Documentation of the PMIP models (Bonfils et al. 1998)

PMIP Documentation for CCM1

State University of New York at Albany: Model NCAR CCM1 (R15 L12) 1992


PMIP Representative(s)

Dr. John E. Kutzbach, IES-Center for Climatic Research, University of Wisconsin, 1225 W. Dayton St. Madison, Wisconsin 53706-1695;Phone: 608 262 0392; Fax: 608 262 5964 ; e-mail:

World Wide Web URL:

Model Designation

NCAR CCM1 (R15 L12) 1992

Model Identification for PMIP


PMIP run(s)

0cal, 21cal

Number of days in each month: 31 28 31 30 31 30 31 31 30 31

Model Lineage

The standard version 1 of the second generation NCAR Community Climate Model (CCM1) was introduced in July of 1987, and included a number of significant changes to the model formulation which were manifested in changes to the simulated climate.

It is identical to the SUNYA model, without for the addition of radiatively active trace gases other than carbon dioxide.

Model Documentation

main reference:

Key documents for the standard CCM1 model are :

Williamson, D.L., J.T. Kiehl, V. Ramanathan, R.E. Dickinson, J.J. Hack, 1987: Description of NCAR Community Climate Model (CCM1). NCAR Technical Note, NCAR/TN-285+STR, 112 pp.

Kiehl et al. (1987) , and Bath et al. (1987a , b ).

Covey, C. and S.L. Thompson (1989). Testing the effects of ocean heat transport on climate. Paleogeography, Paleoclimatology, Paleoecology, 75,331-341.

Kutzbach,J.,R.Gallimore,S.Harrison,P.Behling,R.Selin, and F.Laarif (1998). Climate and biome simulations for the past 21,000 years Quaternary Science Reviews (in press).

Numerical/Computational Properties

Horizontal Representation

Spectral (spherical harmonic basis functions) with transformation to a Gaussian grid for calculation of nonlinear quantities and some physics.

Horizontal Resolution

Spectral rhomboidal 15 (R15), roughly equivalent to 4.5 x 7.5 degrees latitude-longitude.

dim_longitude*dim_latitude: 48*40

Vertical Domain

Surface to 9 hPa; for a surface pressure of 1000 hPa, the lowest atmospheric level is at 991 hPa.

Vertical Representation

Finite-difference sigma coordinates.

Vertical Resolution

There are 12 unevenly spaced sigma levels with the following values: 0.009, 0.025, 0.060, 0.110, 0.165, 0.245, 0.355, 0.500. For a surface pressure of 1000 hPa, 3 levels are below 800 hPa and 5 levels are above 200 hPa.

Computer/Operating System

The PMIP simulation was run on a Cray-YMP computer using a single processor in a UNICOS environment.

Computational Performance

For the PMIP experiment, about 1.2 minutes Cray-YMP computer time per simulated day.


The control experiment was started from an AMIP simulation and for the AMIP experiment, initial conditions for the atmospheric state, soil moisture, and snow cover/depth were specified from the NCAR CCM1 model's standard January initial dataset (cf. Bath et al. 1987a ). The model then was "spun up" for 210 days in a perpetual January mode. The resulting climate state was then taken as the 1 January 1979 starting point for the AMIP simulation.

Time Integration Scheme(s)

Time integration is by a semi-implicit Hoskins and Simmons (1975) scheme with an Asselin (1972) frequency filter. The time step is 30 minutes for dynamics and physics, except for full (at all Gaussian grid points and vertical levels) radiation calculations which are done once every 12 hours (see Solar Constant/Cycles).

Model spinup was from a previously tuned 0k control mixed-layer run for Jan 15th. Model integration for 0k and 21k were then run for 25 years with averages for last 5 years taken for PMIP results.


Orography is smoothed (see Orography). Negative values of atmospheric specific humidity (which arise because of numerical truncation errors in the discretized moisture equation) are filled by horizontal borrowing of moisture in a globally conserving manner. See also Convection.

Sampling Frequency

For the PMIP simulation, the model history is written once every 12 hours.

Dynamical/Physical Properties

Atmospheric Dynamics

Primitive-equation dynamics are expressed in terms of vorticity, divergence, potential temperature, specific humidity, and surface pressure. Energy-conserving vertical finite-difference approximations are utilized (cf. Williamson 1983 , 1988 ).


Fourth-order (Ñ4) horizontal diffusion of vorticity, divergence, temperature, and specific humidity is computed locally on (approximately) constant pressure surfaces in grid-point space, except at stratospheric levels, where second-order (Ñ2) horizontal diffusion is applied (cf. Boville 1984 ).

Stability-dependent vertical diffusion is computed locally in grid-point space at all levels. Cf. Williamson et al. (1987) for further details.

Gravity-wave Drag

Gravity-wave drag is not modeled.

Solar Constant/Cycles

The solar constant is the AMIP-prescribed value of 1365 W/(m2). The orbital parameters and seasonal insolation distribution are calculated after PMIP recommendations. A seasonal, but not a diurnal cycle, in solar forcing is simulated.


The carbon dioxide values for 0k and 21k are 330 ppm and 191 ppm respectively. Atmospheric CO2 was set at the pre-industrial value of 267ppm (based on a lowering from 330ppm Note the change in CO2 radiative forcing is consistent with a decrease in CO2 from 345ppm to 280ppm).

The vertical distribution of zonal-mean mixing ratios of ozone is specified from monthly data of Dütsch (1978) , updated by linear interpolation every 12 hours. The radiative effects of water vapor and oxygen, as well as methane, nitrous oxide, and chlorofluorocarbon compounds CFC-11 and CFC-12 also are included, but not those associated with aerosols (see Radiation).


Shortwave radiation is treated in two spectral intervals--ultraviolet/visible (0.0 to 0.9 micron) and near-infrared (0.9 to 4.0 microns). Shortwave absorption by ozone, water vapor, carbon dioxide, and oxygen is modeled. Direct-beam absorption by water vapor is after the method of Kratz and Cess (1985) ; the reflected-beam absorption (as well as Rayleigh scattering by gases) follows Lacis and Hansen (1974) . Oxygen absorption is treated as in Kiehl and Yamanouchi (1985) , and near-infrared absorption by carbon dioxide is after Sasamori et al. (1972) . Gaseous absorption within clouds is included. Cloud albedo depends on optical depth and solar zenith angle, with multiple scattering effects included.

Longwave radiation is calculated in 5 spectral intervals (with wavenumber boundaries at 0.0, 5.0 x 104, 8.0 x 104, 1.0x105, 1.2 x 105, and 2.2 x 105 m-1). Absorption/emission by water vapor (cf. Ramanathan and Downey 1986 ), carbon dioxide (cf. Kiehl and Briegleb 1991 ), and ozone (cf. Ramanathan and Dickinson 1979 ) is treated; the standard CCM1 radiation code is modified to include absorption/emission by methane, nitrous oxide, and chlorofluorocarbon compounds CFC-11 and CFC-12 (cf. Wang et al. 1991a , b ). The emissivity of nonconvective cloud is a function of diagnostic liquid water content. For purposes of the radiation calculations, cloud is treated as randomly overlapped in the vertical. Cf. Kiehl et al. (1987) and Wang et al. (1991a , b ) for further details. See also Cloud Formation.


Moist convective adjustment after the method of Manabe et al. (1965) performs several functions: removal of negative atmospheric moisture values (operating with a scheme for horizontal borrowing of moisture--see Smoothing/Filling); dry convective adjustment of unsaturated, unstable layers in the model stratosphere, with vertical mixing of moisture; and moist static adjustment of saturated unstable layers and of supersaturated stable layers.

Cloud Formation

Cloud forms in layers where the relative humidity exceeds 100 percent. If the vertical lapse rate of the layer also exceeds the moist adiabatic value, convective cloud forms (see Convection); otherwise, the cloud is nonconvective, and the fractional cloud cover is set to 0.95 in the layer. Convective cloud cover depends on the depth of the vertical instability, with the cloud amount in each layer adjusted so that the total fractional area is at most 0.30. If there is no associated precipitation (see Precipitation), a minimum convective cloud fraction of 0.01 is specified in each layer. Cloud is not allowed to form in the lowest model layer or in the top 3 layers, but clouds form together in the second and third layers above the surface if either of these layers is supersaturated. Cf. Kiehl et al. 1987 for further details. See also Radiation for treatment of cloud-radiative interactions.


Precipitation results from application of convective adjustment (see Convection), if the vertical column is supersaturated with a lapse rate exceeding moist adiabatic. Precipitation also results if the column is supersaturated but with a stable lapse rate. There is no subsequent evaporation of precipitation before it falls to the surface.

The modern run contained a 'bug' that led to an inadvertent failure to correctly update the saturation specific humidity. The bug runs produced a shift of rainfall from convective to large scale and subsequent decrease in total cloud cover and a warmer climate than no-bug runs. The sensitivity of climate (21k minus modern) is, however, not significantly affected by the bug.

Planetary Boundary Layer

The height of the PBL top is assumed to be that of the first level above the surface (sigma = 0.991), except for the calculation of a bulk Richardson number (see Surface Fluxes). In that case, the PBL top is computed from the temperature at the first sigma level but is constrained to be at least 500 m.


For control, after interpolation of 1 x 1-degree Scripps Institution surface height data (cf. Gates and Nelson 1975 ) to the model grid, the data are smoothed using a Gaussian filter with 1.5-degree radius. The resulting heights are transformed into spectral space and truncated at the R15 model resolution. Cf. Pitcher et al. (1983) for further details.

21k Glacial data on coverage and height are taken from Peltier's 1x1 data sealevel was lowered by 106m to account for ice volume of glaciers; Peltier's data was used to determine switches of ocean to land for 21k that resulted from sea level lowering.


Both the 0k and 21k CCM1 PMIP runs were integrated using the Covey-Thompson 50m mixed-layer ocean model. The formulation included a fixed annual mean, zonal mean ocean heat transport in the SST prediction equation to account for poleward heat transport by ocean currents. The actual value used based on tuning (see Covey-Thompson) is about 1/2 the observed transport. Poleward of 60N and S the convergence of heat flux under sea ice is set at 2W/m2. We followed a scheme suggested by Broccoli for PMIP to adjust the ocean heat flux used in the 21k expt (which has reduced ocean area) so that the total convergence of heat transport is unchanged from the control case.

Sea Ice



Sea ice is predicted when the surface temperature falls below 271.2K. The seaice thickness is predicted using a three-layer thermodynamic model following Semtner (see Covey-Thompson for details).

Snow Cover

Precipitation falls as snow if the temperatures of the surface and the first two atmospheric levels above it are all < 0 degrees C. Snow cover is determined from a combination of a monthly latitude-dependent climatology (cf. Bath et al. 1987a ) and prognostic snow accumulation (on land only) that is determined from a budget equation. A surface temperature > 0 degrees C triggers snowmelt, which augments soil moisture (see Land Surface Processes). Snow cover is also depleted by sublimation, which is calculated as part of the surface evaporative flux (see Surface Fluxes).

Surface Characteristics

The surface roughness length is specified as a uniform 0.25 m over land, sea ice, and snow cover, and as 1.0 x 10-3 m over ocean.

Surface albedos for land surfaces are derived from the Matthews (1983) 1 x 1-degree soil/vegetation dataset, but with distinguished vegetation types reduced to 10 and aggregated to the model resolution (see Horizontal Resolution). Land albedo also depends on solar zenith angle and spectral interval (ultraviolet/visible vs near-infrared--see Radiation). Snow cover alters the land albedo; the composite value is determined from equally weighted combinations of the local background albedo and that of the snow (which depends on surface temperature for the diffuse beam and on solar zenith angle for the direct beam). Over the ocean, surface albedos are prescribed to be 0.0244 for the direct-beam (with sun overhead) and 0.06 for the diffuse-beam component of radiation; the direct-beam albedo varies with solar zenith angle. The albedo of ice is a function of surface temperature. Cf. Briegleb et al. (1986) for further details.

Longwave emissivities are set to unity (blackbody emission) for all surface types.

Surface Fluxes

Surface solar absorption is determined from the albedos, and longwave emission from the Planck equation with prescribed surface emissivity of 1.0 (see Surface Characteristics).

Surface fluxes of momentum, sensible heat and moisture are determined from bulk aerodynamic formulae, following the formulation of Deardorff (1972) . Surface drag/exchange coefficients are a function of roughness lengths (see Surface Characteristics) and bulk Richardson number (see Planetary Boundary Layer). For computing these fluxes, the surface wind speed is constrained to be at least 1 m/s.

The surface moisture flux also depends on the evapotranspiration efficiency beta, which is unity over ocean, snow, and sea ice, but which over land is a function of soil moisture (see Land Surface Processes).

Land Surface Processes

Land surface temperature is determined from the balance of surface energy fluxes (see Surface Fluxes) by the diagnostic method of Holloway and Manabe (1971) . (That is, there is no heat diffusion/ storage within the soil.)

Soil moisture is represented by the single-layer "bucket" model of Budyko (1956) and Manabe (1969) , with field capacity a uniform 0.15 m of water. Soil moisture is increased by both precipitation and snowmelt. It is decreased by surface evaporation, which is determined from the product of the evapotranspiration efficiency beta and the potential evaporation from a surface saturated at the local surface temperature/pressure (see Surface Fluxes). Over land, beta is given by the ratio of local soil moisture to the field capacity, with runoff occurring implicitly if this ratio exceeds unity.

Last update November 9, 1998. For further information, contact: Céline Bonfils ( )