Abstract
Abstract. Remote sensing of water colour by ship-mounted sensors represents an important tool for the validation of satellite products and the monitoring of water quality. The recorded radiance from the sea has to be corrected for the surface-reflected radiance from sun and sky in order to obtain the water-leaving radiance. Here the simple case of radiance reflected towards the zenith is studied. A set of observed sky radiance and solar irradiance data from Oslo has been used together with a Gaussian slope distribution for the sea surface in order to estimate the reflected radiance. The spectral range studied is 405–650 nm, the solar zenith angles are in the range 37°–76°, and the wind speeds are up to 10 m s−1. The analysis of the results show that the reflected radiance has to be separated into three contributions: sky radiance and sun rays reflected at the foam-free surface and irradiance reflected by whitecaps and foam. It is then demonstrated that by using four input values, namely the downward irradiance, the sky radiance from the zenith, the solar zenith angle and the wind speed, it is possible to obtain by simple expressions estimates of the reflected radiance that only differ from the former calculated values by relative errors of less than 5%. The analysis also indicates that for the spectral range studied neither the water-leaving radiance nor the surface-reflected radiance can be disregarded relative to the other one in the Case 2 waters of the Oslofjord-Skagerrak area. The results form a first step towards the study of reflected radiance in viewing angles differing from the nadir direction.
Highlights
Radiometric systems mounted on ships of opportunity have in recent years become an important tool for automatic monitoring of water quality
The purpose of the present study is to see how Lr in the Skagerrak-Oslofjord area acts as a function of the solar zenith angle, the wind speed and the wavelength of light, and to determine if it is possible to estimate Lr with acceptable accuracy by indirect methods in the case when Lr may be influenced by both sky and sun glints
The relationship between wind speed and mean square slope found by Cox and Munk (1954a, b) has been used with a one-dimensional Gaussian probability function for the surface slope in order to calculate the radiance from sky and sun reflected towards the zenith
Summary
Radiometric systems mounted on ships of opportunity have in recent years become an important tool for automatic monitoring of water quality. Real-time data are collected from several ferries in Norwegian coastal waters and adjacent areas (http://www.niva.no – Ferrybox monitoring). The analysis of these data require simple and accurate methods for the correction of the reflected radiance. Several studies have been made to develop such methods and to optimize the viewing angles of the radiometers with regard to the sun in order to avoid sun glints, with very satisfactory results. In the NASA protocols (Mueller et al, 2003) it is recommended that the azimuth viewing angle is in the range 90◦–135◦ away from the sun, and that the nadir angle is 40◦–45◦, in order to avoid sun glints. Simple methods that may correct for both sky and sun glints have not yet been established
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