Surface mass and energy process model components¶
The “invisible” model¶
- Options
-surface simple
- Variables
none
- C++ class
pism::surface::Simple
This is the simplest “surface model” available in PISM, enabled using -surface simple
.
Its job is to re-interpret precipitation as climatic mass balance, and to re-interpret
mean annual near-surface (2m) air temperature as the temperature of the ice at the depth
at which firn processes cease to change the temperature of the ice. (I.e. the temperature
below the firn.) This implies that there is no melt. Though primitive, this model
component may be desired in cold environments (e.g. East Antarctic ice sheet) in which
melt is negligible and heat from firn processes is ignored.
Reading top-surface boundary conditions from a file¶
- Options
-surface given
- Variables
ice_surface_temp
,climatic_mass_balance
- C++ class
pism::surface::Given
Note
This is the default choice.
This model component was created to force PISM with sampled (possibly periodic) climate data by reading ice upper surface boundary conditions from a file. These fields are provided directly to the ice dynamics code (see Climate inputs, and their interface with ice dynamics for details).
PISM will stop if variables ice_surface_temp
(ice temperature at the ice surface
but below firn) and climatic_mass_balance
(top surface mass flux into the ice) are
not present in the input file.
A file foo.nc
used with -surface given -surface_given_file foo.nc
may contain
several records. If this file contains one record (i.e. fields corresponding to one time
value only), provided forcing data is interpreted as time-independent. Variables
time
and time_bnds
should specify model times corresponding to individual
records.
For example, to use monthly periodic forcing with a period of 1 year starting at the
beginning of foo.nc
”) with 12 records. The time
variable may contain days since
1980-1-1
”. (It is best to avoid units of “months” and “years” because their meanings
depend on the calendar.) Next, add the time_bounds
variable for the time dimension
with the values time:bounds
attribute accordingly. Now run
pismr -surface given -surface_given_file foo.nc -surface.given.periodic
See Using time-dependent forcing for more information.
Note
This surface model ignores the atmosphere model selection made using the option
-atmosphere
.PISM can handle files with virtually any number of records: it will read and store in memory at most
input
.forcing
.buffer_size
records at any given time (default: 60, or 5 years’ worth of monthly fields).when preparing a file for use with this model, it is best to use the
t,y,x
variable storage order: files using this order can be read in faster than ones using thet,x,y
order, for reasons explained in the User’s Manual.To change the storage order in a NetCDF file, use
ncpdq
:ncpdq -a t,y,x input.nc output.nc
will copy data from
input.nc
intooutput.nc
, changing the storage order tot,y,x
at the same time.
Parameters
Prefix: surface.given.
Elevation-dependent temperature and mass balance¶
- Options
-surface elevation
- Variables
none
- C++ class
pism::surface::Elevation
This surface model component parameterizes the ice surface temperature ice_surface_temp
and the mass balance climatic_mass_balance
as
piecewise-linear functions of surface elevation
The option -ice_surface_temp
(list of 4 numbers) determines the surface
temperature using the 4 parameters
be the temperature gradient. Then
The option -climatic_mass_balance
(list of 5 numbers) determines the surface mass
balance using the 5 parameters
and
be the mass balance gradient in the ablation and in the accumulation area, respectively. Then
The option -climatic_mass_balance_limits
(list of 2 numbers) limits the mass
balance below
Note: this surface model ignores the atmosphere model selection made using the
-atmosphere
option.
Temperature-index scheme¶
- Options
-surface pdd
- Variables
air_temp_sd
,snow_depth
- C++ class
pism::surface::TemperatureIndex
The default PDD model used by PISM, turned on by option -surface pdd
, is based on
[153] and EISMINT-Greenland intercomparison (see [148]).
Our model computes the solid (snow) precipitation rate using the air temperature threshold
with a linear transition. All precipitation during periods with air temperatures above
surface
.pdd
.air_temp_all_precip_as_rain
(default of surface
.pdd
.air_temp_all_precip_as_snow
(default of
For long-term simulations, a PDD model generally uses an idealized seasonal temperature
cycle. “White noise” is added to this cycle to simulate additional daily variability
associated to the vagaries of weather. This additional random variation is quite
significant, as the seasonal cycle may never reach the melting point but that point may be
reached with some probability, in the presence of the daily variability, and thus melt may
occur. Concretely, a normally-distributed, mean zero random temperature increment is added
to the seasonal cycle. There is no assumed spatial correlation of daily variability. The
standard deviation of the daily variability is controlled by configuration parameters with
the prefix surface.pdd.std_dev.
:
file
The name of the file to readair_temp_sd
(standard deviation of air temperature) from.lapse_lat_base
(72 degree_north) standard deviationis is a function of latitude, with valuesurface
.pdd
.std_dev
.value
at this latitude; this value is only active ifsurface
.pdd
.std_dev
.lapse_lat_rate
is nonzerolapse_lat_rate
(0 Kelvin / degree_north) standard deviation is a function of latitude, with rate of change with respect to latitude given by this constantparam_a
(-0.15) Parameter in , with in degrees C. Used only ifsurface
.pdd
.std_dev
.use_param
is set to yes.param_b
(0.66 Kelvin) Parameter in , with in degrees C. Used only ifsurface
.pdd
.std_dev
.use_param
is set to yes.periodic
(no) If true, interpretair_temp_sd
read fromsurface
.pdd
.std_dev
.file
as periodic in timeuse_param
(no) Parameterize standard deviation as a linear function of air temperature over ice-covered grid cells. The region of application is controlled bygeometry
.ice_free_thickness_standard
.value
(5 Kelvin) standard deviation of daily temp variation; = EISMINT-Greenland value [148]
A file foo.nc
used with -surface pdd -pdd_sd_file foo.nc
should contain standard
deviation of near-surface air temperature in variable air_temp_sd
, and the
corresponding time coordinate in variable time
. If -pdd_sd_file
is not set,
PISM uses a constant value for standard deviation, which is set by the
configuration parameter surface
.pdd
.std_dev
.value
. The default value is
Over ice-covered grid cells, daily variability can also be parameterized as a linear
function of near-surface air temperature surface
.pdd
.std_dev
.use_param
configuration flag, and the corresponding
parameters surface
.pdd
.std_dev
.param_a
and
surface
.pdd
.std_dev
.param_b
. This parametrization replaces prescribed standard
deviation values over glacierized grid cells as defined by the mask
variable (see
geometry
.ice_free_thickness_standard
). Default values for the slope
The number of positive degree days is computed as the magnitude of the temperature
excursion above
In PISM there are two methods for computing the number of positive degree days. The first
computes only the expected value, by the method described in [153]. This is
the default when a PDD is chosen (i.e. option -surface pdd
). The second is a Monte
Carlo simulation of the white noise itself, chosen by adding the option -pdd_method
random_process
. This Monte Carlo simulation adds the same daily variation at every point,
though the seasonal cycle is (generally) location dependent. If repeatable randomness is
desired use -pdd_method repeatable_random_process
instead.
![../_images/pdd-model-flowchart.png](../_images/pdd-model-flowchart.png)
Fig. 47 PISM’s positive degree day model.
By default, the computation summarized in Fig. 47 is performed every week.
(This frequency is controlled by the parameter surface
.pdd
.max_evals_per_year
.)
To compute mass balance during each week-long time-step, PISM keeps track of the current
snow depth (using units of ice-equivalent thickness). This is necessary to determine if
melt should be computed using the degree day factor for snow
(surface
.pdd
.factor_snow
) or the corresponding factor for ice
(surface
.pdd
.factor_ice
).
A fraction of the melt controlled by the configuration parameter surface
.pdd
.refreeze
(surface
.pdd
.refreeze_ice_melt
.
Since PISM does not have a principled firn model, the snow depth is set to zero at the
beginning of the balance year. See surface
.mass_balance_year_start_day
. Default is
Our PDD implementation is meant to be used with an atmosphere model implementing a cosine
yearly cycle such as searise_greenland
(section
SeaRISE-Greenland), but it is not restricted to parameterizations
like these.
This code also implements latitude- and mean July temperature dependent ice and snow
factors using formulas (6) and (7) in [145]; set -pdd_fausto
to enable.
The default standard deviation of the daily variability (option -pdd_std_dev
) is
2.53 degrees when -pdd_fausto
is set [145]. See also configuration
parameters with the prefix surface.pdd.fausto.
:
beta_ice_c
(0.015 meter / (Kelvin day)) water-equivalent thickness; for formula (6) in [145]beta_ice_w
(0.007 meter / (Kelvin day)) water-equivalent thickness; for formula (6) in [145]beta_snow_c
(0.003 meter / (Kelvin day)) water-equivalent thickness; for formula (6) in [145]beta_snow_w
(0.003 meter / (Kelvin day)) water-equivalent thickness; for formula (6) in [145]enabled
(false) Set PDD parameters using formulas (6) and (7) in [145]latitude_beta_w
(72 degree_north) latitude below which to use warm case, in formula (6) in [145]
Note that when used with periodic climate data (air temperature and precipitation) that is
read from a file (see section Boundary conditions read from a file), use of
time_stepping
.hit_multiples
is recommended: set it to the length of the climate
data period in years.
This model provides the following scalar:
surface_accumulation_rate
surface_melt_rate
surface_runoff_rate
and these 2D diagnostic quantities (averaged over reporting intervals; positive flux corresponds to ice gain):
surface_accumulation_flux
surface_melt_flux
surface_runoff_flux
This makes it easy to compare the surface mass balance computed by the model to its individual components:
SMB = surface_accumulation_flux - surface_runoff_flux
Parameters
Prefix: surface.pdd.
air_temp_all_precip_as_rain
(275.15 Kelvin) threshold temperature above which all precipitation is rain; must exceedsurface
.pdd
.air_temp_all_precip_as_snow
to avoid division by zero, because difference is in a denominatorair_temp_all_precip_as_snow
(273.15 Kelvin) threshold temperature below which all precipitation is snowfactor_ice
(0.00879121 meter / (Kelvin day)) EISMINT-Greenland value [148]; = (8 mm liquid-water-equivalent) / (pos degree day)factor_snow
(0.0032967 meter / (Kelvin day)) EISMINT-Greenland value [148]; = (3 mm liquid-water-equivalent) / (pos degree day)fausto
.T_c
(272.15 Kelvin) = -1 + 273.15; for formula (6) in [145]fausto
.T_w
(283.15 Kelvin) = 10 + 273.15; for formula (6) in [145]fausto
.beta_ice_c
(0.015 meter / (Kelvin day)) water-equivalent thickness; for formula (6) in [145]fausto
.beta_ice_w
(0.007 meter / (Kelvin day)) water-equivalent thickness; for formula (6) in [145]fausto
.beta_snow_c
(0.003 meter / (Kelvin day)) water-equivalent thickness; for formula (6) in [145]fausto
.beta_snow_w
(0.003 meter / (Kelvin day)) water-equivalent thickness; for formula (6) in [145]fausto
.enabled
(false) Set PDD parameters using formulas (6) and (7) in [145]fausto
.latitude_beta_w
(72 degree_north) latitude below which to use warm case, in formula (6) in [145]firn_compaction_to_accumulation_ratio
(0.75) How much firn as a fraction of accumulation is turned into icefirn_depth_file
The name of the file to read the firn_depth from.interpret_precip_as_snow
(no) Interpret precipitation as snow fall.max_evals_per_year
(52) maximum number of times the PDD scheme will ask for air temperature and precipitation to build location-dependent time series for computing (expected) number of positive degree days and snow accumulation; the default means the PDD uses weekly samples of the annual cycle; see alsosurface
.pdd
.std_dev
.value
method
(expectation_integral
) PDD implementation methodpositive_threshold_temp
(273.15 Kelvin) temperature used to determine meaning of “positive” degree dayrefreeze_ice_melt
(yes) If set to “yes”, refreezesurface
.pdd
.refreeze
fraction of melted ice, otherwise all of the melted ice runs off.std_dev
.file
The name of the file to readair_temp_sd
(standard deviation of air temperature) from.std_dev
.lapse_lat_base
(72 degree_north) standard deviationis is a function of latitude, with valuesurface
.pdd
.std_dev
.value
at this latitude; this value is only active ifsurface
.pdd
.std_dev
.lapse_lat_rate
is nonzerostd_dev
.lapse_lat_rate
(0 Kelvin / degree_north) standard deviation is a function of latitude, with rate of change with respect to latitude given by this constantstd_dev
.param_a
(-0.15) Parameter in , with in degrees C. Used only ifsurface
.pdd
.std_dev
.use_param
is set to yes.std_dev
.param_b
(0.66 Kelvin) Parameter in , with in degrees C. Used only ifsurface
.pdd
.std_dev
.use_param
is set to yes.std_dev
.periodic
(no) If true, interpretair_temp_sd
read fromsurface
.pdd
.std_dev
.file
as periodic in timestd_dev
.use_param
(no) Parameterize standard deviation as a linear function of air temperature over ice-covered grid cells. The region of application is controlled bygeometry
.ice_free_thickness_standard
.std_dev
.value
(5 Kelvin) standard deviation of daily temp variation; = EISMINT-Greenland value [148]
Diurnal Energy Balance Model “dEBM-simple”¶
- Options
-surface debm_simple
- Variables
surface_albedo
- C++ class
pism::surface::DEBMSimple
This PISM module implements the “simple” version of the diurnal energy balance model developed by [157]. It follows [158] and includes parameterizations of the surface albedo and the atmospheric transmissivity that make it possible to run the model in a standalone, prognostic mode.
It is designed to use time-dependent forcing by near-surface air temperature and total (i.e. liquid and solid) precipitation provided by one of PISM’s atmosphere models. The temperature forcing should resolve the annual cycle, i.e. it should use monthly or more frequent temperature records if forced using Boundary conditions read from a file. In cases when only annual temperature records are available we recommend using the Cosine yearly cycle approximation.
Note
We recommend setting time_stepping
.hit_multiples
to the length of the climate
data period in years when forcing dEBM-simple with periodic climate data that are read
from a file.
Similarly to other surface models, the outputs are
ice temperature at its top surface
climatic mass balance (SMB)
dEBM-simple re-interprets near-surface air temperature to produce the ice temperature at its top surface.
SMB is defined as
Solid accumulation is approximated using provided total precipitation and a linear
transition from interpreting it as “all snow” when the air temperature is below
surface
.debm_simple
.air_temp_all_precip_as_snow
(default of surface
.debm_simple
.air_temp_all_precip_as_rain
(default of surface
.debm_simple
.interpret_precip_as_snow
is set.
Note
Part of the precipitation that is interpreted as rain is assumed to run off instantaneously and does not contribute to reported modeled runoff.
A fraction surface
.debm_simple
.refreeze
) of computed melt amount is
assumed to re-freeze:
By default only snow melt is allowed to refreeze; set
surface
.debm_simple
.refreeze_ice_melt
to refreeze both snow and ice melt. To
distinguish between melted snow and ice dEBM-simple keeps track of the evolving snow depth
during a balance year and resets it to zero once a year on the day set using
surface
.mass_balance_year_start_day
.
Let
then the average daily melt rate is approximated by
Here
is the insolation-driven melt contribution representing the net uptake of incoming solar shortwave radiation of the surface during the diurnal melt period, is the temperature-driven melt contribution representing the air-temperature-dependent part of the incoming longwave radiation as well as turbulent sensible heat fluxes, is the negative melt offset representing the outgoing longwave radiation and the air temperature-independent part of the incoming longwave radiation [159].
See Table 36 for details and note that (46) is equation 1 in [157].
The following two sections describe implementations of insolation-driven and temperature-driven melt contributions.
Quantity |
Description |
---|---|
Threshold for the solar elevation angle ( |
|
Fraction of the day during which the sun is above the elevation angle |
|
Transmissivity of the atmosphere |
|
Surface albedo |
|
Mean top of the atmosphere insolation during the part of the day when the sun
is above the elevation angle |
|
“Effective air temperature” computed using provided air temperature forcing and additional stochastic variations used to model the effect of daily temperature variations (see (52), [157] and [153]) |
|
Tuning parameter that controls the temperature influence on melt
( |
|
Tuning parameter that controls the (negative) melt offset
( |
|
Fresh water density ( |
|
Latent heat of fusion ( |
|
Threshold temperature ( |
Insolation-driven melt contribution¶
This term models the influence of the mean insolation during the melt period
Mean top of the atmosphere insolation¶
The mean top of the atmosphere insolation during the part of the day when the sun is above
where
is the solar constantsurface
.debm_simple
.solar_constant
[152], is the ratio of the length of the semimajor axis of the Earth’s orbit to the Earth-Sun distance, is the hour angle when the sun has an elevation angle of at least , is the latitude is the solar declination angle.
In short,
Paleo simulations
Trigonometric expansions for surface
.debm_simple
.paleo
.enabled
to switch to using the “paleo” mode.
In this case
the ratio
is a function of the eccentricity of the Earth’s orbit and the perihelion longitude,the solar declination
is a function of the eccentricity of the Earth’s orbit, the perihelion longitude, and the Earth’s obliquity.
The values of these are set using the following configuration parameters (prefix:
surface.debm_simple.paleo.
):
eccentricity
(0.0167) Eccentricity of the Earth’s orbitobliquity
(23.44 degree) Mean obliquity (axial tilt) of the Earthperihelion_longitude
(102.947 degree) Mean longitude of the perihelion relative to the vernal equinox
Alternatively, PISM can read in scalar time series of variables eccentricity
,
obliquity
, and perihelion_longitude
from a file specified using
surface
.debm_simple
.paleo
.file
.
Note
We provide a script (examples/debm_simple/orbital_parameters.py
) that can be used
to generate time series of these parameters using trigonometric expansions due to
[161] with corrections made by the authors of the GISS GCM ModelE
(expansion coefficients used in orbital_parameters.py
come from the GISS ModelE
version 2.1.2). See https://data.giss.nasa.gov/modelE/ar5plots/srorbpar.html for details.
These expansions are considered to be valid for about 1 million years.
Surface albedo¶
To capture melt processes driven by changes in albedo without requiring a more sophisticated surface process model (including the firn layer, for example), dEBM-simple assumes that the surface albedo is a piecewise linear function of the modeled melt rate.
Here surface
.debm_simple
.albedo_slope
).
In this approach, the albedo decreases linearly with increasing melt from the maximum
value surface
.debm_simple
.albedo_max
) for regions with no melting to the minimum value
surface
.debm_simple
.albedo_min
).
Alternatively, albedo (variable surface_albedo
; no units) can be read from a file
specified using surface
.debm_simple
.albedo_input
.file
.
Note
It is recommended to use monthly records of albedo in
surface
.debm_simple
.albedo_input
.file
.The fresh snow albedo is also treated as a tuning parameter. Default values of
and were obtained by fitting this approximation to the output of a regional climate model.
Atmosphere transmissivity¶
dEBM-simple assumes that the transmissivity of the atmosphere
where surface
.debm_simple
.tau_a_intercept
, surface
.debm_simple
.tau_a_slope
, and
Temperature-driven melt contribution¶
where
The “effective temperature” surface
.debm_simple
.positive_threshold_temp
, usually
Similarly to the PDD Temperature-index scheme, these stochastic variations are assumed to
follow the normal distribution with the mean of zero and the standard deviation
Note
The standard deviation surface
.debm_simple
.max_evals_per_year
.
Here
constant in time and space (the default; set using
surface
.debm_simple
.std_dev
),read from a file containing the two dimensional variable
air_temp_sd
that can be constant in time or time-dependent (units: Kelvin; specify the file name usingsurface
.debm_simple
.std_dev
.file
), orparameterized as a function of air temperature
: with Kelvin.
These mechanisms are controlled by parameters with the prefix surface.debm_simple.std_dev.
:
file
The file to readair_temp_sd
(standard deviation of air temperature) fromparam
.a
(-0.15) Parameter in . Used only ifsurface
.debm_simple
.std_dev
.param
.enabled
is set to yes.param
.b
(0.66 Kelvin) Parameter in . Used only ifsurface
.debm_simple
.std_dev
.param
.enabled
is set to yes.param
.enabled
(no) Parameterize standard deviation as a linear function of air temperature over ice-covered grid cells. The region of application is controlled bygeometry
.ice_free_thickness_standard
.periodic
(no) If true, interpret forcing data as periodic in time
Tuning parameters¶
Default values of many parameters come from [157] and are appropriate for Greenland; their values will need to change to use this model in other contexts. See Table 1 in [159] for parameter values more appropriate in an Antarctic setting and for the description of a calibration procedure that can be used to obtain some of these values.
Parameter |
Equation |
Configuration parameters |
---|---|---|
|
||
|
||
PIK¶
- Options
-surface pik
- Variables
climatic_mass_balance
,lat
(latitude), (degrees north)- C++ class
pism::surface::PIK
This surface model component implements the setup used in [52]. The
climatic_mass_balance
is read from an input (-i
) file; the ice surface
temperature is computed as a function of latitude (variable lat
) and surface
elevation (dynamically updated by PISM). See equation (1) in [52].
Scalar temperature offsets¶
- Options
-surface ...,delta_T
- Variables
delta_T
- C++ class
pism::surface::Delta_T
The time-dependent scalar offsets delta_T
are added to ice_surface_temp
computed by a surface model.
Please make sure that delta_T
has the units of “Kelvin
”.
This modifier is identical to the corresponding atmosphere modifier, but applies offsets at a different stage in the computation of top-surface boundary conditions needed by the ice dynamics core.
Parameters
Prefix: surface.delta_T.
Adjustments using modeled change in surface elevation¶
- Options
-surface ...,elevation_change
- Variables
surface_altitude
(CF standard name),- C++ class
pism::surface::LapseRates
The elevation_change
modifier adjusts ice-surface temperature and surface mass balance
using modeled changes in surface elevation relative to a reference elevation read from a
file.
The surface temperature is modified using an elevation lapse rate
surface
.elevation_change
.temperature_lapse_rate
. Here
Two methods of adjusting the SMB are available:
Scaling using an exponential factor
where
surface
.elevation_change
.smb
.exp_factor
and is the temperature difference produced by applyingsurface
.elevation_change
.temperature_lapse_rate
.This mechanisms increases the SMB by
percent for each degree of temperature increase.To use this method, set
-smb_adjustment scale
.Elevation lapse rate for the SMB
where
surface
.elevation_change
.smb
.lapse_rate
and is the difference between modeled and reference surface elevations.To use this method, set
-smb_adjustment shift
.
Parameters
Prefix: surface.elevation_change.
.
file
Name of the file containing the reference surface elevation field (variableusurf
).periodic
(no) If true, interpret forcing data as periodic in timesmb
.exp_factor
(0 Kelvin-1) Exponential for the surface mass balance.smb
.lapse_rate
(0 (m / year) / km) Lapse rate for the surface mass balance.smb
.method
(shift
) Choose the SMB adjustment method.scale
: use temperature-change-dependent scaling factor.shift
: use the SMB lapse rate.temperature_lapse_rate
(0 K / km) Lapse rate for the temperature at the top of the ice.
Mass flux adjustment¶
- Options
-surface ...,forcing
- Variables
thk
(ice thickness),ftt_mask
(mask of zeros and ones; 1 where surface mass flux is adjusted and 0 elsewhere)- C++ class
pism::surface::ForceThickness
The forcing
modifier implements a surface mass balance adjustment mechanism which
forces the thickness of grounded ice to a target thickness distribution at the end of the
run. The idea behind this mechanism is that spinup of ice sheet models frequently requires
the surface elevation to come close to measured values at the end of a run. A simpler
alternative to accomplish this, namely option -no_mass
, represents an unmodeled,
frequently large, violation of the mass continuity equation.
In more detail, let
(Other details of this equation do not concern us here.) The forcing
modifier causes
where
Option -force_to_thickness_file
identifies the file containing the target ice
thickness field thk
and the mask ftt_mask
. A basic run modifying surface model
given
would look like
pismr -i foo.nc -surface given,forcing -force_to_thickness_file bar.nc
In this case foo.nc
contains fields climatic_mass_balance
and
ice_surface_temp
, as normal for -surface given
, and bar.nc
contains fields
thk
which will serve as the target thickness and ftt_mask
which defines the
map plane area where this adjustment is applied. Option -force_to_thickness_alpha
adjusts the value of
In addition to this one can specify a multiplicative factor -force_to_thickness_ice_free_thickness_threshold
meters of ice;
-force_to_thickness_ice_free_alpha_factor
option to set
Using climate data anomalies¶
- Options
-surface ...,anomaly
- Variables
ice_surface_temp_anomaly
,climatic_mass_balance_anomaly
- C++ class
pism::surface::Anomaly
This modifier implements a spatially-variable version of -surface ...,delta_T
which
also applies time-dependent climatic mass balance anomalies.
See also -atmosphere ...,anomaly
(section Using climate data anomalies), which is
similar but applies anomalies at the atmosphere level.
Parameters
Prefix: surface.anomaly.
The caching modifier¶
- Options
-surface ...,cache
- C++ class
pism::surface::Cache
- See also
This modifier skips surface model updates, so that a surface model is called no more than
every surface
.cache
.update_interval
365-day “years”. A time-step of
This is useful in cases when inter-annual climate variability is important, but one year differs little from the next. (Coarse-grid paleo-climate runs, for example.)
Parameters
Prefix: surface.cache.
update_interval
(10 365days) Update interval (in 365-day years) for the-surface cache
modifier.
Preventing grounding line retreat¶
- Options
-surface ...,no_gl_retreat
- C++ class
pism::surface::NoGLRetreat
This modifier adjust the surface mass balance to prevent the retreat of the grounding line. See Till friction angle optimization for an application.
Note
This modifier adds mass in violation of mass conservation. Save the diagnostic
no_gl_retreat_smb_adjustment
to get an idea about the amount added. Note, though, that this is an imperfect measure: it includes mass added to maintain non-negativity of ice thickness.We assume that the sea level and the bed elevation remain constant throughout the simulation.
This does not prevent grounding line retreat caused by the thinning of the ice due to the melt at the base. Set
geometry
.update
.use_basal_melt_rate
to “false” to ensure that basal melt has no effect on the position of the grounding line
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