Numerical Study Of The Light Propagation Characteristics In The High Turbid Chinese Sea

The Yellow Sea is a semi-enclosed shallow sea on the continental shelf of the West Pacific. Large quantities of terrigenous substances are deposited into the sea by several rivers, such as Huai, Han, and Yalv rivers etc., and accordingly it has come to be seen as a sea area containing the most suspended materials in the world. Under the dynamic process of circulation system, the distribution of the suspended particulate matters has distinct regional and seasonal characteristic. As a result, optical properties of the Yellow Sea are relatively complex and the majority of it belongs to typically Case II waters.The optical properties, including planar PAR (Photosynthetically Available Radiation,400-700nm), scalar PAR, turbidity and concentrations of chlorophyll a ([Chl-a]), were measured across the Yellow Sea during a cruise of the China SOLAS from 19 to 27 March 2005. Based on these data, this paper first analyses the vertical attenuation characteristic of PAR and the factors that affect light attenuation in the water. Then an aquatic optical radiative transfer model (AOMC) is used to simulate the light attenuation characteristic in different sea areas of the Yellow Sea. Relationship between the main light attenuation factors and the apparent optical properties is studied. Main conclusions are as follows:(1) Bio-optical properties of the Yellow Sea:①The distribution of turbidity and the concentration of Chlorophyll a [Chl-a] has distinct regional difference in the sea area. For turbidity, the high value area appeared at inshore stations near Cheng Shantou which located in the northern Yellow Sea and the low value area are those offshore stations located in the middle and southern Yellow Sea. [Chl-a] is lower over the study sea areas and distributes uniformly along the water column. [Chl-a] is higher in the south Yellow Sea than in the north Yellow Sea and so is it in the near shore water than the offshore water.②Turbidity reflects the content of total suspended substances and it has no obvious relationship with [Chl-a] which indicate that non-algal suspended particles contribute most to the turbidity.③Above the euphotic depth, PAR decreases exponentially with both depth and integrated turbidity with high correlation coefficients (R2≥0.98) at most stations. The diffuse attenuation coefficient of PAR, K_d (PAR) is calculated by two kinds of regressive methods, denoted as KT(PAR) and KO(PAR). There is good agreement between KT(PAR) and KO(PAR) at all stations with the relative error△lower than 10% except offshore station Gp station which has a relative high△=12.3%. The result shows that the exponential law is more applicable to inshore shallow sea waters.④Two K_d (PAR) layers are found at the offshore stations, one layer has a maximum value appeared in the surface layer and the second layer, ranging from 20 to 33m, K_d (PAR) approaches an asymptotic.⑤The main factors causing light attenuation are different between stations which show some regional characteristic. At most stations K_d (PAR) has no coherent relationship with [Chl-a] but increase with turbidity which implied that not the phytoplankton but the non-algal particles are the primary light attenuation factors. But at some nearshore stations there is no obvious relationship between K_d (PAR) and turbidity and [Chl-a], and accordingly the controlling factors are complex in inshore waters.(2) It is difficult to study the light attenuation characteristic over the whole sea area with full observed optics data during different season. But the problem can be largely solved by numerical models without limit of time and place. AOMC model developed on the basis of Monte Carlo method. After the necessary validation of exact degree, it is used to study the light attenuation characteristic of the Yellow Sea. The results are showed below:①at most stations, the depth averaged PAR attenuation coefficient computed by AOMC K_dM(PAR) has a high agreement with that calculated by the observed data. The relative errors between K_dM(PAR) and K_dO(PAR) are within the range of 20%.②The vertical distribution of K_dM(PAR) with depth has come to be the layer structure which is similar to the shape of K_dO(PAR). In the near surface layer (1-4m), the modeled values have large differences with the measured value. From 4m down to the euphotic depth the differences between modeled value and measured value are relative small. PAR is influenced by ship shadow and surface waves when measured above 4m which causes the calculation error of K_dO(PAR) and results in the large difference. The environmental factors have little influence on K_d (PAR) below 4m, so the modeled value approaches to the measured value.③The inversion value of PAR derived by an exponential model has small difference with the measured PAR and the lager relative error is less than 45% which indicates that the K_dM(PAR) used in the exponential model has a high degree of exact.④When light propagates from the atmosphere to the ocean which contained a high quantity of suspended particles, the spectrum property of the downwelling irradiance is changed. The spectrum peak value shifts rightward from the blue region to the red region.⑤Sensitive study shows that the non-algal particles are the main factor influencing light propagation in the Yellow Sea and phytoplankton is the secondary factor. Light attenuation by the particles accounts for about 53.3% of average K_d(PAR) and phytoplankton accounts for 27.4%.⑥The non-algal particles have great effect on the spectrum less than 600nm. Compared with other factors, changes in the composition and concentrations of the non-algal particles can more easily alter the light attenuation characteristics, and accordingly have a profound effect on the marine ecosystem.⑦Light attenuation characteristic of the Yellow Sea has distinct regional differences. At the same depth the irradiance was lower in the north Yellow Sea than in the south Yellow Sea, lower in the inshore water than in the offshore water.⑧At present, the optical model used in the marine ecosystem model expresses K_d(PAR) as the sum of light attenuation by water Kw(PAR) and phytoplankton Kph(PAR) which is no longer applicable to the Yellow Sea. On the basis of the AOMC model simulation results and the observed data in the Yellow sea, an simple relationship between turbidity and K_d(PAR) has been derived which can serve as a referenced parameterized optical model for the marine ecosystem model.