• 6.6.2.1 Introduction
• 6.6.2.2 SEAICE configuration, compiling & running
  • 6.6.2.2.1 Compile-time options
  
  
• 6.6.2.3 Run-time parameters
  • 6.6.2.3.1 Enabling the package
  • 6.6.2.3.2 General flags and parameters
  
  
• 6.6.2.4 Description
• 6.6.2.5 Key subroutines
• 6.6.2.6 EXF diagnostics
• 6.6.2.7 Experiments and tutorials that use seaice

6.6.2 SEAICE Package

Authors: Martin Losch, Dimitris Menemenlis, An Nguyen, Jean-Michel Campin, Patrick Heimbach, Chris Hill and Jinlun Zhang

6.6.2.1 Introduction

Package ``seaice'' provides a dynamic and thermodynamic interactive sea-ice model.

CPP options enable or disable different aspects of the package (Section 6.6.2.2). Runtime options, flags, filenames and field-related dates/times are set in data.seaice (Section 6.6.2.3). A description of key subroutines is given in Section 6.6.2.5. Input fields, units and sign conventions are summarized in Section , and available diagnostics output is listed in Section .

6.6.2.2 SEAICE configuration, compiling & running

6.6.2.2.1 Compile-time options

As with all MITgcm packages, SEAICE can be turned on or off at compile time

• using the packages.conf file by adding seaice to it,
• or using genmake2 adding -enable=seaice or -disable=seaice switches
• required packages and CPP options:
  SEAICE requires the external forcing package exf to be enabled; no additional CPP options are required.

Parts of the SEAICE code can be enabled or disabled at compile time via CPP preprocessor flags. These options are set in either SEAICE_OPTIONS.h or in ECCO_CPPOPTIONS.h. Table 6.6.2.2 summarizes these options.

6.6.2.3 Run-time parameters

Run-time parameters are set in files data.pkg (read in packages_readparms.F), and data.seaice (read in seaice_readparms.F).

6.6.2.3.1 Enabling the package

6.6.2.3.2 General flags and parameters

6.6.2.4 Description

[TO BE CONTINUED/MODIFIED]

Sea-ice model thermodynamics are based on Hibler Hibler, III [1980], that is, a 2-category model that simulates ice thickness and concentration. Snow is simulated as per Zhang et al. Zhang et al. [1998]. 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 Semtner [1976] three-layer thermodynamic model that permits heat storage in ice. Recently, the three-layer thermodynamic model has been reformulated by Winton Winton [2000]. 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 Hibler, III [1979], the cavitating fluid (CF) model Flato and Hibler, III [1992], and the elastic-viscous-plastic (EVP) model Hunke and Dukowicz [1997]. 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 Zhang and Rothrock [2000] 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.

6.6.2.5 Key subroutines

Top-level routine: exf_getforcing.F

C     !CALLING SEQUENCE:
c ...
c  seaice_model (TOP LEVEL ROUTINE)
c  |
c  |-- #ifdef SEAICE_CGRID
c  |     SEAICE_DYNSOLVER
c  |     |
c  |     |-- < compute proxy for geostrophic velocity >
c  |     |
c  |     |-- < set up mass per unit area and Coriolis terms >
c  |     |
c  |     |-- < dynamic masking of areas with no ice >
c  |     |
c  |     |

c  |   #ELSE
c  |     DYNSOLVER
c  |   #ENDIF
c  |
c  |-- if ( useOBCS ) 
c  |     OBCS_APPLY_UVICE
c  |
c  |-- if ( SEAICEadvHeff .OR. SEAICEadvArea .OR. SEAICEadvSnow .OR. SEAICEadvSalt )
c  |     SEAICE_ADVDIFF
c  |
c  |-- if ( usePW79thermodynamics ) 
c  |     SEAICE_GROWTH
c  |
c  |-- if ( useOBCS ) 
c  |     if ( SEAICEadvHeff ) OBCS_APPLY_HEFF
c  |     if ( SEAICEadvArea ) OBCS_APPLY_AREA
c  |     if ( SEAICEadvSALT ) OBCS_APPLY_HSALT
c  |     if ( SEAICEadvSNOW ) OBCS_APPLY_HSNOW
c  |
c  |-- < do various exchanges >
c  |
c  |-- < do additional diagnostics >
c  |
c  o

6.6.2.6 EXF diagnostics

Diagnostics output is available via the diagnostics package (see Section 7.1). Available output fields are summarized in Table 6.6.2.6.

Table 6.16:  

6.6.2.7 Experiments and tutorials that use seaice

• Labrador Sea experiment in lab_sea verification directory.

Copyright © 2006 Massachusetts Institute of Technology

Last update 2008-09-06