Journal cover Journal topic
Natural Hazards and Earth System Sciences An interactive open-access journal of the European Geosciences Union
Nat. Hazards Earth Syst. Sci., 17, 861-879, 2017
https://doi.org/10.5194/nhess-17-861-2017
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
13 Jun 2017
High-resolution modelling of atmospheric dispersion of dense gas using TWODEE-2.1: application to the 1986 Lake Nyos limnic eruption
Arnau Folch1, Jordi Barcons1, Tomofumi Kozono2, and Antonio Costa3 1CASE Department, Barcelona Supercomputing Center (BSC), Barcelona, Spain
2Department of Geophysics, Graduate School of Science, Tohoku University, Sendai, Japan
3Istituto Nazionale di Geofisica e Vulcanologia (INGV), Sezione Bologna, Italy
Abstract. Atmospheric dispersal of a gas denser than air can threat the environment and surrounding communities if the terrain and meteorological conditions favour its accumulation in topographic depressions, thereby reaching toxic concentration levels. Numerical modelling of atmospheric gas dispersion constitutes a useful tool for gas hazard assessment studies, essential for planning risk mitigation actions. In complex terrains, microscale winds and local orographic features can have a strong influence on the gas cloud behaviour, potentially leading to inaccurate results if not captured by coarser-scale modelling. We introduce a methodology for microscale wind field characterisation based on transfer functions that couple a mesoscale numerical weather prediction model with a microscale computational fluid dynamics (CFD) model for the atmospheric boundary layer. The resulting time-dependent high-resolution microscale wind field is used as input for a shallow-layer gas dispersal model (TWODEE-2.1) to simulate the time evolution of CO2 gas concentration at different heights above the terrain. The strategy is applied to review simulations of the 1986 Lake Nyos event in Cameroon, where a huge CO2 cloud released by a limnic eruption spread downslopes from the lake, suffocating thousands of people and animals across the Nyos and adjacent secondary valleys. Besides several new features introduced in the new version of the gas dispersal code (TWODEE-2.1), we have also implemented a novel impact criterion based on the percentage of human fatalities depending on CO2 concentration and exposure time. New model results are quantitatively validated using the reported percentage of fatalities at several locations. The comparison with previous simulations that assumed coarser-scale steady winds and topography illustrates the importance of high-resolution modelling in complex terrains.

Citation: Folch, A., Barcons, J., Kozono, T., and Costa, A.: High-resolution modelling of atmospheric dispersion of dense gas using TWODEE-2.1: application to the 1986 Lake Nyos limnic eruption, Nat. Hazards Earth Syst. Sci., 17, 861-879, https://doi.org/10.5194/nhess-17-861-2017, 2017.
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Short summary
Atmospheric dispersal of a gas denser than air can threat the environment and surrounding communities. In complex terrains, microscale winds and local orographic features can have a strong influence on the gas cloud behavior, potentially leading to inaccurate model results if not captured by coarser-scale simulations. We introduce a methodology for microscale wind field characterization and validate it using, as a test case, the CO2 gas dispersal from 1986 Lake Nyos eruption.
Atmospheric dispersal of a gas denser than air can threat the environment and surrounding...
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