Autonomous Above-Water Radiance Measurements from an Offshore Platform: A Field Assessment Experiment

Abstract

An autonomous system for making above-water radiance measurements has been produced by adding a new measurement scenario to a CIMEL CE-318 sun photometer. The new system, called the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Photometer Revision for Incident Surface Measurement (SeaPRISM), combines the normal CE-318 capability for measuring direct sun irradiance and sky radiance, with a new capability for measuring above-water radiance for the retrieval of water-leaving radiance. The system has been extensively tested during several measurement periods over a 1-yr time frame from August 1999 to July 2000 under various sun elevations along with different atmospheric, seawater, and sea-state conditions. The field assessment of the new instrument was conducted at an oceanographic tower located in the northern Adriatic Sea within the framework of measurement campaigns aimed at supporting ocean color calibration and validation activities. Sample data at 440, 500, 670, 870, and 1020 nm were collected at azimuth and zenith angles satisfying the SeaWiFS Ocean Optics Protocols (and successive revisions) for above-water radiance measurements. Specifically, data were collected with azimuth angles of 90° with respect to the sun plane, and with nadir viewing angles of 30°, 40°, and 45° for above-water measurements and of 150°, 140°, and 135° for sky radiance measurements, respectively (the latter are needed for glint correction of the data). The intercomparison between water-leaving radiances computed from SeaPRISM measurements and those obtained from in-water optical profiles taken with the Wire-Stabilized Profiling Environmental Radiometer (WiSPER) system were performed using 113 coincident sets of measurements collected during clear-sky conditions. The SeaPRISM measurements taken at 40° and corrected for glint effects using different methods show the best agreement with WiSPER data. The intercomparisons exhibit average absolute unbiased percent differences, generally lower than 10% at 440 and 500 nm, and lower than 26% at 670 nm. The intercomparison of the water-leaving radiance ratio LW(440)/LW(500) from SeaPRISM data taken at 40° and WiSPER data exhibits average absolute unbiased percent differences lower than 5.6%.