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.
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 short-wave 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."
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 model-data
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.
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.
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 Niņo-Southern 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 ocean-atmosphere-ice interactions, atmospheric
teleconnections between ocean basins, and the impact of upper ocean processes, such as the
“reemergence mechanism,” on climate variability.
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.
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 bathymetry-land 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.
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.