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Overview of recent developments of COSMO-CLM numerics and physical
parameterizations for high resolution simulations
Andreas Will (1) and Ulrich Schättler (2)
(1) Chair of Environmental Meteorology, Brandenburg University of Technology (BTU), P.O. Box 10 13 44, D-03013
Cottbus, Germany, e-mail: will@tu-cottbus.de
(2) Research and Development (FE13), Deutscher Wetterdienst, Offenbach, Germany
1. Motivation
Several RCMs are based on NWP models. They use the
NWP dynamical core and extend the model physics relevant
on long time scales. This is also the case with COSMO-
CLM.
One basic idea of the COSMO-CLM development is that a
model improvement should reduce the inaccuracy of
operational numerical weather prediction and of regional
climate simulations with perfect boundary conditions.
From version 4 on the COSMO-CLM is a unified model for
NWP and RCM designed for high resolution applications of
up to 100m horizontal resolution. In NWP mode the model
is developed and tested at the 2 to 7 km space and the synop
tic time scale. The corresponding RCM mode is used and
evaluated at space scales between 2 and 50 km and
climatological time scales. Both "views"
- the time development of single weather situations (case
studies) investigated in NWP mode and
- the statistics of climate investigated in RCM mode
are regarded as complementary pictures and contribute to
the further improvement of the model system.
The main issue of the presentation is to give an overview of
the COSMO-CLM model system, to present the recent
developments of numerics and of physical parameterizations
and to discuss their relevance for regional climate
simulations.
2. Developments of Model Physics and Numerics
for High Resolution Simulations
The basic non-hydrostatatic model version LM_3 / CLM_3
of the COSMO-CLM uses the leapfrog-dynamical method
as proposed by Klemp-Wilhelmson, moist physics for rain
and snow, 1d radiation scheme of Ritter-Geleyn and a multilayer
deep soil model (see Steppeler et al. (2003) for
details). This model dynamics has the advantage of
efficiency and exhibits an acceptable local accuracy in
NWP (LM) and regional climate mode (CLM) (see Hollweg
et al (2008)).
The model physics of the LM_3 was designed for
resolutions between 10 and 50km and had to be further
developed for applications at much higher resolutions. The
strong relation between the physical parameterizations of
unresolved processes and the simulation of grid scale
phenomena led to the development of a new dyanamical
core in order to achieve higher local accuracy.
schemes for passive tracer fields (water constituents) have
been introduced and the so called "shallow atmosphere
approximation" was removed. Now all advection compo
nents and all coriolis terms are implemented.
At meso-scale resolving simulations also more and more
physical processes are explicitely resolved and/or become
important. In COSMO-CLM the horizontal transport,
evaporation of falling water constituents and a further type
of precipitation (graupel) have been introduced, the cloud
microphysics and the convection scheme have been
modified and a shallow convection parameterization has
been introduced.
The improved numerics (RK dynamics) and higher spatial
resolutions modified the unresolved dynamics, which has
to be parameterized. In COSMO-CLM an explicit parame
terization of the effect of the unresolved orography on the
dynamics has been introduced (Lott and Miller (1997))
and a prognostic turbulent kinetic energy equation is
solved.
At higher resolutions and for climate applications the
heterogenity of the land surface becomes more important.
A lake model has been introduced to calculate the surface
temperature of the grid scale lakes and the albedo of snow
covered evergreen forests has been introduced. The last
improves the model behavior especially in late winter.
The developments have not been evaluated independently
on climatological time scales and/or for different regions
on the globe. Therefore, results will be presented
illustrating the influence of each of the developments for
idealized test cases (numerics), extreme weather
conditions and/or on seasonal to inter-annual time scales.
References
Hollweg, H.-D., U. Boehm, I. Fast, B. Hennemuth, K.
Keuler, E. Keup-Thiel, M. Lautenschlager, S. Legutke,
K.Radtke, B. Rockel, M. Schubert, A. Will, M. Woldt,
C. Wunram, Ensemble simulations over Europe with
the regional climate model CLM forced with IPCC
AR4 global scenarios. M&D Technical Report, No. 3,
145 pp. Max-Planck-Institute for Meteorology, 2008,
www.mad.zmaw.de/projects-at-md/sg-adaptation/
J.Steppeler, G.Doms, H.-W.Bitzer, A.Gassmann,
U.Damrath and G.Gregoric, Meso-gamma scale
forecasts using the nonhydrostatic model LM,
Meteorology and Atmospheric Physics (2003), 82, 75-
96
A new dynamical core based on a non-dissipative 3 rd order
Runge-Kutta time integration method has been introduced
together with 3 rd and 5 th order upwind advection schemes in
order to reduce the phase error. Furthermore, different