IL-GLOBO (1.0) – integrated Lagrangian particle model and Eulerian general circulation model GLOBO: development of the vertical diffusion module

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) consistent 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 fulfills the Well Mixed Condition (WMC) in the general case of a variable density profile. The module is validated through a series of 1-D numerical experiments by assessing its accuracy in maintaining an initially well mixed distribution. A dynamical time-step selection algorithm with constraints related to the shape of the diffusion coefficient profile is developed and gives accurate results, even for strongly peaked diffusivity profiles. Finally, the skills of a linear interpolation and a modified Akima spline interpolation method are compared, showing that the former generally introduces deviations from the WMC, due to the inconsistency between the local value of the diffusion coefficient and its derivatives. The Akima interpolation algorithm, for which the model satisfies the WMC rigorously, has a computational cost within 120% of the linear interpolation algorithm, making it a reasonable option for implementation in the 3-D model.