The Retrieval Of Diffuse Attenuation Coefficient In The Offshore Area Of China

Light is a exponential attenuation along the depth in the sea, due to the effect ofsea water, chlorophyll and suspended particles. Solar radiation reaches the surface,suspended matter in seawater and its absorption, scattering. The attenuation exponentsis the diffuse attenuation coefficient. It is an important parametric in ocean optics.,that determines the growth of marine organism. Meanwhile it is an affecting factorof dynamic oceanography. Diffuse attenuation coefficient id related closely tochlorophyll contents, suspended matter concentration, yellow substances and so on.Based on the optical data collected in the Yellow Sea, the East China Sea and thePearl River Estuary between July2002and February2004, the article the change ruleof downwelling irradiance along the depth and the causes for variation. In situ datawere used to establish new retrieved algorithms to evaluate diffuse attenuationcoefficient at490nm Kd（490） by using the relationship between remote sensingreflectance and Kd（490）. Compared with in situ observation, the retrieval from theempirical algorithm showed the correlation coefficients0.91, the root mean squareerror0.35and the absolute percentage difference28.27%. And comparisons betweenretrieval from the semianalytical method and in situ observation showed0.83,0.48,31.53%respectively. Furthermore, both the empirical algorithm and thesemianalytical method have attained better performance than the existing algorithmsby comparing retrieval with in situ observation. Although the empirical algorithmshowed more precise than the semianalytical method, the semianalytical method ismore independent to the in situ observation than the empirical algorithm during theprocess of algorithm establishment.This paper analyzes the spatiotemporal distribution and seasonal change of diffuse attenuation coefficient, which is derived by using the MODIS-Aqua andMOSIA-Terra data from2002to2011. The results show that Kd（490）has an obviouscharacteristic of seasonal change. The maximum of Kd（490）occur at December andthe minimum at June. The change of diffuse attenuation coefficient keeps coherent withchlorophyll contents and suspended matter concentration, which was affected bysurface runoff, monsoon and ocean current. The spacialtemporal distribution of Kd（490）is obtained by EOF analysis, the first three modes make a contributionaccounting for61.01%，5.10%and2.63%respectively. The first mode assumesabsolute superiority, that shows changing trend of Kd（490）.