Study of attenuation depths for MODIS bands in the Bohai Sea in China

Acta Oceanologica Sinica 2009, Vol.28, No.5, p.39-48 http://www.hyxb.org.cn E-mail: hyxbe@263.net Study of attenuation depths for MODIS bands in the Bohai Sea in China LIU Ying 1,2, LI Guosheng 1 1 Institute of Geographic Sciences and Natural Resources Research, Chinese Academy of Science, Beijing 100101, China 2 Xinjiang Institute of Ecology and Geogrphy, Chinese Academy of Science, Urumqi 830011, China Received 13 October 2009; accepted 29 April 2009 Abstract An attenuation depth is defined for remote sensing purposes as a depth above which 90% of the arising light leaving the water surface is originated. The deeper the attenuation depth, the more information of water is detectable by remote sensing, then the more precise information of water is extracted. Meanwhile, the attenuation depth is helpful to know water layer (by its thickness) from which remote sensing will be able to extract information. A number of investigators are using the moderate resolution imaging spectroradiometer (or MODIS) for remote sensing of ocean color. It is necessary to have a rough idea of the effective attenuation depth of imagery in each of the spectral bands employed by the MODIS. The attenuation depth is directly determined from MODIS data. Though analyzing the spectral distribution of the attenuation depth on 7 August 2003 and the seasonal variation of the attenuation depth (551 nm) in the Bohai Sea indicated that: the spectral distribution of the attenuation depth for the spectral range between 400 to 700 nm is single-peak curve, and it s similar and difference in different regions is consistent with other scholars results of zoning, moreover, it supports the Bohai Sea is Case 2 water; the maximum attenuation depth shifts toward longer wavelengths, liking the red shift, with increase of turbidity of water, just like the maximum attenuation depth in the outside of the northwest coast of the Bohai Sea and the Bohai Strait is at 531nm, in the central of the Bohai Sea is at 551nm, in the region controlled by the Huanghe (Yellow) River, the region impacted by the old Huanghe River, the western side of the Liaodong Bay and the eastern side of the Liaodong Bay is at 555 nm; the seasonal change of the attenuation depth is the largest in the summer, followed by the fall, and the ranking of winter and spring in different regions is distinct. The attenuation depth in different regions is dissimilar: the order of the attenuation depth in different regions from small to big is the region controlled by the Huanghe River or the eastern side of the Liaodong Bay, the western side of the Liaodong Bay, the region impacted by the old Huanghe River, the central of the Bohai Sea or the outside of the northwest coast of the Bohai Sea, the Bohai Strait (except at 412 nm and 645 nm), in which between the region controlled by the Huanghe River and the eastern side of the Liaodong Bay (and between the central of the Bohai Sea and the outside of the northwest coast of the Bohai Sea) it varies in different seasons and different bands. Key words: attenuation depth, MODIS, Bohai Sea 1 Introduction In visible band, the important difference between ocean remote sensing and terra remote sensing is that the former can detect water body information over certain depth under surface (Liang and Chen, 1989). Spectral information in different wavelengths in fact contains information of different thickness of water bodies (Mei et al., 2001). The attenuation depth of different bands in assorted water bodies and even in the same water body varies, so the information of the water body contained in remote sensing data is also distinct (Liang and Chen, 1989). For example, in the same band, in clear waters, its attenuation depth has high value, so it can detect the material composition of the bottom, and measure water depth, but in turbid waters, because its attenuation depth has low value, it is not suitable for detecting them, so it is generally mostly used to detect the concentration of suspended sediment or chlorophyll; in the same waters, in different bands, their attenuation depth is diverse, they representing water bodies information is dissimilar. The attenuation depth shows that information detected by remote sensing comes from how deep water layer be- Foundation item: The National Natural Science Foundation of China with grant numbers 40771030 and 40571020. Corresponding author, E-mail: ligs@igsnrr.ac.cn 1

40 LIU Ying et al. Acta Oceanologica Sinica 2009, Vol. 28, No. 5, P.39-48 low the surface is. The greater the attenuation depth, the more the water information was contained in remote sensing, thus the higher the accuracy of detecting the water information. The attenuation depth can be used for estimating the mean of the associated property (Gordon and Brown, 1975; Gordon and Clark, 1980; Gordon and McCluney, 1975; Gordon, 1978; Sathyendranath and Platt, 1989), selecting band to calculate the concentration of suspended solids and guiding field measurement (in the non-uniform water body, measurements extend at least to the attenuation depth) (Famer et al., 1993; Gordon, 1978) and so on. The attenuation depth is also known as penetration depth or optical depth or information layer (Arst, 2003), and is defined for remote sensing purpose as the depth above which 90% of the diffusely reflected irradiance (excluding specular reflectance) emanates (Gordon and McCluney, 1975). The definition of attenuation depth is beneficial to estimate it and it really provides the bulk of the water body information. It is different from the euphotic depth which is the upper layer of oceans, lakes or rivers with enough light penetration for effective photosynthesis and is usually the depth where downwelling irradiance is equal to 1% from its value at the surface (Arst et al., 1996). Moreover, it is smaller than euphotic zone depth, and represents basically the largest depth the sensor can detect. It is controlled to some extent by the absorption coefficient of the medium (Gordon and McCluney, 1975) and is a function of wavelength for various water types (Jerlov, 1968). The previous study indicated that the maximum attenuation penetration is about 55 m near 475 nm in the clear water of the Sargasso Sea (Jerlov, 1968) and it expected for MSS on ERTS-l is found to be somewhat less than 20 m (Gordon and McCluney, 1975). In the past, the attenuation depth was commonly expressed by Secchi depth. At the beginning of the 19th century, Secchi Disk was lowered into the water until it disappears from view in order to obtain approximate information as to the attenuation depth of sunlight into the water, but this measurement is crude at best because it is not clearly understood how Secchi depth is associated with the attenuation of electromagnetic radiation at different depths (Islam et al., 2004). In the 1970s, Gordon et al. derived theoretical formula of the attenuation depth and applied it (Gordon and McCluney, 1975). In addition, in the 1980s, Liang defined the remote sensing perspective depth, derived its calculation formula and analyzed its relationship with the attenuation depth (Liang and Chen, 1989). The formula Gordon et al. derived for calculating the attenuation depth has been widely used (Arst et al., 1996; Gordon and Boynton, 1998; Gordon and Brown, 1975; Gordon and Clark, 1980; Gordon and McCluney, 1975; Gordon, 1978; Mishra et al., 2005; Sathyendranath and Platt, 1989), and its effectiveness has confirmed whether it s applied to uniform or nonuniform water body. However, it is determined only through direct measurements made in the water with an irradiance meter (Gordon and McCluney, 1975). The Bohai Sea is mostly the Case 2 water and its optical properties are very complicated (Zhou et al., 2005). It is relatively shallow with an average depth of 18.2 m and has sufficient light, so there are higher concentration of chlorophyll pigments and rich in a variety of algae and plankton micro-organisms. While it has vast beach and many inflow rivers, such as the Huanghe River, the Haihe River and the Liaohe River, in which the former two rivers have a large number of suspended sediments and suspended solids of landbased sources, so there are higher concentrations of suspended sediments (Yue et al., 1999; Zhou et al., 2005). Therefore, it is very significant that attenuation depths of MODIS wavelengths were studied in complex optical properties of the Bohai Sea. 2 Methods According to the results of Gordon and McCluney (1975), the attenuation depth can be easily obtained using the QSS model. The water surface leaving radiance over the depth z is given by L Z (µ ) = 4H 0 T (µ, µ ) n(n + 1) 2 1 + µ P ( µ) ω 0 1 ω 0 F {1 exp[ zc(1 ω 0 F )(1 + µ)/µ]}, (1) where µ = cosθ, µ = cosθ, θ =the sensor observation angle (with the zenith), and θ= the underwater incidence angle corresponding to the θ ; n=the refractive index of water; T (µ, µ )=Fresnel transmittance from an angle cos 1 (µ) to cos 1 (µ ); P ( µ)= phase function for scattering through an angle cos 1 (µ) from the incident beam; ω 0 = the ratio of the scattering coefficient b to the beam attenuation coefficient c; F = the fraction of b scattered in the forward direction; H 0 = collimated irradiance from the zenith; µ 2 = 1 n 2 (1 µ 2 )(according to Snell s law).

LIU Ying et al. Acta Oceanologica Sinica 2009, Vol. 28, No.5, P.39-48 41 The total radiance leaving water surface is L Z (µ ) = 4H 0 n(n + 1) 2 T (µ, µ ) 1 + µ P ( µ) ω 0 1 ω 0 F According to the definition of the effective attenuation depth [z 90 (µ )] that the depth above which 90% of the total radiance originates, so or But [L Z90 (µ )]/[L Z (µ )] = 0.9 = 1 exp[ Z 90 (µ )c(1 ω 0 F )(1 + µ)/µ] z 90 (µ )c(1 ω 0 F ) = 2.3µ/(1 + µ). c(1 ω 0 F ) = K(0, ), where K(0, ) is the attenuation coefficient of downwelling irradiance just beneath the surface, so z 90 (µ )(K(0, )) = 2.3µ/(1 + µ). (2) Equation (2) shows that z 90 (µ )(K(0, )) is almost independent of µ, and in fact z 90 (µ )(K(0, )) = 1. (3) Due to z 90 (µ ) is almost independent of µ, the attenuation depth can be defined alternately as R z90 /R = 0.9, where R z is the diffuse reflectance of the ocean due to a surface layer of thickness z and for an axisymmetric incident radiance distribution is given by thus R z = 2π 1 0 L z (µ )µ dµ /H 0, z 90 (K(0, )) 1. (3 ) If K(λ)(m 1 ) is the diffuse attenuation coefficient of wavelength λ of the downward irradiance, then z 90 (λ) = 1/K(λ). (4) For non-uniform water, the diffuse attenuation coefficient is the average over the attenuation depth which is also written as K d (λ). In addition, according to Beer-Lambert law (Jerlov, 1968), irradiance is exponential attenuation with depth increasing, I z = I 0 e Kz where I z =irradiance at water depth z, I 0 =the sea surface irradiance. Generally, the attenuation depth is the depth at which the downwelling in-water irradiance falls to 1/e of its value at the surface. For getting the attenuation depth, it is necessary to firstly obtain the diffuse attenuation coefficient according to Eq.(3 ). The diffuse attenuation coefficient for downwelling irradiance (K d ) is an important property for ocean studies (Lee et al., 2005b). It can be used to classify water types (Jerlov, 1968), and is a critical parameter for the accurate estimation of the light intensity at depth. In addition, it is an apparent optical property, so it varies largely with solar zenith angle, sky and surface conditions, as well as with depth even within the well mixed water column (Lee et al., 2005a). However, studies have shown that it is determined to a large extent by the inherent optical properties of the aquatic medium (e.g., absorption coefficient and volume scattering function) and are not altered obviously by changes in the incident radiation field such as a change in solar elevation (Mishra et al., 2005). For the vast oceans, the estimation of K d (λ) of the surface layer by satellite remote sensing of ocean color is the only practical means to provide repetitive measurements over extended spatial and temporal scales. There are two standard methods used for the derivation of K d (λ) in ocean color remote sensing. Both are based on empirical relationships involving the blue-to-green ratio of ocean color. In Method 1, K d (490)(K d at λ=490 nm) is first estimated from an empirical algorithm based on the relationship between K d (490) and the blue-to-green ratio of water-leaving radiance, Lw, or remote-sensing reflectance, R rs. Then this K d (490) value can be used to estimate K d at other wavelengths from empirical relationships between K d and K d (λ). In Method 2, chlorophyll a concentration (Chl) is estimated from an empirical algorithm based on the blue-to-green ratio of R rs. Then this Chl value is used for the estimation of R rs based on another set of empirical relationships between R rs and Chl (Lee et al., 2005a). In addition, there is Method 3 which is a semi-analytical method to calculate from R rs based on numerical simulations of radiative transfer in the ocean (Lee et al., 2005b). This method quasi-analytically first derives the absorption and backscattering coefficients from

42 LIU Ying et al. Acta Oceanologica Sinica 2009, Vol. 28, No. 5, P.39-48 K rs (λ), and these coefficients are then used as input to a semi-analytical model to estimate the values of K rs (λ). These two standard methods are insufficient to provide an understanding regarding the variation of K rs (λ) and contain large uncertainties in the derived values (Lee et al., 2005b). In addition, they produced satisfactory estimation of K rs (λ) in oceanic waters, but resulted in significant errors in coastal waters (Lee et al., 2005a). While the semi-analytical method provides an improved interpretation about the variation of K rs (λ) and a basis to more accurately determine K rs (λ) (especially using data from remote sensing) (Lee et al., 2005b). Moreover, an important attribute of Method 3 is that its application requires no separation of the entire dataset into subsets of data, such as Case 1 and Case 2 waters, it performed well for both oceanic and coastal waters (Lee et al., 2005a). Furthermore, compared to exact numerical simulation of radiative transfer, its calculation process is more simple and effective. This paper has used just Method 3 to derive K rs (λ) of MODIS wavelengths. Under SeaDAS5.2, K rs (λ) was calculated through the semi-analytical method of Lee et al.(lee et al., 2005b, 2002) which is based on inherent optical property, and then the attenuation depth was calculated based on the formula Gordon et al. derived (Gordon and McCluney, 1975). 3 Results MODIS (Terra/Aqua) data were taken as the remote sensing data source, for its medium spatial resolution, near daily coverage, freely distributed processing software systems, as well as available without charge from several data archives and distributions. These cloud free MODIS images (without or with little cloud or suspicious haze) of the Bohai Sea (37 07 41 00 N, 117 35 122 15 E) in 2003 were selected and downloaded from the ocean color web site (OBPG, 2009). The level 0 which is not extracted, level 1, level 2 (normalized, water-leaving radiances), meteorology & Ozone, and attitude & ephemeris data were downloaded. The atmospheric correction method of Liao et al.(liao et al., 2005) was applied. The SeaDAS5.2 (OBPG, 2008) was used for geometric correction, projection transformation, and extraction of the study area as well as calculating K d (λ) and the attenuation depth. 3.1 Variation of the attenuation depth with wavelength Data of 7 August 2003 was selected for the spectral and regional analysis of the attenuation depth of MODIS wavelengths in the Bohai Sea. The respective attenuation depths for eight wavelengths in the range between 400 to 700 nm are shown in Fig. 1. Beyond 645 nm, the discrepancy of the attenuation depth at different waters was less, then the spatial variability can be hardly seen, which can also be seen from the Jerlov s (Jerlov, 1968) study. In his study, beyond the 600 nm, there is minor difference of the attenuation depth in different water bodies (Fig. 2). Thus the attenuation depths beyond 645 nm is not shown and analyzed. In Fig. 1, NA means not available, which represents apart from mainly land, and also very small region of the attenuation depth was calculated wrong which was caused by the bands saturation and the true signal is unknown over highly turbid coastal waters (Franz, 2006). Data about the attenuation depth of the MODIS wavelengths show that it has considerable spatial variability in the Bohai Sea (Fig. 1). It is increases from the shore to the central of the Bohai Sea. The attenuation depth in the coast has lower value, especially in the Bohai Bay, the Laizhou Bay, and the Liaodong Bay, it is less than 4.5 m, and mostly below 3 m, while it in the central of the Bohai Sea has higher value, particularly in the western region of Liaodong Bay estuary, the region surrounding the central Bohai Sea, and the vicinity of the Bohai Strait, it is larger than that in other regions. According to the zoning of the ocean color spectrum (Zhou et al., 2005) as well as the suspended solids concentration distribution (Jiang et al., 2002, 2004; Qin and Li, 1982; Wang and Li, 2007) in Bohai Sea, together with taking into account points representing different regions selected accuracy and convenience (in the cross-point), seven points that enough represent situation of the Bohai sea were selected to analyze the variation of the attenuation depth with the wavelength of light (Fig. 3). The region controlled by the Huanghe River, the region affected by the old Huanghe River, the outside of the northwest coast of the Bohai Sea, the western side of the Liaodong Bay, the eastern side of the Liaodong Bay, the central of the Bohai Sea and the Bohai Strait were represented by the following Points A, B, C, D, E, F, and G, respectively.

LIU Ying et al. Acta Oceanologica Sinica 2009, Vol. 28, No.5, P.39-48 43 Fig.1. The attenuation depths Z 90 (m) for several wavelengths in the Bohai Sea on 7 August 2003. The attenuation depth of points representing different regions on 7 August 2003 was extracted separately and drawn in Fig. 4. The spectral distribution of the attenuation depth is similar and is a single peak curve. They are basically similar in both the region controlled by the Huanghe River and the eastern side of the Liaodong Bay; in both the region impacted by the old Huanghe River and the western side of the Liaodong Bay, they are basically parallel, except they are cross at 645 nm (their attenuation depth are respectively 2.027, 1.946 m); they also have some similarities in both the outside of the northwest coast of the Bohai Sea and the central of the Bohai Sea, for example, at 531, 551, 555 nm, they almost overlap, at 412, 443, 469, 488, 645 nm, it in the outside of the northwest coast of the Bohai Sea is lower than that in the central of the Bohai Sea; there are obviously difference between the attenuation depth of the Bohai Strait and that of other regions because the former values are generally higher than the latter values, except at 645 nm the attenuation depth because the impact of cloud is not correctly obtained. These similarities or differences are consistent with the zoning of Zhou et al.(2005) and Jiang et al.(2002). In addition, the attenuation depth of all these points of the showed MODIS wavelengths is greater than at least 0.5 m. Moreover, in the same band, the order of the attenuation depth of different regions from small to

44 LIU Ying et al. Acta Oceanologica Sinica 2009, Vol. 28, No. 5, P.39-48 Fig.3. Location of points represents different regions. Fig.2. Variation of Z 90 with wavelength for various water types given by Jerlov [1968]. big is basically the region controlled by the Huanghe River or the eastern side of the Liaodong Bay, the western side of the Liaodong Bay, the region impacted by the old Huanghe River, the central of the Bohai Sea or the outside of the northwest coast of the Bohai Sea, the Bohai Strait (except at 412 nm and 645 nm); at different bands, it is almost like so but it varies between the region controlled by the Huanghe River and the eastern side of the Liaodong Bay (and between the central of the Bohai Sea and the outside of the northwest coast of the Bohai Sea). In Fig. 4, the maximum attenuation depth of the outside of the northwest coast of the Bohai Sea and the Bohai Strait is at 531 nm, that of the central of the Bohai Sea is at 551 nm, while that of the region controlled by the Huanghe River, the region impacted by the old Huanghe River, the western side of the Liaodong Bay, the eastern side of the Liaodong Bay is at 555nm, which shifts toward longer wavelengths (the red) with increase of turbidity of water, and is basically the same as the results of Jerlov (1968) (Fig. 2). Comparison of Fig. 4 with Fig. 2 and the results of Arst (2003) showed except the Bohai Strait is Case 1 water (or ocean water), other regions are all Case 2 water (or coastal water), so it can be further confirmed that the Bohai Sea is a typical region of Case 2 water in China. Fig.4. Spectral distributions of the attenuation depth in the Bohai Sea on 7 August 2003. 3.2 Seasonal variation of the attenuation depth The attenuation depth at 551nm is larger than that at other wavelengths and has more obvious differences between different regions (in Fig. 4), so it was taken as a representative to analyze the seasonal changes of the attenuation depth of the Bohai Sea. The four-day data which have few clouds (but they are not entirely without clouds, for MODIS data of the Bohai Sea are more seriously affected by the cloud) and represent the four or abundance-dry season changes were selected for analysis. These four days are 3 March, 7 August, 15 October., 7 December in 2003, representing spring, summer, fall, and winter, respectively. It is very difficult with naked eye to analyze the seasonal variability of the attenuation depth of the whole Bohai Sea, so those points just like that of analyzing the spectral distribution of the attenuation depth were also selected and their four-days data were extracted (in Table 1), in which Points A and B have

LIU Ying et al. Acta Oceanologica Sinica 2009, Vol. 28, No.5, P.39-48 45 the right value only on 7 August. The season descending order of the attenuation depth of both the western side of the Liaodong Bay and the eastern side of the Liaodong Bay is characterized by summer, fall, spring, winter, while that of the outside of the northwest coast of the Bohai Sea, the central of the Bohai Sea and the Bohai Strait is characterized by summer, fall, winter and spring. Thus, the attenuation depth in summer is the biggest, and fall is in the second, the order of the attenuation depth of spring and winter in different regions varies. In addition, in the same season, the ranking of attenuation depth from small to big in different regions is basically the eastern side of the Liaodong Bay, the western side of the Liaodong Bay, the central of the Bohai Sea, the outside of the northwest coast of the Bohai Sea, the Bohai Strait (but in spring it is the eastern side of the Liaodong Bay, the western side of the Liaodong Bay, the outside of the northwest coast of the Bohai Sea, the central of the Bohai Sea, the Bohai Strait), and in the summer, when all different regions data are available, it is in essence the region controlled by the Huanghe River, the eastern side of the Liaodong Bay, the western side of the Liaodong Bay, the region impacted by the old Huanghe River, the central of the Bohai Sea, the outside of the northwest coast of the Bohai Sea, the Bohai Strait, which is fundamentally similar to the above conclusions of the order of attenuation depth in different regions at the same band. Table 1. Seasonal variation of the attenuation depths (Z 90/m) (MODIS 551 nm) in the Bohai Sea A B C D E F G 7 December 3.141 1.114 0.826 1.844 4.98 3 March 1.494 1.164 1.015 1.543 3.584 7 August 1.852 4.355 6.729 3.679 2.089 6.588 10.33 15 October 3.837 2.709 1.651 2.781 8.621 4 Discussion Like the previous methods described, the attenuation depth and the diffuse attenuation coefficient are subject to the light field, atmospheric conditions and inherent optical properties of water bodies. In this paper, under the assumption that the light field and atmospheric conditions in the whole Bohai Sea (approximately 8 10 4 km 2 in area (Zhou et al., 2005)) were the same, the attenuation depth was analyzed, that is, the change of the attenuation depth of the Bohai Sea with wavelengths and inherent optical property was mainly analyzed. Through analyzing spectral distributions of the attenuation depth of a particular period (7 August 2003) and seasonal variations of the attenuation depth at some band (551 nm) and selecting points represented different regions for analyzing, spectral distributions and spatial-temporal variations of the attenuation depth of the Bohai Sea were essentially obtained. The spectral distribution of the attenuation depth was studied by many researchers (Arst, 2003; Arst et al., 1996; Gordon and McCluney, 1975). It was also studied in the Bohai Sea where it is a single-peak curve wich is basically consistent with that of the related research. It has obvious regional characteristics in the Bohai Sea which reflects the optical complexity of the Bohai Sea. Both the region controlled by the Huanghe River and the eastern side of the Liaodong Bay have more similar water body property, and both are river (the Huanghe River and Liaohe River) inflow affected areas, but land-based sources of material carried by the Huanghe River is more than that by Liaohe River, so the attenuation depth of the region controlled by the Huanghe River should be theoretically smaller than that of the eastern side of the Liaodong Bay, but the real situation is not so for that is reversed at wavelengths less than 469 nm; the spectral distribution of the attenuation depth of the western side of the Liaodong Bay and the region impacted by the old Huanghe River is parallel, and the attenuation depth of the western side of the Liaodong Bay is less than that of the region impacted by the old Huanghe River, which may be partly due to the off-shore distance of Point B is greater than that of Point D and the material of land-based sources reduces along the direction the near-shore to the central of the Bohai Sea, in addition, both the western side of the Liaodong Bay and the region impacted by the old Huanghe River are little affected by river inflow; the attenuation depth of the outside of the northwest coast of the Bohai Sea, the central of the Bohai Sea, and the Bohai Strait also reflects the situation of land-based sources material which reduces along the direction from near-shore to the central of the Bohai Sea, and it can be basically said that the attenuation depth of the outside of the northwest coast of the Bohai Sea and the central of the Bohai Sea is less than that of the Bohai Strait,

46 LIU Ying et al. Acta Oceanologica Sinica 2009, Vol. 28, No. 5, P.39-48 and as to the attenuation depth of the outside of the northwest coast of the Bohai Sea and the central of the Bohai Sea, at 531, 551, 555 nm, the attenuation depth of the outside of the northwest coast of the Bohai Sea is greater than that of the central of the Bohai Sea, but at 412, 443, 469, 488, 645 nm, the attenuation depth of the central of the Bohai Sea is greater than that of the outside of the northwest coast of the Bohai Sea. Many researchers study results show that water composition of the Bohai Sea changes with season (Jiang et al., 2004; Qin and Li, 1982; Wang and Li, 2007).The main water materials of the Bohai Sea are sediment, chlorophyll and yellow substance. No matter what kind of substances increases, the attenuation depth will decrease, because the attenuation depth is inversely proportional to the absorption coefficient (Gordon and McCluney, 1975), and the absorption coefficient is proportional to the concentration of a variety of substances of water bodies (Arst, 2003). The seasonal change of the attenuation depth at 551nm is the largest in the summer, followed by the fall, and the ranking of winter and spring in different regions is distinct. In spring and winter, the chlorophyll content of water bodies is relatively small. However in winter the winds in the Bohai Sea is strong, and major wind directions are many from north, and strong winds whipped up more sediments, then the optical property of water body at this time are mainly decided by the sediment content. The more the sediment content, the smaller the attenuation depth. So the attenuation depth of the western side of the Liaodong Bay and the eastern side of the Liaodong Bay in spring is greater than that in winter. However the attenuation depth of the outside of the northwest coast of the Bohai Sea, the central of the Bohai Sea and the Bohai Strait in winter is greater than that in spring because sediments content in these areas was less and the role of wind lift sand is small, and even if Point C is in the coastal region but where is the bedrock coast with less sediment sources, and meanwhile compared with in winter, in spring plants start growing, then the absorption coefficient of water bodies is larger, so the attenuation depth is smaller. The attenuation depth of Point A and Point B is not correctly obtained, so the rankings of the attenuation depth of other points in different seasons were mainly analyzed. Except in summer, the general order of the attenuation depth from small to big is the eastern side of the Liaodong Bay, the western side of the Liaodong Bay, the central of the Bohai Sea, the outside of the northwest coast of the Bohai Sea, and the Bohai Strait, which is mainly related to the suspended solids content in these regions, such as, in the Bohai Strait, water is more clear, therefore the attenuation depth at the Bohai Strait shows very high value, but in the spring, the attenuation depth of the outside of the northwest coast of the Bohai Sea is less than that of the central of the Bohai Sea. For understanding the above results, further studies are needed, such as the study of water body composition and its content, shape, and size. 5 Conclusions The attenuation depth of the Bohai Sea has obvious regional characteristics. In different regions, the spectral distribution of the attenuation depth is different, but it also has similarities, which basically consists with the zoning of ocean color spectrum (Zhou et al., 2005) and the concentration of suspended sediment (Jiang et al., 2002). The maximum attenuation depth shifts toward longer wavelengths (the red) with the increase of turbidity of water bodies in different parts. Maximum depth attenuation of the outside of the northwest coast of the Bohai Sea and the Bohai Strait is at 531 nm, the central of the Bohai Sea is at 551 nm, the region controlled by the Huanghe River, the region impacted by the old Huanghe River, and the Liaodong Bay is at 555 nm. Though comparing with the spectral distribution of the attenuation depth of other researchers, it is further proved that the Bohai Sea is Case 2 water. In the Bohai Sea, there are seasonal changes of the attenuation depth at 551 nm, in the summer the attenuation depth is the biggest, followed by the autumn, the order of the attenuation depth in spring and winter varies in different regions. In the relatively higher suspended material content regions (like the Liaodong Bay), the descending order of the attenuation depth in different seasons are summer, fall, spring, winter, while in the low suspended solid content regions (like the outside of the northwest coast of the Bohai Sea, the central of the Bohai Sea and the Bohai Strait), it is the summer, fall, winter, spring. In summer, the order of attenuation depth in different regions from small to large is basically the region controlled by the Huanghe River, the eastern side of the Liaodong Bay, the western side of the Liaodong Bay, the region impacted by the old Huanghe River, the central of the

LIU Ying et al. Acta Oceanologica Sinica 2009, Vol. 28, No.5, P.39-48 47 Bohai Sea, the outside of the northwest coast of the Bohai Sea and the Bohai Strait, in the spring, it is almost the same, except that of the outside of the northwest coast of the Bohai Sea and the central of the Bohai Sea is reversed. In fact, in the Bohai Sea, less than 645nm MODIS wavelengths, the order of the attenuation depth in different regions from small to big is the region controlled by the Huanghe River or the eastern side of the Liaodong Bay, the western side of the Liaodong Bay, the region impacted by the old Huanghe River, the central of the Bohai Sea or the outside of the northwest coast of the Bohai Sea, the Bohai Strait (except at 412 nm and 645 nm), but the order of the attenuation depth between the region controlled by the Huanghe River and the eastern side of the Liaodong Bay (and between the central of the Bohai Sea and the outside of the northwest coast of the Bohai Sea) is reverse in different bands and seasons. It is necessary to analyze the field measurements of the water body material for understanding the reason of the difference. The spectral distribution and the regional characteristic of the attenuation depth were mainly analyzed. Many results have been obtained, however filed measurements are also needed for better explaining them. In some case, the attenuation depth reflects the amount of material content of the water body, so it can be used for water clarity analysis, water classification, zoning (as mentioned above, the greater the attenuation depth, the more the water information remote sensing obtained, and the more accurately the material content inversed, so water color studying should separate the entire region into subsets, such as the region that the band of the maximum attenuation depth is close is divided into one type), as well as water body materials content calculation. References Arst H. 2003. Optical Properties and Remote Sensing of Multicomponental Water Bodies. Chichester: Praxis Publishing Arst H, Maekivi S, Kutser T, et al. 1996. Optical investigations of Estonian and Finnish Lakes. Lakes & Reservoirs: Research and Management, 2: 187 198 Farmer C T, Moore C A, Zika R G, et al. 1993. Effects of low and high Orinoco River flow on the underwater light field of the eastern Caribbean Basin. Journal of Geophysical Research, 98(C2): 2279 2288 Franz B.2006. Extension of MODIS ocean processing capabilities to include the 250 & 500-meter land/cloud bands http://oceancolor.gsfc.nasa.gov/docs/modis hires/, 2006-10 23/2008 07 11 Gordon H R, Boynton G C. 1998. Radiance-irradiance inversion algorithm for estimating the absorption and backscattering coefficients of natural waters: vertically stratified water bodies. Applied Optics, 37(18): 3886 3896 Gordon H R, Brown O B. 1975. Diffuse reflectance of the ocean: some effects of vertical structure. Applied Optics, 14(12): 2892 2895 Gordon H R, Clark D K. 1980. Remote sensing optical properties of a stratified ocean: an improved interpretation. Applied Optics, 19(20): 3428 3430 Gordon H R, McCluney W R. 1975. Estimation of the depth of sunlight penetration in the sea for remote sensing. Applied Optics, 14(2): 413 416 Gordon H R. 1978. Remote sensing of optical properties in continuously stratified waters. Applied Optics, 17(12): 1893 1897 Islam A, Gao J, Ahmad W, et al. 2004. A composite DOP approach to excluding bottom reflectance in mapping water parameters of shallow coastal zones from TM imagery. Remote Sensing of Environment, 92(1): 40 51 Jerlov N G. 1968. Optical Oceanography. Amsterdam: Elsevier Science Publications Jiang W, Pohlmann T, Sun J, et al. 2004. SPM transport in the Bohai Sea: field experiments and numerical modeling. Journal of Marine Systems, 44: 175 188 Jiang Wensheng, Su Jian, Yang Hua, et al. 2002. The relationship between SPM concentration and hydrodynamic condition in the Bohai Sea. Acta Oceanologica Sinica (in Chinese), 24(Supp.1): 212 217 Lee Z P, Carder K L, Arnone P R. 2002. Deriving inherent optical properties from water color: a multiband quasi-analytical algorithm for optically deep waters. Applied Optics, 41(27): 5755 5772 Lee Z P, Darecki M, Carder K L, et al. 2005a. Diffuse attenuation coefficient of downwelling irradiance: An evaluation of remote sensing methods. Journal of Geophysical Research, 110, C02017, doi:10.1029/2004jc002573 Lee Z P, Du K P, Arnone R. 2005b. A model for the diffuse attenuation coefficient of downwelling irradiance. Journal of Geophysical Research, 110, C02016, doi:10.1029/2004jc002275 Liang Shunlin, Chen Bingxian. 1989. A study on remote sensing perspective depth into water in visible band. Journal of Nanjing University (Natural Sciences Edition) (in Chinese), 25(2): 322 332 Liao Yingdi, Zhang Wei, Deschamps P Y. 2005. Remote sensing of suspended sediments in China east coastal waters from SeaWiFS data. Journal of Hydrodynamics (in Chinese), 20(5): 558 564

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