Spectral microphysics in weather forecast models
with special emphasis on cloud droplet nucleation
Contribution to the Priority Program SPP 1167 of the DFG
Verena Grützun, Dr. Oswald Knoth, Dr. Martin Simmel, Dr. Ralf Wolke
SPP 1167: "Quantitative Precipitation Forecast"
Forecasting precipitation is one of the most challenging tasks of weather forecast
models. Especially heavy events are difficult to predict correctly for the present models.
Though some improvements were made during the last years
especially regarding the qualitative prediction of precipitation, the quantitative
forecast is still not satisfying. The Priority Program 1167 "Quantitative Precipitation
combines various groups from universities and research institutions to
meet the challenge with a joint and coordinated effort.
Project contribution: "Spectral Microphysics in Weather Forecast Models with Special
Emphasis on Cloud Droplet Nucleation"
(duration: 04/2004 - 03/2008)
In our approach, we combine a Spectral Bin Microphysics model [1,2] with the mesoscale
Lokalmodell (now called COSMO model, Consortium for
), which is the regional part of the forecast system of the German Weather
). Aerosol particles in the
atmosphere have a huge impact on the formation of droplets and thus on the amount of
precipitation. Such particles serve as nuclei for cloud droplets. It depends on on the size and
number of available particles as well as on the chemical composition how many cloud
droplets will evolve and to which size they grow through condensation. Those droplets
grow further through coalescence which finally leads to the formation of rain. Also,
droplet freezing can take place at temperatures below 0°C being dependant, e.g., on
the availability of ice nuclei, which are typically insoluble particles. The presence of the
ice phase promotes the formation of larger particles and, therefore,
precipitation. Thus it is a promising approach to have a closer look on the interaction of
aerosols with clouds which may eventually lead to
improving the quantitative precipitation forecast in operational
models through offering new approaches to the parameterization of cloud
investigating in detail the influence of aerosol particles (their
number and size as well as their chemical composition) on rain formation,
which e.g. makes it possible to analyse the effect of air
pollution on the weather.
LM-SPECS stands for the Lokalmodell (LM)  coupled with a SPEC
[1,2]. The microphysics scheme describes the hydrometeors (liquid
droplets, mixed phase droplets, insoluble aerosol particles) with the help of mass resolved
spectra, each (currently) consisting of 66 size bins. Hereby, the insoluble particles serve
as heterogeneous ice nuclei. The mixed phase droplets consist of an ice core surrounded by a
liquid water shell. For droplets of both types, total and soluble aerosol mass is included as
a prognostic variable. As soon as a liquid droplet freezes, it is shifted to the mixed phase
spectrum, and once a mixed phase particle is fully melted, it is transferred back to the
liquid spectrum. The microphysics was successfully tested
in a box model as well as in a cylinder-symmetric model of the Asai-Kasahara type.
Main features of the microphysics model are:
spectral representation of droplets and of ice particles in (currently) 66 size bins
explicit consideration of soluble and insoluble aerosol mass
particle growth through condensation/evaporation (dynamical)
explicit description of droplet nucleation through dynamic growth (no additional
collision of droplet/droplet, ice/droplet, ice/ice, ice or droplet/insoluble
contact freezing, immersion freezing, freezing/melting
restoration of non-activated particles when droplets evaporate
The figure to the left shows the coupling scheme of the LM-SPECS.
The model works with two time steps. The slow dynamical tendencies of pressure, wind
fields and temperature are calculated first within the framework of the original LM in
a large time step of 10 to 100 s. Then, the microphysical tendencies and the dynamical
tendencies of the moist variables (water vapor mass fraction, bin-wise: number densities,
mass fractions of water, ice, soluble and
insoluble aerosol) are calculated using a time step of 1 s or smaller. For the calculation
of the microphysical tendencies, linearly approximated values of
temperature, pressure and air density are used. At the end of the microphysical timesteps, the
updated value of the water vapor mass fraction and the temperature increment due to
microphysical processes are returned to the LM.
As the microphysics is very sensitive to mass changes, a mass conserving advection is
used in the small time step instead of the original advection of the LM.
LM-SPECS was applied to artificial test cases, such as a heat bubble and a
two-dimensional mountain overflow to test the performance of the new microphysics scheme in a
three dimensional mesoscale framework. First
realistic clouds showed satisfying results and pave the way for realistic case studies.
For the future, the following tasks are planned:
Some improvement of the mixed phase microphysics is under way. Especially, larger particles
will be included. Also, varying particle characteristics with size (e.g., density and terminal
velocity) will be implemented to enable the model to distinguish between the different frozen
Aiming for realistic model runs, some preparation of LM-SPECS is still necessary, for
example the treatment of lateral boundary conditions to nest the model into a coarser one
and the introduction of formation and recovery of aerosol particles.
Improvement of numerical schemes is planned. The current version of LM-SPECS is numerically
very expensive and offers much room for alterations like dynamic load balancing, improved
coupling techniques and a variable grid resolution.
The recent field campaign COPS (June to August 2007)
offers great possibilities for
case studies, as it involves both, detailed measurements and extensive coverage by model
simulations through the project
We will perform case studies on convective single cells using Lidar and in-situ aerosol
measurements for the initialization of the model microphysics and mainly radar data for
comparisons of model results and measurements. Model inter-comparisons offer the possibility to
reveal potential deficiencies in the physical parameterizations of bulk schemes regarding the
inclusions of atmospherical aerosols.
 Diehl et al. (2006), J. Geophys. Res. 111. D07202, doi:10.1029/2005JD005884.
 Simmel and Wurzler (2006), Atmos. Res. 61, 135-148.
 Steppeler et al. (2003), Meteorol. Atmos. Phys. 82, 75-96.
Spectral Microphysics in Forecast Models with Special Emphasis on Cloud Droplet Nucleation
IfT Internal Poster Presentation, 2007.
Poster (pdf, 1.5Mb)
Spectral Microphysics in the regional Forecast Model Lokalmodell
Geophysical Research Abstracts, Vol. 8, 03882, 2006.
Poster (pdf, 1.4Mb)
Spectral Microphysics in the regional Forecast Model Lokalmodell
Biannual report of IfT 2004/2005, pp. 74-76.
V. Grützun, O. Knoth, M. Simmel, Simulation of the influence of aerosol particle
characteristics on clouds and precipitation with LM-SPECS: Model description and first
Atmos. Res., submitted 2007.
V. Grützun, O. Knoth, M. Simmel, R. Wolke, The Role of Aerosol Characteristics in the
Evolution of Clouds and Precipitation
, Proc. of 17th International Conference on Nucleation
and Atmospheric Aerosol (ICNAA), National University of Galway, Ireland, August
13-17, 2007, Ed. Colin O'Dowd and Paul Wagner, Springer, 570-575.
Leibniz Institute for Tropospheric Research
Tel: +49 (0) 341-235-2822
Fax: +49 (0) 341 235-2139
Last change: 2007-11-19