47 research outputs found
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Comparison of elastic-viscous-plastic and viscous-plastic dynamics models using a high resolution Arctic sea ice model
No full textA nonlinear viscous-plastic (VP) rheology proposed by Hibler (1979) has been demonstrated to be the most suitable of the rheologies commonly used for modeling sea ice dynamics. However, the presence of a huge range of effective viscosities hinders numerical implementations of this model, particularly on high resolution grids or when the ice model is coupled to an ocean or atmosphere model. Hunke and Dukowicz (1997) have modified the VP model by including elastic waves as a numerical regularization in the case of zero strain rate. This modification (EVP) allows an efficient, fully explicit discretization that adapts well to parallel architectures. The authors present a comparison of EVP and VP dynamics model results from two 5-year simulations of Arctic sea ice, obtained with a high resolution sea ice model. The purpose of the comparison is to determine how differently the two dynamics models behave, and to decide whether the elastic-viscous-plastic model is preferable for high resolution climate simulations, considering its high efficiency in parallel computation. Results from the first year of this experiment (1990) are discussed in detail in Hunke and Zhang (1997)
What controls primary production in the Arctic Ocean? Results from an intercomparison of five general circulation models with biogeochemistry
No full textAs a part of Arctic Ocean Intercomparison Project, results from five coupled physical and biological ocean models were compared for the Arctic domain, defined here as north of 66.6°N. The global and regional (Arctic Ocean (AO)–only) models included in the intercomparison show similar features in terms of the distribution of present-day water column–integrated primary production and are broadly in agreement with in situ and satellite-derived data. However, the physical factors controlling this distribution differ between the models. The intercomparison between models finds substantial variation in the depth of winter mixing, one of the main mechanisms supplying inorganic nutrients over the majority of the AO. Although all models manifest similar level of light limitation owing to general agreement on the ice distribution, the amount of nutrients available for plankton utilization is different between models. Thus the participating models disagree on a fundamental question: which factor, light or nutrients, controls present-day Arctic productivity. These differences between models may not be detrimental in determining present-day AO primary production since both light and nutrient limitation are tightly coupled to the presence of sea ice. Essentially, as long as at least one of the two limiting factors is reproduced correctly, simulated total primary production will be close to that observed. However, if the retreat of Arctic sea ice continues into the future as expected, a decoupling between sea ice and nutrient limitation will occur, and the predictive capabilities of the models may potentially diminish unless more effort is spent on verifying the mechanisms of nutrient supply. Our study once again emphasizes the importance of a realistic representation of ocean physics, in particular vertical mixing, as a necessary foundation for ecosystem modeling and predictions
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The time-dependent transformed Eliassen balanced vortex model of a tropical cyclone.
No full textThe time-dependent, transformed Eliassen balanced vortex model of a tropical cyclone is analyzed mathematically and integrated numerically. The simulated vortices have characteristics resembling aspects of real tropical cyclones. Based on the principle of potential vorticity invertibility, the baroclinic model vortex evolves on an f-plane, assumes Boussinesq and hydrostatic approximations, and is posed in absolute angular momentum coordinates with circular symmetry. Theoretical analysis of the model equations is used to derive efficient numerical methods. Linear operator theory is applied to elliptic, diagnostic equations for the tangential velocity potential function and the transverse circulation streamfunction, which are then solved by successive line overrelaxation methods. Equations for the potential vorticity and potential temperature are solved using a fourth-order Runge Kutta method. A maximum growth rate estimated for the potential vorticity equation limits the size of the timestep, yet reveals the presence of exponentially growing modes for given profiles of the thermal forcing. Initial potential vorticity and potential temperature distributions are based on a mesocyclone study. Vortex evolution is computed for a specified forcing that emulates condensational heating and a heating function parameterized in terms of the model variables. The specified heating produces a realistic circulation but does not allow the vortex to decay. When a parameterized heating function is used, the transverse circulation requires external forcing through frictionally induced vertical motion at the surface boundary. Mathematical and physical descriptions of the boundary conditions are discussed and compared; the surface conditions are insufficient for maintenance or amplification of the vortex under the parameterized heating. In addition to analyzing physical and dynamical relationships among model variables, the simulations can be used as a 2-D basic state for a perturbation study using 3-D primitive equations. Such an investigation would illuminate nonaxisymmetric features of hurricanes such as rain bands and outflow jets.This item was digitized from a paper original and/or a microfilm copy. If you need
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A numerical study of the western Cosmonaut polynya in a coupled ocean-sea ice model
No full textThe article of record as published may be found at http://dx.doi.org/10.1029/2004JC002858Employing results from a 0.4° , 40-level fully global, coupled ocean–sea ice model, we investigated the role of physical processes emanating from atmosphere, ocean, and ice in the initiation, maintenance, and termination of a sensible heat polynya with a focus on the western Cosmonaut polynya that occurred during May–July 1999. The Cosmonaut polynya first appeared in early May 1999 in the form of an ice-free embayment, transformed into an enclosed polynya on 5–9 July, and disappeared by late July, when the ice from the surrounding regions began to encircle the embayment. Except for the differences in ice concentrations, the time of appearance, size, and shape of the Cosmonaut polynya simulated by the model are in approximate agreement with the Special Sensor Microwave/Imager (SSM/I) observations. Between May and July 1999 the Cosmonaut Sea experienced two synoptic storms, both lasting 5 days. Followed by the passage of the first storm on 12–19 June, there was a remarkable growth in the size of the embayment by 21 103 km2. Associated with this, the sea surface temperature (SST) rose by 0.15° C, the upward heat flux jumped from 5 to 94 W m(-2), and a net freshwater flux into the ocean increased by 2 cm d(-1). By running the model simulation with a 20% wind speed increase, it is demonstrated that the twofold increase in SST and upward heat flux increased the embayment area by 15 X 10(3) km(2) and decreased the ice concentration by approximately 10% from the control run. A similar, but somewhat
weaker wind event that took place on 30 June to 10 July had less influence on the embayment area although the upward heat flux (65 W m(2)) was comparable to the first event. By examining the vertical displacement of the 1.6° C isotherm depth prior to, during, and after these two storms, we demonstrate that the impetus provided by these storms was able to raise the 1.6° C isotherm depth by 30 m through wind-driven mixing, making sufficient oceanic heat input from beneath the mixed layer available
to prevent freezing and/or delay ice formation while ice in the adjacent regions continued to grow. A sudden shift in the ice drift direction from southwest to northeast (3 July) followed by the second storm, accompanied by large air-sea temperature differences, caused the enclosure of the embayment, subsequent formation of the polynya, and its termination.OPP9980607 (NSF)N0001404WR20097 (ONR)National Science FoundationOffice of Naval ResearchDepartment of Energy (CCPP
