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Whitecap Fraction From Satellite Measurements: Algorithm Description
Author(s) -
Anguelova Magdalena D.,
Bettenhausen Michael H.
Publication year - 2019
Publication title -
journal of geophysical research: oceans
Language(s) - English
Resource type - Journals
eISSN - 2169-9291
pISSN - 2169-9275
DOI - 10.1029/2018jc014630
Subject(s) - remote sensing , satellite , radiometer , emissivity , wind speed , environmental science , brightness temperature , microwave , radiometry , sea state , sea surface temperature , meteorology , microwave radiometer , brightness , computer science , geology , geography , physics , optics , telecommunications , astronomy
Many air‐sea interaction processes are quantified in terms of whitecap fraction W because whitecaps are the most visible and direct way of observing breaking waves with air entrainment in the open ocean. Photographs or video images of the sea state collected from towers, ships, and aircrafts have been used over the years to obtain W . Satellite remote sensing of W is a recent development that allows long‐term, consistent observations of whitecapping on a global scale. The method for estimating W uses the variations of ocean surface emissivity at microwave frequencies (6 to 37 GHz) due to presence of sea foam on a rough sea surface. Satellite‐borne microwave radiometers detect these variations at the ocean surface as changes of the brightness temperature T B at the top of the atmosphere. We present the physical and parameterized models, as well as the input data, necessary to calculate W from WindSat T B observations with our W ( T B ) algorithm. We describe the implementation versions through which the W ( T B ) algorithm has developed. We show that satellite‐based W data vary with the frequency and polarization of the T B observations. The wind speed dependence of the W retrievals at horizontal and vertical polarizations shows different behavior and compares differently with in situ W data and existing wind speed parameterizations W ( U 10 ). We discuss the applicability of our methodology to other radiometric data. We indicate possible modifications and tuning of the models in the W ( T B ) algorithm that can help to further improve the accuracy of the satellite W retrievals.