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RESEARCH

Modeling Department Multiphase Processes

Modeling of Cloud Microphysics

Staff

Overview

The detailed description of cloud microphysics is realised with a spectral approach (Simmel et al., 2002; Simmel und Wurzler, 2006). This means that the spectra of hydrometeors (aerosol particles, droplets, ice particles) are resolved according to their mass-size distribution (e.g. 66 size classes for the size range between 1 nm up to several mm). In this way the relevant microphysical processes of the liquid (droplet nucleation, condensation, coalescence, droplet disintegration) and solid (ice nucleation, freezing processes, riming,& ) phases can be computed explicitly without the need for parameterizations.

Fig. 1: Convective cloud with beginning ice phase.

A particular focus is on the description of ice nucleation. Pure water drops freeze in the atmosphere typically at temperatures of -35°C to -40°C or lower. Nevertheless, ice particles are observed at much higher temperatures (sometimes only few degrees below 0°C, often at -15°C or below). These ice particles are formed by heterogeneous ice nucleation processes, involving mostly insoluble particles, so-called ice nuclei (IN). Within the atmosphere four freezing processes are relevant: Deposition-, condensation-, immersion- and contact freezing. Typically, immersion and contact freezing are the most effective of these processes. During immersion freezing an IN within an undercooled drop initiates the freezing process, while during contact freezing an undercooled drop collides with an IN, such that the freezing process is initiated from the outside. Freezing temperatures and -efficiencies strongly depend on the type of the IN. Biological particles are much more efficient IN for immersion freezing than mineral particles or soot. Contact freezing typically starts at higher temperatures, and differences between biological and mineral IN particles are lower (Diehl und Wurzler, 2005; Diehl et al., 2006).
These processes are investigated with sensitivity studies and simulations of realistic situations. Cloud microphysics is mostly computed within air parcel models (partly due to the high computing times caused by the large number of variables and complex process descriptions). Nevertheless the spectral microphysical model as well as the cylinder-symmetric Asai-Kasahara model was also implemented into the Lokalmodell (LM) of the German weather service (DWD).

Grafic: Time evolution of liquid (left) and ice water mixing ratio (right).

Fig. 2: Time evolution of liquid (left) and ice water mixing ratio (right) for a model run using the cylinder-symmetric Asai-Kasahara model with spectral microphysics. .

Current topics

Publication list of the modeling department.

Projects

Priority Program SPP 1167 of the DFG:
Quantitative Precipitation Forecast (Subproject).

Contact: Dr. Martin Simmel

Last change: 2007-02-20