Global coastal attenuation of wind-waves observed with radar altimetry

Marcello Passaro,Florian Seitz,Denise Dettmering,Christian Schwatke,Mark A Hemer,Graham D Quartly

doi:10.1038/s41467-021-23982-4

Abstract

Coastal studies of wave climate and evaluations of wave energy resources are mainly regional and based on the use of computationally very expensive models or a network of in-situ data. Considering the significant wave height, satellite radar altimetry provides an established global and relatively long-term source, whose coastal data are nevertheless typically flagged as unreliable within 30 km of the coast. This study exploits the reprocessing of the radar altimetry signals with a dedicated fitting algorithm to retrieve several years of significant wave height records in the coastal zone. We show significant variations in annual cycle amplitudes and mean state in the last 30 km from the coastline compared to offshore, in areas that were up to now not observable with standard radar altimetry. Consequently, a decrease in the average wave energy flux is observed. Globally, we found that the mean significant wave height at 3 km off the coast is on average 22% smaller than offshore, the amplitude of the annual cycle is reduced on average by 14% and the mean energy flux loses 38% of its offshore value.

Highlights

Coastal studies of wave climate and evaluations of wave energy resources are mainly regional and based on the use of computationally very expensive models or a network of in-situ data
The use of reprocessed time series of coastal altimetry data provide the chance to observe the interaction between waves, Fig. 2 Coastal changes of mean Significant Wave Height (SWH)
Besides the common understanding that significant wave height (SWH) is decreased in the coastal zone, the study quantifies the attenuation of mean state, seasonality, and wave energy flux at an unprecedented resolution that could so far only been achieved using dedicated high-resolution models for regional and local downscalings[25]

Summary

Introduction

Coastal studies of wave climate and evaluations of wave energy resources are mainly regional and based on the use of computationally very expensive models or a network of in-situ data. Such estimation has the advantage of being independent of atmospheric corrections that are needed to estimate the range (distance between the satellite centre of mass and the sea level) This technique has been used to quantify global open ocean mean wave climate, seasonality[9,10], energy flux resources[11], and global historical trends[12]. Other studies identified the potential of using along-track measurements to locally observe variations in the sea state[13,14], but efforts have been restricted to specific regions and were limited by the unreliability of standard altimetry data in the coastal strip This is due to the influence of land and areas with different backscattering characteristics within the satellite footprint[15], which can negatively affect SWH measurements within about 20 km of the coast[16]. This, coupled with a conservative strategy to detect outliers, has brought strong improvements to the quality and the quantity of SWH retrievals

Methods

Results

Conclusion