308
surface pressure, temperature, water vapour and cloud water as prognostic
variables. Except for the water components, the prognostic variables are
represented by spherical harmonics at triangular truncation at wavenumber
42 (T42). In order to reduce truncation errors for the advection of water
vapour and cloud water a semi-Lagrangian scheme is applied (Williamson
and Rasch, 1994). A semi-implicit time stepping scheme is used with a
weak time filter. The time step is 24 minutes for dynamics and physics,
except for radiation which is only calculated every 2nd hour. A hybrid
sigma-pressure coordinate system is used (Simmons and Strufing, 1983)
with 19 irregularly spaced levels up to a pressure level of 10 hPa. The parameterization of radiation has been applied from Morcrette (1991) with
a number of modifications, such as the consideration of additional greenhouse gases (methane, nitrous oxide and 16 different CFCs) as well as
various types of aerosols. Moreover, the water vapour continuum has been
revised to include temperature weighted band averages of e-type absorption and a band dependent ratio of (p-e)-type to e-type continuum absorption (Giorgetta and Wild,1995). The single scattering properties of
cloud droplets and ice-crystal are derived from Mie theory with suitable
adaption to the broad band model (Rockel et al.,1991). The effective radius of droplets and ice crystal is parameterized in terms of the liquid and
ice water content, respectively. The main objective in the development of
the radiation code was to reproduce the satellite and ground based observations of the Earth's radiation within observational constraints while
satisfying a physically stringent theory. Particularly important were the
changes to the continuum absorption, which increased the downward long
wave radiation at the surface by some 10-15 W/m 2 in agreement with
observations. The turbulent transfer of momentum, heat, moisture and
cloud water within and above than atmospheric boundary layer is computed with a higher-order closure scheme. The eddy diffusion coefficients
are calculated as function of the turbulent kinetic energy (Brinkop and
Roeckner, 1995). The effect of orographically excited gravity waves on
the momentum budget is parameterized on the basis of linear theory and
dimensional considerations (Palmer et al., 1986, Miller et aI, 1989). The
vertical structure of the momentum flux induced by the gravity waves is
calculated from a local Richardson number, which describes the onset of
turbulence due to convective instability and the breakdown approaching
a critical level. The parameterization of cumulus convection is based on
the concept of mass flux and comprises the effect of deep, shallow and
surface pressure, temperature, water vapour and cloud water as prognostic
variables. Except for the water components, the prognostic variables are
represented by spherical harmonics at triangular truncation at wavenumber
42 (T42). In order to reduce truncation errors for the advection of water
vapour and cloud water a semi-Lagrangian scheme is applied (Williamson
and Rasch, 1994). A semi-implicit time stepping scheme is used with a
weak time filter. The time step is 24 minutes for dynamics and physics,
except for radiation which is only calculated every 2nd hour. A hybrid
sigma-pressure coordinate system is used (Simmons and Strufing, 1983)
with 19 irregularly spaced levels up to a pressure level of 10 hPa. The parameterization of radiation has been applied from Morcrette (1991) with
a number of modifications, such as the consideration of additional greenhouse gases (methane, nitrous oxide and 16 different CFCs) as well as
various types of aerosols. Moreover, the water vapour continuum has been
revised to include temperature weighted band averages of e-type absorption and a band dependent ratio of (p-e)-type to e-type continuum absorption (Giorgetta and Wild,1995). The single scattering properties of
cloud droplets and ice-crystal are derived from Mie theory with suitable
adaption to the broad band model (Rockel et al.,1991). The effective radius of droplets and ice crystal is parameterized in terms of the liquid and
ice water content, respectively. The main objective in the development of
the radiation code was to reproduce the satellite and ground based observations of the Earth's radiation within observational constraints while
satisfying a physically stringent theory. Particularly important were the
changes to the continuum absorption, which increased the downward long
wave radiation at the surface by some 10-15 W/m 2 in agreement with
observations. The turbulent transfer of momentum, heat, moisture and
cloud water within and above than atmospheric boundary layer is computed with a higher-order closure scheme. The eddy diffusion coefficients
are calculated as function of the turbulent kinetic energy (Brinkop and
Roeckner, 1995). The effect of orographically excited gravity waves on
the momentum budget is parameterized on the basis of linear theory and
dimensional considerations (Palmer et al., 1986, Miller et aI, 1989). The
vertical structure of the momentum flux induced by the gravity waves is
calculated from a local Richardson number, which describes the onset of
turbulence due to convective instability and the breakdown approaching
a critical level. The parameterization of cumulus convection is based on
the concept of mass flux and comprises the effect of deep, shallow and