PMIP Documentation for UIUC11
University of Illinois at Urbana-Champaign: Model UIUC MLAM-PMIP (4x5 L11) 1996
Dr. Michael Schlesinger, Department of Atmospheric Sciences, University of Illinois at Urbana-Champaign, 105 South Gregory Avenue, Urbana, Illinois 61801; Phone: +1 217 333 2192; Fax: +1-217 244 4393; e-mail: schlesin@atmos.uiuc.edu
World Wide Web URL: http://crga.atmos.uiuc.edu
Number of days in each month: 31 28 31 30 31 30 31 31 30 31 30 31
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Schlesinger, M. E., N. G. Andronova, B. Entwistle , A. Ghanem , N. Ramankutty, W. Wang and F. Yang, 1997:
Modeling and Simulation of Climate and Climate Change, In Past and present variability of the
solar-terrestrial system: Measurements, data analysis and theoretical models, Proceedings of the
International School of Physics "Enrico Fermi", Course CXXXIII, 25 June - 5 July 1996, Varrena, Italy. G. Cini Castagnoli and A. Provenzale (Eds.), IOS Press, Amsterdam, p389-429.
secondary reference(s):
The dynamical structure and numerics of the UIUC model, as well as some of its surface schemes are as described in:
Ghan, S. J., J. W. Lingaas, M. E. Schlesinger, R. L. Mobley, and W. L. Gates, A documentation of the OSU two-level atmospheric general circulation model . 1982, Climatic Research Institute, Oregon State University: Corvallis, OR. p. 395.
The parameterizations of radiation, cloud formation, and related physics are discussed in:
Oh, J.-H., Physically-Based General Circulation Model Parameterization
of Clouds and their Radiative Interaction in Department of Atmospheric
Sciences, 1989, Oregon State University: Corvallis. p. 315.
B-grid: Surface pressure, temperature and water-vapor are located in
the center of each grid cell, and the horizontal velocity components are
located at the corners of the cell.
dim_longitude*dim_latitude: 72*46.
The model uses normalized pressure, sigma, as its vertical coordinate, such that the earth's surface is the coordinate surface sigma=1 and the top of the model is the coordinate surface sigma=0.
s = (p - pT)/(pS -
pT), where p is pressure, pT is the constant pressure
at the top of the model, and pS is the surface pressure, which
varies with horizontal location and time.
For a surface pressure of 1000 hPa, 2 levels are below 900 hPa and 4
levels are above 200 hPa.
The model was started from an equilibrium state of a 1*CO2 experiment.
The adiabatic, frictional terms in the governing prognostic equations
are calculated once per hour.
Vertical diffusion of momentum, sensible heat, and moisture operates
at all vertical levels. The diffusion depends on the vertical wind shear,
but not on stability .
A momentum drag is included in the top layer of the model that is proportional
to air density and the square of the velocity (Hansen et. al 1983).
Radiative Transfer. - Upward and downward fluxes of solar (shortwave) radiation are calculated using a two-stream method with the delta-Eddington approximation. Scattering and absorption by both gases and cloud droplets are included. The spectrum of solar radiation is divided into three intervals: (1) 0 to 0.44 µm (2) 0.44 to 0.69 µm and (3) 0.69 to 3.85 µm.The first two intervals are for the treatment of Rayleigh scattering and absorption by ozone and carbon dioxide . The third interval is for water-vapor absorption and is subdivided into six k-distribution intervals . The optical depth and single-scattering albedo for cloud droplets are determined separately for non-ice clouds and for cirrus clouds .
Upward and downward fluxes of longwave (terrestrial) radiation are calculated using a two-stream formulation. Absorption is treated in four spectral bands: (1) 0 - 340 cm-1 and 1380 - 1900 cm-1, for the water-vapor line centers; (2) 340 - 540, 800 - 980, 1100 - 1380, and 1900 - 3000 cm-1 for the water-vapor line wings and part of the water-vapor continuum; (3) 540 - 800 cm-1, for carbon dioxide and part of the water-vapor continuum; and (4) 980 - 1100 cm-1, for ozone and part of the water-vapor continuum. Absorption by the trace gases methane, nitrous oxide, and chlorofluorocarbon compounds CFC-11 and CFC-12 is included in spectral bands 2-4. Pressure-broadening effects are included in the absorption calculations for water vapor , ozone , carbon dioxide , methane and nitrous oxide , and CFC-11 and CFC-12 . Absorption by cloud droplets is treated by an emissivity formulation for non-ice clouds , extratropical clouds and tropical cirrus clouds .
The radiation parameterization includes cloud-cover feedback by calculating separately the radiative fluxes for the cloudy and clear portions of each grid cell, and includes cloud optical-depth feedback by linking the radiative properties to the prognostic cloud-water content. Clouds located in contiguous vertical layers comprise a cloud group. The contiguous cloud layers within each group are overlapped fully in the vertical. The noncontiguous cloud groups, separated from each other by at least one layer of clear air, are overlapped randomly .
The directive radiative forcing of sulfate aerosol due to its backscattering
of incoming solar radiation is included by the model. It is assumed that
the sulfate aerosol particles consist of 75% H2SO4
and 25% H2O, and that their size distribution is log-normal,
with a geometric mean radius of 0.05 µm and a standard deviation
of 2.0 µm . The optical properties of the sulfate aerosol - extinction
efficiency, single-scattering albedo, and asymmetry factor - are computed
once and for all outside the GCM using our Mie scattering model.
Dry-convective adjustment occurs if the temperature lapse rate between any two adjacent vertical layers is absolutely unstable, that is, exceeds the dry-adiabatic lapse rate. If this occurs, the instability is instantaneously removed by adjusting the temperatures of the two layers such that their lapse rate is dry adiabatic. This is done by transferring heat vertically between the layers under the constraint that their total enthalpy is conserved. Dry-convective adjustment is performed from the lowest to the highest model layer, iteratively.
Middle-level convection occurs if the temperature lapse rate between any two adjacent vertical layers is conditionally unstable and the lower-layer air is sufficiently near saturation that it would be positively buoyant if displaced to the higher layer . This condition occurs when the moist static energy of the lower layer exceeds the saturated moist static energy of the upper layer. When the instability exists, an upward convective mass flux occurs between the layers within a convective tower, and a compensating downward mass flux occurs between the layers in the environment outside the convective tower. Because the air within the convective tower is saturated, the convective mass flux therein generates liquid water, part of which is converted into convective precipitation that falls out of the cloud. The subsiding mass flux in the environment modifies the environmental temperature, water vapor and horizontal momentum. This modification of the environment reduces the instability at a rate that depends on the convective mass flux. The latter is calculated such that the instability is removed with an e-folding time of one hour. The fractional cloudiness for middle-level convective cloud is a function of the convective mass flux and the relative humidity of the higher layer.
Penetrating convection occurs if the temperature lapse rate between the PBL and any layer above is conditionally unstable and the PBL air is sufficiently near saturation that it would be positively buoyant if displaced to the higher layer . This condition occurs when the moist static energy of the PBL exceeds the saturated moist static energy of the layer above. Also, the relative humidity of the PBL must equal or exceed 95 percent for penetrative convection to be initiated . The treatment of penetrating convection is essentially the same as the treatment for middle-level convection, except that: (1) as many convective towers may coexist as there are layers above the PBL, one tower extending from the PBL to each layer for which the instability exists; (2) environmental air is entrained into each convective tower from all layers through which it passes, and this mass transport modifies the temperature and water vapor within the cloud, as well as the temperature, water vapor and horizontal momentum within the environment; and (3) when the initiating instability for any cloud tower ceases to exist, the cloudiness at its top level evaporates with a prescribed e-folding time.
Large-scale condensation occurs in a layer not only when the cell is
everywhere saturated, but also when only part of the grid cell is saturated
. The rate of condensation depends on the large-scale convergence rates
of moisture, heat and mass, and the time rate of change of fractional relative
humidity of the layer, U. The latter is determined from U = bUs
+ (1 - b)Uo, where b is the fractional cloudiness, Us
(=1) is the saturated relative humidity within the cloud, (1 - b) is the
cloud-free fraction, and Uo the fractional relative humidity
of the clear air. Closure is achieved by assuming: (1) the moisture convergence
is partitioned between the cloud and clear air in proportion to b and 1-b,
respectively; and (2) Uo = Uoo + b(Uo
- Uoo), where Uoo is the relative humidity at which
condensation can begin. The result is that b = 1 - {(Us - U)/(Us
- Uoo)}1/2, which increases from zero for U = Uoo
to unity for U = Us = 1. Uoo is taken to be 0.90.
Cloud in the PBL (see Planetary Boundary Layer) is diagnostically computed
on the basis of a cloud-topped mixed-layer model .
See also Cloud Formation, Diffusion, Surface Characteristics and Surface
Fluxes
For 6 fix: SSTs and sea-ice prescribed at their present day value, as
in the control run.
Snow cover also alters the surface albedo (see Surface Characteristics).
Snow-free surface albedo is updated monthly by interpolation using values for January, April, July, and October specified from data of Matthews . The albedo of snow-covered surfaces is determined as a linear weighted (by snow depth) interpolation of snow-free and snow-covered values. The albedo of snow is a function of its temperature ; it also depends on solar zenith angle , but not on spectral interval.
Longwave emissivity is specified to be unity (blackbody emission) for
all surfaces.
The turbulent surface fluxes of momentum, sensible heat and moisture
are parameterized by bulk formulas that depend on the differences of the
momentum, temperature and moisture between the ground and surface air,
the surface-air wind speed, and aerodynamic drag and transfer coefficients.
The surface-air wind is taken as a fraction of the winds extrapolated from
the lowest two model layers. Under the assumption that the PBL is well
mixed in potential temperature and moisture, the surface-air temperature
is extrapolated from the temperature at the lowest level (about 80 meters)
with the dry-adiabatic lapse rate (9.8°C/km) and the surface-air moisture
is taken to be the same as that at the lowest atmospheric level. The aerodynamic
drag and transfer coefficients depend on the vertical stability and surface
roughness length, with the same transfer coefficient used for the fluxes
of sensible heat and moisture. The surface moisture flux (see Land Surface
Processes) depends on an evapotranspiration efficiency, taken as unity
over snow, ice and water, and as a function of the soil wetness over land.
The soil wetness is determined from a prognostic budget equation that includes the rates of precipitation, snowmelt, surface evaporation and runoff. Soil wetness is the ratio of the soil moisture content to the field capacity, the latter prescribed for each of the 35 combinations of the AGCM?s 5 soil textures (Sandy; sandy loam; light loam; loamy; and heavy loam, clay) and 7 surface types ((1) Evergreen wood and forest; (2) mixed and deciduous wood, and forest; (3) grassland; (4) cropland; (5) shrub and semi-desert; (6) desert; and (7) tundra, mountain, arctic flora) after data of Vinnikov and Yeserkepova . The evapotranspiration efficiency over land is taken as the minimum of 4/3 the soil wetness and unity. The runoff rate is a nonlinear function of the soil wetness and the combined rates of precipitation and snowmelt. If the predicted soil wetness exceeds unity, the excess moisture is taken as additional runoff.