Monitoring Studies Of Red Tides In The East Sea Using Satellite Data

Timely observation of red tides is critical for all aspects of red tide studies, from early warning, forecasting to understanding of their initiation and dissipation conditions. Ocean color sensors have abilities to detect red tides, especially MEdium Resolution Imaging Spectrometer (MERIS), which equipped with chlorophyll fluorescence 681 nm band and 709 nm band. In addition, ocean color sensors may have the abilities to detect the peak of vertical profile of chlorophyll-a concentration in the sub-surface since only ocean color sensors can penetrate water.A hydro-optical algorithm for the East Sea where red tides occur frequently is developed based on the in-situ data collected during five filed experiments, conducted in the East Sea in February, May, September and November from 2003 to 2007 respectively, and also synthesized with related literature research results. The chlorophyll-a fluorescence model is also added to the algorithm. The comparisons among the remote sensing reflectance (Rrs) of the hydro-optical algorithm, filed measurements and Hydrolight simulation show good agreements. Based on the hydro-optical algorithm, a semi-analytical model using L-M optimization method with multiple groups of concentration vectors is applied for chlorophyll-a retrieval in the East Sea where red tides occur frequently. The values of chlorophyll-a derived from field Rrs and measured from samples are used to compare and validate the model. The correlation coefficient is 0.94, and the root mean square error in percentage (rmse) is 14.5%, which performs better than the model without considering of chlorophyll-a fluorescence effects. It is can be concluded that the semi-analytical model is practical. The model is also applied to MERIS data. The retrieved chlorophyll-a concentration shows a better result than algal₂ product, which is the operational product in Caseâ…¡water provided by European Space Agency (ESA)3000 groups of data are simulated with the hydro-optical algorithm developed for the East Sea where red tides occur frequently to study the relationship between values of Fluorescence Line Height FLH665-681-753, FLH665-681-709, Maximum Chlorophyll Index MCI665-709-753, MCI681-709-753, peak and chlorophyll-a concentration. It is found that MCI681-709-753 performs best for the intense red tide detection in the East Sea. The criterion for monitoring red tides in the East Sea is also proposed. The satellite monitoring results of two cases have a good agreement with the bulletins published by State Oceanic Administration (SOA).180 groups of numerical simulations of radiative transfer are used to study the effects of a non-uniform vertical profile of Chl-a on Rrs during the developing period of evolution of a red tide with Hydrolight 4.1 using the IOP models derived from the hydro-optical model developed for the East Sea where red tides occur frequently and Gaussian function that describes the chlorophyll concentration profile. We estimate a range of four parameters of Gaussian function for the East Sea where red tides occur frequently according to field observations. The simulations of a non-uniform vertical profile of Chl-a are compared with homogeneous one. Effects of four parameters of Guassian function on Rrs are discussed respectively. It can be concluded that changes of parameters of a non-uniform vertical profile show effects on green-yellow bands, however, few effects on values of FLH and MCI.