Phillip J. Wolfram

Diagnosing Isopycnal Diffusivity in an Eddying, Idealized Midlatitude Ocean Basin via Lagrangian, in Situ, Global, High-Performance Particle Tracking (LIGHT)

AbstractIsopycnal diffusivity due to stirring by mesoscale eddies in an idealized, wind-forced, eddying, midlatitude ocean basin is computed using Lagrangian, in Situ, Global, High-Performance Particle Tracking (LIGHT). Simulation is performed via LIGHT within the Model for Prediction across Scales Ocean (MPAS-O). Simulations are performed at 4-, 8-, 16-, and 32-km resolution, where the first Rossby radius of deformation (RRD) is approximately 30 km. Scalar and tensor diffusivities are estimated at each resolution based on 30 ensemble members using particle cluster statistics. Each ensemble member is composed of 303 665 particles distributed across five potential density surfaces. Diffusivity dependence upon model resolution, velocity spatial scale, and buoyancy surface is quantified and compared with mixing length theory. The spatial structure of diffusivity ranges over approximately two orders of magnitude with values of O(105) m2 s−1 in the region of western boundary current separation to O(103) m2 s−1...

A thickness-weighted average perspective of force balance in an idealized circumpolar current

AbstractThe exact, three-dimensional, thickness-weighted averaged (TWA) Boussinesq equations are used to diagnose eddy–mean flow interaction in an idealized circumpolar current (ICC). The force exerted by mesoscale eddies on the TWA velocity is expressed as the divergence of the Eliassen–Palm flux tensor. Consistent with previous findings, the analysis indicates that the dynamically relevant definition of the ocean surface layer is composed of the set of buoyancy coordinates that ever reside at the ocean surface at a given horizontal position. The surface layer is found to be a physically distinct object with a diabatic and force balance that is largely isolated from the underlying adiabatic region in the interior. Within the ICC surface layer, the TWA meridional velocity is southward/northward in the top/bottom half and has a value near zero at the bottom. In the top half of the surface layer, the zonal forces due to wind stress and meridional advection of potential vorticity act to accelerate the TWA zo...

Quantifying residual, eddy, and mean flow effects on mixing in an idealized circumpolar current

AbstractMeridional diffusivity is assessed for a baroclinically unstable jet in a high-latitude idealized circumpolar current (ICC) using the Model for Prediction across Scales Ocean (MPAS-O) and the online Lagrangian in Situ Global High-Performance Particle Tracking (LIGHT) diagnostic via space–time dispersion of particle clusters over 120 monthly realizations of O(106) particles on 11 potential density surfaces. Diffusivity in the jet reaches values of O(6000) m2 s−1 and is largest near the critical layer supporting mixing suppression and critical layer theory. Values in the vicinity of the shelf break are suppressed to O(100) m2 s−1 because of the presence of westward slope front currents. Diffusivity attenuates less rapidly with depth in the jet than both eddy velocity and kinetic energy scalings would suggest. Removal of the mean flow via high-pass filtering shifts the nonlinear parameter (ratio of the eddy velocity to eddy phase speed) into the linear wave regime by increasing the eddy phase speed v...

Longstanding Design: Roman Engineering of Aqueduct Arcades

Abstract Ancient Roman engineers are famous for their use of arcades, or arched bridges, to elevate roads and aqueducts across valleys. The Romans first designed and built arches using stone blocks and then later changed to special concrete construction. In this paper, we illustrate how early unmortared arches could have been built using geometric design principles. This simplified analysis is sufficient to demonstrate, via retrospective analysis, the need for reconstruction efforts for arches violating these design criteria. Well-preserved remnants at the Simian Bridge and Vallon des Arcs arcades, located in southern France near Arles, are excellent examples of Roman arch engineering and are used as case studies to understand geometric arch design in practice. These arcades are vital parts of a larger aqueduct system used to provide the water-powered industrial Barbegal Mill and city of Arles with water in the first century A.D. Analysis of currently standing arches implies that Roman engineers may have relied extensively upon geometrical rules of thumb for design of these structures. In particular, modern stability analysis demonstrates the effectiveness of geometrical rules of thumb in determining structural failure requiring reconstruction at the Simian Bridge. The reconstruction that exists at the Simian bridge shows a shift from stone blocks to concrete. This shift may have occurred as a result of the Roman engineers’ increased awareness of geometrical criteria for arch design.