Sunglint images of current gradients at high resolution: Critical angle and directional observing strategy

Type Article
Date 2018-10
Language English
Author(s) Rascle Nicolas2, 4, Nouguier FredericORCID1, 3, Chapron Bertrand1, Ocampo-Torres Francisco J.2
Affiliation(s) 1 : Univ Brest, CNRS, IRD, Ifremer,LOPS,IUEM, Brest, France.
2 : Ctr Invest Cient & Educ Super Ensenada, Dept Oceanog Fis, Ensenada, Baja California, Mexico.
3 : Univ Toulon & Var, CNRS, INSU, IRD,MIO, UM 110, F-83957 La Garde, France.
Source Remote Sensing Of Environment (0034-4257) (Elsevier Science Inc), 2018-10 , Vol. 216 , P. 786-797
DOI 10.1016/j.rse.2018.06.011
WOS© Times Cited 1
Keyword(s) Sea surface roughness, Surface current gradients, Submesoscale turbulence, Wave-current interaction, Surfactants, Sun glint imagery, SAR images, Multi-angle radiometers, Surface slope distribution, Mean square slope

High-resolution satellite images of the ocean surface in and around the sunglint often provide unique observations of sub-mesoscale upper ocean surface processes. Local anomalies of wind, waves, currents or surfactants appear on the images as local anomalies of brightness. A quantitative interpretation of those brightness anomalies must relate them to slope properties of the wave field, which are to the lowest order described by the mean square slope (mss).

The prevailing paradigm for such interpretation is that of the critical zenith angle. It states that, for sub-critical zenith view angle, brightness and mss anomalies have opposite signs, and this defines the so-called inversion region. This prevailing paradigm implicitly builds on the assumption that the mss decomposition between upwind and crosswind components is conserved. The mss anomalies are then isotropic and can be reduced to a scalar (i.e. one-dimensional) quantity. In such a case, one single sunglint image would be sufficient to retrieve the mss anomaly. This isotropic case likely applies for surface wave changes induced by varying wind speed or by surfactants.

Yet, satellite and airborne observations at multiple view angles recently revealed anisotropic mss anomalies, e.g. with mss increase in the upwind direction and decrease in the crosswind direction. This anisotropic behavior likely characterizes wave modulations by anisotropic surface current gradients. This paper details the expected properties of such anisotropic mss modulations. It is shown that: 1) The classical concept of critical angle does not systematically hold, neither for frontal current shear nor for internal wave divergence. 2) At least two sunglint images at different zenith and azimuth angles are needed to retrieve the mss anomalies, and a single observation is not sufficient. 3) A satellite with radiometers looking at multiple zenith angles is capable of providing a geometry favorable to retrieve mss anomalies. An illustration is discussed with internal waves observed by the Multi-angle Imaging SpectroRadiometer (MISR), where the upwind and crosswind components of the retrieved mss anomalies are anisotropic. Those results provide guidelines to interpret available observations and to help refine strategy for future satellite missions.


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