Pacific Northwest National Laboratory: Model PNNL CCM2 (T42 L18) 1997a

Contact Information

Experimental Implementation

Model Output Description

Model Characteristics

Contact Information

Modeling Group

AMIP Representative(s)

• Steven Ghan
• Mail address: PO Box 999,  Pacific Northwest National Laboratory, Richland Washington 99352, USA
• Phone: +1-509-372-6169
• Fax: +1-509-372-6168
• Internet e-mail: steve.ghan@pnl.gov
• Web address: http://www.pnl.gov/atmos_sciences/as_clim2.html

Experimental Implementation

Simulation Period

Earth Orbital Parameters

Calendar

Radiative Boundary Conditions

Ocean/Surface Boundary Conditions

Orography/Land-Sea Mask

• Raw orography is obtained from the U.S. Navy dataset with resolution of 10 minutes arc on a latitude/longitude grid (cf. Joseph 1980 [28]). These data are area-averaged to a 1 x 1-degree grid, interpolated to a T119 Gaussian grid, spectrally truncated to the model's T42 Gaussian grid, and then spectrally filtered to reduce the amplitude of the smallest scales.

• The subgrid-scale orographic variances required for the gravity wave drag parameterization 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.

 
• Orography is smoothed. Because advection of moisture is treated by semi-Lagrangian transport (SLT) scheme negative specific humidity values are avoided. In cases where negative mixing ratios would result from application of the countergradient term in the parameterization of nonlocal vertical diffusion of moisture in the planetary boundary layer (PBL) (see Surface Fluxes), the countergradient term is not calculated. In addition, at each 20-minute time step a "fixer" is applied to the surface pressure and water vapor so that the global average mass and moisture are conserved (cf. Williamson and Rasch 1994) [45].

 

Atmospheric Mass

Spinup/Initialization

Computer/Operating System

Computational Performance

Model Output Description

Calculation of Standard Output Variables

• Calculation of percentage time that a pressure surface is below ground follows Boer et al. (1986)[42].

• The monthly mean temperature and moisture tendencies are supplied on model levels, without interpolation to WMO standard pressure levels.

• Cloud optical properties include the grid-cell mean values of the cloud water/ice, the extinction coefficient (the cloud optical thickness/layer depth), and the cloud emittance.

• Calculation of 10 m winds, 2 m specific humidity, and the 2 m temperature follows Geleyn (1988)[27].

• The calculation of mean sea-level pressure follows the method of Trenberth et al. (1993)[44].

• The calculation of clear-sky radiation and cloud radiative forcing follows Potter et al. (1992)[43].

• Potential vorticity is not supplied.

• The calculation of planetary boundary layer height follows the method of Holtslag and Boville (1993) [9].

Sampling Procedures

Interpolation Procedures

• The monthly means of atmospheric variablesare derived from samples interpolated to standard pressure surfaces at each time step.

• Treatment of variables on pressure surfaces below ground follows the recommended procedure of Trenberth et al. (1993)[44].

Output Data Structure/Format/Compression

Model Characteristics

AMIP II Model Designation

Model Lineage

Model Documentation

Numerical/Computational Properties

Horizontal Representation

Horizontal Resolution

Spectral triangular 42 (T42), roughly equivalent to 2.8 x 2.8 degrees latitude-longitude.

Vertical Domain

Vertical Representation

Vertical Resolution

Time Integration Scheme(s)

Smoothing/Filling

Dynamical/Physical Properties

Equations of State

Diffusion

• In the troposphere, linear biharmonic (Del⁴) horizontal diffusion (with coefficient 1 x 10¹⁶ m⁴/s) is applied to divergence and vorticity on hybrid sigma-pressure surfaces, and to temperature on first-order constant pressure surfaces (requiring that biharmonic diffusion of surface  pressure also be calculated on the Gaussian grid). In the stratosphere, linear second-order (Del²) diffusion is applied to the same variables at the  top three levels (with diffusivities increasing with height from 2.5 x 10⁵ to 7.5 x 10⁵ m² /s). In the top model layer, diffusion is enhanced by a  factor of 10³ on all spectral wave numbers that violate the Courant-Friedrichs-Lewy (CFL) numerical stability criterion, based on the maximum wind speed.

• Above the planetary boundary layer (PBL) a second-order, stability-dependent local formulation of the vertical diffusion of momentum, heat, and moisture is adopted (cf. Smagorinsky et al. 1965 [17]). The mixing length is taken to be a constant 30 m, and the diffusivity is as given by  Williamson et al. (1987) [1] for unstable and neutral conditions and by Holtslag and Beljaars (1989) [33] 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) [9] (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)).

Gravity Wave Drag

Chemistry

Radiation

• Shortwave scattering/absorption is parameterized by the delta-Eddington approximation of Joseph et al. (1976) [20] and Coakley et al. (1983)[21] applied in 18 spectral intervals, as described by Briegleb (1992) [5]. (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.) Cf. Hack et al. (1993) [3] for further details.

•  Longwave absorption by ozone and carbon dioxide is treated by a broad-band absorptance technique, following Ramanathan and Dickinson (1979) [23] and Kiehl and Briegleb (1991) [7]. 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) [24]. Cf. Hack et al. (1993) [3] for further details.

• Convective clouds are treated as radiatively transparent. The shortwave optical properties of large-scale clouds for the delta-Eddington approximation (optical depth, single scattering albedo, and asymmetry factor) are  parameterized in terms of the cloud liquid water and ice water paths, the droplet effective radius, and the crystal effective size. The droplet effective radius is parameterized in terms of the cloud liquid water content (diagnosed from the prognostic total moisture mixing ratio) and a prescribed droplet number concentration. The ice crystal effective size is parameterized in terms of the prognostic ice water content and ice crystal number concentration. Longwave broad-band emissivity of large-scale cloud is a negative exponential function of liquid water path, with the cloud ice absorption coefficient specified as 1x10^-4 m²/Kg. Large-scale cloud fills the entire grid square (i.e., cloud fraction is 1, with full vertical overlap). Cf. Ghan et al. (1997)[39] for further details. See also Cloud Formation.

Convection

• If the atmosphere is moist adiabatically unstable, temperature/moisture column profiles are adjusted by a mass-flux convective parameterization (cf. Hack 1994) [8]. The scheme utilizes a three-layer model that provides for convergence and entrainment in the lowest subcloud layer, cloud  condensation and rainout in the middle layer, and limited detrainment in the top layer. This scheme is applied by working upward from the surface  on three contiguous layers, and shifting up successively one layer at a time until the whole column is stabilized.

•  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 (increased from 3600 seconds in the standard NCAR CCM2 model to 5400 seconds in order to better simulate cloud radiative forcing); 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 then are 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).

Cloud Formation

• Large-scale cloud formation is determined by the prognostic scheme of Ghan et al. (1997)[39] under the assumption that the maximum relative humidity is 100%. Cloud water mixing ratio r_c is diagnosed from two prognostic variables:

1) total moisture mixing ratio r_w = r_v + r_{c ,} where r_v is the water vapor mixing ratio

2) a condensation-conserved temperature T_c = T - Lr_c/c_p, where L is the latent heat of vaporization and c_p is the specific heat at constant pressure.
 

• Additional prognostic variables include the cloud ice mixing ratio r_i and number concentration N_i , while the cloud liquid droplet number is prescribed. Large-scale cloud is assumed to fill the grid square (i.e., cloud fraction 1, with full vertical overlap). The treated microphysical processes include: condensation of water vapor and evaporation of cloud water and rain; nucleation of ice crystals; vapor deposition and sublimation of cloud ice and snow; autoconversion and accretion of cloud water; aggregation and collection of cloud ice; melting of ice and snow; riming on ice and snow; and gravitational settling of ice; rain, and snow. Saturation of water vapor with respect to ice vs liquid water is clearly distinguished and the Bergeron-Findeisen process is explicitly represented.

• Convective cloud base and top are determined by the vertical extent of moist instability (see Convection). Because convective cloud is treated as radiatively transparent, a sub-grid scale fraction is not computed.  See also Radiation and Precipitation.

Precipitation

• Subgrid-scale precipitation is generated in unstable conditions by the moist convective scheme.

• The grid-scale precipitation (including snowfall) rate is determined diagnostically from conservation equations (neglecting tendency and advection terms) for rain and snow that apply to the prognostic large-scale cloud formation scheme. Subsequent evaporation of falling precipitation is not simulated.

Planetary Boundary Layer

Sea Ice

Snow Cover

• Snowfall is diagnosed from the vertical integral of snow formation in clouds, with instantaneous melting (conserving latent heat) when the temperature exceeds 0 C. Icefall is expressed as the product of the predicted ice mixing ratio and the bulk terminal velocity of ice. See also Cloud Formation.

 
• Continental snow cover is treated as in the BATS1e land surface scheme (cf. Dickinson et al. (1993)[40]). The sub-gridscale fractional snow cover varies inversely as the product of the surface roughness (unmodified by snow cover) and the snow density, which depends on the age of the snow. The albedo also depends on the snow age, as well as on the solar zenith angle.  Sublimation is parameterized through the surface drag formulation.  Surface snowmelt is diagnosed thermodynamically from the latent energy required to balance the surface heating at 0 C. The presence of snow cover also affects the temperature of the underlying soil. See also Surface Characteristics, Surface Fluxes, and Land Surface Processes.

 
• Climatological snow cover is prescribed on sea ice, and affects the albedo, the roughness and wetness, and the thermodynamics of the surface (see Sea Ice).

Surface Characteristics

• The roughness length is a uniform 1 x 10^-4 m  over ocean, and 0.04 m over ice surfaces. Over continents, roughness lengths for the bare-soil fraction of each grid box are distinguished from those over vegetation, where 18 vegetation/land cover types are defined (cf. Dickinson et al. (1993)[40]). For each type, specified vegetation parameters include (among others) a constant stem area index (SAI), and a seasonally varying (according to subsoil temperature) leaf area index (LAI) and vegetation fraction (reduced when there is snow cover). Soil type is also specified according to 12 texture classes and 8 color classes.  See also Land Surface Processes and Surface Fluxes.

• The albedo of ice is a function of surface temperature. Over ocean, surface albedos are prescribed to be 0.025 for the direct-beam (with sun overhead--varies with solar zenith angle) and 0.06 for the diffuse-beam  component of radiation. For each of the 18 vegetation/land cover types, the albedo is specified for visible (< 0.7 microns) and near-infrared (> 0.7 microns) wavelengths  regimes. Over bare soil, the albedo varies according to the 8 soil color classes; it also decreases linearly with increasing soil moisture to a minimum value.

 
• The longwave emissivity is set to unity (blackbody emission) for all surfaces. Cf. Briegleb et al. (1986)[6], Briegleb (1992)[5], Dickinson et al. (1986 [31], 1993 [40]), and Hack et al. 1993[3] for further details.

Surface Fluxes

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

• Turbulent vertical eddy fluxes of momentum, heat, and moisture within the surface layer are expressed as bulk formulae, following Monin-Obukhov similarity theory. The  values of wind, temperature, and humidity required for the bulk formulae are take from those at the lowest atmospheric level (sigma = 0.993), but are interpolated  to near- surface values and modified further within the vegetation canopy. The drag and transfer coefficients in the bulk formulae are functions of roughness  length and stability (bulk Richardson number), following the method of Louis et al. (1981)[32] for neutral and  unstable conditions, and Holtslag and Beljaars (1989)[33] for stable conditions. The surface fluxes over the vegetated fraction of a grid box differ from those over the bare-soil fraction, owing to different roughnesses and temperatures, as well as the evaporation of canopy- intercepted moisture and transpiration.  See also Surface Characteristics and Land Surface Processes.

• Above the surface layer, and within the PBL under unstable conditions, mixing of heat and moisture (but not of  momentum) is formulated as nonlocal vertical diffusion by eddies with length scales of the order of the PBL depth (cf. Deardorff 1972[34]). Under these conditions, a countergradient term that is a function of the surface flux, a convective vertical velocity scale, and the PBL height is added  to the eddy diffusivity coefficient of heat and moisture. Within the stable and neutral PBL (and under all conditions for momentum), the same  stability-dependent local vertical diffusion as is utilized in the model's free atmosphere applies (see Diffusion). Cf. Holtslag and Boville (1993)[9] for further details.

Land Surface Processes

• The single-story vegetation canopy intercepts a fraction of the precipitation, which can both drip to the surface below and evaporate at the potential rate to the atmosphere above. Transpiration of soil moisture through dry portions of the canopy also complements the evaporation from the bare-soil fraction of each grid box. The stomatal resistance is varied to ensure that transpiration does not exceed water flow to the vegetation roots.  The temperature of the canopy also is predicted. .

• Soil temperature is predicted by force-restore calculations in soil layers corresponding to the diurnal and annual penetration depths, with the thermal effects of snow cover also taken into account. Subsurface thermal diffusivity and heat capacity are functions of soil moisture and texture. Modified thermal properties are specified for frozen soil.

• Soil moisture is represented by 3 parameters: soil water in an upper layer, in the rooting zone, and in the total soil column, where the depth of the upper layer and of the rooting zone depend on vegetation type (see Surface Characteristics). Hydraulic properties (e.g. porosity, saturated conductivity, and minimum soil water suction) are associated with each of the 12 soil texture classes.  Sources of soil moisture include rainfall, snow melt, and water dripping from the vegetation canopy; sinks include evaporation from bare soil, transpiration through the vegetation canopy, surface runoff, and gravitational drainage.

• Surface runoff is proportional to the product of the net moisture sources and the fourth power of the ratio of soil moisture to the local field capacity. If the subsurface temperature is below freezing, surface runoff is increased. Within the soil, transport of water between layers occurs in proportion to the relative saturation of each layer. Gravitational drainage from the deepest layer is expressed in terms of hydraulic conductivity, which depends on the degree of saturation of the entire soil column. See also Surface Characteristics and Surface Fluxes.

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