ceaice_split_version/ceaice_part1/ceaice_global.tex

\section{Global Ocean and Sea Ice Simulation}
\label{sec:globalmodel}

One example application of the MITgcm sea ice model is the
eddy-admitting, global 
ocean and sea ice state estimates, which are being generated by the Estimating
the Circulation and Climate of the Ocean, Phase II (ECCO2) project
\citep{menemenlis05}.  One particular, unconstrained ECCO2 simulation, labeled
cube76, provides the baseline solution and the lateral boundary conditions for
all the numerical experiments carried out in \refsec{arcticmodel}.
\reffig{cube76marsepice} shows representative sea ice results from this
simulation.
\begin{figure*}[t]
  \centering
  \includegraphics[angle=0,width=\widefigwidth]{\fpath/cube76marsepice}
  \caption{Effective sea ice thickness distribution (color, in meters)
    averaged over the years 1992--2002 from an eddy-admitting, global
    ocean and sea ice simulation. The ice edge estimated as the 15\%
    isoline of modeled ice concentration is drawn as a white dashed
    line. The white solid line marks the ice edge, defined as
    the 15\% isoline of ice concentrations, retrieved
    from passive microwave satellite data for comparison.  The top row
    shows the results for the Arctic Ocean and the bottom row for the
    Southern Ocean; the left column shows distributions for March
    and the right column for September.}
  \label{fig:cube76marsepice}
\end{figure*}

The simulation is integrated on a cubed-sphere grid, permitting
relatively even grid spacing throughout the domain and avoiding polar
singularities \citep{adcroft04:_cubed_sphere}. Each face of the cube comprises
510 by 510 grid cells for a mean horizontal grid spacing of 18\,km. There are
50 vertical levels ranging in thickness from 10 m near the surface to
approximately 450 m at a maximum model depth of 6150~m. The model employs the
rescaled vertical coordinate ``z$^*$'' \citep{adcroft04} 
with partial-cell formulation of \citet{adcroft97:_shaved_cells}, 
which permits accurate representation of the
bathymetry. Bathymetry is from the S2004 (W.~Smith, unpublished) blend of the
\citet{smi97} and the General Bathymetric Charts of the Oceans (GEBCO) one
arc-minute bathymetric grid.
In the ocean, the non-linear equation of state of \citet{jac95} is
used. Vertical mixing follows \citet{lar94} but with meridionally and vertically
varying background vertical diffusivity; at the surface, vertical diffusivity
is $4.4\times 10^{-6}$~m$^2$~s$^{-1}$ at the Equator, $3.6\times
10^{-6}$~m$^2$~s$^{-1}$ north of 70\degN, and $1.9\times
10^{-5}$~m$^2$~s$^{-1}$ south of 30\degS\ and between 30\degN\ and
60\degN, with sinusoidally varying values in between these latitudes;
vertically, diffusivity increases to $1.1\times 10^{-4}$~m$^2$~s$^{-1}$ at a
depth of 6150 m as per \citet{bry79}.  A 7th-order monotonicity-preserving
advection scheme \citep{dar04} is employed and there is no explicit horizontal
diffusivity.  Horizontal viscosity follows \citet{lei96} but is modified to sense
the divergent flow \citep{kem08}. The global ocean model is coupled to a sea
ice model in a configuration similar to the case C-LSR-ns (see
\reftab{experiments} in
Section~\ref{sec:arcticmodel}).  The values of open water, dry ice, wet ice,
dry snow, and wet snow albedos are, respectively, 0.15, 0.88, 0.79, 0.97, and
0.83.  These values are relatively high compared to observations and they were chosen
to compensate for deficiencies in the surface boundary conditions and to
produce realistic sea ice extent (\reffig{cube76marsepice}).

The simulation is initialized in January 1979 from rest and from
temperature and salinity
fields derived from the Polar Science Center Hydrographic Climatology (PHC)
3.0 \citep{ste01a}. Surface boundary conditions are derived from the European
Centre for Medium-Range Weather
Forecasts (ECMWF) 40 year re-analysis (ERA-40) \citep{upp05}. Six-hourly
surface winds, temperature, humidity, downward short- and long-wave
radiation, and precipitation are converted to heat, freshwater, and wind
stress fluxes using the 
%\citet{large81,large82} 
\citet{larg-yeag:04}
bulk formulae.  Shortwave
radiation decays exponentially with depth as per \citet{pau77}. Low frequency
precipitation has been adjusted using the pentad (5-day) data from the Global
Precipitation Climatology Project \citep[GPCP,][]{huf01}.  The time-mean river
run-off from \citet{lar01} is applied globally, except in the Arctic Ocean
where monthly mean river runoff based on the Arctic Runoff Data Base (ARDB)
and prepared by P. Winsor (personal communication, 2007) is specified.
%Additionally, where there is open water, there is a relaxation to the
%monthly-mean climatological sea
%surface salinity values from PHC 3.0 with a relaxation time scale of 101 days.

The remainder of this article discusses results from forward
sensitivity experiments in a regional Arctic Ocean model, which
operates on a sub-domain of, and which obtains open boundary
conditions from, the cube76 simulation just described.

%%% Local Variables: 
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1	\section{Global Ocean and Sea Ice Simulation}
2	\label{sec:globalmodel}
3
4	One example application of the MITgcm sea ice model is the
5	eddy-admitting, global
6	ocean and sea ice state estimates, which are being generated by the Estimating
7	the Circulation and Climate of the Ocean, Phase II (ECCO2) project
8	\citep{menemenlis05}. One particular, unconstrained ECCO2 simulation, labeled
9	cube76, provides the baseline solution and the lateral boundary conditions for
10	all the numerical experiments carried out in \refsec{arcticmodel}.
11	\reffig{cube76marsepice} shows representative sea ice results from this
12	simulation.
13	\begin{figure*}[t]
14	\centering
15	\includegraphics[angle=0,width=\widefigwidth]{\fpath/cube76marsepice}
16	\caption{Effective sea ice thickness distribution (color, in meters)
17	averaged over the years 1992--2002 from an eddy-admitting, global
18	ocean and sea ice simulation. The ice edge estimated as the 15\%
19	isoline of modeled ice concentration is drawn as a white dashed
20	line. The white solid line marks the ice edge, defined as
21	the 15\% isoline of ice concentrations, retrieved
22	from passive microwave satellite data for comparison. The top row
23	shows the results for the Arctic Ocean and the bottom row for the
24	Southern Ocean; the left column shows distributions for March
25	and the right column for September.}
26	\label{fig:cube76marsepice}
27	\end{figure*}
28
29	The simulation is integrated on a cubed-sphere grid, permitting
30	relatively even grid spacing throughout the domain and avoiding polar
31	singularities \citep{adcroft04:_cubed_sphere}. Each face of the cube comprises
32	510 by 510 grid cells for a mean horizontal grid spacing of 18\,km. There are
33	50 vertical levels ranging in thickness from 10 m near the surface to
34	approximately 450 m at a maximum model depth of 6150~m. The model employs the
35	rescaled vertical coordinate ``z$^*$'' \citep{adcroft04}
36	with partial-cell formulation of \citet{adcroft97:_shaved_cells},
37	which permits accurate representation of the
38	bathymetry. Bathymetry is from the S2004 (W.~Smith, unpublished) blend of the
39	\citet{smi97} and the General Bathymetric Charts of the Oceans (GEBCO) one
40	arc-minute bathymetric grid.
41	In the ocean, the non-linear equation of state of \citet{jac95} is
42	used. Vertical mixing follows \citet{lar94} but with meridionally and vertically
43	varying background vertical diffusivity; at the surface, vertical diffusivity
44	is $4.4\times 10^{-6}$~m$^2$~s$^{-1}$ at the Equator, $3.6\times
45	10^{-6}$~m$^2$~s$^{-1}$ north of 70\degN, and $1.9\times
46	10^{-5}$~m$^2$~s$^{-1}$ south of 30\degS\ and between 30\degN\ and
47	60\degN, with sinusoidally varying values in between these latitudes;
48	vertically, diffusivity increases to $1.1\times 10^{-4}$~m$^2$~s$^{-1}$ at a
49	depth of 6150 m as per \citet{bry79}. A 7th-order monotonicity-preserving
50	advection scheme \citep{dar04} is employed and there is no explicit horizontal
51	diffusivity. Horizontal viscosity follows \citet{lei96} but is modified to sense
52	the divergent flow \citep{kem08}. The global ocean model is coupled to a sea
53	ice model in a configuration similar to the case C-LSR-ns (see
54	\reftab{experiments} in
55	Section~\ref{sec:arcticmodel}). The values of open water, dry ice, wet ice,
56	dry snow, and wet snow albedos are, respectively, 0.15, 0.88, 0.79, 0.97, and
57	0.83. These values are relatively high compared to observations and they were chosen
58	to compensate for deficiencies in the surface boundary conditions and to
59	produce realistic sea ice extent (\reffig{cube76marsepice}).
60
61	The simulation is initialized in January 1979 from rest and from
62	temperature and salinity
63	fields derived from the Polar Science Center Hydrographic Climatology (PHC)
64	3.0 \citep{ste01a}. Surface boundary conditions are derived from the European
65	Centre for Medium-Range Weather
66	Forecasts (ECMWF) 40 year re-analysis (ERA-40) \citep{upp05}. Six-hourly
67	surface winds, temperature, humidity, downward short- and long-wave
68	radiation, and precipitation are converted to heat, freshwater, and wind
69	stress fluxes using the
70	%\citet{large81,large82}
71	\citet{larg-yeag:04}
72	bulk formulae. Shortwave
73	radiation decays exponentially with depth as per \citet{pau77}. Low frequency
74	precipitation has been adjusted using the pentad (5-day) data from the Global
75	Precipitation Climatology Project \citep[GPCP,][]{huf01}. The time-mean river
76	run-off from \citet{lar01} is applied globally, except in the Arctic Ocean
77	where monthly mean river runoff based on the Arctic Runoff Data Base (ARDB)
78	and prepared by P. Winsor (personal communication, 2007) is specified.
79	%Additionally, where there is open water, there is a relaxation to the
80	%monthly-mean climatological sea
81	%surface salinity values from PHC 3.0 with a relaxation time scale of 101 days.
82
83	The remainder of this article discusses results from forward
84	sensitivity experiments in a regional Arctic Ocean model, which
85	operates on a sub-domain of, and which obtains open boundary
86	conditions from, the cube76 simulation just described.
87
88	%%% Local Variables:
89	%%% mode: latex
90	%%% TeX-master: "ceaice_part1"
91	%%% End: