2:30 PM - 2:45 PM
[PPS07-14] Characteristics of the relationship between wind fields and surface stress with high-resolution large eddy simulations for the Martian atmosphere
Keywords:Mars, Atmospheric Boundary Layer, High Resolution Large Eddy Simulatiion, Vortex Extraction, Surface Stress, Dust Lifting
1. Introduction
Dust in the Martian atmosphere has a great influence on the temperature structure. Dust is lifted from the ground by the wind in the atmospheric boundary layer. An important point of dust lifting is the existence of required minimum stress threshold value. In many studies using Martian atmospheric general circulation models (MGCMs), dust lifting by the subgrid-scale wind such as dust devils is parameterized (Kahre et al., 2006). However, in the scheme considering dust devils, surface dust flux is estimated from thermodynamical consideration, not from a consideration based on circulation structures. Actually, in MGCMs using these parameterization schemes, the interhemispheric dust transportation process has not been successfully produced. (Mulholland et al., 2013). In order to deepen understanding of dust lifting process. it would be effective to examine the circulation structures producing strong wind stress.
Circulation structures with the subgrid-scale size of MGCMs have been obtained by using high-resolution large eddy simulations (LESs). Nishizawa et al. (2016) studied the Martian atmospheric boundary layer using LES calculations with high resolution up to 5 m. They found that many isolated vortices that may correspond dust devils emerged, and that the mode of the number density distribution of vortex depends on the resolution.
We have been investigating the relationship between the wind fields and the surface stress field that determines surface dust flux. We use the data obtained by Nishizawa et al. (2016). We previously reported that the wind stress values exceeding 0.03 Pa which is the threshold value of dust lifting appear in the 5 m resolution result. Some points of high surface stress are accompanied by vortex structures and other points are not (Murahashi et al., 2018). In this study, we clarify the number of points of high surface stress associated with vortex quantitatively.
2. Data
We use the data obtained by Nishizawa et al. (2016) which utilized SCALE-LES ver. 3 developed by RIKEN / AICS. The values of model parameters are those of Mars. The model domain is 19.2 km × 19.2 km × 21 km. The resolution is ranging from 100 m to 5 m. The heating rate and the surface temperature are given externally from a vertical one-dimensional simulation by Odaka et al. (2001). Horizontally periodic boundary conditions are adopted. Except for the 5 m resolution run, the vertical temperature profile of the initial state is obtained from Odaka et al. (2001) and tiny random perturbations are added. Except for 5 m resolution run, integrations are performed for 1 day from 0:00 (local time). For the 5 m resolution run, integration is performed for 1 hour from the result at 14:00 obtained by the 10 m resolution run. In this study, we use the data at 14:30. Vortex extraction methods same as Nishizawa et al. (2016) are used.
3. Result
In this presentation, we show the characteristics of the circulation structures associated with high surface stresses. We also investigate whether the characteristics of the relationship between wind fields and surface stress fields change for resolutions other than 5 m.
Dust in the Martian atmosphere has a great influence on the temperature structure. Dust is lifted from the ground by the wind in the atmospheric boundary layer. An important point of dust lifting is the existence of required minimum stress threshold value. In many studies using Martian atmospheric general circulation models (MGCMs), dust lifting by the subgrid-scale wind such as dust devils is parameterized (Kahre et al., 2006). However, in the scheme considering dust devils, surface dust flux is estimated from thermodynamical consideration, not from a consideration based on circulation structures. Actually, in MGCMs using these parameterization schemes, the interhemispheric dust transportation process has not been successfully produced. (Mulholland et al., 2013). In order to deepen understanding of dust lifting process. it would be effective to examine the circulation structures producing strong wind stress.
Circulation structures with the subgrid-scale size of MGCMs have been obtained by using high-resolution large eddy simulations (LESs). Nishizawa et al. (2016) studied the Martian atmospheric boundary layer using LES calculations with high resolution up to 5 m. They found that many isolated vortices that may correspond dust devils emerged, and that the mode of the number density distribution of vortex depends on the resolution.
We have been investigating the relationship between the wind fields and the surface stress field that determines surface dust flux. We use the data obtained by Nishizawa et al. (2016). We previously reported that the wind stress values exceeding 0.03 Pa which is the threshold value of dust lifting appear in the 5 m resolution result. Some points of high surface stress are accompanied by vortex structures and other points are not (Murahashi et al., 2018). In this study, we clarify the number of points of high surface stress associated with vortex quantitatively.
2. Data
We use the data obtained by Nishizawa et al. (2016) which utilized SCALE-LES ver. 3 developed by RIKEN / AICS. The values of model parameters are those of Mars. The model domain is 19.2 km × 19.2 km × 21 km. The resolution is ranging from 100 m to 5 m. The heating rate and the surface temperature are given externally from a vertical one-dimensional simulation by Odaka et al. (2001). Horizontally periodic boundary conditions are adopted. Except for the 5 m resolution run, the vertical temperature profile of the initial state is obtained from Odaka et al. (2001) and tiny random perturbations are added. Except for 5 m resolution run, integrations are performed for 1 day from 0:00 (local time). For the 5 m resolution run, integration is performed for 1 hour from the result at 14:00 obtained by the 10 m resolution run. In this study, we use the data at 14:30. Vortex extraction methods same as Nishizawa et al. (2016) are used.
3. Result
In this presentation, we show the characteristics of the circulation structures associated with high surface stresses. We also investigate whether the characteristics of the relationship between wind fields and surface stress fields change for resolutions other than 5 m.