4.11 Measurement of Sea Wind

4.11 Measurement of Sea Wind

Measurement of the sea wind is not made directly but indirectly from two processes, that is, the microwave scattering from the sea surface and relationship between sea wind and wave height.There are two methods for measurement of sea wind.

a. to estimate the backscattering coefficient using a microwave scatterometer
b. to measure the brightness temperature using a microwave radiometer

The following three models are used to estimate the backscattering coefficient in the case of a microwave scatterometer.

a. specular point model
b. Bragg model
c. composite surface model

Figure 4.11.1 shows the relationship between backscattering cross section and the incident angle which has been obtained from the actual measurement. The specular point model can be applied to the region A in the figure, with the incident angle from 0 to 25, where the sea clutter or sea roughness is much larger than the microwave wavelength. In this case, the backscattering coefficient is proportional to the resultant probability density of x and y components of the gradient.

The Bragg model can be applied to the region B in the figure with the incident angle larger than 25, where Ą decreases very gently except near 90. This is called Bragg scattering which can be seen under the condition when the wavelengths of microwave and the sea wave have a similar spectrum. The ideal condition for Bragg scattering has the range from 25 to 65 with the wavelength of the capillary wave being from 1 cm to a few cm.

However, the actual sea surface is a composite of capillary waves and gravity waves for which the composite surface model has been developed but not yet verified theoretically. The second procedure is to estimate the sea surface condition from the backscattering coefficient . Figure 4.11.2 shows the correlation between variance of wave slope, S, and the wind velocity measured by Cox and Munk, which can be applied for the specular point model when is given as a function of S. In the case of >25, it is found that the sea wind is proportional to the spectral density, but the models are still under development. Figure 4.11.3 shows the distribution of wind velocity and wind direction that was measured by SASS (Seasat-A Satellite Scatterometer) for the typhoon on Oct. 2, 1978. In the case of the microwave radiometer, the sea wind can be computed from the brightness temperature using the fact that the emissivity is a function of complex permittivity with parameters of salinity, sea clutter, sea temperature and bubbles. Two algorithms have been developed by Wentz and Wilheit for Seasat- SMMR for the sea wind velocity.