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Atmospheric dispersal of a gas denser than\udair can threat the environment and surrounding communities if the terrain and\udmeteorological conditions favour its accumulation in topographic depressions,\udthereby reaching toxic concentration levels. Numerical modelling of\udatmospheric gas dispersion constitutes a useful tool for gas hazard\udassessment studies, essential for planning risk mitigation actions. In\udcomplex terrains, microscale winds and local orographic features can have a\udstrong influence on the gas cloud behaviour, potentially leading to\udinaccurate results if not captured by coarser-scale modelling. We introduce a\udmethodology for microscale wind field characterisation based on transfer\udfunctions that couple a mesoscale numerical weather prediction model with a\udmicroscale computational fluid dynamics (CFD) model for the atmospheric\udboundary layer. The resulting time-dependent high-resolution microscale wind\udfield is used as input for a shallow-layer gas dispersal model (TWODEE-2.1)\udto simulate the time evolution of CO2 gas concentration at different\udheights above the terrain. The strategy is applied to review simulations of\udthe 1986 Lake Nyos event in Cameroon, where a huge CO2 cloud released by a\udlimnic eruption spread downslopes from the lake, suffocating thousands of\udpeople and animals across the Nyos and adjacent secondary valleys. Besides\udseveral new features introduced in the new version of the gas dispersal code\ud(TWODEE-2.1), we have also implemented a novel impact criterion based on the\udpercentage of human fatalities depending on CO2 concentration and exposure\udtime. New model results are quantitatively validated using the reported\udpercentage of fatalities at several locations. The comparison with previous\udsimulations that assumed coarser-scale steady winds and topography\udillustrates the importance of high-resolution modelling in complex terrains

High-resolution modelling of atmospheric dispersion of dense gas using TWODEE-2.1: application to the 1986 Lake Nyos limnic eruption