Modeling of very low frequency motions during RIPEX

Abstract

Numerical computations are used to explain the presence of very low frequency motions (VLFs), with frequencies less than 0.004 Hz, in the rip current velocity signals observed during the Rip‐current field Experiment (RIPEX) field experiment. Observations show that the VLF motions are most intense within the surfzone and then quickly taper off in the offshore direction. By comparing computed VLF intensity (URMS,vlf) distributions in both the cross‐shore and alongshore direction with observations in a qualitative sense, the most important contributions to the VLF dynamics are established. VLF motions at neighboring rip‐channels are seen to interact in the computations, with stronger surfzone intensity for increasing bathymetric variation. The intermittent forcing by spatially varying wave groups is essential in obtaining the correct URMS,vlf distribution in the cross‐shore direction, suggesting this is the predominant mechanism responsible for the generation of the VLF motions observed during RIPEX. Computations also suggest that VLF motions can occasionally propagate offshore but are mostly confined to the surfzone corresponding to surfzone eddies. A quantitative comparison shows good correspondence between model computations and measurements of URMS,vlf with a model skill of O(0.7), with generally increased (decreased) URMS,vlf during mean low (high) water levels.