Rainer Bleck

A comparison of data-parallel and message-passing versions of the Miami Isopycnic Coordinate Ocean Model (MICOM)

Abstract A two-pronged effort to convert a recently developed ocean circulation model written in Fortran-77 for execution on massively parallel computers is described. A data-parallel version was developed for the CM-5 manufactured by Thinking Machines, Inc., while a message-passing version was developed for both the Cray T3D and the Silicon Graphics ONYX workstation. Since the time differentiation scheme in the ocean model is fully explicit and does not require solution of elliptic partial differential equations, adequate machine utilization has been achieved without major changes to the original algorithms. We developed a partitioning strategy for the message passing version that significantly reduces memory requirements and increases model speed. On a per-node basis (a T3D node is one Alpha processor, a CM-5 node is one Sparc chip and four vector units), the T3D and CM-5 are found to execute our “large” model version consisting of 511 × 511 horizontal mesh points at roughly the same speed.

Ocean Modeling in Isopycnic Coordinates

Electronic computers were made available to civilian researchers in the late 1940s, shortly after World War II. Numerical Weather Prediction — at that time the repetitive solution of a 2-dimensional elliptic partial differential equation describing the evolution of the height of a mid-tropospheric isobaric surface — was among the first mathematical problems tackled with that new technology.

Jet Streak Dynamics and Geostrophic Adjustment Processes During the Initial Stages of Lee Cyclogenesis

Abstract A numerical experiment is described which explores the relationship between upper-level potential vorticity advection and cyclogenesis on the leeward side of mountain barriers. A multilevel primitive equation model framed in isentropic coordinates is used to simulate the growth of a wave disturbance on the cold front associated with a preexisting “parent” cyclone. The effect of a mountain barrier placed in the path of the advancing cold front and the effect of an enhanced upper-level jet streak on the growth rate of the disturbance are investigated. Enhancement of the jet streak is accomplished by altering the geostrophic potential vorticity in a region upstream of the mountain barrier and solving for the corresponding man and geostrophic velocity field. The experiment suggests a strong connection between the intensity of the jet weak impinging on the barrier and the pressure fall in the lee. We also find that the strongest leeside pressure fall in this experiment is not accompanied by a conversi...

The Inclusion of a Surface Mixed Layer in a Large-Scale Circulation Model

Abstract This paper is concerned with a numerical model that combines an adiabatic, baroclinic interior ocean with a thermodynamically active mixed layer of variable depth. The interior ocean is specified in terms of isopycnic coordinates. An adjustment procedure that permits matching of these two domains during periods of mixed layer shallowing is described. Discussion of the models formulation is followed by a presentation of some preliminary model results.

Study of Transport Fluctuations and Meandering of The Florida Current using an Isopycnic Coordinate Numerical Model

Abstract An isopycnic coordinate numerical model, using the Flux-Corrected Transport algorithm to control isopycnal outcropping and intersection with the ocean bottom, is configured in a channel with the bottom topography of the Florida Straits at 27 ° N. Bulk parameters determined from analysis of observations in the Subtropical Atlantic Climate Studies (STACS) program are combined with a dynamical initialization procedure, generating a Florida Current-like mass/flow pattern. Ten isopycnal layers and 2 km horizontal grid spacing resolve this cross-sectional flow. The channel is created by duplicating the cross-section in the downstream direction. Investigation focuses on whether the transport fluctuations on time scales of a few days to several weeks and the meandering observed in the Florida Current may be either locally forced by the wind or due to inherent dynamical instabilities. When a single cross-section is forced with spatially constant but temporally fluctuating downstream wind stress, the barotropic transport response exhibits an almost 90 ° phase lag to the wind. The response amplitude is slightly less than directly proportional to the amplitude and period of the forcing. The baroclinic response, defined as the difference between the transport flucutations above and below 200 m depth, exhibits the almost 90 ° phase lag to the wind, but has an order of magnitude less amplitude than the barotropic. In the three-dimensional channel, when the cross-stream density gradient is strong enough, perturbations with wavelengths greater than 60 km exhibit substantial amplification. The current is most unstable to wavelengths of a little more than 200 km when using the dynamically initialized fields and 150 km when using analyzed STACS Pegasus data to initialize. Events of wave amplification are of limited duration, however, and leave the basic structure of the current unchanged. In corresponding flat bottom channel experiments, the perturbation continues to amplify until the baroclinic structure has been substantially modified. From an analysis of energy conversions it is concluded that the primary mechanism of wave amplification in all cases is the release of baroclinic instability.

Development and Evaluation of a Global Version of the Miami Isopycnic-Coordinate Ocean Model. Final report

The objective of this project was to test the ability of the Miami Isopycnic-Coordinate Ocean Model (MICOM) to simulate the global ocean circulation, setting the stage for the models incorporation into coupled global climate models. An existing basin-scale model will be expanded to global domain; suitable atmospheric forcing fields, including precipitation and river runoff, will be selected; the modeling of ayssal flow will be improved by incorporating compressibility and particularly thermobaric effects; a sea-ice model will be added; parameterization options will be explored for subgrid-scale deep convection; parallel coarse- and fine-mesh simulations will be carried out to investigate the impact of grid resolution; the sensitivity of the models solution to magnitude of vertical (diapycnal) exchange coefficient will be studied; and long-term trends in meridional heat transport and water-mass properties in model solutions will be documented and interpreted.