**PMIP Documentation for GFDL**

**Geophysical Fluid Dynamics Laboratory
: Model GFDL CDG (R30 L20) 1997**

World Wide Web URL: http://www.gfdl.noaa.gov/~ajb/

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

dim_longitude*dim_latitude : 96*80

Second-order vertical diffusion with coefficients derived from mixing-length
considerations is applied to momentum, heat, and moisture is applied at
all levels up to a height of about 5 km. The vertical diffusion is not
stability-dependent.

Longwave radiation follows the method of Rodgers and Walshaw (1966) , as modified by Stone and Manabe (1968) . Absorption by water vapor, carbon dioxide, and ozone is included. The 6.3-micron band, the rotation band, and the continuum of water vapor are subdivided into 19 subintervals, two of which contain the 15-micron carbon dioxide and 9.6-micron ozone absorption bands, with transmissions of the latter two gases multiplied together in overlapping bands. A random model is used to represent the absorptivity for each subinterval, and the Curtis-Godson approximation is used to estimate the effective pressure for absorption. The temperature dependence of line intensity is also incorporated. Carbon dioxide absorptivity is obtained from data of Burch et al. (1961) , with its temperature dependence following the model of Sasamori (1959) . The ozone absorptivity is obtained from data of Walshaw (1957) .

Cloud-radiative interactions are treated as described by Wetherald and
Manabe (1988) and Wetherald et al. (1991) . Cloud optical properties (absorptivity/
reflectivity in the ultraviolet-visible and near-infrared intervals, and
emissivity in the longwave) depend on cloud height (high, middle, and low)
and thickness. The values of shortwave properties are assigned following
Rodgers (1967a) ; in the longwave, the emissivity of thin high clouds (above
10.5 km) is prescribed as 0.6 after Kondratiev (1972) , while all other
clouds are treated as blackbodies (emissivity = 1.0). No partial cloudiness
is accounted for in each grid box; clouds, therefore, are treated as fully
overlapped in the vertical. See also Cloud Formation.

Over oceans, the surface albedo depends on solar zenith angle (cf. Payne 1972 ). The albedo of sea ice is a function of ice thickness and surface temperature. Albedos for snow-free land are obtained from the modern albedo data base of CLIMAP Project Members (1981) and do not depend on solar zenith angle or spectral interval.

Snow cover modifies the local background albedo of the surface according to its depth, following Holloway and Manabe (1971) . The albedo also depends on surface temperature, with higher albedos at lower temperatures.

Longwave emissivity is prescribed as unity (blackbody emission) for
all surfaces.

Surface turbulent eddy fluxes follow Manabe (1969). The momentum flux is proportional to the product of a drag coefficient, the wind speed, and the wind velocity vector at the lowest atmospheric level. Surface sensible heat flux is proportional to the product of a transfer coefficient, the wind speed at the lowest atmospheric level, and the vertical difference between the temperature at the surface and that of the lowest level. The drag and transfer coefficients are functions of surface roughness length (see Surface Characteristics) but are not stability-dependent.

The surface moisture flux is the product of potential evaporation and
evapotranspiration efficiency beta. Potential evaporation is proportional
to the product of the same transfer coefficient as for the sensible heat
flux, the wind speed at the lowest atmospheric level, and the difference
between the specific humidity at the lowest level and the saturated specific
humidity for the local surface temperature and pressure. The evapotranspiration
efficiency beta is prescribed to be unity over oceans, snow, and ice surfaces;
over land, beta is a function of the ratio of soil moisture to the constant
field capacity (see Land Surface Processes).

Soil moisture is represented by the single-layer "bucket" model of Manabe (1969) , with field capacity everywhere 0.15 m. Soil moisture is increased by precipitation and snowmelt; it is depleted by surface evaporation, which is determined from a product of the evapotranspiration efficiency beta and the potential evaporation from a surface saturated at the local surface temperature and pressure (see Surface Fluxes). Over land, beta is given by the ratio of local soil moisture to a critical value that is 75 percent of field capacity, and is set to unity if soil moisture exceeds this value. Runoff occurs implicitly if soil moisture exceeds the field capacity.

Last update November 9, 1998. For further information, contact: Céline Bonfils (pmipweb@lsce.ipsl.fr )