E. Becker, M. Schlutow, H. Koernich, and B. Wolf
Representation of transport and orographic gravity waves
in a mechanistic climate model
We introduce a new mechanistic climate model from the
surface to the lower thermosphere and present a few
examples that may be useful for the S-RIP
when using the model in nudged mode.
The model is based on a standard spectral dynamical
core, but includes a new tracer transport scheme.
The main ideas are 1) using a new monotonic
function of the tracer as prognostic variable to
ensure positive definite concentrations, and
2) defining the mass correction as a horizontally
uniform multiplication of the tracer in grid space,
giving rise to corrections of the zero-wavenumber
spectral components at each layer.
The mass correction is generalized to include also
physical sources and sinks, allowing to include an
energy preserving water vapor cycle.
Idealized age tracer simulations show an increase
of age of air with increasing vertical Schmidt number.
This effect can be shown to result
from decreased vertical mixing in the troposphere.
The model furthermore utilizes an idealized radiative
transfer scheme and includes the full surface budget
by means of a swamp ocean model with prescribed
lateral heat-flux convergence. Since all components
of the model are energy conserving by definition,
including the parameterizations of gravity waves and
turbulence, the radiation budget at the top of the
atmosphere is equilibrated independently from tuning
(Knoepfel and Becker, 2001, JQSRT).
The classical McFarlane-scheme for orographic gravity
waves is extended to ensure consistent scale
interaction with the vertical diffusion.
This follows the idea of Becker and McLandress
(2009, JAS) to modify the Doppler-spread parameterization
of non-orographic gravity waves, which is
included as well. We find that the circulation
in the middle atmosphere is quite sensitive to the
details of the orographic gravity-wave scheme.
In particular, applying the vertical diffusion induced
by orographic gravity waves to the resolved flow,
and to the parameterized non-orographic
gravity waves as well, results in a stronger polar
night jet during NH winter and a corresponding global
response of the Interhemispheric Coupling.
In the future, the model may be used in nudged mode
in order to study the sensitivity of the simulated
tracer transport/mixing and the middle
atmospheric circulation to 1) the nudged scales
and 2) the model resolution (including very high
resolution with resolved non-orographic gravity waves).
Such experiments may help to assess the scales down to
which the respective reanalysis is dynamically
consistent with free running models.
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