Simulation of Aerosol Effects on Mixed-Phase Deep Convection: A Comparison of Spectral and Two-Moment Parameterized Microphysics
Abstract
The simulation of aerosol-cloud interaction is a challenge for numerical modelers in meteorology on all spatial scales. Recent observations indicate a strong anthropogenic influence on convective clouds and precipitation by aerosol modification, but unfortunately most cloud resolving models (CRMs) are not capable to investigate aerosol-cloud effects. When using parameterized microphysics, a main problem is the formulation of the autoconversion rate describing the production of raindrops by binary collisions of cloud droplets. The widely used Kessler-type threshold formulation is not able to describe any aerosol-cloud effects since in Kessler's parameterization the autoconversion rate is a function of cloud water content alone. Recently Seifert and Beheng (2001) derived a two-moment scheme for warm phase coagulation processes including a new autoconversion parameterization directly from the stochastic collection equation. Based on this warm phase scheme Seifert (2002) developed a two-moment mixed-phase cloud scheme that predicts the evolution of number as well as mass concentration of cloud droplets, raindrops, cloud ice, snow and graupel. The nucleation of cloud droplets is explicitly described as a function of supersaturation by a simple adjustment procedure to a prescribed activation spectrum, which depends on aerosol characteristics. Another approach to simulate aerosol-cloud interactions is the spectral formulation of cloud microphysics. For each particle type taken into account the size distribution function is represented by a number of discrete size bins. This approach has the clear advantage of being a more general representation of the relevant physical processes and the different physical properties of particles of different sizes. These models are capable to describe aerosol-cloud effects properly. Unfortunately this approach suffers from the large computational effort, especially in three-dimensional models. Using the Hebrew University Cloud Model (HUCM), which includes the most detailed spectral microphysics model available today, we present a comparison of the two different modeling approaches. The study is based on two very different test cases: a single deep convective cloud and a squall line simulation. The sensitivity of these cloud systems to changes of the aerosol activation spectrum is investigated. The comparison shows that the new two-moment bulk-scheme is able to describe aerosol effects on the development of deep convective clouds, but reveals also some limitations of the bulk-scheme. Our conclusion is that advanced two-moment bulk-schemes can provide a computationally efficient approach to study aerosol-cloud interaction. Seifert and Beheng (2001): Atmos. Res, 59-60, 265-281 Seifert(2002): PhD Thesis, University Karlsruhe
- Publication:
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EGS - AGU - EUG Joint Assembly
- Pub Date:
- April 2003
- Bibcode:
- 2003EAEJA.....8799S