1. SUMMARY PAGE ECHAM3 ======================== 1.1 SHORT DESCRIPTION AND MAIN AUTHORS ---------------------------------------- The ECHAM model has been developed from the ECMWF model (cycle 31, November 1988). It contains several changes, mostly in the parameterization, in order to adjust the model for climate simulations. The reference resolution is T42, but the model is set up to use resolutions in the range T21 to T106. Long term integrations have so far only been done for T21, T42 and T63. The lay-out is as follows (** indicates changes from the original ECMWF model): 1.1.1 Numerical solution --------------------------- + prognostic variables: vorticity, divergence, temperature, log surface pressure, water vapour, ** cloud water + horizontal representation: spectral transform, triangular truncation (T21/T42/T63/T106) + vertical representation: hybrid coordinate system, second order finite differences, 19 layers + time integration: semi-implicit; leap frog with time filter, ** Dt = 40 min (T21), Dt = 24 min (T42), Dt = 15 min (T63), Dt = 12 min (T106) 1.1.2 Surface boundary conditions ------------------------------------- + SST and sea-ice: blended data set (Reynolds, 1988) and Alexander and Mobley (1976) (ECHAM1,ECHAM2) COLA/CAC AMIP SST and Sea-Ice Dataset (ECHAM3) + orography: mean terrain heights computed from high resolution US Navy data set land-sea mask: from US Navy data set + roughness length sea: Charnock formula, modified after Miller et al. (1992) sea-ice: constant (0.01 m) land: function of vegetation and orography (variance) + vegetation: fraction of grid area covered by vegetation based on Wilson and Henderson - Sellers(1985) data + albedo: ** sea: function of solar zenith angle bare land: satellite data (Geleyn and Preuss (1983)) ** sea ice: function of temperature (Robock,1980) ** land ice: function of temperature (Robock, 1980; Kukla and Robinson, 1980) ** snow: function of temperature and fractional forest area (Robock, 1980; Matthews, 1983) 1.1.3 Physical parameterization ---------------------------------- + ** radiation (Hense et al.,1982; Rockel et al, 1991; Eickerling, 1989) two-stream approximation six spectral intervals in the terrestrial part four spectral intervals in the solar part gaseous absorbers: H2O, CO2 and O3 (CO2 and O3 prescribed) aerosols: prescribed clouds: computed cloud optical depth and cloud cover emissivity: function of cloud water path (Stephens, 1978) continuum absorption: included cloud overlap: maximum for contiguous clouds layer and random otherwise diurnal cycle: included radiation time step: 2 hours + ** clouds (Sundquist,1978; Roeckner and Schlese,1985; Roeckner et.al,1991) cloud water transport equation subgrid-scale condensation and cloud formation with different thresholds for convective and stratiform clouds (Xu and Krueger, 1991) temperature dependent partitioning of liquid/ice phase (Matveev, 1984) rain formation by auto-conversion of cloud droplets (Sundquist, 1978) sedimentation of ice crystals (Heymsfield, 1977) evaporation of cloud water evaporation of precipitation + convection (Tiedtke, 1989) mass flux scheme for deep, shallow and mid-level convection clouds are represented by a bulk model and include updraft and downdraft mass fluxes convective momentum transport is parameterized according to Schneider and Lindzen (1976) Evaporation of rain is parameterized according to Kessler (1969) stratocumulus convection is parameterized as a vertical diffusion process with enhanced eddy diffusion coefficients (Tiedtke et al., 1988) + planetary boundary layer (Louis, 1979) surface fluxes of momentum, heat, moisture and cloud water are calculated from Monin-Obukhov similarity theory with transfer coefficients depending on roughness length and Richardson number above the surface layer: eddy diffusivity approach with coefficients depending on wind shear, thermal stability and mixing length above the PBL: vertical diffusion only for unstable stratification ** cloud water ** ,moist" Richardson number (Brinkop, 1991; 1992) + ** land-surface processes (Sellers et al.,1986; Blondin,1989; Dümenil and Todini,1992) heat transfer: diffusion equation solved in a 5-layer model with zero heat flux at the bottom (10 m ) water budget equation for three reservoirs: soil moisture, interception reservoir (vegetation), snow vegetation effects: stomatal control on evapotranspiration and interception of rain and snow run-off scheme: based on catchment considerations including sub-grid scale variations of field capacity over inhomogeneous terrain sea-ice temperature calculated from surface energy budget + ** horizontal diffusion (Laursen and Eliasen, 1989) scale selective operator applied beyond a threshold wave number + gravity wave drag (Palmer et al, 1986; Miller et al, 1989) surface stress due to gravity waves, which are exited by stably stratified flow over irregular terrain is calculated from linear theory and dimensional considerations orographic forcing prescribed as a directionally dependent sub-grid scale orographic variance computed from the high resolution US Navy data set vertical structure of momentum flux induced by gravity waves calculated from a local wave Richardson number, which describes the onset of turbulence due to convective instability and the turbulent breakdown approaching a critical level the GWD scheme is not used at T21 resolution 1.2 ACKNOWLEDGEMENTS ---------------------- The ECHAM climate model has been developed from the ECMWF model (therefore the first part of its name: EC) and a comprehensive parametrisation package developed at Hamburg (therefore the abbreviation HAM). This documentation has been written in a similar fashion: A substantial part is based on the ECMWF documentation, which then had been modified to describe the newly implemented subroutines and the changes necessary for climate experiments.