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[MGI33-P02] Development of a global non-hydrostatic Martian atmospheric model: simulation with topography
Keywords:Mars, Atmosphere, Global Simulation, Non-hydrostatic, High-resolution, Topography
In the Martian atmosphere, dust storms in various scales—from dust devils of tens to hundreds of meter scales, to local dust storms of several tens of kilometer-scale, and to global dust storm—have been observed. However, interactions between these scales are unknown. In addition, since Mars has a thin atmosphere and no ocean, the temperature difference between day and night is large and vertical convection should play an essential role in the Martian meteorology, but it is also unknown. To investigate these mysteries, global atmospheric simulations with horizontal resolution as high as few kilometers are required. Additionally, in order to explicitly simulate vertical convection, it is necessary to solve the governing equations without assuming the hydrostatic balance.
We are developing a non-hydrostatic global Martian atmospheric model (Martian SCALE-GM) targeting to perform high-resolution simulations described above on the supercomputer Fugaku. SCALE-GM (http://r-ccs-climate.riken.jp/scale/) is being developed by using the dynamical core of NICAM (Tomita and Satoh, 2005; Satoh et al., 2008; Satoh et al., 2014), a non-hydrostatic model using a finite volume method in the icosahedral grid systems, that has been used for simulations of Earth atmosphere and climate. We are developing Martian SCALE-GM by incorporating constants and physical process modules of the Martian atmosphere. The Martian physical modules are taken from DCPAM (https://www.gfd-dennou.org/library/dcpam/), an existing pan-planetary atmospheric general circulation model (GCM). DCPAM is a traditional, hydrostatic GCM using a spectral method for horizontal discretization.
We have ported a Martian atmospheric radiation process and a dust process from DCPAM to SCALE-GM and performed a high-resolution calculation with 1.9 km grid-intervals. In this simulation, the topography was omitted. The Martian surface is very rugged and the thin Martian atmosphere is strongly affected by heat and momentum fluxes from the surface. Therefore, considering the topography is essential for studying the weather and climate of Mars. In this study, we attempted to introduce the Martian topography in a high-resolution simulation with Martian SCALE-GM.
In order to represent the Martian topography in the model, we used a terrain-following coordinate for the vertical coordinate and the high-resolution elevation data from NASA's Mars Global Surveyor/Mars Orbiter Laser Altimater observations. The elevation data were reduced in gradient to match the horizontal and vertical resolution of the model. As a result of the trial, it was found that the calculation of SCALE-GM with a large-scale terrain in the Mars setting tends to be numerically unstable. In the terrain-following coordinate system, it is required to keep the terrain gradient less than half of the aspect ratio of the grid at the bottom of the model for numerical stability (Mahrer, 1984), but even if this condition is satisfied, the calculation became unstable. We found that the instability was caused by a large local vertical gradient of velocity and temperature near the surface. Increasing the vertical resolution to adequately represent this large vertical gradient cannot be a solution, because the terrain gradient must be further relaxed in order to satisfy the aforementioned condition. If the vertical transport by the turbulent mixing process is increased, the calculation is stabilized but the heat is carried vertically by the turbulent mixing and the vertical convection is not represented. Thus, we introduced a higher-order viscosity in the vertical direction to eliminate the local vertical gradient. With the higher-order vertical viscosity, we have realized a global Martian atmospheric simulation which includes the topography and explicitly represented the vertical convection.
We are developing a non-hydrostatic global Martian atmospheric model (Martian SCALE-GM) targeting to perform high-resolution simulations described above on the supercomputer Fugaku. SCALE-GM (http://r-ccs-climate.riken.jp/scale/) is being developed by using the dynamical core of NICAM (Tomita and Satoh, 2005; Satoh et al., 2008; Satoh et al., 2014), a non-hydrostatic model using a finite volume method in the icosahedral grid systems, that has been used for simulations of Earth atmosphere and climate. We are developing Martian SCALE-GM by incorporating constants and physical process modules of the Martian atmosphere. The Martian physical modules are taken from DCPAM (https://www.gfd-dennou.org/library/dcpam/), an existing pan-planetary atmospheric general circulation model (GCM). DCPAM is a traditional, hydrostatic GCM using a spectral method for horizontal discretization.
We have ported a Martian atmospheric radiation process and a dust process from DCPAM to SCALE-GM and performed a high-resolution calculation with 1.9 km grid-intervals. In this simulation, the topography was omitted. The Martian surface is very rugged and the thin Martian atmosphere is strongly affected by heat and momentum fluxes from the surface. Therefore, considering the topography is essential for studying the weather and climate of Mars. In this study, we attempted to introduce the Martian topography in a high-resolution simulation with Martian SCALE-GM.
In order to represent the Martian topography in the model, we used a terrain-following coordinate for the vertical coordinate and the high-resolution elevation data from NASA's Mars Global Surveyor/Mars Orbiter Laser Altimater observations. The elevation data were reduced in gradient to match the horizontal and vertical resolution of the model. As a result of the trial, it was found that the calculation of SCALE-GM with a large-scale terrain in the Mars setting tends to be numerically unstable. In the terrain-following coordinate system, it is required to keep the terrain gradient less than half of the aspect ratio of the grid at the bottom of the model for numerical stability (Mahrer, 1984), but even if this condition is satisfied, the calculation became unstable. We found that the instability was caused by a large local vertical gradient of velocity and temperature near the surface. Increasing the vertical resolution to adequately represent this large vertical gradient cannot be a solution, because the terrain gradient must be further relaxed in order to satisfy the aforementioned condition. If the vertical transport by the turbulent mixing process is increased, the calculation is stabilized but the heat is carried vertically by the turbulent mixing and the vertical convection is not represented. Thus, we introduced a higher-order viscosity in the vertical direction to eliminate the local vertical gradient. With the higher-order vertical viscosity, we have realized a global Martian atmospheric simulation which includes the topography and explicitly represented the vertical convection.