ALBERT

Description: Direct numerical simulations are used to investigate the nature of fully resolved small-scale convection and its role in large-scale circulation in a rotating f -plane rectangular basin with imposed surface temperature difference. The large-scale circulation has a horizontal geostrophic component and a deep vertical overturning. This paper focuses on convective circulation with no wind stress, and buoyancy forcing sufficiently strong to ensure turbulent convection within the thermal boundary layer (horizontal Rayleigh numbers Ra ∼ 10 12 -10 13 ). The dynamics are found to depend on the value of a convective Rossby number, RoΔT , which represents the strength of buoyancy forcing relative to Coriolis forces. Vertical convection shifts from a mean endwall plume under weak rotation (R oΔT 〉 10 -1 ) to 'open ocean' chimney convection plus mean vertical plumes at the side boundaries under strong rotation (R oΔT 〈 10 -1 ). The overall heat throughput, horizontal gyre transport and zonally integrated overturning transport are then consistent with scaling predictions for flow constrained by thermal wind balance in the thermal boundary layer coupled to vertical advection-diffusion balance in the boundary layer. For small Rossby numbers relevant to circulation in an ocean basin, vertical heat transport from the surface layer into the deep interior occurs mostly in 'open ocean' chimney convection while most vertical mass transport is against the side boundaries. Both heat throughput and the mean circulation (in geostrophic gyres, boundary currents and overturning) are reduced by geostrophic constraints.. © 2019 Cambridge University Press.

Description: Convection in a rotating rectangular basin with differential thermal forcing at one horizontal boundary is examined using laboratory experiments. The experiments have an imposed heat flux boundary condition, are at large values of the flux Rayleigh number ( based on the box length ), use water with Prandtl number and have a small depth to length aspect ratio. The results show the conditions for transition from non-rotating horizontal convection governed by an inertial-buoyancy balance in the thermal boundary layer, to circulation governed by geostrophic flow in the boundary layer. The geostrophic balance constrains mean flow and reduces the heat transport as Nusselt number , where is the convective Rossby number, is the imposed buoyancy flux and is the Coriolis parameter. Thus flow in the geostrophic boundary layer regime is governed by the relative roles of horizontal convective accelerations and Coriolis accelerations, or buoyancy and rotation, in the boundary layer. Experimental evidence suggests that for more rapid rotation there is another transition to a regime in which the momentum budget is dominated by fluctuating vertical accelerations in a region of vortical plumes, which we refer to as a 'chimney' following related discussion of regions of deep convection in the ocean. Coupling of the chimney convection in the region of destabilising boundary flux to the diffusive boundary layer of horizontal convection in the region of stabilising boundary flux gives heat transport independent of rotation in this 'inertial chimney' regime, and the new scaling . Scaling analysis predicts the transition conditions observed in the experiments, as well as a further 'geostrophic chimney' regime in which the vertical plumes are controlled by local geostrophy. When 〈 [CDATA] Ro, the convection is also observed to produce a set of large basin-scale gyres at all depths in the time-averaged flow. © 2017 Cambridge University Press.

Description: Data have been obtained for a wave-driven current system in a closed basin. Owing to interaction of the longshore current with the sidewall a strong rip current was generated. The velocity distribution over the depth of the rip current appeared to be more or less uniform. The current system has been modelled by means of a mathematical model. The effect of bottom topography, bottom friction, convection and turbulent viscosity on the current system has been investigated. The conclusions are that the rip current is dominated by convection and that the bottom topography plays a role in the convergence and divergence of the streamlines. The order of magnitude of the velocities is largely determined by the bottom-friction coefficient. The velocity field is modified by viscosity. First, turbulent viscosity entrains fluid in the longshore current and into the rip current, secondly it permits turbulent boundary layers and thirdly it is responsible for the existence of closed streamlines outside the breaker zone. Finally the model and the conclusions are extrapolated to prototype conditions. © 1986, Cambridge University Press. All rights reserved.

Description: The fluvial development of the Roer river in the southeastern Netherlands and western Germany is presented for the Late Pleniglacial, Late-glacial and Early Holocene periods. Reconstruction of fluvial-style changes is based on geomorphological and sedimentological analysis. Time control comes from correlation to the pollen-based biochronostratigraphic framework of the Netherlands combined with independent optically stimulated luminescence (OSL) ages. At the Pleniglacial to Late-glacial transition a system and channel pattern change occurred from an aggrading braided to an incising meandering system. Rapid rates of meander migration, as established for the Late-glacial by optical dating, were likely related to the sandy nature of the substratum and the Late-glacial incision of the Meuse that resulted in a higher river gradient in the downstream part of the Roer. In the Roer valley the Younger Dryas cooling is not clearly reflected by a fluvial system response, but this may also be related to Holocene erosion of Younger Dryas fluvial forms. An important incision and terrace formation was established at the Younger Dryas to Early Holocene transition, probably related to forest recovery, reduced sediment supply and base-level lowering of the Meuse. The results of this study show a stepwise reduction in the number of channel courses from a multi-channel braided system in the Pleniglacial, to a double meander-belt system in the Late-glacial and a single-channel meandering system in the Early Holocene. The results show that the forcing factors of fluvial-system change in the Roer valley are climate change (precipitation, permafrost and vegetation) and downstream base-level control by the Meuse.