AMIP I/AMIP II Model Differences: Model UKMO  HadAM3 (2.5x3.75 L19 ) 1998

AMIP II Model Designation

Most Similar AMIP I Model

AMIP I/AMIP II Model Differences

AMIP II Model Designation

UKMO HadAM3 (2.5x3.75 L19) 1998

Most Similar AMIP I Model

AMIP I/AMIP II Model Differences

• Smoothing/Filling
• Diffusion
• Gravity Wave Drag
• Chemistry
• Radiation
• Convection
• Cloud Formation
• Precipitation
• Planetary Boundary Layer
• Snow Cover
• Surface Characteristics
• Surface Fluxes
• Land Surface Processes

Smoothing/Filling

Diffusion

Gravity Wave Drag

Chemistry

• In addition to the radiatively active gases (water vapor, carbon dioxide, and ozone) of the AMIP I model,  oxygen (O₂, 209500 ppm), methane (CH₄, 1650 ppbv), nitrous oxide (N₂O, 306 ppbv), and chlorofluorocarbons (CFC-11, 222 pptv and CFC-12, 382 pptv) are included. The AMIP I ozone data also are replaced by the recommended zonal-mean monthly climatology of Wang et al. (1995)[59].

• Also in contrast to the AMIP I model, radiatively active aerosols are included, with fixed background concentrations specified as in Cusack et al. (1998)[49].

 

Radiation

• The shortwave fluxes are calculated by a two-stream method, where the spectrum is divided into 6 bands with the following boundaries (units of microns): 0.20-0.32; 0.32-0.69 (2 bands);  0.69-1.19; 1.19-2.38; and 2.38-5.00. Within each band, data are treated using correlated k-distribution methods (cf. Cusack et al. 1999[60]). Gaseous absorption data are derived from HITRAN92 (cf. Rothman et al. 1992[75]), except for ozone where the data are derived from LOWTRAN7 (cf. Kneizys et al. 1988[76]). Rayleigh scattering by gases and scattering/absorption by a climatological background aerosol also are represented. See also Chemistry.

• The longwave fluxes also are calculated by a two-stream method, where the spectrum is divided into 8 bands with the following boundaries (units of m^-1): 0-40000; 40000-55000; 55000-80000 (excluding 59000-75000); 59000-75000; 80000-120000 (excluding 99000-112000); 99000-112000; 120000-150000; and 150000-300000. Gaseous absorption data are derived from HITRAN92 and continuum absorption by water vapor is treated using version 2.1 of the CKD continuum model after Clough et al. 1989[77]. Cf. Cusack et al. (1999)[60] and Edwards (1996)[62] for further details.

• Cloud-radiative interactions are treated consistently in the shortwave and longwave. The radiative properties of clouds are parameterized separately for water and ice as functions of condensed water content and effective radius. These parameterizations follow the functional forms of Slingo (1989)[20], but with newly derived coefficients for each shortwave/longwave spectral band. The effective radii of water droplets are calculated following Martin et al. (1994)[61] , while the effective radius of ice particles (treated as spheres) is fixed at 30 microns. Stratiform and convective cloud are distinguished, with the latter treated as a vertical tower. Clouds in adjacent layers are assumed to overlap maximally, and otherwise to overlap randomly, with the vertical coherence of stratiform and convective cloud accounted for. Mixed phase clouds are represented by horizontally adjacent regions of ice and water cloud. Cf. Edwards and Slingo (1996)[50] for further details.  See also Convection and Cloud Formation.

Convection

Cloud Formation

Precipitation

• The temperature criteria for determining whether the large-scale precipitation condensate is liquid or ice have changed: the condensate is treated as liquid at temperatures above 0 deg C, and as ice below -9 deg C, with the liquid/ice fraction at intermediate temperatures determined by linear interpolation (cf. Gregory and Morris 1996[52]).

• The representation of evaporation/sublimation of falling liquid/frozen precipitation follows the method of Gregory (1995)[51].

Planetary Boundary Layer

Snow Cover

Surface Characteristics

• An effective momentum roughness length is included to represent unresolved orographic form drag (cf. Milton and Wilson 1996[56]).

• Three soil texture types--fine, medium, and coarse--are represented, based on the 1x1-degree map of Wilson and Henderson-Sellers (1985)[37]. Mean textures for each 2.5x3.75-degree grid box are calculated as weighted means of sand, silt, and clay fractions.

• Geographically varying fields of the vegetation parameters are derived by assigning typical values to the 23 landcover classes of the 1 x 1-degree data of Wilson and Henderson-Sellers (1985)[37]. Effective parameters for each grid box are calculated as area-weighted means for all parameters, except for the roughness length which is aggregated using a blending height concept (cf. Mason 1988[55]). Those vegetation parameters in common with the AMIP I land-surface scheme (snow-free albedo, deep-snow albedo, roughness length, canopy interception capacity, and infiltration enhancement factor) are taken from Buckley and Warrilow (1988)[38]. These, along with parameters required by the new land-surface scheme (leaf area index and canopy height), are documented by Cox et al. (1999)[48]. See also Surface Fluxes and Land Surface Processes.

Surface Fluxes

• The atmospheric evaporative demand is met first by sublimation from snow which occurs at the potential rate (i.e. aerodynamic resistance only), following a bulk formulation.

• The canopy is represented as a single-story "big leaf" whose height, leaf area index, and water storage capacity vary with vegetation type. Canopy interception is determined assuming the sub-gridscale rainfall intensity follows an exponential distribution (cf. Dolman and Gregory 1992[34]). Evaporation from the wet canopy occurs at the potential rate, where the wet fractional area  varies linearly with the canopy storage. (A non-zero wet area also is prescribed on bare soil to allow for ponding effects.) Evaporation from bare soil is limited by aerodynamic and soil surface resistance (a constant 100 s m^-1) and by a ``beta'' factor that depends on the volumetric moisture content of the top soil layer: beta is 0 if moisture content is less than a wilting point and is 1 if greater than a critical point, while beta varies linearly for intermediate moisture contents. The wilting and critical points (corresponding, respectively, to soil water suctions of 1.5 MPa and 0.033 MPa) vary geographically according to soil texture types.

• The rooting depth for transpiration is assumed to be 1 meter (the top 3 soil layers) for all vegetation types except trees, which can draw moisture from the entire 3-meter soil column. Transpiration also is limited by a bulk stomatal resistance, which is  calculated by the coupled canopy conductance-photosynthesis scheme of Cox et al. (1998)[47] that includes a representation of carbon fluxes.  The bulk resistance depends on surface temperature, vapor pressure deficit, photosynthetically active radiation (PAR), carbon dioxide concentration, and soil moisture availability.  See also Surface Characteristics and  Land Surface Processes.

Land Surface Processes

• The vegetated and bare-soil fractions of a grid box are both at the diagnosed skin temperature. The subsurface temperatures are updated using a discretized form of the heat diffusion equation that is applied in four vertical layers (thicknesses 0.1, 0.25, 0.65, and 2.0 m, with depth), subject to a zero-flux lower boundary condition. Effects of soil water phase changes on heat capacity are incorporated by the approach of Williams and Smith 1989[63], in which the maximum unfrozen water at a given temperature is derived from the soil water suction curve (cf. Black and Tice 1988[64]). The effective soil thermal conductivity is calculated as a function of water and ice content after Farouki (1981)[65].

 
• Soil hydrology is based on a finite difference approximation to the Richards equation (cf. Richards 1931[66]) with the same vertical discretization as for soil thermodynamics. The moisture within a layer is incremented by the net incoming/outgoing Darcian fluxes, and by transpiration. The dependencies of soil water suction and hydraulic conductivity on soil moisture follow Clapp and Hornberger (1978)[67], with parameters derived from the Wilson and Henderson-Sellers (1985)[37] soil texture dataset using the regression relations of Cosby et al. (1984)[68].

 
• Surface runoff occurs whenever the grid-box mean rainfall intensity (computed from an exponential sub-gridscale distribution--cf. Dolman and Gregory 1992[34]) exceeds the hydraulic conductivity for prescribed saturated moisture values that depend on the soil texture types. Drainage also occurs from the base of the soil column at a rate equal to the hydraulic conductivity of the deepest soil layer (i.e. a "free drainage" lower boundary condition is applied); however, lateral flow and recharge from below are not represented. See also Surface Characteristics and Surface Fluxes.
 

References

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