Abstract

Doppler scatterometry is a promising new technique for the simultaneous measurement of ocean surface currents and winds. These measurements have been recommended by the recent US NRC Decadal Review for NASA as being priority variables for the coming decade of Earth observations. In addition, currents and winds are useful for many applications, including assessing the operating conditions for oil platforms or tracking the dispersal of plastic or oil by surface currents and winds. While promising, Doppler scatterometry is relatively new and understanding the measurement characteristics is an important area of research. To this end, Chevron sponsored the deployment of DopplerScatt, a NASA/JPL Ka-band Doppler scatterometer, over instrumented sites located at the edge of a Gulf of Mexico Loop Current Eddy (LCE). In addition to in situ measurements, coincident synoptic maps of surface currents were collected by the Areté ROCIS instrument, an optical current measurement system. Here we report on the results of this experiment for both surface currents and winds. Surface current comparisons show that the Ka-band Current Geophysical Model Function (CGMF) needs to include wind drift currents, which could not be estimated with prior data sets. Once the CGMF is updated, ROCIS and DopplerScatt show good agreement for surface current speeds, but, at times, direction differences on the order of 10° can occur. Remote sensing optical and radar data agree better among themselves than with ADCP currents measured at 5 m depth, showing that remote sensing is sensitive to the the currents in top 1 m of the ocean. The LCE data provided a unique opportunity to study the effects of surface currents and stability conditions on scatterometer winds. We show that, like Ku-band, Ka-band estimates of winds are related to neutral winds (and wind stress) and are referenced relative to the moving frame provided by the current. This is useful for the study of air-sea interactions, but must be accounted for when using scatterometer winds for weather prediction.

Highlights

  • Ocean surface currents and winds are essential climate variables that govern the flux of heat and gases between the ocean and the atmosphere

  • We present the first steps towards answering these questions using a validation experiment for the DopplerScatt instrument undertaken in partnership between Jet Propulsion Laboratory (JPL), Chevron, and Areté over a Loop Current eddy, named “Quantum Eddy”, conducted in March, 2018

  • The wind inversion process minimizes the differences between observed values of σ0 and those predicted by an empirical wind GMF, which depends on wind speed and observation direction

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Summary

Introduction

Ocean surface currents and winds are essential climate variables that govern the flux of heat and gases between the ocean and the atmosphere. The importance of these variables has been highlighted by the latest US National Academy of Sciences Decadal Survey report for NASA [1], which identifies these variables as a suitable target for an Earth Explorer mission in the decade. The Sub-Mesoscale Ocean Dynamics Experiment (S-MODE), selected in 2018 as a NASA Earth Ventures Suborbital (EVS) Mission, will use the DopplerScatt instrument, along with other airborne and in situ sensors, to improve our understanding of the ocean sub-mesoscales

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