Space‐time structure of long ocean swell fields

Abstract

[1] The space-time structure of long-period ocean swell fields is investigated, with particular attention given to features in the direction orthogonal to the propagation direction. This study combines space-borne synthetic aperture radar (SAR) data with numerical model hindcasts and time series recorded by in situ instruments. In each data set the swell field is defined by a common storm source. The correlation of swell height time series is very high along a single great circle path with a time shift given by the deep water dispersion relation of the dominant swells. This correlation is also high for locations situated on different great circles in entire ocean basins. Given the Earth radius R, we define the distance from the source Ra and the transversal angle b so that a and b would be equal the colatitude and longitude for a storm centered on the North Pole. Outside of land influence, the swell height field at time t, H ss (a, b,t) is well approximated by a function H ss,0 (t -Ra/C g )/ ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi sin ð Þ ð Þ p times another function r 2 (b), where C g is a representative group speed. Here r 2 (b) derived from SAR data is very broad, with a width at half the maximum that is larger than 70°, and varies significantly from storm to storm. Land shadows introduce further modifications so that in general r 2 is a function of b and a. This separation of variables and the smoothness of the H ss field, allows the estimation of the full field of H ss from sparse measurements, such as wave mode SAR data, combined with one time series, such as that provided by a single buoy. A first crude estimation of a synthetic H ss field based on this principle already shows that swell hindcasts and forecasts can be improved by assimilating such synthetic observations.

Limitation of SAR Quasi-Linear Inversion Data on Swell Climate: An Example of Global Crossing Swells

“…Spectra are partitioned by the method found in [20], with at most one wind-sea component and five swell components remaining. It should be noted that the partitioning methods of [18] and [20] give equivalent results [21]. The accuracy of wave parameters of the model output has been verified against measurements from buoys and altimeters, both showing good agreement (e.g., [19,21,22]).…”

“…It should be noted that the partitioning methods of [18] and [20] give equivalent results [21]. The accuracy of wave parameters of the model output has been verified against measurements from buoys and altimeters, both showing good agreement (e.g., [19,21,22]). The data can be downloaded from the IFREMER FTP server, where more detailed information is available [23].…”

“…To investigate the extent to which the SAR records would be impacted by such problems, the spectral distances between the 1st swell in the SAR data and the 1st swell in the WW3 data are calculated using the definition in [21]:…”

Three-dimensional modelling of wave-induced current from the surf zone to the inner shelf

“…One can find more information about the parameterizations proposed by this version in Ardhuin et al (2010). This model has been validated using in-situ and remote sensing data (Ardhuin et al, 2008aDelpey et al, 2010).…”

Ocean swell within the kinetic equation for water waves

“…We also reveal a remarkable feature of collapsing the swell spectra onto an angular distribution that depends weakly on initial angular spreading. Such universality can be of great value for modeling swell and developing methods for its monitoring (Delpey et al, 2010).…”