The MODIS/Aqua and MODIS/Terra instruments are sensitive to the polarization of the incoming radiance. This polarization sensitivity will affect the accuracy of the retrieved ocean products if not properly accounted for. The polarization of the upwelling spectral radiance distribution in the ocean has only been measured in a few occasions, and we have just begun a study of this. Our early measurements show that the upwelling spectral radiance distribution can be highly polarized, up to 55% in certain directions in clear water. Since both the polarization sensitivity of the instruments and the polarization of the water varies spectrally, this may affect the ocean color products even if they are determined by a spectral ratio of water leaving radiance bands. Thus it is important to understand how this polarization varies in the ocean.
With our current funding, we have built an instrument to measure the upwelling spectral linear polarization in the ocean. This proposal will continue our efforts in making this measurement in varied oceanic and coastal environments. We will combine this experimental work with numerical modeling of the upwelling polarized radiance distribution. Once we have a validated model of the polarized radiance distribution we will work on techniques to modify the current ocean color algorithms to correct for the errors due to polarization.
We will also continue our efforts at understanding the effect of the upwelling radiance distribution (often called the BRDF or f/Q correction) on the ocean color measurement. We have been working on validating the most common model of this effect for open ocean and Case I waters. We have some data currently of this effect in Case II, but we will be working on techniques to make this correction in coastal waters. Currently the application of the f/Q correction to the MODIS/Aqua ocean color products has helped reduce seasonal differences between MODIS and SeaWIFS. However, the correction has not been as successful in coastal waters. We need a new model for these areas, and this new model will be based on our radiance distribution measurements.
