CESM1.0: Notable Improvements
It is important to note that CCSM4 is a subset of CESM1. Althought CESM1 supersedes CCSM4, users can run all CCSM4 experiments from the CESM1 code base. As in CCSM4, CESM1 contains totally new infrastructure capabilities that permit new flexibility and extensibility to address the challenges involved in earth system modeling. An integral part of CESM1 is the implementation of a coupling architecture that takes a completely new approach with respect to the high-level design of the system. The CESM1 coupling infrastructure now provides the ability to use a single code base in a start-to-end development cycle; from model parameterization development that might only require a single processor, to performing ultra high resolution simulations on HPC platforms using tens-of-thousands of cores. The CESM1 coupling architecture also provides "plug and play" capability of data and active components and includes a user-friendly scripting system and informative timing utilities. Together, these tools enable a user to create a wide variety of "out-of-the-box" experiments for different model configurations and resolutions and also to determine the optimal load balance for those experiments to ensure maximal throughput and efficiency. CESM1 is also targeting much higher resolutions than any previous CCSM coupled model and efforts have been made to reduce the memory footprint and to improve scaling in all components.
CAM version 5 (CAM5) has been modified substantially with a range of enhancements and improvement in the representation of physical processes since version 4 (CAM4). A new moist turbulence scheme explicitly simulates stratus-radiation-turbulence interactions, making it possible to simulate full aerosol indirect effects within stratus. A new shallow convection scheme uses a realistic plume dilution equation and closure that accurately simulates the spatial distribution of shallow convective activity. Computation of an updraft vertical velocity now allows for aerosol-cumulus interactions. The revised cloud macrophysics scheme provides a more transparent treatment of cloud processes and imposes full consistency between cloud fraction and cloud condensate. Stratiform microphysical processes are represented by a prognostic, two-moment formulation for cloud droplet and cloud ice, and liquid mass and number concentrations. The scheme allows ice supersaturation and features activation of aerosols to form cloud drops and ice crystals. The radiation scheme has been updated to the Rapid Radiative Transfer Method for GCMs (RRTMG) and employs an efficient and accurate correlated-k method for calculating radiative fluxes and heating rates. RRTMG has an extensive spectral representation of the water vapor continuum and offline agreement with line-by-line calculations is significantly improved. The 3-mode modal aerosol scheme (MAM3) has been implemented and provides internally mixed representations of number concentrations and mass for Aitkin, accumulation and course aerosol modes. A full inventory of observationally based aerosol emission mass and size is provided in standard available datasets. These major physics enhancements permit new research capability for assessing the impact of aerosol on cloud properties. In particular, they provide a physically based estimate of the impact of anthropogenic aerosol emissions on the radiative forcing of climate by clouds.
As of CESM1.0.3, CAM5 physics has been updated to CAM5.1 and now uses CLM-CN in fully coupled experiments. A new finite volume low-resolution option (2.5 by 3.33 deg) is available which halves the run time compared to the 2 deg configuration. This option has not been scientifically validated yet. Diagnostic enhancements include the addition of the CFMIP Observation Simulator Package (COSP) for both CAM4 and CAM5, the option to interpolate history output from unstructured grid experiments (e.g. CAM-SE) to a namelist-defined regular lat-lon grid, and the capability to output columns along a specified track mimicking satellite orbits.
The CAM Chemistry Model (CAM-CHEM) Chemistry in CAM is now fully interactive and implemented in CESM; in particular, emissions of biogenic compounds and deposition of aerosols to snow, ice, ocean and vegetation are handled through the coupler. The released version of CAM-chem in CESM is using the recently-developed superfast chemistry (in collaboration with P. Cameron-Smith from LLNL and M. Prather from UCI) to perform centennial scale simulations at a minor cost increase over the base CAM4. These simulations use the recently developed 1850-2005 emissions created in support of CMIP5.
As of CESM1.0.3, CAM-chem is available with full tropospheric and stratospheric chemistry, including new emissions, updates to photolysis rate calculation and polar-stratospheric cloud chemistry (also in WACCM) for a better representation of stratospheric ozone in the 26 level version of CAM. In addition, a new wet removal scheme (from J. Neu, JPL and M. Prather, UC Irvine) is accessible through a namelist switch. The capability of outputting model results along satellite and aircraft track is available. All these features are available in CAM with online dynamics and in the version in which the meteorological fields are specified following meteorological analyses. Multiple simulations spanning the last 15-20 years were performed (compsets available) and evaluated against surface, aircraft and satellite observations.
WACCM4 incorporates several improvements and enhancements over the previous version (3.1.9). It can be run coupled to the POP2 and CICE CESM model components. The model's chemistry module has been updated according to the latest JPL-2006 recommendations; a quasi-biennial oscillation may be imposed (as an option) by relaxing the winds to observations in the Tropics; heating from stratospheric volcanic aerosols is now computed explicitly; the effects of solar proton events are now included; the effect of unresolved orography is parameterized as a surface stress (turbulent mountain stress) leading to an improvement in the frequency of sudden stratospheric warmings; and gravity waves due to convective and frontal sources are parameterized based upon the occurrence of convection and the diagnosis of regions of frontogenesis in the model.
As of CESM1.0.3, WACCM provides the ability to output history at specified times and gridpoints along satellite and aircraft observational tracks, greatly enhancing the ability to validate simulations against observations. WACCM may also be driven by specified dynamics (SD-WACCM) from meteorological analyses in the troposphere and stratosphere to reproduce the winds and temperatures of specific periods in the observational data set. WACCM may also be run with specified chemistry (SC-WACCM), using tracer fields from previous runs, allowing significantly faster model throughput.
As of CESM1.0.4, WACCM-X is available as an option to extend the top boundary of the atmosphere model to the upper thermosphere (2.5x10-9 hPa, or ~500 km), compared to the normal upper boundary in the middle stratosphere for CAM, and in the lower thermosphere for WACCM. As in the regular configuration of WACCM, the photochemistry is interactive with dynamics through transport and exothermic heating. Neutral and ion species (O+, NO+, O2+, N2+ and N+) are self-consistently resolved in the model from the Earth’s surface to the upper thermosphere. It should be noted that ions in WACCM-X are currently still transported as a neutral species. Diffusive separation of major neutral species (N2, O2, and O) becomes important above the homopause, and is treated separately from the minor species separation in WACCM, along with the dependence of specific heats, gas constants and mean molecular weights on the variable composition. At the upper boundary in the upper thermosphere, most atmospheric species are in diffusive equilibrium, and the vertical gradients of temperature and winds are near zero. An exception is atomic hydrogen, for which an empirical uniform flux has been specified in the model. WACCM-X increases the number of vertical levels from 66 to 81, with a vertical resolution of half a scale height throughout the thermosphere. WACCM-X is supported and scientifically validated for the standard WACCM horizontal resolution of 1.9°x2.5° coupled to a data ocean model on the same horizontal grid. Compositional, thermal and wind structures, as well as some important tidal components have been compared with observations, an empirical model (NRLMSISE-00), and TIME-GCM. More detailed model description is given in Liu et al. (Thermosphere extension of the Whole Atmosphere Community Climate Model, J. Geophys. Res.,doi:10.1029/2010JA015586, 2010).
CLM has been modified substantially and includes several new capabilities, input datasets, and parameterization updates. The model is extended with a carbon-nitrogen (CN) cycle model that is prognostic in carbon and nitrogen as well as vegetation phenology. A transient landcover change capability, including wood harvest, is introduced and the dynamic vegetation model is merged with CN (CNDV). An urban model (CLMU) is added and the BVOC model is replaced with the MEGAN model. The hydrology scheme is updated with a TOPMODEL-based runoff model, a simple groundwater model, a new frozen soil scheme, a new soil evaporation parameterization, and a corrected numerical solution of the Richards equation. The snow model incorporates SNICAR - which includes aerosol deposition, grain-size dependent snow ageing, and vertically resolved snowpack heating - as well as new snow cover fraction and snow burial fraction parameterizations. CLM4 also includes a new canopy integration scheme, new canopy interception scaling, and a representation of organic soil thermal and hydraulic properties. The ground column is extended to ~50-m depth by adding 5 bedrock layers (15 total layers). New surface datasets based on MODIS products have been derived, providing a basis for the transient land cover datasets. To improve global energy conservation, runoff is split into separate liquid and ice water streams that are passed separately to the ocean model. Glacier landunits can be partitioned into multiple elevation classes, with a distinct surface mass balance computed for each class. The surface mass balance is passed to the dynamic ice sheet model via the coupler and downscaled to the ice sheet grid.
As of CESM1.0.3, a prognostic crop model (based on AGROIBIS) is included as an option. The crop model currently includes four crop types: corn, spring-temperate cereal, winter-temperate cereal (not tested), and soybean. Tropical crops are not currently represented. An irrigation model is also now available as an option. Currently, irrigation only works for generic crops (i.e., not prognostic crops).
The CESM sea ice component is now CICE, the Los Alamos Sea Ice Model, sometimes referred to as the Community Ice CodE. The main areas of enhancement fall into two categories: physics and computation. The scientific enhancements include new tracers, a new shortwave radiative transfer scheme, a melt pond scheme, and aerosol deposition, all applied to the snow and sea ice. The new computational enhancements include: more flexible computational decomposition strategies, high resolution support, parallel input / output, and OpenMP threading capability.
As of the CESM1.0.4 release, users are able to set up ice-only and ocean-ice cases forced with the COREv2 interannual forcing datasets (DIAF and GIAF compsets, respectively).
The CESM1 active ocean model, POP2, now includes an optional ocean ecosystem model. This model can be used as a component of the global carbon cycle model in CESM. It also allows enables a feedback from biogeochemistry to the ocean physics via pronostically computed chlorophyll impacting the depth profile of short-wave absorption in the ocean. The base ocean model is identical to the one released in CCSM4.
As of the CESM1.0.4 release, users are able to set up ocean-only and ocean-ice cases forced with the COREv2 interannual forcing datasets (CIAF and GIAF compsets, respectively).
CESM1 includes a new ice sheet component. This is a preliminary implementation; several upgrades are planned in the near future. The dynamic ice sheet model is Glimmer, the Community Ice Sheet Model (Glimmer-CISM), an open-source code developed by researchers from various institutions in the U.S. and U.K. Glimmer-CISM has been configured to simulate the dynamic evolution of the Greenland ice sheet, given a surface mass balance received from CLM (for multiple elevation classes) and downscaled to the ice sheet grid. The version currently implemented is Glimmer-CISM 1.6. This version uses the so-called shallow-ice approximation, which is valid in the slow-moving interior of ice sheets but not for ice streams and ice shelves. The code will be upgraded later in 2010 to Glimmer-CISM 2.0, which will include a higher-order dynamics scheme that is valid in all parts of an ice sheet. At present, the coupling with CLM is one-way in the sense that the land model provides a surface mass balance for ice sheets, but the new ice sheet geometry is not returned to CLM. Two-way coupling will be supported in the future. Other planned improvements include a parallel implementation, coupling to the POP ocean model, and support for Antarctic ice sheet simulations.
The CESM1 data models have been completely rewritten. They are now parallelized and share significant amounts of source code. The new data models have created a natural hierarchy in the system and methods for reading and interpolating data have been established that can easily be reused.
Data Atmosphere Model Interannual Forcing
As of the CESM1.0.4 release, we now have the capability of running interannually forced ocean-only (CIAF compset), ice-only (DIAF compset), and ocean-ice (GIAF compset) cases with the COREv2 datasets from years 1948-2007.