Short Term Forecast of the Quality of Water in New York Coastal Zone Using Multispectral Satellite Data

Abstract

One of the possible and probably most effective ways of solving the problem of monitoring the quality of water in ocean coastal areas is to use satellite measurements (remote sensing). Analysis of temporal and spatial structure of the inhomogeneities in coastal waters is an important step for the development of a model of the impurity transport and pollution forecast, but direct contact sensors cannot cover the necessary scales. Therefore, it is very important that WLR (normalized water leaving radiance) measurements made by satellites SeaWiFS, MODIS, and occasionally ASTER be used and analyzed. One of the most informative hydro-optical parameters is known to be the color index defined as the ratio of the WLR values in two spectral ranges, i.e. I(/spl lambda//sub 1/, /spl lambda//sub 2/) = WLR(/spl lambda//sub 1/)/WLR(/spl lambda//sub 2/). When calculating the color index I(/spl lambda//sub 1/, /spl lambda//sub 2/), partial compensation of the multiplicative measurement errors is taking place. For this reason, the influence of such factors as the visual angle, illumination of the ocean surface, sky color, etc. is substantially weakened, which enhances the informativity of the optical data on the upper ocean layer. This property of the color index is the basic prerequisite of its use aimed at determining multiple parameters characterizing the quality of water. Preliminary color index analyses to determine water characteristics in the New York coastal zone have shown that this parameter is quite relevant in this respect. Main mesoscale disturbances of the color index are concentrated near the coastline. Their spectral analysis has revealed two basic scales of such disturbances. The first one corresponds to the dimensions of heterogeneities within 20-30 km, the second one to 9-11 km. The first type of disturbances is identified as mesoscale eddies since their dimension corresponds to 2/spl pi/R/sub b/ (R/sub b/ is the baroclinic deformation radius). The second type of color index disturbances, with dimensions 9-11 km, manifests itself as the structures that move along the Lond Island coast in the western direction. The coherence of such disturbances quickly decreases with the distance from the coastline. These properties of the disturbances are characteristic for the entrapped waves and, as shown by the analysis, their parameters correspond well enough to the dispersion curves of the barotropic shelf waves. The role of such disturbances in the exchange processes remains so far not clear, and it may be a subject for further research. Consecutive digital images of the color index in the New York coastal zone may be a source of information on the development and dynamics of the mesoscale eddies. On this basis, methods of forecast of water quality changes may be developed.