Tost, Holger: Global Modelling of Cloud, Convection and Precipitation Influences on Trace Gases and Aerosols. - Bonn, 2006. - Dissertation, Rheinische Friedrich-Wilhelms-Universität Bonn.
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author = {{Holger Tost}},
title = {Global Modelling of Cloud, Convection and Precipitation Influences on Trace Gases and Aerosols},
school = {Rheinische Friedrich-Wilhelms-Universität Bonn},
year = 2006,
note = {In atmospheric chemistry modelling increasing attention has been paid to the effects of aqueous phase chemistry in the last decade. Even though wet deposition is a considerable sink in the atmospheric budgets of several trace species, it has been parameterised with highly simplified approaches. With increasing computer capacity global atmospheric chemistry and climate models have become more comprehensive allowing more detailed consideration of aqueous phase chemistry. The chemical processes within clouds and precipitation droplets do not only contribute to sinks for gas phase constituents, they can alter the chemical composition because reaction pathways can be quite different from the gas phase. Products, formed in the droplets, can be released into the gas phase and undergo reactions with other gaseous species. The droplets futhermore provide a surface for heterogeneous surface reactions. Additionally, precipitation can vertically redistribute gas phase trace species due to uptake and subsequent release back into the gas phase.
An adequate simulation of the hydrological cycle is a basic requirement for cloud and precipitation chemistry modelling. Since convective clouds cannot be resolved by the coarse grid size of global models, atmospheric convection is described with parameterisations. From a variety of available schemes for the calculation of convection, four have been selected and implemented into the atmospheric chemistry general circulation model ECHAM5/MESSy. The effects of the alternative convection schemes on the hydrological cycle and consequently on the simulated climate system have been analysed. A strong dependence of the precipitation distribution on the selected convection parameterisation has become apparent from the comparison with global observations. A similar comparison for water vapour columns obtains consistent results. The temperature distribution can be sensitively modified, resulting in substantial local average surface temperature changes, while the overall energy budget of the atmosphere is hardly affected. Generally, convection is of major importance for the distribution of trace species in the atmosphere, owing to enhanced vertical transport, the production of reactive nitrogen species in the upper troposphere by lightning, and scavenging and wet deposition. In this thesis these processes are analysed with different convection schemes and different descriptions of the scavenging process. To also address multiphase chemistry the new scavenging submodel SCAV has been developed, and has been applied in several studies under idealised as well as realistic conditions, including comprehensive gas and aqueous phase chemistry. The main factor that alters wet deposition distributions as a result of replacing the convection scheme is the modified precipitation distribution, both in the occurence and the intensity of rain events. The observed wet deposition patterns of nitrate, ammonia, and sulphate are captured accurately by the model with respect to the location and the approximate total amount. Comparisons of the model results with observed vertical profiles of trace gases show that the most comprehensive aqueous phase chemistry representation reproduces the mixing ratios and distributions of gaseous compounds most realistically.},

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