Department of Numerical Mathematics (of the Russian Academy of Sciences): Model DNM A5407.V1 (4x5 L7) 1991
Dr. V. Galin, Department of Numerical Mathematics, Russian Academy of Sciences, Leninsky Prospect, 32 A, Moscow 117334, Russia; Phone: +7-095-938-1808; Fax: +7-095-938-1808; e-mail: galin@inm.ras.ru
DNM A5407.V1 (4x5 L7) 1991
The DNM model was initially developed in the early 1980s by G. I. Marchuk and collaborators (cf. Marchuk et al. 1984)
[1].
Key documentation of the DNM model is provided by Marchuk et al. (1984)
[1]. The radiation scheme is described by
Feigelson (1984)
[2], Podolskaya and Rivin (1988)
[3], and Galin (1984)
[4]. The treatment of turbulent fluxes in the planetary boundary layer (PBL) follows Lykossov (1990)
[5] and Kazakov and Lykossov (1982)
[6].
Second-order finite differences on a shifted C-grid (cf. Arakawa and Lamb 1977)
[7] with conservation of total atmospheric mass, energy, and potential enstrophy.
4 x 5-degree latitude-longitude grid.
Surface to about 71 hPa. For a surface pressure of 1000 hPa, the lowest
atmospheric level is at about 929 hPa.
Finite differences in sigma coordinates.
There are 7 regularly spaced sigma levels. For a surface pressure of 1000 hPa,
one level is below 800 hPa and one level is above 200 hPa.
The AMIP simulation was run on a Cray 2 computer using a single processor in a
UNICOS environment.
For the AMIP experiment, about 3 minutes Cray 2 computation time per simulated
day.
For the AMIP simulation, initial conditions of the model atmosphere, soil
moisture, and snow cover/depth for 1 January 1979 are determined from a
simulation of October through December 1978, starting from arbitrary initial
conditions, but with the ocean surface temperatures and sea ice extents
prescribed to be the same as the AMIP boundary conditions for January 1979.
Time integration is by the Matsuno scheme, with time steps of 6 minutes for
dynamics, 3 hours for radiation, and 1 hour for all other model physics.
Orography is smoothed (see
Orography). Atmospheric temperature, specific humidity, and u-v winds are filtered at latitudes poleward of 50 degrees.
Negative values of atmospheric specific humidity are filled, with conservation
of the mass of water vapor in the vertical column.
For the AMIP simulation, the model history is written every 6 hours.
Primitive-equation dynamics are expressed in terms of u-v winds, temperature,
specific humidity, surface pressure, and vertical motion.
- Second-order nonlinear horizontal diffusion after Marchuk et al. (1984)
[1] is applied to the winds, temperature, and specific humidity on sigma surfaces.
- Vertical diffusion includes use of a turbulent kinetic energy equation and
non-gradient transport of momentum, heat, and moisture after the method of Lykossov (1990)
[5] in the PBL (see
Planetary Boundary Layer). Vertical diffusion is not applied above the PBL.
Gravity-wave drag is not modeled.
The solar constant is the AMIP-prescribed value of 1365 W/(m^2). Both seasonal
and diurnal cycles in solar forcing are simulated.
The carbon dioxide concentration is the AMIP-prescribed value of 345 ppm.
Zonally averaged total column ozone concentration is specified from the data of Koprova and Uranova (1978)
[8], with the zonal
vertical profile computed as in Lacis and Hansen (1974)
[9]. Radiative effects of water vapor are also included, but not the effects of aerosols (see
Radiation).
- Shortwave radiation is computed after Manabe and Strickler (1964)
[10] and Lacis and Hansen (1974)
[9] in two spectral bands--an ultraviolet/visible (UVV) band from 0.2 to 0.9 micron, and a near
infrared (NIR) band from 0.9 to 4.0 microns. In the UVV, shortwave radiation is absorbed by ozone, but not by water vapor. Rayleigh scattering and multiple
cloud reflection effects also are taken into account following Lacis and Hansen
(1974)
[9]. In the NIR, the radiation is absorbed by water vapor and clouds, but the radiative effects of aerosols are not included; the direct-beam flux and the flux reflected by clouds and by the Earth's surface are distinguished. NIR scattering by water vapor and cloud droplets is not treated.
- Longwave radiation is computed by the method of Feigelson (1984)
[2]. The integral transmission functions (including pressure-broadening effects) of carbon dioxide and water vapor are after Podolskaya and Rivin (1988)
[3], but the radiative effects of the water vapor continuum are not taken into account. The longwave transmission function of ozone is calculated by the method of Raschke (1973)
[11]. The longwave optical properties of
clouds are obtained from Rodgers (1967)
[12]. Longwave emissivity of high cloud is prescribed as 0.5, but middle and low clouds are treated as blackbodies (emissivity of 1.0). For purposes of the radiation calculations, clouds are assumed to be randomly overlapped in the vertical. See also
Cloud Formation.
- Penetrative convection is simulated after the method of Kuo (1974)
[13], and is assumed to occur only in the presence of conditionally unstable layers in the vertical and large-scale net
moisture convergence in the horizontal. The associated convective cloud formed at the lifting condensation level is assumed to dissolve instantaneously through lateral mixing, thereby imparting heat and moisture to the environment. 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 parameter b is determined after Anthes (1977)
[14] as a cubic function of the ratio of the mean relative humidity of the cloud layer to a prescribed critical relative humidity threshold value; if the cloud relative humidity is less than the threshold, b is set to 1 (no heating of the environment).
- Shallow convection is simulated inasmuch as a moist convective adjustment is carried out in the PBL only (see
Planetary Boundary Layer).
A subgrid-scale convective cloud fraction is not explicitly calculated. The
gridscale cloud fraction is based on the relative humidity diagnostic of
Smagorinsky (1960)
[15] with a threshold
humidity of 100 percent required for condensation. These clouds form at three levels (low, middle and high cloud), and are taken to be half the thickness of a sigma layer. Eight fractional cases are distinguished in each grid box: a clear-sky fraction; fractions covered only by high, middle, and low cloud, respectively; fractions covered by high and middle cloud, by high and low cloud, and by middle and low cloud; and a fraction that is covered by cloud at all levels (cf. Galin 1984
[4] for further details). See also
Radiation for
cloud-radiative interactions.
Grid-scale precipitation is equal to the amount of condensation necessary to
return a supersaturated layer to 100 percent relative humidity. There is no
subsequent evaporation of falling precipitation. Convective precipitation is
determined from the specification of the Kuo moistening parameter b (see
Convection).
- The top of the PBL is assumed to be at the height of the lowest
atmospheric vertical level (at sigma = 0.929). The PBL is treated physically as
two sublayers: a lower constant-flux sublayer, and an upper mixed transitional sublayer. In the lower sublayer, with top at height h = 70 m, the turbulent momentum, heat, and moisture fluxes are calculated following Monin-Obukhov similarity theory (see
Surface Fluxes). The wind at height h is taken to be the same as that at the PBL top. The temperature at height h is determined from a continuity equation for the equivalent potential temperature flux. The specific humidity at height h is given by the product of saturated specific humidity at this temperature, and of the relative humidity, which is computed from the arithmetic mean of the inverse values at the PBL top and at the surface.
- In the upper sublayer, the wind speed is constant, but the turning of the
wind vector with height depends on latitude and surface type. In this upper
sublayer there is also nongradient diffusion of heat, moisture, and momentum.
See also Diffusion, Surface Fluxes, and Land Surface Processes.
The 1 x 1-degree topographic height data of Gates and Nelson (1975)
[16] are smoothed by averaging over each 4 x 5-degree grid box. Then a 9-point filter is applied to further smooth data from surrounding grid cells. Finally, a Fourier filter is applied poleward of 50 degrees latitude to eliminate high-frequency orographic variance.
AMIP monthly sea surface temperatures are prescribed, with daily values
determined by linear interpolation.
Monthly AMIP sea ice extents are prescribed, and the ice depth is taken to be a
uniform 2 m. Snow is allowed to accumulate on sea ice (see Snow Cover). The top surface temperature of the ice is predicted from the balance
of surface energy fluxes (see Surface Fluxes) including conduction heating from below. The conduction is a function of the temperature gradient across the ice, with the bottom surface temperature fixed at -2 degrees C for other surface types.
Precipitation falls as snow if the surface air temperature is < 0 degrees C.
The snow depth is determined prognostically on land, continental ice, and sea
ice from the moisture budget equation. Snow, which covers the whole of a grid box (i.e., no fractional coverage), affects the surface albedo (see
Surface Characteristics). New snow also decreases the heat conductivity of bare soil up to 33 percent, but with no further decrease occurring as snow accumulates (see
Land Surface Processes). Sublimation of snow is calculated as part of the surface evaporative flux (see Surface Fluxes). Melting of snow, which contributes to soil moisture, occurs whenever the surface air temperature is > 0 degrees C.
- The surface roughness length is assumed to be different for momentum than for the heat and moisture fluxes (see
Surface Fluxes). The momentum
roughness length over the ocean is a function of the surface stress given by
the Charnock (1955)
[17] relation, with coefficient 0.14. Over ice surfaces, the roughness is prescribed to be a uniform 0.01 m. Over land, the roughness length is a spatially variable function of orography (see
Orography). The roughness lengths for surface heat and moisture fluxes are the same, and are equated to the momentum roughness length weighted by a function of the Reynolds number. Observed data reviewed by Garratt (1977)
[18] are used to specify this function (cf. Kazakov and Lykossov 1982
[6] for details).
- Surface albedos are prescribed as 0.1 over oceans, 0.6 over ice, 0.2 over
bare soil. For snow-covered areas (see
Snow Cover), the albedo is a
linear function of water-equivalent snow depth, with values ranging between 0.2 and 0.6. These albedos are not a function of solar zenith angle or spectral
interval.
- Longwave emissivity is prescribed as unity (i.e., blackbody emission) fo
all surfaces.
- Surface solar absorption is determined from specified constant albedos,
and longwave emission from the Planck equation with prescribed constant
emissivity of 1.0 (see
Surface Characteristics).
- Surface turbulent eddy fluxes of momentum, heat, and moisture are
simulated by a bulk aerodynamic approximation, with drag and transfer
coefficients calculated following Monin-Obukhov similarity theory. Empirical
functions of Businger et al. (1971)
[19] are matched with a -1/3 power-law dependence for strong instability (cf. Kazakov and Lykossov 1982)
[6]; these are used as universal functions of roughness length and of thermal stability to define the vertical profiles of wind, temperature, and humidity in the constant-flux surface layer (see Planetary Boundary Layer). When the wind speed at the top of the surface layer (at height h = 70 m) > 15 m/s, the ocean surface heat and moisture fluxes are augmented by the evaporation of sea spray, which is modeled after Borisenkov and Kuznetsov (1978)
[20], based on observations of
Monahan (1968)[21]. Cf. Kazakov and Lykossov (1980)
[22] for further details.
- The surface moisture flux depends on the near-surface wind speed and on the specific humidity difference between the top and bottom of the surface
layer, which is a function of surface relative humidity (see Planetary Boundary Layer). Over oceans, sea ice, and snow the surface relative humidity is prescribed as 100 percent, while over land it is a function of the soil moisture (see Land Surface Processes).
- Soil temperature is computed from a surface energy balance with heat
storage in a 1-meter layer, and with deep soil temperature at 1 meter
prescribed from data of Legates (1987)
[23].
- Soil moisture is predicted by a single-layer "bucket" model with uniform 0.15 m field capacity after the method of Budyko (1956)
[24], but with deep soil moisture also prescribed from monthly estimates of Mintz and Serafini (1981)
[25]. The surface relative humidity is a function of the ratio of soil moisture to field capacity (see Surface Fluxes).
Runoff occurs implicitly if this ratio exceeds unity.
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Last update May 28, 1996. For further information, contact: Tom Phillips (phillips@tworks.llnl.gov )LLNL Disclaimers
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