Abstract
This paper examines the impact of ocean surface swell waves on near-coastal L-band high-resolution synthetic aperture radar (SAR) data collected using the National Aeronautics and Space Administration’s (NASA) Soil Moisture Active/Passive (SMAP) radar at 40° incidence angle. The two-scale model and a more efficient off-nadir approximation of the second-order small-slope-approximation are used for co- and cross-polarized backscatter normalized radar cross-section (NRCS) predictions of the ocean surface, respectively. Backscatter NRCS predictions are modeled using a combined wind and swell model where wind-driven surface roughness is characterized using the Durden–Vesecky directional spectrum, while swell effects are represented through their contribution to the long wave slope variance (mean-square slopes, or MSS). The swell-only MSS is numerically computed based on a model defined using the JONSWAP spectrum with parameters calculated using the National Data Buoy Center and Wave Watch III data. The backscatter NRCS model is further refined to include fetch-limited and low-wind corrections. The results show an improved agreement between modeled and observed HH-polarized backscatter NRCS when swell effects are included and indicate a relatively larger swell impact on L-band compared to higher radar frequencies. Preliminary investigations into the potential swell retrieval capabilities in the form of excess MSS are encouraging, however further refinements are required to make broadly applicable conclusions.
Highlights
Accepted: 12 January 2022The generation and dissipation of ocean surface waves at the air–sea boundary layer involves many complex and often non-linear processes at various spatial and temporal timescales [1,2]
The percentage of predictions within ±1 dB of Soil Moisture Active/Passive (SMAP) measurements increases from 39% for the wind-only model to 65% for the combined wind and swell model with the largest gains observed at low wind speeds
This paper presented an investigation of the impact of swell waves on National Aeronautics and Space Administration (NASA)’s SMAP
Summary
The generation and dissipation of ocean surface waves at the air–sea boundary layer involves many complex and often non-linear processes at various spatial and temporal timescales [1,2]. Physics-based backscatter NRCS forward modeling studies typically rely on wind-driven ocean wave spectra to characterize rough surface effects. The impact of swell waves on ERS-1/2 C-band SAR backscatter NRCS measurements has been investigated through. Reference [13] further examines the impact of swell on both C- and Ku-band near normal incidence backscatter for multiple polarizations, and finds swell contributions to be relatively more important at low wind speeds These studies provide insight into the impact of swell waves, continued analyses into the effects of swell on radar oceanography are well motivated to characterize its effects. Insights gained from such a study further the understanding of the impact of swell waves on L-band backscatter NRCS, and can be leveraged to observe/model other ocean features that are visible in high-resolution L-band SAR imagery. Laboratory (JPL) https://smap.jpl.nasa.gov/resources/59/smap-taking-data-from-orbit, Date Accessed: 15 May 2021
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