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1 mlosch 1.1 \section{Sea Ice Model Formulation}
2     \label{app:model}
3    
4     \ml{[All of this (and more) should go into the documentation, but we
5     can remove a large part of the text because it is completely
6     redundant. I leave it in for now \ldots]}
7    
8     \subsection{Dynamics}
9     \label{app:dynamics}
10    
11     The momentum equation of the sea-ice model is
12     \begin{equation}
13     \label{eq:momseaice}
14     m \frac{D\vek{u}}{Dt} = -mf\vek{k}\times\vek{u} + \vtau_{air} +
15     \vtau_{ocean} - m \nabla{\phi(0)} + \vek{F},
16     \end{equation}
17     where $m=m_{i}+m_{s}$ is the ice and snow mass per unit area;
18     $\vek{u}=u\vek{i}+v\vek{j}$ is the ice velocity vector;
19     $\vek{i}$, $\vek{j}$, and $\vek{k}$ are unit vectors in the $x$, $y$, and $z$
20     directions, respectively;
21     $f$ is the Coriolis parameter;
22     $\vtau_{air}$ and $\vtau_{ocean}$ are the wind-ice and ocean-ice stresses,
23     respectively;
24     $g$ is the gravity accelation;
25     $\nabla\phi(0)$ is the gradient (or tilt) of the sea surface height;
26     $\phi(0) = g\eta + p_{a}/\rho_{0} + mg/\rho_{0}$ is the sea surface
27     height potential in response to ocean dynamics ($g\eta$), to
28     atmospheric pressure loading ($p_{a}/\rho_{0}$, where $\rho_{0}$ is a
29     reference density) and a term due to snow and ice loading \citep{campin08};
30     and $\vek{F}=\nabla\cdot\sigma$ is the divergence of the internal ice
31     stress tensor $\sigma_{ij}$. %
32     Advection of sea-ice momentum is neglected. The wind and ice-ocean stress
33     terms are given by
34     \begin{align*}
35     \vtau_{air} = & \rho_{air} C_{air} |\vek{U}_{air} -\vek{u}|
36     R_{air} (\vek{U}_{air} -\vek{u}), \\
37     \vtau_{ocean} = & \rho_{ocean}C_{ocean} |\vek{U}_{ocean}-\vek{u}|
38     R_{ocean}(\vek{U}_{ocean}-\vek{u}), \\
39     \end{align*}
40     where $\vek{U}_{air/ocean}$ are the surface winds of the atmosphere
41     and surface currents of the ocean, respectively; $C_{air/ocean}$ are
42     air and ocean drag coefficients; $\rho_{air/ocean}$ are reference
43     densities; and $R_{air/ocean}$ are rotation matrices that act on the
44     wind/current vectors.
45    
46     For an isotropic system the stress tensor $\sigma_{ij}$ ($i,j=1,2$) can
47     be related to the ice strain rate and strength by a nonlinear
48     viscous-plastic (VP) constitutive law \citep{hibler79, zhang97}:
49     \begin{equation}
50     \label{eq:vpequation}
51     \sigma_{ij}=2\eta(\dot{\epsilon}_{ij},P)\dot{\epsilon}_{ij}
52     + \left[\zeta(\dot{\epsilon}_{ij},P) -
53     \eta(\dot{\epsilon}_{ij},P)\right]\dot{\epsilon}_{kk}\delta_{ij}
54     - \frac{P}{2}\delta_{ij}.
55     \end{equation}
56     The ice strain rate is given by
57     \begin{equation*}
58     \dot{\epsilon}_{ij} = \frac{1}{2}\left(
59     \frac{\partial{u_{i}}}{\partial{x_{j}}} +
60     \frac{\partial{u_{j}}}{\partial{x_{i}}}\right).
61     \end{equation*}
62     The maximum ice pressure $P_{\max}$, a measure of ice strength, depends on
63     both thickness $h$ and compactness (concentration) $c$:
64     \begin{equation}
65     P_{\max} = P^{*}c\,h\,e^{[C^{*}\cdot(1-c)]},
66     \label{eq:icestrength}
67     \end{equation}
68     with the constants $P^{*}$ and $C^{*}$. The nonlinear bulk and shear
69     viscosities $\eta$ and $\zeta$ are functions of ice strain rate
70     invariants and ice strength such that the principal components of the
71     stress lie on an elliptical yield curve with the ratio of major to
72     minor axis $e$ equal to $2$; they are given by:
73     \begin{align*}
74     \zeta =& \min\left(\frac{P_{\max}}{2\max(\Delta,\Delta_{\min})},
75     \zeta_{\max}\right) \\
76     \eta =& \frac{\zeta}{e^2} \\
77     \intertext{with the abbreviation}
78     \Delta = & \left[
79     \left(\dot{\epsilon}_{11}^2+\dot{\epsilon}_{22}^2\right)
80     (1+e^{-2}) + 4e^{-2}\dot{\epsilon}_{12}^2 +
81     2\dot{\epsilon}_{11}\dot{\epsilon}_{22} (1-e^{-2})
82     \right]^{\frac{1}{2}}.
83     \end{align*}
84     The bulk viscosities are bounded above by imposing both a minimum
85     $\Delta_{\min}=10^{-11}\text{\,s}^{-1}$ (for numerical reasons) and a
86     maximum $\zeta_{\max} = P_{\max}/\Delta^*$, where
87     $\Delta^*=(5\times10^{12}/2\times10^4)\text{\,s}^{-1}$. For stress
88     tensor computation the replacement pressure $P = 2\,\Delta\zeta$
89     \citep{hibler95} is used so that the stress state always lies on the
90     elliptic yield curve by definition.
91    
92     In the so-called truncated ellipse method the shear viscosity $\eta$
93     is capped to suppress any tensile stress \citep{hibler97, geiger98}:
94     \begin{equation}
95     \label{eq:etatem}
96     \eta = \min\left(\frac{\zeta}{e^2},
97     \frac{\frac{P}{2}-\zeta(\dot{\epsilon}_{11}+\dot{\epsilon}_{22})}
98     {\sqrt{(\dot{\epsilon}_{11}+\dot{\epsilon}_{22})^2
99     +4\dot{\epsilon}_{12}^2}}\right).
100     \end{equation}
101    
102     In the current implementation, the VP-model is integrated with the
103     semi-implicit line successive over relaxation (LSOR)-solver of
104     \citet{zhang98}, which allows for long time steps that, in our case,
105     are limited by the explicit treatment of the Coriolis term. The
106     explicit treatment of the Coriolis term does not represent a severe
107     limitation because it restricts the time step to approximately the
108     same length as in the ocean model where the Coriolis term is also
109     treated explicitly.
110    
111     \citet{hunke97}'s introduced an elastic contribution to the strain
112     rate in order to regularize Eq.\refeq{vpequation} in such a way that
113     the resulting elastic-viscous-plastic (EVP) and VP models are
114     identical at steady state,
115     \begin{equation}
116     \label{eq:evpequation}
117     \frac{1}{E}\frac{\partial\sigma_{ij}}{\partial{t}} +
118     \frac{1}{2\eta}\sigma_{ij}
119     + \frac{\eta - \zeta}{4\zeta\eta}\sigma_{kk}\delta_{ij}
120     + \frac{P}{4\zeta}\delta_{ij}
121     = \dot{\epsilon}_{ij}.
122     \end{equation}
123     %In the EVP model, equations for the components of the stress tensor
124     %$\sigma_{ij}$ are solved explicitly. Both model formulations will be
125     %used and compared the present sea-ice model study.
126     The EVP-model uses an explicit time stepping scheme with a short
127     timestep. According to the recommendation of \citet{hunke97}, the
128     EVP-model is stepped forward in time 120 times within the physical
129     ocean model time step (although this parameter is under debate), to
130     allow for elastic waves to disappear. Because the scheme does not
131     require a matrix inversion it is fast in spite of the small internal
132     timestep and simple to implement on parallel computers
133     \citep{hunke97}. For completeness, we repeat the equations for the
134     components of the stress tensor $\sigma_{1} =
135     \sigma_{11}+\sigma_{22}$, $\sigma_{2}= \sigma_{11}-\sigma_{22}$, and
136     $\sigma_{12}$. Introducing the divergence $D_D =
137     \dot{\epsilon}_{11}+\dot{\epsilon}_{22}$, and the horizontal tension
138     and shearing strain rates, $D_T =
139     \dot{\epsilon}_{11}-\dot{\epsilon}_{22}$ and $D_S =
140     2\dot{\epsilon}_{12}$, respectively, and using the above
141     abbreviations, the equations\refeq{evpequation} can be written as:
142     \begin{align}
143     \label{eq:evpstresstensor1}
144     \frac{\partial\sigma_{1}}{\partial{t}} + \frac{\sigma_{1}}{2T} +
145     \frac{P}{2T} &= \frac{P}{2T\Delta} D_D \\
146     \label{eq:evpstresstensor2}
147     \frac{\partial\sigma_{2}}{\partial{t}} + \frac{\sigma_{2} e^{2}}{2T}
148     &= \frac{P}{2T\Delta} D_T \\
149     \label{eq:evpstresstensor12}
150     \frac{\partial\sigma_{12}}{\partial{t}} + \frac{\sigma_{12} e^{2}}{2T}
151     &= \frac{P}{4T\Delta} D_S
152     \end{align}
153     Here, the elastic parameter $E$ is redefined in terms of a damping timescale
154     $T$ for elastic waves \[E=\frac{\zeta}{T}.\]
155     $T=E_{0}\Delta{t}$ with the tunable parameter $E_0<1$ and
156     the external (long) timestep $\Delta{t}$. \citet{hunke97} recommend
157     $E_{0} = \frac{1}{3}$.
158    
159     Our discretization differs from \citet{zhang98, zhang03} in the
160     underlying grid, namely the Arakawa C-grid, but is otherwise
161     straightforward. The EVP model, in particular, is discretized
162     naturally on the C-grid with $\sigma_{1}$ and $\sigma_{2}$ on the
163     center points and $\sigma_{12}$ on the corner (or vorticity) points of
164     the grid. With this choice all derivatives are discretized as central
165     differences and averaging is only involved in computing $\Delta$ and
166     $P$ at vorticity points.
167    
168     For a general curvilinear grid, one needs in principle to take metric
169     terms into account that arise in the transformation of a curvilinear
170     grid on the sphere. For now, however, we can neglect these metric
171     terms because they are very small on the \ml{[modify following
172     section3:] cubed sphere grids used in this paper; in particular,
173     only near the edges of the cubed sphere grid, we expect them to be
174     non-zero, but these edges are at approximately 35\degS\ or 35\degN\
175     which are never covered by sea-ice in our simulations. Everywhere
176     else the coordinate system is locally nearly cartesian.} However, for
177     last-glacial-maximum or snowball-earth-like simulations the question
178     of metric terms needs to be reconsidered. Either, one includes these
179     terms as in \citet{zhang03}, or one finds a vector-invariant
180     formulation for the sea-ice internal stress term that does not require
181     any metric terms, as it is done in the ocean dynamics of the MITgcm
182     \citep{adcroft04:_cubed_sphere}.
183    
184     Lateral boundary conditions are naturally ``no-slip'' for B-grids, as
185     the tangential velocities points lie on the boundary. For C-grids, the
186     lateral boundary condition for tangential velocities is realized via
187     ``ghost points'', allowing alternatively no-slip or free-slip
188     conditions. In ocean models free-slip boundary conditions in
189     conjunction with piecewise-constant (``castellated'') coastlines have
190     been shown to reduce in effect to no-slip boundary conditions
191     \citep{adcroft98:_slippery_coast}; for sea-ice models the effects of
192     lateral boundary conditions have not yet been studied.
193    
194     Moving sea ice exerts a stress on the ocean which is the opposite of
195     the stress $\vtau_{ocean}$ in Eq.\refeq{momseaice}. This stess is
196     applied directly to the surface layer of the ocean model. An
197     alternative ocean stress formulation is given by \citet{hibler87}.
198     Rather than applying $\vtau_{ocean}$ directly, the stress is derived
199     from integrating over the ice thickness to the bottom of the oceanic
200     surface layer. In the resulting equation for the \emph{combined}
201     ocean-ice momentum, the interfacial stress cancels and the total
202     stress appears as the sum of windstress and divergence of internal ice
203     stresses: $\delta(z) (\vtau_{air} + \vek{F})/\rho_0$, \citep[see also
204     Eq.\,2 of][]{hibler87}. The disadvantage of this formulation is that
205     now the velocity in the surface layer of the ocean that is used to
206     advect tracers, is really an average over the ocean surface
207     velocity and the ice velocity leading to an inconsistency as the ice
208     temperature and salinity are different from the oceanic variables.
209    
210     %\subparagraph{boundary conditions: no-slip, free-slip, half-slip}
211     %\begin{itemize}
212     %\item transition from existing B-Grid to C-Grid
213     %\item boundary conditions: no-slip, free-slip, half-slip
214     %\item fancy (multi dimensional) advection schemes
215     %\item VP vs.\ EVP \citep{hunke97}
216     %\item ocean stress formulation \citep{hibler87}
217     %\end{itemize}
218    
219     \subsection{Thermodynamics}
220     \label{app:thermodynamics}
221    
222     In its original formulation the sea ice model \citep{menemenlis05}
223     uses simple thermodynamics following the appendix of
224     \citet{semtner76}. This formulation does not allow storage of heat,
225     that is, the heat capacity of ice is zero. Upward conductive heat flux
226     is parameterized assuming a linear temperature profile and together
227     with a constant ice conductivity. It is expressed as
228     $(K/h)(T_{w}-T_{0})$, where $K$ is the ice conductivity, $h$ the ice
229     thickness, and $T_{w}-T_{0}$ the difference between water and ice
230 mlosch 1.2 surface temperatures. This type of model is often refered to as a
231 mlosch 1.1 ``zero-layer'' model. The surface heat flux is computed in a similar
232 mlosch 1.2 way to that of \citet{parkinson79} and \citet{manabe79}.
233 mlosch 1.1
234     The conductive heat flux depends strongly on the ice thickness $h$.
235     However, the ice thickness in the model represents a mean over a
236     potentially very heterogeneous thickness distribution. In order to
237     parameterize a sub-grid scale distribution for heat flux
238     computations, the mean ice thickness $h$ is split into seven thickness
239     categories $H_{n}$ that are equally distributed between $2h$ and a
240     minimum imposed ice thickness of $5\text{\,cm}$ by $H_n=
241     \frac{2n-1}{7}\,h$ for $n\in[1,7]$. The heat fluxes computed for each
242     thickness category is area-averaged to give the total heat flux
243     \citep{hibler84}.
244    
245     \ml{[This is Ian Fenty's work and we may want to remove this paragraph
246     from the paper]: %
247     The atmospheric heat flux is balanced by an oceanic heat flux from
248     below. The oceanic flux is proportional to
249     $\rho\,c_{p}\left(T_{w}-T_{fr}\right)$ where $\rho$ and $c_{p}$ are
250     the density and heat capacity of sea water and $T_{fr}$ is the local
251     freezing point temperature that is a function of salinity. Contrary to
252     \citet{menemenlis05}, this flux is not assumed to instantaneously melt
253     or create ice, but a time scale of three days is used to relax $T_{w}$
254     to the freezing point.}
255 mlosch 1.2 %
256 mlosch 1.1 The parameterization of lateral and vertical growth of sea ice follows
257     that of \citet{hibler79, hibler80}.
258    
259     On top of the ice there is a layer of snow that modifies the heat flux
260 mlosch 1.2 and the albedo \citep{zhang98}. Snow modifies the effective
261     conductivity according to
262     \[\frac{K}{h} \rightarrow \frac{1}{\frac{h_{s}}{K_{s}}+\frac{h}{K}},\]
263     where $K_s$ is the conductivity of snow and $h_s$ the snow thickness.
264     If enough snow accumulates so that its weight submerges the ice and
265     the snow is flooded, a simple mass conserving parameterization of
266     snowice formation (a flood-freeze algorithm following Archimedes'
267     principle) turns snow into ice until the ice surface is back at $z=0$
268     \citep{leppaeranta83}.
269 mlosch 1.1
270     Effective ice thickness (ice volume per unit area,
271     $c\cdot{h}$), concentration $c$ and effective snow thickness
272     ($c\cdot{h}_{s}$) are advected by ice velocities:
273     \begin{equation}
274     \label{eq:advection}
275     \frac{\partial{X}}{\partial{t}} = - \nabla\cdot\left(\vek{u}\,X\right) +
276     \Gamma_{X} + D_{X}
277     \end{equation}
278     where $\Gamma_X$ are the thermodynamic source terms and $D_{X}$ the
279     diffusive terms for quantities $X=(c\cdot{h}), c, (c\cdot{h}_{s})$.
280     %
281     From the various advection scheme that are available in the MITgcm
282     \citep{mitgcm02}, we choose flux-limited schemes
283     \citep[multidimensional 2nd and 3rd-order advection scheme with flux
284     limiter][]{roe85, hundsdorfer94} to preserve sharp gradients and edges
285     that are typical of sea ice distributions and to rule out unphysical
286     over- and undershoots (negative thickness or concentration). These
287     scheme conserve volume and horizontal area and are unconditionally
288     stable, so that we can set $D_{X}=0$. \ml{[do we need to proove that?
289     can we proove that? citation?]}
290    
291     There is considerable doubt about the reliability of such a simple
292     thermodynamic model---\citet{semtner84} found significant errors in
293     phase (one month lead) and amplitude ($\approx$50\%\,overestimate) in
294     such models---, so that today many sea ice models employ more complex
295     thermodynamics. A popular thermodynamics model of \citet{winton00} is
296     based on the 3-layer model of \citet{semtner76} and treats brine
297     content by means of enthalphy conservation. This model requires in
298     addition to ice-thickness and compactness (fractional area) additional
299     state variables to be advected by ice velocities, namely enthalphy of
300     the two ice layers and the thickness of the overlying snow layer.
301     \ml{[Jean-Michel, your turf: ]Care must be taken in advecting these
302     quantities in order to ensure conservation of enthalphy. Currently
303     this can only be accomplished with a 2nd-order advection scheme with
304     flux limiter \citep{roe85}.}
305    
306     %%% Local Variables:
307     %%% mode: latex
308     %%% TeX-master: "ceaice"
309     %%% End:

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