Spectral microphysics in weather forecast models
with special emphasis on cloud droplet nucleation

Overview

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 Forecast" 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 small-scale modelling), which is the regional part of the forecast system of the German Weather Service (DWD). 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 microphysics and
• 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.

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LM-SPECS

LM-SPECS stands for the Lokalmodell (LM) [3] coupled with a SPECtral Bin Cloud MicrophysicS [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 parameterization)
• collision of droplet/droplet, ice/droplet, ice/ice, ice or droplet/insoluble particles
• contact freezing, immersion freezing, freezing/melting
• restoration of non-activated particles when droplets evaporate

Coupling
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.
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First results

• First look on realistic clouds (in preparation)
• Heat bubble test case
• Idealized mountain overflow

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Future plans

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 hydrometeors.
• 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 D-PHASE. 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.

References
[1] Diehl et al. (2006), J. Geophys. Res. 111. D07202, doi:10.1029/2005JD005884.
[2] Simmel and Wurzler (2006), Atmos. Res. 61, 135-148.
[3] Steppeler et al. (2003), Meteorol. Atmos. Phys. 82, 75-96.

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Further Reading

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. Abstract (pdf)   Poster (pdf, 1.4Mb)

Spectral Microphysics in the regional Forecast Model Lokalmodell
Biannual report of IfT 2004/2005, pp. 74-76. Report

Publications

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 results. 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. home

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Contact

Verena Grützun

Leibniz Institute for Tropospheric Research Permoserstraße 15 04318 Leipzig, Germany Email: gruetzuntropos.de Tel: +49 (0) 341-235-2822 Fax: +49 (0) 341 235-2139 www.tropos.de