Download Ocean Model Working Group

Community Climate System Model (CCSM)
Project Goal
The primary goal of the Community Climate System Model (CCSM) project is to
develop a state-of-the-art climate model and to use it to perform the best possible
science to understand climate variability and global change. We will strive to
build a CCSM community of users who are interested in participating in this
project.
CCSM Science Plan and Strategic Business Plan
The Community Climate System Model (CCSM) project office produced two
documents during this past year. The CCSM Science Plan 2004-2008 was
approved by the Scientific Steering Committee and released in June 2003. It
describes a program for the mitigation of CCSM biases, as well as applications of
the CCSM to science questions of concern to the Intergovernmental Panel on
Climate Change (IPCC) and of the U.S. Climate Change Science Program. The
plan also outlines new directions for the development of the CCSM to include
much more comprehensive treatment of the biogeochemical processes that
operate to determine the chemical and physical characteristics of the climate
system and to make the CCSM more broadly applicable to the needs of decision
makers.
The project office also produced a CCSM Strategic Business Plan that outlines
the staffing needs of scientists and software engineers and computer resource
requirements necessary to execute the CCSM Science Plan.
The major preoccupation of the CCSM project, as a whole, was to diagnose and
decrease the biases that were evident in the long-term integration of CCSM2.
The objective was to produce a version of CCSM that had the highest fidelity
possible for use in the Fourth Assessment Report (FAR) of the IPCC. This
objective involved a reconsideration of the parameterization of cloud processes
in the atmospheric component and adjustments in the land surface model in
respect to the treatment of snow covered surfaces. As is the case in a coupled
model, such changes in the parameterizations of one component had effects on
the behavior of the other components. There were also adjustments to the
ocean model and the sea ice model to achieve balanced and much less biased
simulations of the properties of the climate system for present forcing conditions.
In addition, the project has produced a version of CCSM that has much higher
resolution in the atmospheric component, T85 as compared to T42, while holding
to the same physical parameterizations in the two versions. This will enable a
systematic exploration of the effects of resolution on climate simulations and
climate sensitivity, provide better measures of the uncertainty associated with
simulations of global warming, and give more detail on the potential changes for
the assessment community.
Atmospheric Component
The Atmosphere Model Working Group (AMWG) has undertaken a number of
developments to address model biases, improve performance and portability,
improve the physical representation of various processes, introduce
representations for missing physical processes, and provide more flexibility in
modes of interaction with other components of CCSM. Many of these
modifications were discussed in presentations at the June 2003 annual CCSM
workshop in Breckenridge, Colorado. We refer to the model outlined at that
workshop as the Community Atmosphere Model (CAM2.X). It is anticipated that
it will be released as CAM3 near the end of the calendar year 2003.
These developments had two motivations:
1) to continue the normal evolutionary process pursued by members of the
AMWG to improve the representation of atmospheric processes and
reduce model biases; and
2) to accelerate the effort taking place to prepare the model for use in the
upcoming IPCC assessment activity.
Improvements to the physics include:
1) A substantially revised prognostic cloud water parameterization that
includes separate phases for ice and liquid condensate, advection and
sedimentation of condensate, and a consistent treatment of condensate in
the microphysics and radiative transfer parameterizations. The latent heat
of fusion is now included in all aspects of the thermodynamics, involving
the phase transformation of water substances. The model now conserves
energy exactly in all physical parameterizations. The status of both phases
of water is now communicated to other components of the climate system.
The shallow/frontal convective parameterization now interacts more
closely with the prognostic cloud parameterization by detraining
condensate directly into the stratiform clouds.
2) A significant improvement was made to the representation of direct shortwave aerosol forcing. An aerosol distribution for sulfate, dust, sea salt, and
carbonaceous aerosols is now included in the model. This distribution has
an annual variation, but it is the same from year-to-year. There is also an
optional package for the prognostic representation of these aerosols.
3) The long- and short-wave radiative transfer parameterizations have been
revised to include more recent characterizations of water vapor absorption
and aerosol scattering and absorption (in the short-wave) and absorption
and emission by greenhouse gases (in the long-wave).
These modifications have been evaluated in stand-alone CAM runs and included
in coupled CCSM runs with newer versions of the other components of the
climate system. The model simulations represent a substantial improvement over
CAM2. In particular, the warm bias present in the CAM2 arctic simulations has
been remedied, the cold bias in the tropical tropopause temperature has been
significantly reduced, and the cloud response to tropical sea surface temperature
(SST) variations is now significantly more realistic. Each of these improvements
address deficiencies identified by the AMWG at the 2002 annual CCSM
workshop as a problem requiring attention by the CAM community. We
anticipate the release of a complete set of model source code, documentation,
initial and boundary datasets, and control integrations at a variety of horizontal
resolutions, complete with diagnostic analyses to the community via the Web in
late 2003 or early 2004. The code has been tested on a variety of different
computer architectures, and significant improvements have been made to
performance and portability. One of the configurations of the model released at
that time will match that to be used for the simulations for the upcoming IPCC
effort. The distribution will also contain configurations useful for runs at other
resolutions with a variety of dynamical "cores."
Land Component
The Land Model Working Group (LMWG) undertook several projects to reduce
prominent biases in the Community Land Model (CLM2). A new under-canopy
turbulence scheme was adopted to reduce the excessively warm daytime ground
temperatures in sparsely vegetated areas. A new parameterization of fractional
snow cover on the ground was proposed to improve the low fractional snow
cover in CLM even with deep snow packs. The proposed parameterization uses
different relationships between snow depth and fractional snow cover during the
snow accumulation and snow melt phases. The transition between these two
phases was problematic and only the accumulation phase was accepted for
implementation in CLM. This new parameterization increased the fractional
snow cover on the ground and therefore increased surface albedo in the arctic
during winter. This cooled surface temperature and helped eliminate a prominent
high-latitude winter warm bias in CCSM2. However, the parameterization was
not formally adopted for the next version of CCSM because the surface cooling
led to excessive sea ice in the arctic.
Active research was undertaken to understand and improve other known biases
in the model related to high evaporation of intercepted water. Downscaling of
rainfall within a grid cell is thought to be key to improving the interception of
rainfall. The implementation of sunlit and shaded leaves in CLM2 is also
deficient. Changes to this and to the Vmax parameter that controls
photosynthesis and stomatal conductance are needed to improve the simulation
of gross primary production and also alleviate some of the low transpiration bias
in CLM2. Runoff generation based on a topographic index was also advocated,
but not yet adopted for CLM.
New capabilities being developed for CLM include biogeochemistry (carbon and
nitrogen cycles, mineral aerosols [see Biogeochemistry Working Group section]),
dynamic vegetation, prognostic canopy air space, water isotopes, and land cover
and land use change, including an urban land cover parameterization. During
the last half of the year, much time was spent developing a vector version of
CLM. When finished, this will provide a single code for scalar and vector
platforms, will maintain the scientific functionality of the model, will be portable to
various machines, and will not significantly degrade performance on existing
supported platforms.
Biogeochemistry Working Group
One of the main accomplishments this year of the Biogeochemistry Working
Group (BGCWG) has been the incorporation of active land, ocean, and
atmosphere carbon cycle modules into the CCSM1 physical framework (CCSM1
carbon-climate model). The land biogeochemistry module is based on a merging
of Carnegie-Ames-Stanford Approach (CASA) biogeochemistry and Land
Surface Model (LSM) biogeophysics, with additional dynamic allocation and
prognostic leaf phenology. The ocean module is based on a full-depth carbon,
phosphorus, and oxygen model developed for the Ocean Carbon Model
Intercomparison Project (OCMIP-2), with the addition of fully prognostic
production and an active iron cycle.
Much of the work of the last year has focused on the integration of the carbon
dynamics with the coupled model physics following a sequential spin-up strategy.
Biases in the coupled CCSM physical solutions can introduce large drifts in
land/ocean/atmosphere carbon inventories, and thus gradual adjustments are
required before the full integration of the atmospheric CO2 with the physics
through the radiation terms. Several multicentury spin-up runs (land-atmosphere
and land-ocean-atmosphere) have been completed with the new land-ocean
biogeochemistry modules and are under analysis.
Considerable effort also has been devoted within the BGCWG to the
development of more sophisticated biogeochemistry components for the land
and ocean within later versions of the CCSM. A fairly sophisticated marine
ecosystem model has been implemented within an uncoupled version of the
CCSM2 ocean physical model. The ecosystem model includes multiple element
cycles (C, P, N, Si, Fe, O) and multiple plankton functional groups (picoplankton,
diatoms, diazotrophs, calcifiers). Multidecade-long simulations have been
conducted to explore the upper ocean behavior of the systems.
In conjunction with the LMWG, a new land biogeochemistry model is being
developed within the CLM2 biogeophysical framework. The model explicitly
includes nitrogen dynamics and has been extensively tested against data at
individual sites, with preliminary work underway on regional and global modeldata comparisons.
Work has been completed on a suite of past, present, and future atmospheric
dust simulations within CCSM. Predictions of future dust levels are particularly
important because they allow for the study of the impact of changing land surface
processes on ocean biogeochemistry, as well as radiative feedbacks. The
results of the study suggest that “natural” aerosols have very strong responses to
human interactions and should be more carefully studied for the climate and
biogeochemical impact.
Polar Climate Working Group
Members of the Polar Climate Working Group (PCWG) have performed a
number of simulations and analyses using CCSM2, including analysis of the
1000-year climate simulation and 1% per year increasing CO2 integrations.
Others have continued model development and identifying areas for model
improvements. Model analyses include the relation between high-latitude storm
tracks and model biases, such as the position of the Siberian high and weak
cyclogenesis near the Antarctic peninsula, variability of Antarctic sea ice and its
interactions with the ocean and atmosphere, controls on the location of the sea
ice edge, and investigations of polar amplification in CCSM2. Other simulations
highlight the performance of CCSM components and parameterizations; for
example, the single column model version of CCSM simulated variables at the
Surface Heat Exchange Budget for the Arctic (SHEBA) site on scales of a few
days to one year, with the exception of the cloud fraction. In response, members
of the AMWG have made changes to the cloud physics parameterizations that
improve polar simulations. Ongoing model sensitivity studies tested the effects
of interactive sea ice and ocean model components within the CCSM framework,
coupled model sensitivity to resolution of the ice thickness distribution, and
sensitivity of the polar atmospheric circulation to horizontal resolution. Additional
single column ice-ocean simulations of SHEBA conditions have been performed
to investigate ice-ocean coupling issues and improved parameterizations of
summertime lead conditions.
Several new features have been added to the sea ice model, including an
incremental remapping advection algorithm that includes open water advection,
non-zero sea ice reference salinity (with respect to ice-ocean exchanges),
correction of wind and ice-ocean stress terms for the free drift regime, and a few
minor bug fixes.
Ocean Model Working Group
The ocean component of CCSM has been upgraded in several ways. The
absorption of solar radiation in the upper ocean is now governed by spatially and
monthly dependent global fields of specified chlorophyll distributions derived from
satellite ocean color observations. Compared to the previous constant
absorption scheme, regions of high primary productivity have warmer SSTs and
unproductive regions lower, even though the net solar surface radiation is
unchanged. The numerics of the K-Profile Parameterization (KPP) vertical mixing
scheme were improved to remove a shallow mixing bias. A more efficient
barotropic solver has been implemented. A simple diurnal solar cycle was tested
and is now a run time option, which results in a significant reduction in the cold
bias of coupled solutions in the equatorial Pacific SST.
The very warm (>5°C) SST biases along the west coasts of South America,
South Africa, and California have been investigated. These regions are
physically similar in their abrupt near-coast orography, upwelling favorable
equatorward coastal winds, and non-precipitating stratus clouds, which suggests
a common cause for the biases. Numerical experiments indicate that these
biases are more the cause of, rather than a passive response to, long standing
model deficiencies in the Intertropical Convergence Zones (ITCZs). In a global
coupled model experiment, near coastal ocean temperatures above 500m off
South Africa and South America were forced to remain close to observations.
There was a marked improvement in the ITCZ structures in hemispheres of both
the Atlantic and Pacific. In the Atlantic, the SSTs both south and north of the
equator are improved by as much as 5°C, and the precipitation simulations are
also improved. In the Pacific the effects are felt as far away as New Guinea,
where there is a significant increase in precipitation accompanied by a
contraction of the spurious "double ITCZ" across the central and eastern South
Pacific. There is also a marked improvement in the distribution of rainfall across
the North Pacific ITCZ, with a bigger fraction falling in the east. These
experiments demonstrate how local active regions can have long-range
influences and the importance of getting good simulations of the processes in
those areas.
Climate Variability Working Group
A 15-member ensemble of CAM2 simulations was performed where observed
SST and sea ice concentrations were specified over the global oceans for the
period 1950-2002. These Atmosphere Model Intercomparison Project (AMIP)
experiments allow detailed investigations of the time-varying behavior of the
simulated atmosphere. For example, CAM simulates the long-term precipitation
trends over Africa quite well (see highlight). The CCSM community has begun to
use these AMIP experiments to investigate the atmospheric response to El NiñoSouthern Oscillation (ENSO), the role of anomalous SST in drought and flood
episodes, and decadal variability and trends of natural climate phenomena, such
as the Pacific Decadal Oscillation (PDO) and the North Atlantic Oscillation
(NAO). To explore higher frequency phenomena, a large number of daily and
subdaily fields were archived as requested by several users. The daily, as well as
monthly, data are available via the Web at
http://www.cgd.ucar.edu/~asphilli/cam2.0.1/.
A new SST and sea ice dataset has been prepared for the AMIP simulations,
based on a merger of historical SSTs (1871-1999) reconstructed from ship
observations by the Hadley Centre with more recent (1982-to-present) SST
analyses produced from in situ and satellite data. This SST product is continually
updated and made available for community use through NCAR. It is already
being utilized by the Geophysical Fluid Dynamics Laboratory (GFDL) in their
AMIP experiments, which will aid in model comparison efforts.
An upper ocean model, which simulates the temperature, salinity, and depth of
the surface mixed layer, was coupled to CAM and the thermodynamic
component of the sea ice model. The model has been designed such that the
ocean and sea ice can be active in some regions, while SST and sea ice are
specified over the remainder of the globe. An extended control integration (>150
years) has been performed in which the mixed layer and sea ice models are
active over the global oceans. The climate from this model configuration is
stable and provides a reasonable representation of SST and sea ice extent over
many parts of the world’s oceans. The model is being used to study local oceanatmosphere-ice interactions, atmospheric teleconnections between ocean
basins, and the impact of upper ocean processes, such as the “reemergence
mechanism,” on climate variability.
Climate Change Working Group
The Climate Change Working Group (CCWG) is currently running and analyzing
results from two global coupled climate models. One, the Parallel Climate Model
(PCM), has the Community Climate Model (CCM3.2) atmosphere (T42, 18L) and
a version of the Parallel Ocean Program (POP) ocean (2/3 degree down to 1/2
degree in the equatorial tropics), with an elastic-viscous-plastic (EVP) sea ice
model and LSM. This model has been run in a large number of 20th and 21st
century climate experiments with various forcings. For 20th century climate, this
model currently has the largest number of ensemble simulations of forced climate
experiments of any model in the world. Each of five forcings (solar, volcano,
ozone, greenhouse gases, and sulfate aerosol) individually, along with eight
additional experiments with forcings in various combinations, has been run for
four ensemble members each. Additionally, the PCM has been run for future
climate change scenarios including five member ensembles of A Consortium for
the Application of Climate Impact Assessments (ACACIA) business-as-usual and
stabilization scenarios, and single experiments for the five IPCC marker
scenarios (A2, B2, A1B, B1, A1FI). The PCM has also been run with idealized
forcing with CO2 increasing at 1% per year compounded and stabilized at
doubling and quadrupling CO2 for roughly 150 years. PCM runs in progress
include the new FAR IPCC climate change commitment scenarios, as well as a
suite of land surface change experiments. Signals from all of these experiments
can be evaluated through the use of a 1500-year control run with the PCM.
The second model currently actively being run by the CCWG is CCSM2.0.
Components in CCSM2.0 include CAM2 (T42, 26L), a version of the POP ocean
(1 degree down to 1/2 degree in the equatorial tropics, 40L), EVP sea ice, and
CLM. This model was released in mid-2002 and has been run for a 1000-year
control run that is currently being extended to evaluate unforced natural climate
variability. Additional experiments with CCSM2.0 include several 1% per year
CO2 increase simulations and a slab ocean run to evaluate equilibrium climate
sensitivity. A major new set of experiments was run with CCSM2.0 to participate
in a Coupled Model Intercomparison Project (CMIP) coordinated experiment to
evaluate processes that contribute to variability and change of the thermohaline
circulation, alternatively called Meridional Overturning Circulation (MOC). Three
experiments were performed. In the first, 0.1 Sverdrup (Sv) of freshwater was
added to an area of the North Atlantic for 100 years, followed by 100 years when
the anomalous freshwater flux was turned off. This experiment allows
quantification of the slow down and recovery of the MOC in response to an
anomalous freshwater forcing. In a second experiment, partial coupling of the
freshwater flux was performed by forcing a control experiment with the
freshwater flux from a 1% per year CO2 increase experiment. In the third
experiment, also a partially coupled run, the freshwater flux from the control run
was used in a 1% per year CO2 increase run. These three experiments show
that the MOC slows due to an anomalous freshwater flux, but the processes that
contribute to that slowing in a climate change simulation are dominated by
anomalous heat fluxes from the atmosphere.
Paleoclimate Working Group
The Paleoclimate Working Group [with special thanks to E. Brady, Climate
Change Research (CCR) Section; J. Davis, Los Alamos National Laboratory
(LANL); S. Levis, Terrestrial Sciences Section (TSS); C. Shields (CCR); R. Smith
(LANL); M. Vertenstein (TSS); and S. Yeager (Oceanography Section)]
developed tools to create the required CCSM input datasets for past
geographies. Continental boundaries and land surface conditions were vastly
different than present in the deep past. These tools use the ocean bathymetryland topography and vegetation biomes reconstructed by the researcher for
his/her particular time period. A series of programs takes these boundary
conditions and creates all the CCSM input datasets, including new continental
boundaries, topography, bathymetry, plant functional types, and runoff flow
directions and its input into the ocean. These tools have been used to create the
input datasets for a Cretaceous simulation and were demonstrated at the June
2003 annual CCSM workshop. The University of California-Santa Cruz
paleoclimate modeling group has created an Eocene configuration.
This figure shows an example of land topography and ocean bathymetry (m) for
a time period in the deep past. Tools created under the auspices of the
Paleoclimate Working Group allow paleoclimate users to set up the CCSM for
their boundary conditions.
A set of long coupled simulations with the Climate System Model version 1
(CSM1) has been made available to the paleoclimate research community.
These include a Last Glacial Maximum (LGM) simulation (300 years) with large
continental ice sheets over North America and Europe, lowered sea level, and
reduced atmospheric greenhouse gases and a series of Holocene simulations
(11 ky BP- 540 years, 8.5 ky BP-1000 years, 6 ky BP-100 years, 3.5 ky BP-100
years) investigating the role of Milankovitch solar anomalies on the coupled
climate system. These simulations are being use to support research at NCAR,
University of Wisconsin, Duke University, and Lamont-Doherty Earth
Observatory.