Abstract
Abstract. The development and validation of the vertical diffusion module of IL-GLOBO, a Lagrangian transport model coupled online with the Eulerian general circulation model GLOBO, is described. The module simulates the effects of turbulence on particle motion by means of a Lagrangian stochastic model (LSM) consistently with the turbulent diffusion equation used in GLOBO. The implemented LSM integrates particle trajectories, using the native σ-hybrid coordinates of the Eulerian component, and fulfils the well-mixed condition (WMC) in the general case of a variable density profile. The module is validated through a series of 1-D offline numerical experiments by assessing its accuracy in maintaining an initially well-mixed distribution in the vertical. A dynamical time-step selection algorithm with constraints related to the shape of the diffusion coefficient profile is developed and discussed. Finally, the skills of a linear interpolation and a modified Akima spline interpolation method are compared, showing that both satisfy the WMC with significant differences in computational time. A preliminary run of the fully integrated 3-D model confirms the result only for the Akima interpolation scheme while the linear interpolation does not satisfy the WMC with a reasonable choice of the minimum integration time step.
Highlights
Global- scale transport is recognised as an important issue in air pollution and climate change studies
In the Lagrangian framework, the tracer transport is described by integrating the kinematic equation of motion for fluid “particles” in a given flow velocity field, provided by, e.g. a meteorological model
Within IL-GLOBO, the Lagrangian equations are integrated in the same coordinate system used in the Eulerian model
Summary
Global- (or hemispheric-) scale transport is recognised as an important issue in air pollution and climate change studies. The stochastic component can be added to the particle position to give the Lagrangian equivalent of the Eulerian advection-diffusion equation This kind of model is usually called a random displacement model (RDM) and is suitable for dispersion over long timescales. FLEXPART (FLEXible PARTicle dispersion model) (Stohl et al, 2005) and HYSPLIT (Hybrid Single Particle Lagrangian Integrated Trajectory Model) (Draxler and Hess, 1998), which are highly flexible and can be used in a variety of situations Both are compatible with different input types (usually provided by meteorological services like the European Centre for Medium-Range Weather Forecasts (ECMWF)), relying on their own parameterisation for fields not available from the meteorological model output.
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