clm5.0/doc/source/tech_note/Glacier/CLM50_Tech_Note_Glacier.rst

.. _rst_Glaciers:

Glaciers
========

This chapter describes features of CLM that are specific to coupling to
an ice sheet model (in the CESM context, this is the CISM model;
:ref:`Lipscomb and Sacks (2012)<LipscombSacks2012>` provide
documentation and user’s guide for CISM). General information
about glacier land units can be found elsewhere in this document (see
Chapter :numref:`rst_Surface Characterization, Vertical Discretization,
and Model Input Requirements` for an overview).

.. _Glaciers summary of CLM5.0 updates relative to CLM4.5:

Summary of CLM5.0 updates relative to CLM4.5
--------------------------------------------

Compared with CLM4.5 (:ref:`Oleson et al. 2013 <Olesonetal2013>`),
CLM5.0 contains substantial improvements in its capabilities for
land-ice science. This section summarizes these improvements, and the
following sections provide more details.

- All runs include multiple glacier elevation classes over Greenland and
  Antarctica and compute ice sheet surface mass balance in those
  regions.

- A number of namelist parameters offer fine-grained control over
  glacier behavior in different regions of the world (section
  :numref:`Glacier regions`). (The options used outside of Greenland and
  Antarctica reproduce the standard CLM4.5 glacier behavior.)

- CLM can now keep its glacier areas and elevations in sync with CISM
  when running with an evolving ice sheet. (However, in typical
  configurations, the ice sheet geometry still remains fixed throughout
  the run.)

- The downscaling to elevation classes now includes downwelling longwave
  radiation and partitioning of precipitation into rain vs. snow
  (section :numref:`Multiple elevation class scheme`).

- Other land units within the CISM domain undergo the same downscaling
  as the glacier land unit, and surface mass balance is computed for the
  natural vegetated land unit. This allows CLM to produce glacial
  inception when running with an evolving ice sheet model.

- There have also been substantial improvements to CLM's snow physics,
  as described in other chapters of this document.

.. _Overview Glaciers:

Overview
--------

CLM is responsible for computing two quantities that are passed to the
ice sheet model:

#. Surface mass balance (SMB) - the net annual accumulation/ablation of
   mass at the upper surface (section 
   :numref:`Computation of the surface mass balance`)

#. Ground surface temperature, which serves as an upper boundary
   condition for CISM's temperature calculation

The ice sheet model is typically run at much higher resolution than CLM
(e.g., :math:`\sim`\ 5 km rather than :math:`\sim`\ 100 km). To improve
the downscaling from CLM’s grid to the ice sheet grid, the glaciated
portion of each grid cell is divided into multiple elevation classes
(section :numref:`Multiple elevation class scheme`). The above
quantities are computed separately in each elevation class. The CESM
coupler then computes high-resolution quantities via horizontal and
vertical interpolation, and passes these high-resolution quantities to
CISM.

There are several reasons for computing the SMB in CLM rather than in
CISM:

#. It is much cheaper to compute the SMB in CLM for :math:`\sim`\ 10
   elevation classes than in CISM. For example, suppose we are
   running CLM at a resolution of :math:`\sim`\ 50 km and CISM at
   :math:`\sim`\ 5 km. Greenland has dimensions of about 1000 x 2000 km.
   For CLM we would have 20 x 40 x 10 = 8,000 columns, whereas for
   CISM we would have 200 x 400 = 80,000 columns.

#. We can use the sophisticated snow physics parameterization already in
   CLM instead of implementing a separate scheme for CISM. Any
   improvements to CLM are applied to ice sheets automatically.

#. The atmosphere model can respond during runtime to ice-sheet surface
   changes (even in the absence of two-way feedbacks with CISM). As
   shown by :ref:`Pritchard et al. (2008)<Pritchardetal2008>`, runtime
   albedo feedback from the ice sheet is critical for simulating
   ice-sheet retreat on paleoclimate time scales. Without this feedback
   the atmosphere warms much less, and the retreat is delayed.

#. The improved SMB is potentially available in CLM for all glaciated
   grid cells (e.g., in the Alps, Rockies, Andes, and Himalayas), not
   just those which are part of ice sheets.

In typical runs, CISM is not evolving; CLM computes the SMB and sends it
to CISM, but CISM's ice sheet geometry remains fixed over the course of
the run. In these runs, CISM serves two roles in the system:

#. Over the CISM domain (typically Greenland in CESM2), CISM dictates
   glacier areas and topographic elevations, overriding the values on
   CLM's surface dataset. CISM also dictates the elevation of
   non-glacier land units in its domain, and only in this domain are
   atmospheric fields downscaled to non-glacier land units. (So if you
   run with a stub glacier model - SGLC - then glacier areas and
   elevations will be taken entirely from CLM's surface dataset, and no
   downscaling will be done over non-glacier land units.)

#. CISM provides the grid onto which SMB is downscaled. (If you run with
   SGLC then SMB will still be computed in CLM, but it won't be
   downscaled to a high-resolution ice sheet grid.)

It is also possible to run CESM with an evolving ice sheet. In this
case, CLM responds to CISM's evolution by adjusting the areas of the
glacier land unit and each elevation class within this land unit, as well
as the mean topographic heights of each elevation class. Thus, CLM's
glacier areas and elevations remain in sync with CISM's. Conservation of
mass and energy is done as for other landcover change (see Chapter
:numref:`rst_Transient Landcover Change`).

.. _Glacier regions:

Glacier regions and their behaviors
-----------------------------------

The world's glaciers and ice sheets are broken down into a number of
different regions (four by default) that differ in three respects:

#. Whether the gridcell's glacier land unit contains:

   a. Multiple elevation classes (section :numref:`Multiple elevation
      class scheme`)

   b. Multiple elevation classes plus virtual elevation classes

   c. Just a single elevation class whose elevation matches the
      atmosphere's topographic height (so there is no adjustment in
      atmospheric forcings due to downscaling).

#. Treatment of glacial melt water:

   a. Glacial melt water runs off and is replaced by ice, thus keeping
      the column always frozen. In the absence of a dynamic ice sheet
      model, this behavior implicitly assumes an infinite store of
      glacial ice that can be melted (with appropriate adjustments made
      to ensure mass and energy conservation). This behavior is
      discussed in more detail in section :numref:`Computation of the
      surface mass balance`.

   b. Glacial melt water remains in place until it refreezes - possibly
      remaining in place indefinitely if the glacier column is in a warm
      climate. With this behavior, ice melt does not result in any
      runoff. Regions with this behavior cannot compute SMB, because
      negative SMB would be meaningless (due to the liquid water on top
      of the ice column). This behavior produces less realistic glacier
      physics. However, it avoids the negative ice runoff that is needed
      for the "replaced by ice" behavior to conserve mass and energy (as
      described in section :numref:`Computation of the surface mass
      balance`). Thus, in regions where CLM has glaciers but the
      atmospheric forcings are too warm to sustain those glaciers, this
      behavior avoids persistent negative ice runoff. This situation can
      often occur for mountain glaciers, where topographic smoothing in
      the atmosphere results in a too-warm climate. There, avoiding
      persistent negative ice runoff can be more important than getting
      the right glacier ice physics.

#. Treatment of ice runoff from snow capping (as described in section
   :numref:`Runoff from glaciers and snow-capped surfaces`). Note that this
   is irrelevant in regions with an evolving, two-way-coupled ice sheet
   (where the snow capping term is sent to CISM rather than running off):

   a. Ice runoff from snow capping remains ice. This is a crude
      parameterization of iceberg calving, and so is appropriate in
      regions where there is substantial iceberg calving in reality.

   b. Ice runoff from snow capping is melted (generating a negative
      sensible heat flux) and runs off as liquid. This matches the
      behavior for non-glacier columns. This is appropriate in regions
      that have little iceberg calving in reality. This can be important
      to avoid unrealistic cooling of the ocean and consequent runaway
      sea ice growth.

The default behaviors for the world's glacier and ice sheet regions are
described in :numref:`Table Glacier region behaviors`. Note that the
standard CISM grid covers Greenland plus enough surrounding area to
allow for ice sheet growth and to have a regular rectangular grid. We
need to have the "replaced by ice" melt behavior within the CISM domain
in order to compute SMB there, and we need virtual elevation classes in
that domain in order to compute SMB for all elevation classes and to
facilitate glacial advance and retreat in the two-way-coupled
case. However, this domain is split into Greenland itself and areas
outside Greenland so that ice runoff in the Canadian archipelago (which
is inside the CISM domain) is melted before reaching the ocean, to avoid
runaway sea ice growth in that region.

.. _Table Glacier region behaviors:

.. table:: Glacier region behaviors

 +---------------+---------------+---------------+---------------+
 | Region        | Elevation     | Glacial melt  | Ice runoff    |
 |               | classes       |               |               |
 +===============+===============+===============+===============+
 | Greenland     | Virtual       | Replaced by   | Remains ice   |
 |               |               | ice           |               |
 +---------------+---------------+---------------+---------------+
 | Inside        | Virtual       | Replaced by   | Melted        |
 | standard CISM |               | ice           |               |
 | grid but      |               |               |               |
 | outside       |               |               |               |
 | Greenland     |               |               |               |
 | itself        |               |               |               |
 +---------------+---------------+---------------+---------------+
 | Antarctica    | Multiple      | Replaced by   | Remains ice   |
 |               |               | ice           |               |
 +---------------+---------------+---------------+---------------+
 | All others    | Single        | Remains in    | Melted        |
 |               |               | place         |               |
 +---------------+---------------+---------------+---------------+

.. note::

   In regions that have both the ``Glacial melt = Replaced by ice`` and the ``Ice runoff =
   Melted`` behaviors (by default, this is just the region inside the standard CISM grid
   but outside Greenland itself): During periods of glacial melt, a negative ice runoff is
   generated (due to the ``Glacial melt = Replaced by ice`` behavior); this negative ice
   runoff is converted to a negative liquid runoff plus a positive sensible heat flux (due
   to the ``Ice runoff = Melted`` behavior). We recommend that you limit the portion of
   the globe with both of these behaviors combined, in order to avoid having too large of
   an impact of this non-physical behavior.

.. _Multiple elevation class scheme:

Multiple elevation class scheme
-------------------------------

The glacier land unit contains multiple columns based on surface
elevation. These are known as elevation classes, and the land unit is
referred to as *glacier\_mec*. (As described in section :numref:`Glacier
regions`, some regions have only a single elevation class, but they are
still referred to as *glacier\_mec* land units.) The default is to have 10
elevation classes whose lower limits are 0, 200, 400, 700, 1000, 1300,
1600, 2000, 2500, and 3000 m. Each column is characterized by a
fractional area and surface elevation that are read in during model
initialization, and then possibly overridden by CISM as the run
progresses. Each *glacier\_mec* column within a grid cell has distinct ice
and snow temperatures, snow water content, surface fluxes, and SMB.

The atmospheric surface temperature, potential temperature, specific
humidity, density, and pressure are downscaled from the atmosphere's
mean grid cell elevation to the *glacier\_mec* column elevation using a
specified lapse rate (typically 6.0 deg/km) and an assumption of uniform
relative humidity. Longwave radiation is downscaled by assuming a linear
decrease in downwelling longwave radiation with increasing elevation
(0.032 W m\ :sup:`-2` m\ :sup:`-1`, limited to 0.5 - 1.5 times the
gridcell mean value, then normalized to conserve gridcell total energy)
:ref:`(Van Tricht et al., 2016)<VanTrichtetal2016>`. Total precipitation
is partitioned into rain vs. snow as described in Chapter
:numref:`rst_Surface Characterization, Vertical Discretization, and
Model Input Requirements`. The partitioning of precipitation is based on
the downscaled temperature, allowing rain to fall at lower elevations
while snow falls at higher elevations.

This downscaling allows lower-elevation columns to undergo surface
melting while columns at higher elevations remain frozen. This gives a
more accurate simulation of summer melting, which is a highly nonlinear
function of air temperature.

Within the CISM domain, this same downscaling procedure is also applied
to all non-urban land units. The elevation of non-glacier land units is
taken from the mean elevation of ice-free grid cells in CISM. This is
done in order to keep the glaciated and non-glaciated portions of the
CISM domain as consistent as possible.

In contrast to most CLM subgrid units, glacier\_mec columns can be
active (i.e., have model calculations run there) even if their area is
zero. These are known as "virtual" columns. This is done because the ice
sheet model may require a SMB for some grid cells where CLM has zero
glacier area in that elevation range. Virtual columns also facilitate
glacial advance and retreat in the two-way coupled case. Virtual columns
do not affect energy exchange between the land and the atmosphere.

.. _Computation of the surface mass balance:

Computation of the surface mass balance
---------------------------------------

This section describes the computation of surface mass balance and
associated runoff terms. The description here only applies to regions
where glacial melt runs off and is replaced by ice, not to regions where
glacial melt remains in place. Thus, by default, this only applies to
Greenland and Antarctica, not to mountain glaciers elsewhere in the
world. (See also section :numref:`Glacier regions`.)

The SMB of a glacier or ice sheet is the net annual
accumulation/ablation of mass at the upper surface. Ablation is defined
as the mass of water that runs off to the ocean. Not all the surface
meltwater runs off; some of the melt percolates into the snow and
refreezes. Accumulation is primarily by snowfall and deposition, and
ablation is primarily by melting and evaporation/sublimation. CLM uses a
surface-energy-balance (SEB) scheme to compute the SMB. In this scheme,
the melting depends on the sum of the radiative, turbulent, and
conductive fluxes reaching the surface, as described elsewhere in this
document.

Note that the SMB typically is defined as the total accumulation of ice
and snow, minus the total ablation. The SMB flux passed to CISM is the
mass balance for ice alone, not snow. We can think of CLM as owning the
snow, whereas CISM owns the underlying ice.  Fluctuations in snow depth
between 0 and 10 m water equivalent are not reflected in the SMB passed
to CISM. In transient runs, this can lead to delays of a few decades in
the onset of accumulation or ablation in a given glacier column.

SMB is computed and sent to the CESM coupler regardless of whether and
where CISM is operating. However, the effect of SMB terms on runoff
fluxes differs depending on whether and where CISM is evolving in
two-way-coupled mode. This is described by the variable
*glc\_dyn\_runoff\_routing*. (This is real-valued in the code to handle
the edge case where a CLM grid cell partially overlaps with the CISM
grid, but we describe it as a logical variable here for simplicity.) In
typical cases where CISM is not evolving, *glc\_dyn\_runoff\_routing*
will be false everywhere; in these cases, CISM's mass is not considered
to be part of the coupled system. In cases where CISM is evolving and
sending its own calving flux to the coupler, *glc\_dyn\_runoff\_routing*
will be true over the CISM domain and false elsewhere.

Any snow capping (section :numref:`Runoff from glaciers and snow-capped
surfaces`) is added to :math:`q_{ice,frz}`. Any liquid water (i.e.,
melted ice) below the snow pack in the glacier column is added to
:math:`q_{ice,melt}`, then is converted back to ice to maintain a
pure-ice column. Then the total SMB is given by :math:`q_{ice,tot}`:

.. math::
   :label: 13.1

   q_{ice,tot} = q_{ice,frz} - q_{ice,melt}

CLM is responsible for generating glacial surface melt, even when
running with an evolving ice sheet. Thus, :math:`q_{ice,melt}` is always
added to liquid runoff (:math:`q_{rgwl}`), regardless of
*glc\_dyn\_runoff\_routing*. However, the ice runoff flux depends on
*glc\_dyn\_runoff\_routing*. If *glc\_dyn\_runoff\_routing* is true,
then CISM controls the fate of the snow capping mass in
:math:`q_{ice,frz}` (e.g., eventually transporting it to lower
elevations where it can be melted or calved). Since CISM will now own
this mass, the snow capping flux does *not* contribute to any runoff
fluxes generated by CLM in this case.

If *glc\_dyn\_runoff\_routing* is false, then CLM sends the snow capping
flux as runoff, as a crude representation of ice calving (see also
sections :numref:`Runoff from glaciers and snow-capped surfaces` and
:numref:`Glacier regions`). However, this ice runoff flux is reduced by
:math:`q_{ice,melt}`. This reduction is needed for conservation; its
need is subtle, but can be understood with either of these explanations:

- When ice melts, we let the liquid run off and replace it with new
  ice. That new ice needs to come from somewhere to keep the coupled
  system in water balance. We "request" the new ice from the ocean by
  generating a negative ice runoff equivalent to the amount we have
  melted.

- Ice melt removes mass from the system, as it should. But the snow
  capping flux also removes mass from the system. The latter is a crude
  parameterization of calving, assuming steady state - i.e., all ice
  gain is balanced by ice loss. This removal of mass due to both
  accumulation and melt represents a double-counting. Each unit of melt
  indicates that one unit of accumulation should not have made it to the
  ocean as ice, but instead melted before it got there. So we need to
  correct for this double-counting by removing one unit of ice runoff
  for each unit of melt.

For a given point in space or time, this reduction can result in
negative ice runoff. However, when integrated over space and time, for
an ice sheet that is near equilibrium, this just serves to decrease the
too-high positive ice runoff from snow capping. (The treatment of snow
capping with *glc\_dyn\_runoff\_routing* false is based on this
near-equilibrium assumption - i.e., that ice accumulation is roughly
balanced by :math:`calving + melt`, integrated across space and time.
For glaciers and ice sheets that violate this assumption, either because
they are far out of equilibrium with the climate or because the model is
being run for hundreds of years, there are two ways to avoid the
unrealistic ice runoff from snow capping: by running with an evolving,
two-way-coupled ice sheet or by changing a glacier region's ice runoff
behavior as described in section :numref:`Glacier regions`.)

In regions where SMB is computed for glaciers, SMB is also computed for
the natural vegetated land unit. Because there is no ice to melt in this
land unit, it can only generate a zero or positive SMB. A positive SMB
is generated once the snow pack reaches its maximum depth. When running
with an evolving ice sheet, this condition triggers glacial inception.