PMIP Documentation for UGAMP
The UK Universities' Global Atmospheric Modelling Programme: Model UGAMP UGCM Version 2 (T42 L19) 1994
Dr. Buwen Dong, Department of meteorology, University of Reading, Reading RG6 6BB, U.K.; Phone: 44 0118 9875123 ext:4384; Fax: 44 0118 9318316; e-mail: email@example.com ;
World Wide Web URL: http://ugamp.nerc.ac.uk
Number of days in each month: 31 28 31 30 31 30 31 31 30 31 30 31
The 21fix and 21cal simulations were documented by :
Dong B. W. and P. J. Valdes, 1998: Simulations of the Last Glacial Maximum climates using a general circulation model: Prescribed versus computed Sea Surface temperatures. Climate dynamics, 14, 571-591.
and 6fix was documeted by :
Hall, N.M.J. and P. J. Valdes, 1997: A GCM simulation of the climate 6000 years ago. J. Clim., 10, 3-17.
Valdes P. J., and N. M. J. Hall, 1994: Mid-latitude depressions during the ice age. NATO ASI Volume on Long Term Climatic Variations-Data and Modelling. J. C. Duplessy, Ed., 511-531.
Dong, B., and P. J. Valdes, 1995:Sensitivity studies of northern hemisphere glaciation using an atmospheric general circulation model. J. Climate, 8, 2471-2496.
Simmons, A. J., D. M. Burridge, M. Jarraud, C. Girard, and W. Wergen, 1989: The ECMWF medium-range prediction models: Development of the numerical formulations and the impact of increased resolution. Meteor. Atmos. Phys., 40, 28-60.
Subsequent modifications are described by Slingo et al. (1994) and references therein:
Slingo, J. M., M. Blackburn, A. Betts, R. Brugge, K. Hodges, B. Hoskins, M. Miller, L. Steenman-Clark, and J. Thuburn, 1994: Mean climate and transience in the tropics of the UGAMP GCM: Sensitivity to convection parameterization. Quart. J. Roy. Meteor. Soc., 120, 881-922
Documentation for the ECMWF(cycle 27) predecessor model is provided
by Tiedtke et al. (1988) : Tiedtke, M., W. A. Heckley and J. M. Slingo,
1988: Tropical forecasting at ECMWF: The influence of physical parameterization
on the mean structure of forecasts and analyses. Quart. J. Roy. Meteor.
Soc., 114, 639-664
The UGAMP GCM uses a hybrid coordinate in the vertical. The half-level values are given by
where P(0) is a constant pressure. This coordinate is identical to the usual
sigma when A(K+1/2)=0, and in general equals sigma when p(s)=p(0)*eta=p/p(0)
at levels where coordinate surfaces are surfaces of constant pressure. Values
of eta in between half-levels are given by
for p in the range between p(k-1/2) and p(k+1/2).
The value for p(0) is 101325 Pa. A(k+1/2) and B(k+1/2) are as following
k A(k+1/2) B(k+1/2)
0 0.000000 0.0000000000
1 2000.000000 0.0000000000
2 4000.000000 0.0000000000
3 6046.110595 0.0003389933
4 8267.927560 0.0033571866
5 10609.513232 0.0130700434
6 12851.100169 0.0340771467
7 14698.498086 0.0706498323
8 15861.125180 0.1259166826
9 16116.236610 0.2011954093
10 15356.924115 0.2955196487
11 13621.460403 0.4054091989
12 11101.561987 0.5249322253
13 8127.144155 0.6461079479
14 5125.141747 0.7596983769
15 2549.969411 0.8564375573
16 783.195032 0.9287469142
17 0.000000 0.9729851852
18 0.000000 0.9922814815
19 0.000000 1.000000000
Second-order vertical diffusion is applied below a hybrid model coordinate
level of 0.650 to parameterize the PBL (see Planetary Boundary Layer).
In addition, the TVD vertical advection scheme (see Vertical Representation)
includes some dissipation of kinetic energy where sharp changes in gradient
For clear-sky conditions, shortwave radiation is modeled by a two-stream formulation in two spectral wavelength intervals (0.25 to 0.68 micron and 0.68 to 4.0 microns), using a photon path distribution method to separate the contributions of scattering and absorption processes to radiative transfer. Rayleigh scattering and Mie scattering/absorption by five aerosol types (see Chemistry) are treated by a delta-Eddington approximation.
The clear-sky longwave scheme employs a broad-band flux emissivity method in six spectral intervals between wavenumbers 0 and 2.6 x 105 m-1, with continuum absorption by water vapor included between wavenumbers 3.5 x 104 to 1.25 x 105 m-1. The temperature/pressure dependence of longwave gaseous absorption follows Morcrette et al. (1986) . Aerosol absorption is also modeled by an emissivity formulation.
Shortwave scattering and absorption by cloud droplets are treated by a delta-Eddington approximation; radiative parameters include optical thickness, single-scattering albedo linked to cloud liquid water path, and prescribed asymmetry factor. Cloud types are distinguished by also defining shortwave optical thickness as a function of effective droplet radius. Clouds are treated as graybodies in the longwave, with emissivity depending on cloud liquid water path after Stephens (1978) . Longwave scattering by cloud droplets is neglected, and droplet absorption is modeled by an emissivity formulation in terms of the cloud liquid water path. For purposes of the radiation calculations, clouds of different types are treated as randomly overlapped in the vertical; convective cloud and the same type of nonconvective cloud in adjacent layers are treated as fully overlapped.
The full radiation calculations are performed every 3 hours on a reduced
horizontal grid (every fourth point in longitude only), but with effective
transmissivities and emissivities returned on the T42 Gaussian grid (see
Horizontal Resolution). For intermediate time steps, the effective transmissivities
are scaled by the instantaneous incoming solar radiation to represent correctly
the diurnal cycle; the effective emissivities are scaled by the instantaneous
Planck function to treat temperature variations. However, the influence
of clouds remains fixed between full-radiation steps. See also Cloud Formation.
The temperature and humidity reference profiles for deep convection are based on relevant observational data (cf. Betts 1986 ). The temperature reference profile is a lapse rate that is slightly unstable with respect to the wet virtual adiabat below the freezing level, and that returns at cloud top to the moist adiabat of the cloud base. For energy conservation, this reference profile is corrected (with a second iteration) in order to remove the vertically integrated difference between the total moist enthalpy of the environment and that of the reference profile. The humidity reference profile is derived from the temperature reference by linearly interpolating between the humidities for specified values of subsaturation pressure deficit at cloud base, freezing level, and cloud top. Below the cloud base, cooling/drying by convective downdrafts is parameterized by specifying reference profiles for air parcels originating near 850 hPa that descend at constant subsaturation and equivalent potential temperature.
Nonprecipitating shallow convection is parameterized as a mixing of
enthalpy and moisture of air below cloud base with air at and just above
the capping inversion top. The reference profile is a mixing line structure
joining the conserved saturation pressure and potential temperature points
of all mixtures of the two sources of air (cf. Betts 1983 , 1986 ). Reference
temperature and humidity profiles are computed after specifying a partial
degree of mixing within the cloud, and mixing that is a function of the
inversion strength at cloud top. Cf. Betts and Miller (1993) and Slingo
et al. (1994) for further details. See also Cloud Formation and Precipitation.
The fraction of shallow convective cloud (typically about 0.30) is related to the moisture tendencies within the cloud layer (cf. Betts and Miller 1993 ). The fraction of deep convective cloud (ranging between 0.20 to 0.80) is determined from the scaled convective precipitation rate (see Precipitation). If deep convective cloud forms above 400 hPa and the fractional area is > 0.4, anvil cirrus and shallow convective cloud also form.
Stratiform cloud is present only when the local relative humidity is
> 80 percent, the amount being a quadratic function of this humidity excess.
Low stratiform cloud is absent in regions of grid-scale subsidence, and
the amount of low and middle stratiform cloud is reduced in dry downdrafts
around subgrid-scale convective clouds. Low cloud forms below a temperature
inversion if the relative humidity is > 60 percent, the cloud amount depending
on this humidity excess and the inversion strength. See also Radiation
for treatment of cloud-radiative interactions.
In the absence of convective adjustment, precipitation also results
from gridscale condensation when the local specific humidity exceeds the
saturated value at the ambient temperature and pressure; the amount of
precipitate depends on the new equilibrium specific humidity resulting
from the accompanying latent heat release. Before falling to the surface,
grid-scale precipitation must saturate all layers below the condensation
level by evaporation. Melting of falling snow (see Snow Cover) occurs for
air temperatures > +2 degrees C.
In 21k simulations, the anomalies of 21k and 0k orography based on Peltier's
reconstruction was added to UGAMP 0k orography. The land-sea mask is also
based on Peltier's reconstruction. The ice sheet reconstruction for 21k
is from Peltier et al. (1994).
For 6 fix: SSTs and sea-ice prescribed at their present day value, as in the control run.
For 21 fix: SSTs and sea-ice: The change in SSTs (LGM minus present-day) given by CLIMAP (1981), available at NGDC rather than the LGM absolute values in order to avoid differences due to differences in present day climatologies, was used. To obtain seasonally varying SSTs and sea ice edge from data for February and August, a simple sinusoidal variations, with extrems in February and August, is used.
For computed SSTs experiments, the model was coupled to a mixed layer ocean model (Dong and Valdes, 1995), with prescribed, seasonally varying ocean heat flux transport. The ocean heat transport is diagnosed seperately over ocean and sea ice from the contral simulation.
For 21 cal: The 21 kyr BP experiments with computed SSTs and sea ice
should be performed in the same way as the CO2 experiments. Typically this
means that a coupled atmosphere-mixed layer ocean model will be used with
prescribed present day ocean heat flux.
For 21fix, sea ice edge of CLIMAP data is used for February and August.
Roughness length is prescribed as 1.0 x 10-3 m over sea ice. Over open ocean the roughness is computed from the surface wind stress following Charnock (1955) , but it is constrained to be at least 1.5 x 10-5 m. The roughness length over land is prescribed as a blended function of local orographic variance (Tibaldi and Geleyn 1981 ), vegetation (Baumgartner et al. 1977 ), and urbanization (from the U.S. Navy data set described by Joseph 1980 ) that is interpolated to the model grid; the logarithm of local roughness length is also smoothed by the same Gaussian filter used for orography (see Orography).
Annual means of satellite-observed surface albedo (range 0.07 to 0.80) from data of Preuss and Geleyn (1980) and Geleyn and Preuss (1983) are interpolated to the model grid and smoothed by the same Gaussian filter as used for orography (see Orography). Snow cover alters this background albedo, with a limiting value of 0.80 for snow depths > 0.01 m equivalent water. Sea ice albedo is prescribed as 0.55, and ocean albedo as 0.07. All albedos are also functions of solar zenith angle.
Longwave emissivity is prescribed as 0.996 for all surfaces. See also
Sea Ice, Snow Cover, Surface Fluxes, and Land Surface Processes.
Surface turbulent eddy fluxes are simulated as stability-dependent diffusive processes, following Monin-Obukhov similarity theory. Fluxes of momentum/heat/moisture are calculated from bulk formulae that include the product of a drag/transfer coefficient, the low-level wind speed, and the vertical difference between winds/dry static energy/specific humidity at the surface and their values at the lowest atmospheric level (996 hPa for a surface pressure of 1000 hPa). The low-level wind speed includes an imposed minimum of 3 m/s and an additional 3 m/s (added quadratically) in the presence of convection. (The former quantity increases surface fluxes in the limit of low wind speed, while the latter accounts for subgrid-scale convective circulations--cf. Slingo et al. 1994 .) The surface drag/exchange coefficients are functions of stability (bulk Richardson number) and roughness length (see Surface Characteristics) following the formulation of Louis (1979 ) and Louis et al. (1981) . The same transfer coefficient is used for the surface heat and moisture fluxes.
The surface moisture flux is also equivalent to the potential evaporation
from a saturated surface multiplied by an evapotranspiration efficiency
factor beta (cf. Budyko 1974 ). The factor beta is specified as unity over
oceans and regions of dew formation (where the lowest atmospheric level
is supersaturated); otherwise, beta varies with the snow cover and soil
moisture content (see Snow Cover and Land Surface Processes).
Soil moisture also obeys a diffusion equation (with diffusivity one-seventh that of the heat diffusivity). The upper boundary condition is specified from the combined rainfall and snowmelt, and from surface evaporation that is reduced by the presence of (fractional) snow cover. Runoff occurs if the soil moisture exceeds the layer capacity (scaled according to thickness: 0.02 m for the surface layer and 0.10 m for the middle and 0.333 m for the bottom layer). The evapotranspiration efficiency factor beta (see Surface Fluxes) is a composite of values determined for the snow-covered and bare-land fractions of a grid box. For snow-covered surfaces (see Snow Cover), beta is unity. Over bare land, beta is the ratio of the surface layer moisture to a prescribed fraction (0.75) of field capacity, but is constrained to be at most unity. There is also a temperature-dependent correction to account for limitation of evaporation due to lack of shortwave radiation.