Physics-Based Forward Modeling of Ocean Surface Swell Effects on SMAP L1-C NRCS Observations

Area of Science:

• Oceanography
• Remote Sensing
• Electromagnetics

Background:

• Synthetic Aperture Radar (SAR) is crucial for monitoring ocean surface conditions.
• Understanding the influence of ocean surface swell waves on SAR data is vital for accurate interpretation.
• Existing models may not fully capture the complex interactions between swell and radar backscatter.

Purpose of the Study:

• To investigate the impact of ocean surface swell waves on near-coastal L-band SAR data.
• To refine backscatter Normalized Radar Cross-Section (NRCS) predictions by incorporating swell effects.
• To explore the potential for retrieving swell information from SAR data.

Main Methods:

• Utilized the two-scale model and an off-nadir approximation of the second-order small-slope approximation for NRCS predictions.
• Developed a combined wind and swell model, characterizing wind-driven roughness with the Durden-Vesecky spectrum and swell with mean-square slopes (MSS).
• Numerically computed swell-only MSS using the JONSWAP spectrum with data from the National Data Buoy Center and Wave Watch III, incorporating fetch-limited and low-wind corrections.

Main Results:

• Inclusion of swell effects improved agreement between modeled and observed HH-polarized backscatter NRCS.
• Observed a relatively larger impact of swell on L-band SAR data compared to higher radar frequencies.
• Preliminary swell retrieval investigations using excess MSS showed encouraging results.

Conclusions:

• Ocean swell waves are a significant factor influencing L-band SAR data, necessitating their inclusion in models.
• The study demonstrates improved NRCS modeling accuracy with swell incorporation.
• Further research is needed to fully develop and validate swell retrieval capabilities from SAR data.