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DRAFT CAM Development Plan

version 1.0 [19 June 2006]


Timeline, Science Objectives, Metrics for Scientific Quality of Simulation, Physics, Dynamics, Software Engineering,
Getting a Contribution into CAM/CSM

Tentative Timeline for the development of CAM4

CAM4 time schedules are controlled in part by the overall development of CCSM4 (which is still under discussion). Here is our best guess at the CAM4 timeline. For convenience, the annual CCSM meeting is identified as occurring in June of each year, and the annual CAM meeting as occurring in February.

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Below are suggested science objectives for CAM4.

  1. A Reduction of the biases (also see Metrics for Scientific Quality of Simulation below) present in the CAM that impede its utility in simulations involving a coupling of the basic components of the climate system, e.g.:
  2. A state of the art representation of processes required to assess Aerosol Indirect Effects. The underlying goal is to explore the relative contribution of natural and anthropogenic aerosol influences on the climate system
  3. Improvements to our basic understanding of cloud feedbacks
  4. Improvements to representation and understanding of subgridscale (convective, and turbulent) transport of heat, moisture, momentum, trace constituents and their role in the the general circulation
  5. Better understanding of the sensitivity of atmospheric simulations to vertical and horizontal resolution

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A key goal of the development effort is to quantify simulation biases objectively by constructing measures (metrics) to compare simulations directly against observational estimates of the same field. A series of metrics (one or two numbers) is to be developed that can be run on any simulation to provide a measure of agreement between the model and real world of a particular field (e.g. precipitation) or phenomena (e.g. ENSO). All CCSM users are invited to submit candidate metrics (AMWG Diagnostics) for discussion by the AMWG and possible inclusion into the standard package.

Please note that the Metrics page above is used to evaluate the comprehensive model simulation. Individual component processes (e.g. convection, or dynamics) can and will use specific procedures to evaluate that component individually. See for example the tests of dynamical cores discussed in Dynamics

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Component Improvements required in CAM4

To attain the science goals articulated above we believe that certain issues merit particular attention. These issues can be catagorized in a number of ways, but we have decided to do so in terms of Physics, Dynamics and Software Engineering

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The term "Physics" loosely refers to all processes in the model that are not "Dynamics" (see Dynamics). These include all diabatic terms in the evolution equations, source/sink terms in equations describing the evolution of atmospheric constituents, and parameterizations of subgridscale (unresolvable) processes.

Our overall goal for the physical parameterizations employed in CAM is a better intellectual and theoretical basis for the representation of processes in the model. Our current focus is on the following list of physical parameterizations: Boundary Layer,Convection,Cloud Fraction,Sub-Columns,Radiation,Microphysics,Gravity Waves,Aerosols and Chemistry

We have currently identified:

We welcome the active involvement of others. Anyone in the AMWG can be added to the list and included in development wiki access

Boundary Layer


U Washington Boundary Layer scheme is a candidate parameterization. Boundary layer might pace progress on other components. Because of the dependancy of other components on the boundary layer, this may be our highest priority.

A discussion of the scientific requirements for any boundary layer parameterization and basics of development progress will be contained in a separate PBL requirements document

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A separate convection development document addresses how candidate convection schemes will be established. This process will be more involved because of the large number of people and candidate schemes involved. All who are interested are urged to contact the internal contacts listed.

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Cloud Fraction


Several options are on the table for adjustments to the cloud fraction scheme. These include a Modified version of the current Slingo Scheme for consistency (no empty cloud), and a simplified Tompkins PDF moment scheme for vapor. Contact Gettelman for more details.

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Independent Sub-Columns


A proposal exists to use independent sub-columns (Monte Carlo ICA-McICA) for moist physics. At least: radiation, microphysics (Gettelman/Neale), probably (eventually) corelated-k radiation. It may build off of total water pdfs in tompkins cloud fraction scheme (simplifies closure for microphysics) Consider acceleration options for ICA if we go this route.

Options include schemse by Pincus, Collins (deterministic), or Raisenen formulations. A small group is working on this effort to get the infrastructure into the moist physics. Please contact the internal contacts for more information.

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Ideally, we will utilize a consistent treatment of radiation and physical processes in CAM, for example, it would desirable to have the microphysical formulation for stratiform and convective clouds, as well as the scavenging formulations, and photolysis rates for any photochemistry use the same cloud overlap, and scattering assumptions

Alternative RT formulations that have been put forward are:

'Beyond correlated-k'. Link to Requirements web page

The AMWG will run or orchestrate an evaluation of these codes against benchmarks, and interested parties should contact Collins for more details.

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Currently we are developing a size resolved (2 moment bulk) microphysics code for implementation into CAM. This will also include a radiation interface (Collins/Gettelman/Rasch/Mitchell) and an Aerosol nucleation interface (Gettelman/Ghan). The microphysics development has been proceding with a coordinated group which has its own web site. The basic approach is outlined in a Roadmap and progress can be found on the Microphysics Swiki

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Gravity Waves

Contact: Sassi/Richter

Mountain drag formulations may be developed to modify the gravity wave scheme currently in the model.

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A first aerosol framework has been developed from the MATCH and CAM2 formulation developed by Phil Rasch, Mary Barth, Bill Collins, and Natalie Mahowald. This aerosol framework has been undergoing continuous development over the last 5 years. It is a bulk formulation that predicts only size segregated mass concentrations for externally mixed aerosols.

There has also been a parallel development track occuring in the MOZART offline transport model by X-X Tie, J.-F. Lamarque, Peter Hess and others over the last 5 years. It uses a similar framework, although in differs in several subtle and important ways.

Each of the two frameworks have advantages and disadvantages compared to the other. These is an active effort taking place to merge the two efforts, and connect the resulting formulation to the cloud drop activation parameterization of Ghan and colleagues, to provide a near term parameterization with the best attributes of both the MATCH and MOZART frameworks as well as the ability tocrudely represent the first and second indirect effect. We will refer to this parameterization as the INTERIM aerosol formulation.

There are several options ranging from a version of the Ghan droplet scheme that connects with the current aerosol treatment to detailed 7 mode schemes. The latter may be coupled to MOZART chemistry and aerosols. The goal is to characterize direct and indirect effects, or at least create the infrastructure in the code to do so. We hope to do this in each version of CAM as it evolves. As we add each piece of new functionality (eg new turbulence parameterization, new dynamical cores, new subgridscale vertical velocity, new cloud overlap, new PDF clouds, new McICA) we can then test the direct and indirect effects

The ultimate goal is to link the aerosol framework (existing, interim and proposed options) to the new cloud Microphysics and to ensure this treatment is as consistent as possible.

DIAGNOSTICS: We are importing AEROCOM framework

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Chemistry includes two pieces: WACCM4 and Tropospheric Chemistry


Contact: Gettelman/Sassi/(Garcia)

How high do we go? What do we incorporate for an upper atmosphere in the operational version of CCSM4? Is it WACCM, or MACCM, or a reduced-lid version of MACCM? This has huge implications for the computational cost. [Who wants to test this? There is a paper here]

Tropopsheric Chemistry

Contact: (Lamarque) Carbon/Nitrogen cycles [this might be a CCSM issue]

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The dynamical core refers to the collection of numerical methods that are used to represent the solution to the equations of motion (Navier Stokes equations) and the evolution equations for tracer transport. There are currently three dynamical cores that can be used in CAM and CCSM: a solution method based upon a combination of spectral, finite difference, and semi-lagrangian methods framed using the evolution equations in an Eulerian framework, called the "Eulerian" core; a solution method using the same combination of techniques but expressed in a Lagrangian framework, called the "semi-Lagrangian" method, and a control volume formulation in which the equations are expressed in flux form called the "finite volume" methods.

Each of these techniques currently use a rectilinear (approximately uniform in latitude and longitude) grid. We are currently modifying the CAM to allow its use with alternative grid distributions (for example the "cubed sphere" or "icosehedral" grids) that resolve the sphere more uniformly.

Before doing traditional simulations with a full model it is necessary to test the underlying dynamics of the model (the dynamical core). A test procedure has been developed to make sure a core is doing what we think it is doing. A proposed procedure is described in Dynamics Evaluations

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There are a variety of issues that need to be considered in any candidates for the next generation of CAM.

We have developed a number of documents to assist in understanding the steps used in integrating any new formulations into the model. These documents describe:

For information on these issues please see Software Engineering Issues

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Getting A Contribution into CAM-CSM

This section defines the process that is required to allow a contribution to be considered for the next generation of CAM/CSM. They will be made more specific after initial discussions with the community.

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