Abstract
Ocean surface currents and winds are tightly coupled essential climate variables, and, given their short time scales, observing them at the same time and resolution is of great interest. DopplerScatt is an airborne Ka-band scatterometer that has been developed under NASA’s Instrument Incubator Program (IIP) to provide a proof of concept of the feasability of measuring these variables using pencil-beam scanning Doppler scatterometry. In the first half of this paper, we present the Doppler scatterometer measurement and processing principles, paying particular attention to deriving a complete measurement error budget. Although Doppler radars have been used for the estimation of surface currents, pencil-beam Doppler Scatterometry offers challenges and opportunities that require separate treatment. The calibration of the Doppler measurement to remove platform and instrument biases has been a traditional challenge for Doppler systems, and we introduce several new techniques to mitigate these errors when conical scanning is used. The use of Ka-band for airborne Doppler scatterometry measurements is also new, and, in the second half of the paper, we examine the phenomenology of the mapping from radar cross section and radial velocity measurements to winds and surface currents. To this end, we present new Ka-band Geophysical Model Functions (GMFs) for winds and surface currents obtained from multiple airborne campaigns. We find that the wind Ka-band GMF exhibits similar dependence on wind speed as that for Ku-band scatterometers, such as QuikSCAT, albeit with much greater upwind-crosswind modulation. The surface current GMF at Ka-band is significantly different from that at C-band, and, above 4.5 m/s has a weak dependence on wind speed, although still dependent on wind direction. We examine the effects of Bragg-wave modulation by long waves through a Modululation Transfer Function (MTF), and show that the observed surface current dependence on winds is consistent with past Ka-band MTF observations. Finally, we provide a preliminary validation of our geophysical retrievals, which will be expanded in subsequent publications. Our results indicate that Ka-band Doppler scatterometry could be a feasible method for wide-swath simultaneous measurements of winds and currents from space.
Highlights
The two-way interaction between ocean surface currents and ocean winds is an important component of the ocean-atmosphere system
DopplerScatt differs from spaceborne scatterometers in having only one polarization and one antenna beam. This limitation would lead to unacceptable azimuth ambiguities, but we show in Section 2.6 that, following the spirit of Mouche et al [13], the surface current radial velocity information can be used to obtain unambiguous wind directions
In April 2017, DopplerScatt participated in the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment (CARTHE) Submesoscale Processes and Lagrangian Analysis on the Shelf (SPLASH) campaign, covering the Mississippi River plume and Barataria Bay, Louisiana, for eight days of data collection
Summary
The two-way interaction between ocean surface currents and ocean winds is an important component of the ocean-atmosphere system. At small space and time scales, the interaction of winds and surface currents becomes tighter as winds can drive inertial oscillations or aid in the formation of mesoscale fronts (e.g., [4]), where significant vertical ocean motion can occur, leading to enhanced mixing. For these reasons, it is very desirable to be able to obtain simultaneous synoptic measurements of ocean surface currents and winds. Fois et al [10] showed that a Doppler system amenable to the ASCAT architecture could be implemented by correlating the Doppler shift from opposite sense linear frequency modulated pulses (i.e., chirps)
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