PMIP Documentation for CCM3
National Center for Atmospheric Research: Model NCAR CCM3 (T42 L18) 1992
World Wide Web URL: http://www.ncar.ucar.edu/
Number of days in each month: 31 28 31 30 31 30 31 31 30 31 30 31
The model is identical to latest AMIP model exept for different initial
conditionsand the Earth's orbital parameters.
Bonan, G.B. (1996). A land surface model (LSM version 1.0) for ecological, hydrological, and atmospheric studies: technical description and user's guide, NCAR Tech. Note NCAR/TN-417+STR, Boulder, CO. 150 pp.
Key documents are the NCAR CCM2 model description by the user's guide by Bath et al. (1992) and by:
Hack, J.J., Boville, B.A., Briegleb, B.P., Kiehl, J.T., Rasch, P.J.,and Williamson, D.L. (1993).
Description of the NCAR Community, Climate Model (CCM2), NCAR Tech. Note NCAR/TN-382+STR, Boulder, CO
Various aspects of the simulated climate with prescribed climatological sea surface temperatures are described by:
Kiehl, J.T. (1994). Sensitivity of a GCM climate simulation to differences in continental versus maritime cloud drop size. J. Geophys. Res. 99, 23107-23115.
Hack, J.J., Boville, B.A., Kiehl, J.T., Rasch, P.J., and Williamson, D.L. (1994). Climate statistics from the National Center for Atmospheric Research community climate model CCM2. J. Geophys. Res. 99, 20785-20813. [11,12]
Other papers that provide details on particular model features include
Briegleb (1992) , Briegleb et al. (1986) , and Kiehl and Briegleb (1991)
on the radiation parameterizations,Hack (1994) on the convection scheme,
Holtslag and Boville (1993) on the simulation ofboundary-layer diffusion,
and Williamson and Rasch (1994) on the semi-Lagrangian transport scheme.
Model datasets available for analysis atNCAR (including those from the
AMIP simulation) are summarized by Williamson (1993) .
For a surface pressure of 1000 hPa, 4 levels are below 800 hPa and 7
levels are above 200 hPa.
For 6fix, the experiment was started using modern, spun-up initial conditions
obtained from the modern control run.
time-splitting procedure. The overall time step is 20 minutes for dynamics
and physics, with radiation circulations performed every hour. Cf. Hack
et al. (1993) for further details.
Above the PBL (see Planetary Boundary Layer) a second-order, stability-dependent local formulation of the vertical diffusion of momentum, heat, and moisture is adopted (cf. Smagorinsky et al. 1965 ). The mixing length is taken to be a constant 30 m, and the diffusivity is as given by Williamson et al. (1987) for unstable and neutral conditions and by Holtslag and Beljaars (1989) for stable conditions. Above the surface layer, but within the PBL under unstable conditions, mixing of heat and moisture (but not of momentum) is formulated as nonlocal diffusion, following Holtslag and Boville (1993) --see Surface Fluxes.
Horizontal and vertical diffusion are calculated implicitly via time
splitting apart from the solution of the semi-implicit dynamical equations
(see Time Integration Scheme(s)).
There is an incorporation of radiative properties of ice clouds. Shortwave scattering/absorption is parameterized by the delta-Eddington approximation of Joseph et al. (1976) and Coakley et al. (1983) applied in 18 spectral intervals, as described by Briegleb (1992) . (These include 7 intervals between 0.20 and 0.35 micron to capture ozone Hartley-Huggins band absorption and Rayleigh scattering; 1 interval between 0.35 to 0.70 micron to capture Rayleigh scattering and ozone Chappius-band and oxygen B-band absorption; 7 intervals between 0.70 and 5.0 microns to capture oxygen A-band and water vapor/liquid absorption; and 3 intervals between 2.7 and 4.3 microns to capture carbon dioxide absorption.) Following Slingo (1989) , the shortwave optical properties of clouds for the delta-Eddington approximation (optical depth, single-scattering albedo, asymmetry factor) are specified for 4 spectral ranges (with boundaries at 0.25, 0.69, 1.19, 2.38, and 4.0 microns). These properties depend on the specified effective droplet radius (10 microns) and the liquid water path (LWP), which is a prescribed nonlinear function of latitude and height (cf. Kiehl et al. 1994) .
Longwave absorption by ozone and carbon dioxide is treated by a broad-band
absorptance technique, following Ramanathan and Dickinson (1979) and Kiehl
and Briegleb (1991) . A Voigt line profile (temperature) dependence is
added to the pressure broadening of the absorption lines. Absorption by
water vapor (and its overlap with that of ozone and carbon dioxide) are
modeled as in Ramanathan and Downey (1986) . Longwave broad-band emissivity
of clouds is a negative exponential function of LWP, with all clouds assumed
to be randomly overlapped in the vertical. Cf. Hack et al. (1993) for further
details. See also Cloud Formation.
The parameterization is based on simplified equations for the three-layer moist static energy that include (among other terms) the convective mass flux, a "penetration parameter" beta (ranging between 0 and 1) that regulates the detrainment of liquid water, and temperature and moisture perturbations furnished by the PBL parameterization (see Planetary Boundary Layer, Diffusion, and Surface Fluxes). Other free parameters in the scheme include minimum values for beta, for the vertical gradient of moist static energy, and for the depth of precipitating convection; a characteristic convective adjustment time scale; and a cloud-water to rain-water autoconversion coefficient. The parameter beta is determined by iteration, subject to constraints that it and the vertical gradient of moist static energy be at least their minimum values, that the convective mass flux be positive, and that the detrainment layer not be supersaturated. The profiles of convective mass flux, temperature, and moisture are then obtained, and the total convective precipitation rate is calculated by vertical integration of the convective-scale liquid water sink.
If a layer in the stratosphere (i.e., at the top three vertical levels)
is dry adiabatically unstable, the temperature is adjusted so that stability
is restored under the constraint that sensible heat be conserved. Whenever
two layers undergo this dry adjustment, the moisture is also mixed in a
conserving manner. (In the model troposphere, vertical diffusion provides
stabilizing mixing, and momentum is mixed as well--see Diffusion). If a
layer is supersaturated but stable, nonconvective condensation and precipitation
result (see Precipitation).
Convective cloud base and top are determined by the vertical extent of moist instability (see Convection). In each vertical column, the total fractional cloud amount is a logarithmic function of the convective precipitation rate, but is constrained to be between 0.2 and 0.8. The convective cloud fraction in each layer is determined assuming the cloud is distributed randomly in the vertical. For subsequent diagnosis of the fractional amount of nonconvective cloud (see below), the layer relative humidity is reduced proportional to the fraction of convective cloud present.
In regions of upward vertical motion, the fraction of low-level layer cloud is a quadratic function of the difference between the reduced relative humidity (see above) and a constant threshold value (90 percent). The fraction of midlevel and high-level layer cloud is a quadratic function of the difference between the reduced relative humidity and a threshold value that is a linear function of the squared Brunt-Vaisalla frequency (i.e., it is proportional to the vertical stability).
The fraction of marine stratus/stratocumulus is a function of the strength
of the associated low-level inversion and the reduced relative humidity.
Cf. Hack et al. (1993) for further details. See also Radiation. There are
improved diagnosis of cloud optical properties (maritime versus continental
effective radius (Kiehl, 1994), liquid water path) and an incorporation
of evaporation of stratiform precipitation.
The subgrid-scale orographic variances required for the gravity-wave
drag parameterization (see Gravity-wave Drag) are also obtained from the
U.S. Navy dataset. For the spectral T42 model resolution, the variances
are first evaluated on a 2 x 2-degree grid, assuming they are isotropic.
Then the variances are binned to the T42 Gaussian grid (i.e., all values
whose latitude and longitude centers fall within each Gaussian grid box
are averaged together), and are smoothed twice with a 1-2-1 spatial filter.
Values over ocean are set to zero.
LSM replaces the CCM2 specification of surface wetness, prescribed snow cover and prescribed surface albedos. LSM also replaces CCM2 fluxes over land with a parameterization that includes hydrological and ecological processes ( e.g. soil water, phenology, stomatal physiology, interception of water by plants).
The land surface model (LSM version 1) is a one-dimensional model of energy, momentum, water, and CO2 exchange between the atmosphere and land, accounting for ecological differences among vegetation types, hydraulic and thermal differences among soil types, and allowing for multiple surface types including lakes and wetlands within a grid cell. Vegetation effects are included by allowing for twelve plant types that differ in leaf and stem areas, root profile, height, leaf dimension, optical properties, stomatal physiology, roughness length, displacement height, and biomass. These 12 plant types are combined to form 28 different vegetated surfaces, each comprised of multiple plant types and bare ground so that, for example, a mixed broadleaf deciduous and needleleaf evergreen forest consists of patches of broadleaf deciduous trees, needleleaf evergreen trees, and bare ground. Lakes and wetland, if present, form additional patches. Soil effects are included by allowing thermal properties (heat capacity, thermal conductivity) and hydraulic properties (porosity, saturated hydraulic conductivity, saturated matric potential, slope of retention curve) to vary as functions of percent sand and percent clay. Soils also differ in color, which affects soil albedos. Consequently, each grid cell in the domain of interest is assigned a surface type, a fraction covered by lakes, a fraction covered by wetlands, a soil texture (percent sand, percent silt, percent clay), and a soil color.
Major features of the model are:
* prescribed time-varying leaf and stem areas
* absorption, reflection, and transmittance of solar radiation, accounting for the different optical properties of vegetation, soil, water, snow, and ice
* absorption and emission of longwave radiation allowing for emissivities less than one sensible and latent heat fluxes, partitioning latent heat into canopy evaporation, soil evaporation, and transpiration
* turbulent transfer above and within plant canopies
* vegetation and ground temperatures that balance the surface energy budget (net radiation, sensible heat, latent heat, soil heat)
* stomatal physiology and CO2 fluxes
* interception, throughfall, and stemflow
* snow hydrology
* infiltration and runoff
* temperatures for a six-layer soil column using a heat diffusion equation that accounts for phase change
* soil water for the same six-layer soil column using a one-dimensional conservation equation that accounts for infiltration input, gravitational drainage at the bottom of the column, evapotranspiration losses, and vertical water flow based on head gradients temperatures for six-layer deep and shallow lakes accounting for eddy diffusion and convective mixing
In coupling to the atmospheric model, the land surface model provides to the atmospheric model, at every time step, surface albedos (direct beam and diffuse for visible and near-infrared wavebands), upward longwave radiation, sensible heat flux, latent heat flux, water vapor flux, and surface stresses. The atmospheric model provides to the land model, at every time step, incident solar radiation (direct beam and diffuse for visible and near-infrared wavebands), incident longwave radiation, convective and large-scale precipitation, and lowest model level temperature, wind, specific humidity, pressure, and height