat surface
exhibits a rather high reflectivity (! as shown in figure 16.7 for 37 GHz where (! is shown for two
temperatures according to the parameterisation of Klein and Swift (1977). The reflectivities
increase monotonically at horizontal polarization; the vertically polarized component shows a
minimum at Brewster's angles between 75° and 80°. By virtue of Kirchhoff's law and in view
of the high opacity of sea water at microwave frequencies the surface emissivity! is related to
the reflectivity by c: = 1 - (!. Several models have been developed describing the change of (! as
function of wind speed or friction velocity. Pandey and Kakar (1982) proposed the form
(!p(u) = [(!sp - Ll(!rp(u)](l - f(u)) + (!fpf(u)
(16.27)
where (!sp is the specular reflectivity of the smooth sea surface at polarization p, Ll(!rp( u) is the
change of the reflectivity due to tilted, not foam covered surface facets, (!fp is the reflectivity
of foam, and f( u) is the fractional foam coverage depending on wind speed u. The wind speed
dependence of Ll(!rp( u) is a linear one while f varies with higher powers of u. Detailed descriptions of (16.27) in terms of temperature, salinity, incidence angle and frequency have been
optimized for the use with radiometers operating at different scan angles and frequencies for the
Scanning Multichannel Microwave Radiometer (SMMR) that was flown on the Nimbus-7 satellite (Francis et al., 1983; Wentz 1983) and for the Special Sensor Microwave/Imager (SSM/I)
being flown as part of the DMSP satellites (Schliissel and Luthardt, 1991; Wentz, 1992; Rosenkranz, 1992). With the knowledge about the wind dependent reflectivity of the sea surface it is
straight forward to calculate the brightness of the sea surface for a given atmospheric/oceanic
situation and subsequently the brightness temperature as measured by a satellite radiometer.
Such calculations are used for the development of remote-sensing methods seeking for the windspeed retrieval from a given set of spectrally distributed brightness tmperatures measured from
satellites. Figure 16.8 shows the simulated brightness temperatures for the SSM/I channels at
varying wind speed for a mid-latitude summmer situation. Especially, at horizontal polarization the brighness temperature can vary by 40 K within the wind speed range a to 30 m/s.
However, the actual atmospheric situation induces further brightness-temperature changes that
mask the relationship with the wind speed as shown for the brightness temperatures at 37 GHz,
horizontal polarization (Figure 16.9). Consequently, multi-channel retrieval techniques have to
linstead of" emissivity" often the term" emittance" is used.
449
on it the polarization by the surface reflection decreases due to different mechanisms (Wentz,
1992):
• Surface waves with wavelengths greater than the wavelength of the radiation interacting
with the surface change the local incidence angle due to tilted surface facets. This effects a
mixture of horizontal and vertical polarizations by which the polarization of the reflected
radiation is decreased (Stogryn, 1967).
• Whitecapping on the sea surface increases the reflectivity at visible wavelengths and
decreases the reflectivity at infrared and microwave frequencies (Koepke, 1986; Stogryn,
1972).
• Surface ripples being small compared to the wavelength of the incident radiation cause
diffraction of the radiation. Considering capillary waves as the shortest surface waves this
effect is only relevant for radiation exceeding millimetre wavelengths (Wentz, 1975).
The three effects have been parameterised in terms of the rms slope of the large undulations, a
fractional whitecap coverage and the correlation length of the small-scale ripples which all can in
turn be described, more or less, as function of wind speed or, better, friction velocity (Cox and
Munk, 1954; Monahan and O'Muircheartaigh, 1986; Mitsuyasu and Honda, 1982). The surface
roughness especially modifies the reflectivity at microwave frequencies where the flat surface
exhibits a rather high reflectivity (! as shown in figure 16.7 for 37 GHz where (! is shown for two
temperatures according to the parameterisation of Klein and Swift (1977). The reflectivities
increase monotonically at horizontal polarization; the vertically polarized component shows a
minimum at Brewster's angles between 75° and 80°. By virtue of Kirchhoff's law and in view
of the high opacity of sea water at microwave frequencies the surface emissivity! is related to
the reflectivity by c: = 1 - (!. Several models have been developed describing the change of (! as
function of wind speed or friction velocity. Pandey and Kakar (1982) proposed the form
(!p(u) = [(!sp - Ll(!rp(u)](l - f(u)) + (!fpf(u)
(16.27)
where (!sp is the specular reflectivity of the smooth sea surface at polarization p, Ll(!rp( u) is the
change of the reflectivity due to tilted, not foam covered surface facets, (!fp is the reflectivity
of foam, and f( u) is the fractional foam coverage depending on wind speed u. The wind speed
dependence of Ll(!rp( u) is a linear one while f varies with higher powers of u. Detailed descriptions of (16.27) in terms of temperature, salinity, incidence angle and frequency have been
optimized for the use with radiometers operating at different scan angles and frequencies for the
Scanning Multichannel Microwave Radiometer (SMMR) that was flown on the Nimbus-7 satellite (Francis et al., 1983; Wentz 1983) and for the Special Sensor Microwave/Imager (SSM/I)
being flown as part of the DMSP satellites (Schliissel and Luthardt, 1991; Wentz, 1992; Rosenkranz, 1992). With the knowledge about the wind dependent reflectivity of the sea surface it is
straight forward to calculate the brightness of the sea surface for a given atmospheric/oceanic
situation and subsequently the brightness temperature as measured by a satellite radiometer.
Such calculations are used for the development of remote-sensing methods seeking for the windspeed retrieval from a given set of spectrally distributed brightness tmperatures measured from
satellites. Figure 16.8 shows the simulated brightness temperatures for the SSM/I channels at
varying wind speed for a mid-latitude summmer situation. Especially, at horizontal polarization the brighness temperature can vary by 40 K within the wind speed range a to 30 m/s.
However, the actual atmospheric situation induces further brightness-temperature changes that
mask the relationship with the wind speed as shown for the brightness temperatures at 37 GHz,
horizontal polarization (Figure 16.9). Consequently, multi-channel retrieval techniques have to
linstead of" emissivity" often the term" emittance" is used.