.  and by Sugi et al. (1989) . .
. Soil moisture is initialized according to estimates of Willmott et al. (1985) , and snow cover/depth according to data of Dewey (1987) .  time filter (cf. Jarraud et al. 1982) . The length of the time step is not fixed, but is reset every 6 hours to satisfy the Courant-Friedrichs-Lewy (CFL) condition for the advection terms. Shortwave radiation is recalculated hourly, and longwave radiation every 3 hours. Orography). When the atmospheric moisture content of a grid box becomes negative due to spectral truncation, its value is reset to zero without any other modification of the local or global moisture budgets.  for hybrid vertical coordinates.
- Fourth order linear (del^4) horizontal diffusion is applied to
vorticity, divergence, temperature, and specific humidity on the hybrid
vertical surfaces, but with a first-order correction of the temperature and
moisture equations to approximate diffusion on constant-pressure surfaces
(thereby reducing spurious mixing along steep mountain slopes). Diffusion
coefficients are chosen so that the enstrophy power spectrum coincides with
that expected from two-dimensional turbulence theory.
- Stability-dependent vertical diffusion of momentum, heat, and moisture in the planetary boundary layer (PBL) as well as in the free atmosphere follows the Mellor and Yamada (1974)  level-2 turbulence closure scheme. The eddy diffusion coefficient is diagnostically determined from a mixing length formulated after the method of Blackadar (1962). See also Planetary Boundary Layer and Surface Fluxes.
- Shortwave radiation is parameterized differently for wavelengths <0.9
micron (visible) and >0.9 micron (near-infrared). In the visible, absorption
by ozone, Rayleigh scattering by air molecules, and Mie scattering by cloud
droplets are treated. In the near-infrared, water vapor absorption is modeled
after Lacis and Hansen (1974)
Near-infrared scattering and absorption by cloud droplets are calculated by the
delta-Eddington approximation with constant single-scattering albedo.
- Longwave absorption by water vapor, ozone, and carbon dioxide is determined from transmission functions of Rodgers and Walshaw (1966) , Goldman and Kyle (1968) , and Houghton (1977) , respectively; pressure broadening effects are also included. Continuum absorption by water vapor is treated by the method of Roberts et al. (1976) . Transmission in four spectral bands (with boundaries at 4.0 x 10^3, 5.5 x 10^4, 8.0 x 10^4, 1.2 x 10^5, and 2.2 x 10^5 m^-1) includes overlapping effects of different absorbers. Longwave emissivity of cirrus cloud is set at 0.80, and that of all other clouds at 1.0 (blackbody emission). For purposes of the radiation calculations, all clouds are assumed to be randomly overlapped in the vertical. Cf. Sugi et al. (1989)  for further details. See also Cloud Formation.
- A modified Kuo (1974)
 parameterization is used to simulate deep convection. The criteria for the occurrence of convection include conditionally unstable stratification and positive moisture convergence between the cloud base and top. The cloud base is
at the lifting condensation level for surface air, and the top is at a level
where the cloud and environmental temperatures are identical. The cloud
temperature is determined from the moist adiabatic lapse rate modified by
height-dependent entrainment, as proposed by Simpson and Wiggard (1969)
. In a vertical column, the total moisture available from convergence is divided between a fraction b that moistens the environment and the remainder (1 - b) that contributes to the latent heating (rainfall) rate. The moistening paramenter b is a cubic function of the ratio of the mean relative humidity of the cloud layer to a prescribed critical relative
humidity threshold value (70 percent); if cloud relative humidity is less than
this threshold, b is set to unity (no heating of the environment).
- Shallow convection occurs where the vertical stratification is conditionally unstable but moisture convergence is negative. It is parameterized by enhancing the vertical diffusion coefficients after the method of Tiedtke (1983) .
- Over land, the 12 vegetation/surface types of the Simple Biosphere (SiB)
model of Sellers et al. (1986)
specified at monthly intervals.
- The local roughness length over land varies monthly according to
vegetation type (cf. Dorman and Sellers 1989)
; it decreases with increasing snow depth,
the minimum value being 5 percent of that without snow cover. The surface
roughness of sea ice is a uniform 1 x 10^-3 m. Over oceans, the
roughness length for momentum is a function of the surface wind stress after
, while the roughness
length for surface heat and moisture fluxes is specified as a constant 1.52 x
10^-4 m (cf. Kondo 1975)
- Over land, surface albedos vary monthly according to seasonal changes in
vegetation (cf. Dorman and Sellers 1989)
. The albedo is specified separately
for visible (0.0-0.7 micron) and near-infrared (0.7-4.0 microns) spectral
intervals, and is also a function of solar zenith angle. Following Sellers et
, snow cover alters the surface albedo. Over oceans and sea ice,
albedos are functions of solar zenith angle but are independent of spectral
- Longwave emissivity is prescribed to be unity (blackbody emission) for all surfaces. See also Surface Fluxes and Land Surface Processes.
- Solar absorption at the surface is determined from the albedo, and
longwave emission from the Planck equation with prescribed emissivity of 1.0
(see Surface Characteristics).
- The representation of turbulent surface fluxes of momentum, heat, and
moisture follows Monin-Obukhov similarity theory as expressed by bulk
formulae. The wind, temperature, and humidity required for these formulae are
taken to be the values at the lowest atmospheric level (at 995 hPa for a
surface pressure of 1000 hPa). The associated drag/transfer coefficients are
functions of the surface roughness (see Surface Characteristics) and
vertical stability, following Louis et al. (1981)
- Over vegetated surfaces, the temperature and specific humidity of the vegetation canopy space of the SiB model of Sellers et al. (1986)  are used as surface atmospheric values. Over land, the surface moisture flux includes evapotranspiration from dry vegetation (reflecting the presence of stomatal and canopy resistances) as well as direct evaporation from the wet canopy and from bare soil (see Land Surface Processes).
- Land surface processes are simulated by the SiB model of Sellers et al.
, as implemented by Sato et al. (1989a , b ).
Vegetation in each grid box may consist both of ground cover and an upper-story
canopy, with the spatial pattern of the ground cover varying monthly. Within
the canopy, evaporative fluxes are computed by the Penman-Monteith method (cf. Monteith 1973)
. Evapotranspiration from
dry leaves includes the detailed modeling of stomatal and canopy resistances.
Direct evaporation from the wet canopy and from bare soil is also treated (see
Surface Fluxes). Precipitation interception by the canopy (with
large-scale and convective precipitation distinguished) is simulated, and
infiltration of moisture into the ground is limited to less than the local
hydraulic conductivity of the soil.
- Soil temperature is predicted in four layers by the force-restore method of Deardorff (1978) . Soil liquid moisture is predicted from budget equations in three layers, and snow and soil ice in four layers. This moisture is increased by infiltrated precipitation and snowmelt, and is depleted by evapotranspiration and direct evaporation. Both surface runoff and deep runoff from gravitational drainage are simulated. See also Surface Characteristics and Surface Fluxes.
Last update April 19, 1996. For further information, contact:Tom Phillips ( firstname.lastname@example.org)