Optimization and validation of atmospheric advective and diffusive transport simulations

Lagrangian particle dispersion models are indispensable tools to study atmospheric transport processesbased on the flow of individual air parcels.In operational uses cases they are used to simulate the spread of radionuclides or volcanic emissionsin emergency situations.Different Lagrangian particle dispersion models have been developed for those studies, like FLEXPART or HYSPLIT.It is very important that the models are verified and their errors are estimated so that the results are reliable.In this thesis the advection and diffusion part of the new Lagrangian particle dispersionmodel Massive Parallel Trajectory Calculations (MPTRAC) was verified in idealized test cases and inreal atmospheric conditions and optimized in terms of accuracy and performance. Analytical test cases were applied to the model to validate the advection.The implementation and accuracy of integration schemes of different order,the error caused by the linear interpolation of the wind fields and the used coordinate system were discussed.All implementations lead to correct and very accurate results. A notable difference concerning the accuracy was only determined between the first order Euler method and other methods of higher order. Runge-Kutta methods of the order 2 to 4 and the Petterssen scheme led to very similar results, indicating that an order of larger than 2 can not increase the accuracy significantly.This is partly caused by the linear interpolation of the meteorological data. Even if tests have shown that the error caused by this linear interpolation is small, higher order methods may need a better interpolation to provide benefits.Tests for real atmospheric conditions confirm these results, i.g., the midpoint method was found to be the most performant integration scheme. However, a small time step is required to yieldsmall deviations from the reference solution. For the tests with real atmospheric conditions the atmosphere was separated into regions with similar conditions and the resulting transport deviations are analyzed in detail. Simulation errors in the stratosphere and in the tropics are significantly smaller than in the troposphere or at high latitudes.The diffusion scheme of MPTRAC consists of three components, a random horizontal displacement, a random vertical displacement, and a mesoscale diffusion that depends on the variation of the wind field around an air parcel.All three components were tested individually without advection and produced results that correspond to analytical solutions.However, it is difficult to predict a solution under real atmospheric conditions, because the wind field has a significant influence. A sensitivity test was done, to get an overview of the impact of the diffusion components on the transport of air parcels.The mesoscale diffusion has the strongest influence, if default parameters are used, followed by the vertical diffusion.The horizontal diffusion has only a very small impact and does not seem to influence the transport simulations significantly.In conclusion, the model MPTRAC can be used to simulate the advection and dispersion of air parcels correctly and efficiently.Simulations with altitudes below the free troposphere require relatively small time steps and the diffusion model isprobably too simplistic, but the main task of the model are simulations in the free troposphere and stratosphere where the model shows good results