Study Of Chlorophyll Distribution In Bohai Sea Based On MODIS Data And Hydrodynamic Model

To ensure the coordinated development of society,economics and ecological environment in the Bohai Sea region,it is imperative to enhance the understanding of the marine system so as to achieve the unity of development and protection.Phytoplankton is one of the fundamental components of marine food webs.Chlorophyll is an indicator of phytoplankton biomass,which reflects the primary productivity of the ocean.It also has significant influences on the global carbon cycle.Therefore,further study of the temporal and spatial variations of chlorophyll can promote understanding of oceanic physical-ecological environment.In this paper,data sources of remote sensing images,wind fields and pressure fields were firstly introduced,as well as overview of the concerned area and data preprocessing.In addition to radiometric calibration and geometric correction for MODIS remote sensing images,a series of processing such as image cropping and striping noise removal were performed.The wind field and pressure field data from NCEP were interpolated where necessary.Based on the SWIR atmospheric correction algorithm,the method of piecewise linear fitting was used to modify the parameter c in the expression of atmospheric correction factor?(?_i,?_j),and the original SWIR algorithm was improved.Then the original and improved algorithms were applied to the calculations in the Bohai Sea.It is shown that the overall result of the improved SWIR atmospheric correction algorithm is smaller than that of the original algorithm,and the relative difference in the whole Bohai Sea is mostly between 1%and 15%.The improvement of results varies for the remote sensing data of different seas in different periods and of different bands.After atmospheric correction,chlorophyll concentration was calculated and analyzed.With the Clack three-channel algorithm which can be well applied in Bohai Sea and SWIR atmospheric correction algorithm,the MODIS imagery data of the Bohai Sea in 2013 were calculated.The results show that the chlorophyll concentration decreases from offshore area to open ocean.The chlorophyll concentration is higher in winter and spring,and is lower in summer and autumn.Except in winter,Bohai Bay,Laizhou Bay,central Bohai Sea and Bohai Strait reach a high value of chlorophyll concentration in May and reach a low value from June to August(in summer),while the variation of Liaodong Bay is insignificant in the whole year.The Euler residual currents in the Bohai Sea were numerically simulated by the hydrodynamic model based on the shallow water equation and the results were used to further analyze the chlorophyll concentration.It is indicated that the residual currents in the Bohai Sea are weak and the capacity of seawater exchange is poor,which is favorable for the accumulation of some substances like nutrients and then leads to the high value of chlorophyll concentration.The direction of residual currents can influence the transport of nutrients,and thus indirectly affect the distribution of chlorophyll concentration.Due to the monsoon climate in the Bohai Sea,the variations of the wind stress in different quarters have had great influence on the flow of sea water and mass transport.Therefore,the driving of wind should be considered.