Abstract
Satellite altimetry is a unique system that provides repeated observations of significant wave height (SWH) globally, but its measurements could be contaminated by lands, slicks, or calm water with smooth surface. In this study, capability of subwaveform retrackers against 20 Hz Jason-2 measurements is examined in the calm Celebes Sea. Distances between contamination sources and Jason-2 observation points can be determined using sequentially assembled adjacent waveforms (radargram). When no contamination sources are present within a Jason-2 footprint, subwaveform retrackers are in excellent agreement with the Sensor Geophysical Data Records (SGDR) MLE4 retracker that uses full-length waveforms, except that Adaptive Leading Edge Subwaveform (ALES) retracker has a positive bias in a calm sea state (SWH < 1 m), which is not unusual in the Celebes Sea. Meanwhile, when contamination sources exist within 4.5 km from Jason-2 observation points, SGDR occasionally estimates unrealistically large SWH values, although they could be partly eliminated by sigma0 filters. These datasets are then compared with WAVEWATCH III model, resulting in good agreement. The agreement becomes worse if swells from the Pacific is excluded in the model, suggesting constant presence of swells despite the semi-enclosed nature. In addition, outliers are found related with locally-confined SWH events, which could be inadequately represented in the model.
Highlights
Satellite altimeters transmit microwave pulses toward the sea surface below and measure “waveforms”, i.e., time series of the power of received backscattered echoes (Figure 1a)
The different correlation coefficients in [16] and [17] would be due to discrepancy of the quality of the altimeter data, not the resolution of WAVEWATCH III version 3.14 (WW3) models. These results suggest that the capability of Jason-2 retrackers for significant wave height (SWH) can be assessed by correlation coefficients with WW3 models, as a consistency with wave fields numerically determined from the NCEP wind fields
The estimated SWH datasets are first compared with the original Sensor Geophysical Data Records (SGDR) SWH data that use full-length waveforms
Summary
Satellite altimeters transmit microwave pulses toward the sea surface below and measure “waveforms”, i.e., time series of the power of received backscattered echoes (Figure 1a). Under the presence of sea surface waves, microwave pulse signals reflected at the wave crests reach back to the satellite earlier than ones reflected at the wave troughs This temporal discrepancy is represented as the leading edge rise time, i.e., duration of the leading edge slope of the waveform, and enables to measure significant wave height (SWH). The Brown mathematical model is most commonly used as the idealized model in open oceans, but it assumes homogeneous reflectance within a footprint of an altimeter [3] This assumption, tends to be broken in coastal areas since sources of inhomogeneous reflectance (e.g., ships, lands, slicks, or calm water in small bays) are often present within a footprint. Altimeter measurements in coastal areas could be contaminated by such sources within a footprint
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