6.7 Sea Ice Package: ``seaice''

Package ``seaice'' provides a dynamic and thermodynamic interactive sea-ice model. Sea-ice model thermodynamics are based on Hibler [28], that is, a 2-category model that simulates ice thickness and concentration. Snow is simulated as per Zhang et al. [54]. Although recent years have seen an increased use of multi-category thickness distribution sea-ice models for climate studies, the Hibler 2-category ice model is still the most widely used model and has resulted in realistic simulation of sea-ice variability on regional and global scales. Being less complicated, compared to multi-category models, the 2-category model permits easier application of adjoint model optimization methods.

Note, however, that the Hibler 2-category model and its variants use a so-called zero-layer thermodynamic model to estimate ice growth and decay. The zero-layer thermodynamic model assumes that ice does not store heat and, therefore, tends to exaggerate the seasonal variability in ice thickness. This exaggeration can be significantly reduced by using Semtner's [46] three-layer thermodynamic model that permits heat storage in ice. Recently, the three-layer thermodynamic model has been reformulated by Winton [53]. The reformulation improves model physics by representing the brine content of the upper ice with a variable heat capacity. It also improves model numerics and consumes less computer time and memory. The Winton sea-ice thermodynamics have been ported to the MIT GCM; they currently reside under pkg/thsice. At present pkg/thsice is not fully compatible with pkg/seaice and with pkg/exf. But the eventual objective is to have fully compatible and interchangeable thermodynamic packages for sea-ice, so that it becomes possible to use Hibler dynamics with Winton thermodyanmics.

The ice dynamics models that are most widely used for large-scale climate studies are the viscous-plastic (VP) model [27], the cavitating fluid (CF) model [14], and the elastic-viscous-plastic (EVP) model [32]. Compared to the VP model, the CF model does not allow ice shear in calculating ice motion, stress, and deformation. EVP models approximate VP by adding an elastic term to the equations for easier adaptation to parallel computers. Because of its higher accuracy in plastic solution and relatively simpler formulation, compared to the EVP model, we decided to use the VP model as the dynamic component of our ice model. To do this we extended the alternating-direction-implicit (ADI) method of Zhang and Rothrock [55] for use in a parallel configuration.

The sea ice model requires the following input fields: 10-m winds, 2-m air temperature and specific humidity, downward longwave and shortwave radiations, precipitation, evaporation, and river and glacier runoff. The sea ice model also requires surface temperature from the ocean model and third level horizontal velocity which is used as a proxy for surface geostrophic velocity. Output fields are surface wind stress, evaporation minus precipitation minus runoff, net surface heat flux, and net shortwave flux. The sea-ice model is global: in ice-free regions bulk formulae are used to estimate oceanic forcing from the atmospheric fields.

Copyright © 2002 Massachusetts Institute of Technology