AbstractsPhysics

From turbulence to cloud formation - modelling the aerosol-cloud interactions

by Juha Tonttila




Institution: University of Helsinki
Department: Department of Physics; Finnish Meteorological Institute
Year: 2015
Keywords: meteorologia
Record ID: 1143083
Full text PDF: http://hdl.handle.net/10138/153370


Abstract

Clouds, aerosols and the interactions between them are some of the most important uncertainties in climate modelling. The scales of spatial variability related to clouds are generally too small to be resolved using a typical climate model grid resolution. This work comprises studies about the small-scale variability of the vertical wind component, which significantly contributes to the process of cloud droplet formation. In addition, more elaborate methods for describing the small-scale variability of cloud properties in climate models are developed. The key questions that are investigated include: 1) What are the statistical properties of the turbulent vertical wind variability in the boundary layer and can they be represented accurately by atmospheric models? 2) How does parameterizing the small-scale variability in cloud microphysical processes affect the simulated cloud properties in climate models? 3) How does accounting for the small-scale variability in cloud properties affect the model-based estimates of the aerosol indirect radiative effects? The most important tool used in this work was the ECHAM5-HAM2 aerosol-climate model. The model simulates not only the atmospheric circulation and thermodynamics, but also the global distribution of aerosols and the physical processes between particles that affect the aerosol particle population. This allows the model to represent the interactions between clouds and aerosols. In addition, parts of this work also make use of measurement data based on remote sensing methods as well as high-resolution output from a numerical weather prediction model. The results show that the small-scale variability of the vertical wind associated with cloud droplet formation must be parameterized even in models with relatively high grid resolution. This highlights especially the importance of such methods for lower-resolution climate models. The variability of vertical wind can be described using a probability density function (PDF), the shape of which may vary significantly depending on the atmospheric conditions. The intricacies of the PDF include many uncertainties which can only be reduced by more extensive observations. With a simplified representation of the vertical velocity PDF, a new version of the climate model is constructed in this work, which can be used to study the climate effects due to the small-scale variability in vertical wind and clouds. It is noted that earlier methods that try to account for the variability in vertical velocity and cloud formation are somewhat insufficient. More attention should be paid on treating the small-scale variability self-consistently for entire chains of processes rather than separately for individual processes. This was accomplished in this work with the newly developed method, comprising the chain of processes from cloud formation to radiative transfer. The new method has a strong impact on the number of cloud droplets and drizzle formation as compared to the default model version, where the small-scale variaiblity of clouds is not as…