Yeon S. Chang

Improving Oceanic Overflow Representation in Climate Models: The Gravity Current Entrainment Climate Process Team

Abstract Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model dev...

Wave-formed sand ripples at Duck, North Carolina

A recently developed acoustic multiple transducer array was utilized to measure small-scale bed forms in the nearshore and inner shelf regions at Duck, North Carolina. Two populations of wave-formed ripples were observed: short wave ripples (SWR) with heights ranging from 3 mm to 2 cm and lengths ranging from 4 to 25 cm and long wave ripples (LWR) with heights ranging from 3 mm to 6 cm and lengths ranging from 35 to 200 cm. The SWR were only present sometimes, and their presence or absence was determined by a critical value of the near-bed mobility number. The SWR were highly dynamic, sometimes flattening during wave groups and reforming over several incident wave periods. The LWR, in contrast, were almost always present. They were longer and lower relief than predicted by models or generally observed previously. Both SWR and LWR were often observed to migrate shoreward but were rarely observed to migrate seaward. The dimensions of the SWR, when they were present, were predictable by the Nielsen [1981] model or the Wiberg and Harris [1994] model to within approximately a factor of 2.

Entrainment and suspension of sediments into a turbulent flow over ripples

We have analysed the trajectories of small particles released into a turbulent flow over a wavy wall in order to clarify the role of coherent structures in controlling the suspension and entrainment of sediments when ripples are present. LES was used to simulate the turbulent flow, while the motion of individual particles was calculated in a Lagrangian framework considering the effects of drag, lift and added mass. The results show that the process of suspension can be divided into two phases: the formation of a cloud in the lee of the ripples and the subsequent ejection of the particles across the shear layer capping the sediment cloud. Strong vortical structures on the upslope are shown to control the amounts of sediment entrained along the upslope, which feed the cloud. The implications for modelling are discussed.

Modeling unsteady turbulent flows over ripples: Reynolds‐averaged Navier‐Stokes equations (RANS) versus large‐eddy simulation (LES)

(1) In this paper we consider the problem of modeling a turbulent pulsating boundary layer over ripples. We compare the results of two modeling strategies, Wilcoxs kw Reynolds-Averaged Navier-Stokes equations (RANS) and large-eddy simulation (LES) employing the Lagrangian dynamic eddy viscosity model. The geometry and parameters employed are relevant to nearshore oceanic flows, and the results are discussed in relation to the problem of sediment transport. Generally, RANS and LES agree well only with regard to the vertical profiles of the streamwise component of the velocity. Large discrepancies were found in all the other quantities considered (e.g., vertical velocity, turbulent kinetic energy, and Reynolds stress). In particular, RANS severely underpredicted the magnitude of the Reynolds stress and overpredicted the amplitude of the oscillations in the vertical velocity. We also found that often the trends exhibited by RANS and LES when the frequency and/or amplitude of the driving conditions was varied were at odds. Since comparison with available experiments indicates that LES is able to accurately model this kind of flows, we conclude that the RANS model is not appropriate to model the suspension and transport of sediment under conditions similar to the ones presented in this study. INDEX TERMS: 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4558 Oceanography: Physical: Sediment transport; 3210 Mathematical Geophysics: Modeling; 3220 Mathematical Geophysics: Nonlinear dynamics; 3230 Mathematical Geophysics: Numerical solutions; KEYWORDS: turbulence, numerical modeling, sediment transport

Numerical Simulation of the Red Sea Outflow Using HYCOM and Comparison with REDSOX Observations

Abstract The outflow of warm, salty, and dense water from the Red Sea into the western Gulf of Aden is numerically simulated using the Hybrid Coordinate Ocean Model (HYCOM). The pathways of the modeled overflow, temperature, salinity, velocity profiles from stations and across sections, and transport estimates are compared to those observed during the 2001 Red Sea Outflow Experiment. As in nature, the simulated outflow is funneled into two narrow channels along the seafloor. The results from the three-dimensional simulations show a favorable agreement with the observed temperature and salinity profiles along the channels. The volume transport of the modeled overflow increases with the increasing distance from the southern exit of the Bab el Mandeb Strait due to entrainment of ambient fluid, such that the modeled transport shows a reasonable agreement with that estimated from the observations. The initial propagation speed of the outflow is found to be smaller than the estimated interfacial wave speed. The...