8.6.3. SHELFICE Package¶
Authors: Martin Losch, JeanMichel Campin
8.6.3.1. Introduction¶
pkg/shelfice provides a thermodynamic model for basal melting underneath floating ice shelves.
CPP options enable or disable different aspects of the package (Section 8.6.3.2). Runtime options, flags, filenames and fieldrelated dates/times are described in Section 8.6.3.3. A description of key subroutines is given in Section 8.6.3.5. Available diagnostics output is listed in Section 8.6.3.6.
8.6.3.2. SHELFICE configuration¶
As with all MITgcm packages, pkg/shelfice can be turned on or off at compile time:
using the
packages.conf
file by addingshelfice
to it,or using genmake2 adding
enable=shelfice
ordisable=shelfice
switches
pkg/shelfice does not require any additional packages, but it will only work with conventional vertical \(z\)coordinates (pressure coordinates are not implemented). If you use it together with vertical mixing schemes, be aware that nonlocal parameterizations are turned off, e.g., such as pkg/kpp.
Parts of the pkg/shelfice code can be enabled or disabled at compile time via CPP preprocessor flags. These options are set in SHELFICE_OPTIONS.h:
CPP Flag Name 
Default 
Description 

#undef 
include code for enhanced diagnostics and debug output 

#define 
include code for for simplified ISOMIP thermodynamics 

#define 
allow friction velocitydependent transfer coefficient following Holland and Jenkins (1999) [HJ99] 
8.6.3.3. SHELFICE runtime parameters¶
pkg/shelfice is switched on/off at runtime by setting useSHELFICE to .TRUE.
in file data.pkg
.
Runtime parameters are set in file data.shelfice
(read in pkg/shelfice/shelfice_readparms.F),as listed below.
The data file specifying underice topography of ice shelves (SHELFICEtopoFile) is in meters; upwards is positive, and as for the bathymetry files, negative values are required for topography below the sealevel. The data file for the pressure load anomaly at the bottom of the ice shelves SHELFICEloadAnomalyFile is in pressure units (Pa). This field is absolutely required to avoid large excursions of the free surface during initial adjustment processes, obtained by integrating an approximate density from the surface at \(z=0\) down to the bottom of the last fully dry cell within the ice shelf, see (8.28). Note however the file SHELFICEloadAnomalyFile must not be \(p_{top}\), but \(p_{top}g\sum_{k'=1}^{n1}\rho_{0}\Delta{z}_{k'}\), with \(\rho_{0}=\) rhoConst, so that in the absence of a \(\rho^{*}\) that is different from \(\rho_{0}\), the anomaly is zero.
Parameter 
Group 
Default 
Description 

SHELFICE_PARM01 
FALSE 
use simplified ISOMIP thermodynamics on/off flag 

SHELFICE_PARM01 
FALSE 
use conservative form of temperature boundary conditions on/off flag 

SHELFICE_PARM01 
FALSE 
use simple boundary layer mixing parameterization on/off flag 

SHELFICE_PARM01 
FALSE 
with SHELFICEboundaryLayer, allow to use realFW flux 

SHELFICE_PARM01 
FALSE 
with SHELFICEboundaryLayer, compute uStar from uVel,vVel averaged over top Dz thickness 

SHELFICE_PARM01 
' ' 
initial geopotential anomaly 

SHELFICE_PARM01 
' ' 
filename for underice topography of ice shelves 

SHELFICE_PARM01 
' ' 
filename for mass of ice shelves 

SHELFICE_PARM01 
' ' 
filename for mass tendency of ice shelves 

SHELFICE_PARM01 
' ' 
filename for spatially varying transfer coefficients 

SHELFICE_PARM01 
334.0E+03 
latent heat of fusion (J/kg) 

SHELFICE_PARM01 
2000.0E+00 
specific heat capacity of ice (J/kg/K) 

SHELFICE_PARM01 
917.0E+00 
(constant) mean density of ice shelf (kg/m^{3}) 

SHELFICE_PARM01 
1.0E04 
transfer coefficient (exchange velocity) for temperature (m/s) 

SHELFICE_PARM01 
5.05E03 \(\times\) SHELFICEheatTransCoeff 
transfer coefficient (exchange velocity) for salinity (m/s) 

SHELFICE_PARM01 
1.54E06 
temperature diffusion coefficient of the ice shelf (m^{2}/s) 

SHELFICE_PARM01 
20.0E+00 
(constant) surface temperature above the ice shelf (^{o}C) 

SHELFICE_PARM01 
slip along bottom of ice shelf on/off flag 

SHELFICE_PARM01 
linear drag coefficient at bottom ice shelf (m/s) 

SHELFICE_PARM01 
quadratic drag coefficient at bottom ice shelf (nondim.) 

SHELFICE_PARM01 
1 
select form of quadratic drag coefficient (nondim.) 

SHELFICE_PARM01 
FALSE 
recalculate ice shelf mass at every time step 

SHELFICE_PARM01 
FALSE 
if SHELFICEmassStepping = TRUE, exclude freshwater flux contribution 

SHELFICE_PARM01 
FALSE 
use advectivediffusive heat flux into ice shelf instead of default diffusive heat flux 

SHELFICE_PARM01 
FALSE 
use velocity dependent exchange coefficients (Holland and Jenkins 1999 [HJ99]) 

SHELFICE_PARM01 
FALSE 
use old uStar averaging expression 

SHELFICE_PARM01 
FALSE 
write ice shelf state to file on/off flag 

SHELFICE_PARM01 
dump frequency (s) 

SHELFICE_PARM01 
write snapshot using MNC on/off flag 
8.6.3.4. SHELFICE description¶
In the light of isomorphic equations for pressure and height coordinates, the ice shelf topography on top of the water column has a similar role as (and in the language of Marshall et al. (2004) [MAC+04], is isomorphic to) the orography and the pressure boundary conditions at the bottom of the fluid for atmospheric and oceanic models in pressure coordinates. The total pressure \(p_{tot}\) in the ocean can be divided into the pressure at the top of the water column \(p_{top}\), the hydrostatic pressure and the nonhydrostatic pressure contribution \(p_{NH}\):
with the gravitational acceleration \(g\), the density \(\rho\), the vertical coordinate \(z\) (positive upwards), and the dynamic seasurface height \(\eta\). For the open ocean, \(p_{top}=p_{a}\) (atmospheric pressure) and \(h=0\). Underneath an iceshelf that is assumed to be floating in isostatic equilibrium, \(p_{top}\) at the top of the water column is the atmospheric pressure \(p_{a}\) plus the weight of the iceshelf. It is this weight of the iceshelf that has to be provided as a boundary condition at the top of the water column (in runtime parameter SHELFICEloadAnomalyFile). The weight is conveniently computed by integrating a density profile \(\rho^*\), that is constant in time and corresponds to the seawater replaced by ice, from \(z=0\) to a “reference” iceshelf draft at \(z=h\) (Beckmann et al. (1999) [BHT99]), so that
Underneath the ice shelf, the “seasurface height” \(\eta\) is the deviation from the “reference” iceshelf draft \(h\). During a model integration, \(\eta\) adjusts so that the isostatic equilibrium is maintained for sufficiently slow and large scale motion.
In MITgcm, the total pressure anomaly \(p'_{tot}\) which is used for pressure gradient computations is defined by substracting a purely depth dependent contribution \(g\rho_{0}z\) with a constant reference density \(\rho_{0}\) from \(p_{tot}\). (8.25) becomes
and after rearranging
with \(p'_{tot} = p_{tot} + g\,\rho_0\,z\) and \(p'_{top} = p_{top}  g\,\rho_0\,h\). The nonhydrostatic pressure contribution \(p_{NH}\) is neglected in the following.
In practice, the ice shelf contribution to \(p_{top}\) is computed by integrating (8.26) from \(z=0\) to the bottom of the last fully dry cell within the ice shelf:
where \(n\) is the vertical index of the first (at least partially) “wet” cell and \(\Delta{z_{k'}}\) is the thickness of the \(k'\)th layer (counting downwards). The pressure anomaly for evaluating the pressure gradient is computed in the center of the “wet” cell \(k\) as
where \(H(k'k)=1\) for \(k'<k\) and \(0\) otherwise.
Setting SHELFICEboundaryLayer =.TRUE.
introduces a simple boundary layer that reduces the potential
noise problem at the cost of increased vertical mixing. For this purpose
the water temperature at the \(k\)th layer abutting ice shelf
topography for use in the heat flux parameterizations is computed as a
mean temperature \(\overline{\theta}_{k}\) over a boundary layer of
the same thickness as the layer thickness \(\Delta{z}_{k}\):
where \(h_{k}\in[0,1]\) is the fractional layer thickness of the \(k\)th layer (see Figure 8.11). The original contributions due to ice shelfocean interaction \(g_{\theta}\) to the total tendency terms \(G_{\theta}\) in the timestepping equation \(\theta^{n+1} = f(\theta^{n},\Delta{t},G_{\theta}^{n})\) are
for layers \(k\) and \(k+1\) (\(c_{p}\) is the heat capacity). Averaging these terms over a layer thickness \(\Delta{z_{k}}\) (e.g., extending from the ice shelf base down to the dashed line in cell C) and applying the averaged tendency to cell A (in layer \(k\)) and to the appropriate fraction of cells C (in layer \(k+1\)) yields
(8.33) describes averaging over the part of the grid cell \(k+1\) that is part of the boundary layer with tendency \(g_{\theta,k}^*\) and the part with no tendency. Salinity is treated in the same way. The momentum equations are not modified.
8.6.3.4.1. Threeequations thermodynamics¶
Freezing and melting form a boundary layer between ice shelf and ocean. Phase transitions at the boundary between saline water and ice imply the following fluxes across the boundary: the freshwater mass flux \(q\) (\(<0\) for melting); the heat flux that consists of the diffusive flux through the ice, the latent heat flux due to melting and freezing and the heat that is carried by the mass flux; and the salinity that is carried by the mass flux, if the ice has a nonzero salinity \(S_I\). Further, the position of the interface between ice and ocean changes because of \(q\), so that, say, in the case of melting the volume of sea water increases. As a consequence salinity and temperature are modified.
The turbulent exchange terms for tracers at the iceocean interface are generally expressed as diffusive fluxes. Following Jenkins et al. (2001) [JHH01], the boundary conditions for a tracer take into account that this boundary is not a material surface. The implied upward freshwater flux \(q\) (in mass units, negative for melting) is included in the boundary conditions for the temperature and salinity equation as an advective flux:
where tracer \(X\) stands for either temperature \(T\) or salinity \(S\). \(X_b\) is the tracer at the interface (taken to be at freezing), \(X\) is the tracer at the first interior grid point, \(\rho\) is the density of seawater, and \(\gamma_X\) is the turbulent exchange coefficient (in units of an exchange velocity). The left hand side of (8.34) is shorthand for the (downward) flux of tracer \(X\) across the boundary. \(T_b\), \(S_b\) and the freshwater flux \(q\) are obtained from solving a system of three equations that is derived from the heat and freshwater balance at the ice ocean interface.
In this socalled threeequationmodel (e.g., Hellmer and Olbers (1989) [HO89], Jenkins et al. (2001) [JHH01]) the heat balance at the iceocean interface is expressed as
where \(\rho\) is the density of seawater, \(c_{p} = 3974 \, \text{J kg}^{1} \text{K}^{1}\) is the specific heat capacity of water and \(\gamma_T\) the turbulent exchange coefficient of temperature. The value of \(\gamma_T\) is discussed in Holland and Jenkins (1999) [HJ99]. \(L = 334000 \, \text{J kg}^{1}\) is the latent heat of fusion. \(\rho_{I} = 920 \, \text{kg m}^{3}\), \(c_{p,I} = 2000 \, \text{J kg}^{1} \text{K}^{1}\), and \(T_{S}\) are the density, heat capacity and the surface temperature of the ice shelf; \(\kappa=1.54\times10^{6} \, \text{m}^2 \text{s}^{1}\) is the heat diffusivity through the iceshelf and \(h\) is the iceshelf draft. The second term on the right hand side describes the heat flux through the ice shelf. A constant surface temperature \(T_S=20^{\circ}\text{C}\) is imposed. \(T\) is the temperature of the model cell adjacent to the icewater interface. The temperature at the interface \(T_{b}\) is assumed to be the insitu freezing point temperature of seawater \(T_{f}\) which is computed from a linear equation of state
with the salinity \(S_{b}\) and the pressure \(p_{b}\) (in dBar) in the cell at the icewater interface. From the salt budget, the salt flux across the shelf iceocean interface is equal to the salt flux due to melting and freezing:
where \(\gamma_S = 5.05\times10^{3}\gamma_T\) is the turbulent salinity exchange coefficient, and \(S\) and \(S_{b}\) are defined in analogy to temperature as the salinity of the model cell adjacent to the icewater interface and at the interface, respectively. Note, that the salinity of the ice shelf is generally neglected (\(S_{I}=0\)). (8.35) to (8.37) can be solved for \(S_{b}\), \(T_{b}\), and the freshwater flux \(q\) due to melting. These values are substituted into expression (8.34) to obtain the boundary conditions for the temperature and salinity equations of the ocean model. This formulation tends to yield smaller melt rates than the simpler formulation of the ISOMIP protocol because the freshwater flux due to melting decreases the salinity which raises the freezing point temperature and thus leads to less melting at the interface. For a simpler thermodynamics model where \(S_b\) is not computed explicitly, for example as in the ISOMIP protocol, (8.34) cannot be applied directly. In this case (8.37) can be used with (8.34) to obtain:
This formulation can be used for all cases for which (8.37) is valid. Further, in this formulation it is obvious that melting (\(q<0\)) leads to a reduction of salinity.
The default value of SHELFICEconserve =.FALSE.
removes the contribution \(q\, ( X_{b}X )\) from
(8.34), making the boundary conditions for temperature
nonconservative.
8.6.3.4.2. Solving the threeequations system¶
There has been some confusion about the threeequations system, so we document the solution in the code here: We use (8.36) \(T_{b} = a_{0} S_{b} + \epsilon_{4}\) to eliminate \(T_{b}\) from (8.35) and find an expression for the freshwater flux \(q\):
to be substituted into (8.37):
where the abbrevations \(\epsilon_{1} = c_{p} \rho \gamma_{T}\), \(\epsilon_{2} = \rho L \gamma_{S}\), \(\epsilon_{3} = \frac{\rho_{I} c_{p,I} \kappa}{h}\), \(\epsilon_{4}=b_{0}p + c_{0}\), \(\epsilon_{q} = \epsilon_{1}\,(\epsilon_{4}  T) + \epsilon_{3}\,(\epsilon_{4}  T_{S})\) have been introduced. The quadratic equation in \(S_{b}\) is solved and the smaller nonnegative root is used. In the MITgcm code, the ice shelf salinity \(S_{I}\) is always zero and the quadratic equation simplifies to
With \(S_b\), the boundary layer temperature \(T_b\) and the melt rate \(q\) are known through (8.36) and (8.38).
8.6.3.4.3. ISOMIP thermodynamics¶
A simpler formulation follows the ISOMIP protocol. The freezing and melting in the boundary layer between ice shelf and ocean is parameterized following Grosfeld et al. (1997) [GGD97]. In this formulation (8.35) reduces to
and the fresh water flux \(q\) is computed from
In order to use this formulation, set runtime parameter useISOMIPTD =.TRUE.
in
data.shelfice
.
8.6.3.4.4. Exchange coefficients¶
The default exchange coefficents \(\gamma_{T/S}\) are constant and
set by the runtime parameters SHELFICEheatTransCoeff and
SHELFICEsaltTransCoeff (see
Table 8.19). If
SHELFICEuseGammaFrict =.TRUE.
, exchange coefficients
are computed from drag laws and friction velocities estimated from
ocean speeds following Holland and Jenkins (1999)
[HJ99]. This computation can be modified using runtime
parameters and user is referred to S/R
pkg/shelfice/shelfice_readparms.F for details.
8.6.3.5. Key subroutines¶
The main routine is shelfice_thermodynamics.F but note that /pkg/shelfice routines are also called when solving the momentum equations.
C !CALLING SEQUENCE:
C ...
C FORWARD_STEP :: Step forward a timestep ( AT LAST !!! )
C ...
C  DO_OCEANIC_PHY :: Control oceanic physics and parameterization
C ...
C   SHELFICE_THERMODYNAMICS :: main routine for thermodynamics
C with diagnostics
C ...
C  THERMODYNAMICS :: theta, salt + tracer equations driver.
C ...
C   EXTERNAL_FORCING_T :: Problem specific forcing for temperature.
C   SHELFICE_FORCING_T :: apply heat fluxes from ice shelf model
C ...
C   EXTERNAL_FORCING_S :: Problem specific forcing for salinity.
C   SHELFICE_FORCING_S :: apply fresh water fluxes from ice shelf model
C ...
C  DYNAMICS :: Momentum equations driver.
C ...
C   MOM_FLUXFORM :: Flux form mom eqn. package ( see
C ...
C    SHELFICE_U_DRAG :: apply drag along ice shelf to uequation
C with diagnostics
C ...
C   MOM_VECINV :: Vector invariant form mom eqn. package ( see
C ...
C    SHELFICE_V_DRAG :: apply drag along ice shelf to vequation
C with diagnostics
C ...
C o
8.6.3.6. SHELFICE diagnostics¶
Diagnostics output is available via the diagnostics package (see Section 9). Available output fields are summarized as follows:
++++
<Name>Levsgrid< Units >< Tile (max=80c)
++++
SHIfwFlx 1 SM kg/m^2/s Ice shelf fresh water flux (positive upward)
SHIhtFlx 1 SM W/m^2 Ice shelf heat flux (positive upward)
SHIUDrag 30 UU m/s^2 U momentum tendency from ice shelf drag
SHIVDrag 30 VV m/s^2 V momentum tendency from ice shelf drag
SHIForcT 1 SM W/m^2 Ice shelf forcing for theta, >0 increases theta
SHIForcS 1 SM g/m^2/s Ice shelf forcing for salt, >0 increases salt
8.6.3.7. Experiments and tutorials that use shelfice¶
See the verification experiment isomip for example usage of pkg/shelfice.