baroclinic channel

description

Baroclinic channel tasks implements variants of the Baroclinic Eddies test case from Ilicak et al. (2012).

Polaris includes includes 5 baroclinic channel test cases. All test cases have at least 2 steps, init, which defines the mesh and initial conditions for the model, and some variation on forward (given another name in many test cases to distinguish multiple forward runs), which performs time integration of the model.

mesh

The domain is zonally periodic with solid northern and southern boundaries. The domain size is 160 km in the zonal dimension by 500 km in the meridional dimension. These dimensions are set in the config file by lx and ly. Variants of the test case are available at 1-km, 4-km and 10-km horizontal resolution.

vertical grid

By default, all tests have 20 vertical layers of 50-m uniform thickness. The domain bottom is flat.

# Options related to the vertical grid
[vertical_grid]

# the type of vertical grid
grid_type = uniform

# Number of vertical levels
vert_levels = 20

# Depth of the bottom of the ocean
bottom_depth = 1000.0

# The type of vertical coordinate (e.g. z-level, z-star)
coord_type = z-star

# Whether to use "partial" or "full", or "None" to not alter the topography
partial_cell_type = None

# The minimum fraction of a layer for partial cells
min_pc_fraction = 0.1

initial conditions

Salinity is constant throughout the domain (at 35 PSU). The initial temperature is cooler in the southern half of the domain than in the north, with a gradient between the two halves that is sinusoidally perturbed in the meridional direction.

First the temperature in the depth dimension is defined:

\[\begin{split} T_0(z) = T_b + (T_s - T_b)(z - z_b)/z_b\\ y_{perturb}(x) = \Delta y_{perturb} \sin(6 \pi x/l_x)\\ \end{split}\]

where \(y_{perturb}\) corresponds to the config option perturb_width, \(T_s\) to surface_temperature and \(T_b\) to bottom_temperature. The surface temperature is warmer than at depth. \(z_b\) is the bottom depth.

A fraction is defined to scale the horizontal temperature perturbations, which are uniform with depth:

\[\begin{split} f(y) = \begin{cases} 1 &\text{ if } y < y_{mid} - y_{perturb}\\ 1 - (y - (y_{mid} - y{perturb}))/\Delta y_{perturb} & \text{ if } y \ge y_{mid} - y_{perturb}\\ 0 & \text{otherwise} \end{cases} \end{split}\]

where \(\Delta y_{perturb}\) corresponds to the perturbation width in the y-dimension defined by gradient_width_dist or gradient_width_fraction * ly. \(y_{mid} = \frac{1}{2} l_y\).

The temperature field is constructed using this fraction of the temperature_difference, \(\Delta T\), and a fraction of \(\Delta T_{crest}\), hard-coded as 0.3, applied over a more limited x-extent.

\[\begin{split} T_0(y) = \begin{cases} T_0(z) - \Delta T \: f(y) + \Delta T_{crest} \: (1-(y - (y_{mid} - y_{crest}(x))/\frac{1}{2} \Delta y_{perturb}) & \text{ if } y_{min}(x) \le y \le y_{max}(x); \frac{4}{6} l_x \le x \le \frac{5}{6} l_x \\ T_0(z) - \Delta T \: f(y) & \text{otherwise} \end{cases} \end{split}\]

where

\[\begin{split} y_{crest}(x) = ~ & \frac{1}{2} \Delta y_{perturb} \sin\left[\pi (x - \frac{4}{6} l_x)/(\frac{1}{6} l_x)\right], \\ y_{min}(x) = ~ & y_{mid} - y_{crest}(x) - \frac{1}{2}\Delta y_{perturb}, \\ y_{max}(x) = ~ & y_{mid} - y_{crest}(x) + \frac{1}{2}\Delta y_{perturb}. \end{split}\]

The flow is initially stationary and the Coriolis parameter is uniform across the domain and defined in the config file by coriolis_parameter.

forcing

N/A

time step and run duration

The time step is determined by the config options dt_per_km so that the time step is proportional to the resolution. By default, a 10 km-resolution test has a time step of 5 min. Run duration will be specified for each test case. Run duration will be discussed for individual test cases.

config options

All 5 test cases share the same set of config options:

# config options for baroclinic channel testcases
[baroclinic_channel]

# the size of the domain in km in the x and y directions
lx = 160.0
ly = 500.0

# time step per resolution (s/km), since dt is proportional to resolution
dt_per_km = 30

# barotropic time step per resolution (s/km), since btr_dt is proportional to
# resolution
btr_dt_per_km = 1.5

# Logical flag that determines if locations of features are defined by distance
# or fractions. False means fractions.
use_distances = False

# Viscosity values to test for rpe test case
viscosities = 1, 5, 10, 20, 200

# Temperature of the surface in the northern half of the domain.
surface_temperature = 13.1

# Temperature of the bottom in the northern half of the domain.
bottom_temperature = 10.1

# Difference in the temperature field between the northern and southern halves
# of the domain.
temperature_difference = 1.2

# Fraction of domain in Y direction the temperature gradient should be linear
# over. Used when use_distances = False.
gradient_width_frac = 0.08

# Width of the temperature gradient around the center sin wave. Default value
# is relative to a 500km domain in Y. Used when use_distances = True.
gradient_width_dist = 40e3

# Salinity of the water in the entire domain.
salinity = 35.0

# Coriolis parameter for entire domain.
coriolis_parameter = -1.2e-4

The default domain size (lx and ly) is designed to be consistent with the literature, but can be modified by users to suit their needs.

The config options dt_per_km and btr_dt_per_km are used to determine a time steps that is consistent with a given resolution. Changing these config options is likely to break the restart, since it assumes a time step of 5 minutes at 10 km, consistent with the default dt_per_km. A user can safely modify dt_per_km and btr_dt_per_km to control the time step for any other baroclinic channel tests.

All units are mks, with temperature in degrees Celsius and salinity in PSU.

default

description

ocean/baroclinic_channel/10km/default is the default version of the baroclinic eddies test case for a short (15 min) test run and validation of prognostic variables for regression testing.

mesh

See baroclinic channel. Currently, only 10-km horizontal resolution is supported.

baroclinic channel

description

mesh

vertical grid

initial conditions

forcing

time step and run duration

config options

default

description

mesh

vertical grid

initial conditions

forcing

time step and run duration

config options

cores

decomp

description

mesh

vertical grid

initial conditions

forcing

time step and run duration

config options

cores

thread

description

mesh

vertical grid

initial conditions

forcing

time step and run duration

config options

cores

restart

description

mesh

vertical grid

initial conditions

forcing

time step and run duration

config options

cores

rpe

mesh

vertical grid

initial conditions

forcing

time step and run duration

config options

cores