Ka-Band Doppler Scatterometry over a Loop Current Eddy

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.

Near‐Surface Oceanic Kinetic Energy Distributions From Drifter Observations and Numerical Models

“…An examination of the vertical structure of near‐surface currents will aid our understanding of the air‐sea exchange of heat, momentum, and gases and of the dispersal of pollutants and biologically important tracers (Elipot & Wenegrat, 2021). The vertical structure of velocity has important implications for the ongoing S‐MODE airborne mission and proposed satellite missions focusing on surface ocean velocity measurements (Ardhuin et al., 2019; Rodriguez et al., 2020). Such missions will need information on the frequency dependence of vertical structure in order to interpret the implications of surface current measurements for subsurface oceanic conditions.…”

“…Quantitative mapping of low‐ and high‐frequency motions is important for satellite missions including the SWOT mission (Morrow et al., 2019), planned for launch in 2022, which will measure SSH at high resolution in two‐dimensional swaths. Remote sensing missions focused on measuring near‐surface ocean velocities, such as the existing airborne Sub‐Mesoscale Ocean Dynamics Experiment (S‐MODE) mission (Rodriguez et al., 2020), and proposed velocity‐measuring satellite missions (Ardhuin et al., 2019; Rodriguez et al., 2020), now under the umbrella name “Odysea,” will benefit from quantification of high‐ and low‐frequency KE as well. Because HYCOM and MITgcm LLC4320 are widely used, it is important to compare these models to observations.…”

Frequency dependence of near-surface oceanic kinetic energy from drifter observations and global high-resolution models

“…Quantitative mapping of low-and high-frequency motions is important for satellite missions including the Surface Water Ocean Topography (SWOT) mission (Morrow et al, 2019), planned for launch in 2022, which will measure SSH at high resolution in two-dimensional swaths. Remote sensing missions focused on measuring nearsurface ocean velocities, such as the existing airborne Sub-Mesoscale Ocean Dynamics Experiment (S-MODE) mission (Rodriguez et al, 2020), the proposed Sea surface KInematics Multiscale monitoring mission (SKIM; Ardhuin et al, 2019) and the proposed Winds And Currents Mission (WACM; Rodriguez et al, 2020), will benefit from quantification of high-and low-frequency KE as well.…”

“…A better quantification of the vertical structure of near-surface currents will aid our understanding of the air-sea exchange of heat, momentum, and gases, and of the dispersal of pollutants and biologically important tracers (Elipot & Wenegrat, 2021). The vertical structure of velocity has important implications for the ongoing S-MODE airborne mission and proposed satellite missions focusing on surface ocean velocity measurements (Ardhuin et al, 2019;Rodriguez et al, 2020). These missions will need information on the frequency dependence of vertical structure in order to interpret the implications of surface current measurements for subsurface oceanic conditions.…”

A Ka-Band Wind Geophysical Model Function Using Doppler Scatterometer Measurements from the Air-Sea Interaction Tower Experiment

“…Given the importance of these measurements, NASA funded an airborne Ka-band Doppler scatterometer, called Doppler-Scatt, under the NASA Instrument Incubator Program (IIP) and Airborne Instrument Technology transition (AITT) program [19]. DopplerScatt has successfully demonstrated its ability to sense submesoscale ocean vector winds and currents [20]. Its measurements are an essential contribution in the ongoing NASA Earth Ventures Suborbital-3 Submesoscale Ocean Dynamics Experiment (S-MODE) mission [21].…”