14:45 〜 15:00
[PPS06-15] 再解析データを用いたMY28グローバルダストストーム(GDS)発生時の冬極域昇温に関する研究
キーワード：火星大気大循環、再解析データ、グローバルダストストーム
A global dust storm (GDS) occurring every few Mars years is one of the most spectacular events in the Martian atmosphere. Previous studies showed that during the GDS, temperature inversion in the winter polar region is considerably stronger than in normal years without GDS events. Previous studies using the Martian general circulation models (MGCM) (e.g., Forget et al., 1999) show that the temperature enhancement is likely induced by adiabatic heating associated with the markedly intensified downwelling in the winter polar region. However, the general circulation reproduced by MGCM may not necessarily be realistic, although MGCM is useful to clarify what kind of and how mechanical forcings contribute to the characteristics of each event. In the present study, a case study was made for the GDS in MY28 using recently available long-term reanalysis dataset, EMARS, based on the transformed Eulerian mean (TEM) equation theory. The TEM equations, which describe the interaction between waves and mean flow, are commonly used for the analysis on the Earth middle atmosphere dynamics.
By comparing the residual mean circulation during the GDS with the climatology, changes in the general circulation in MY28 associated with the GDS event were quantitatively investigated. It is shown that during the GDS in MY28, the temperature enhancement is observed with its maximum value of 68 K at z=50 km around the north pole and that the downward branch of the circulation is strengthened in the winter polar region. The mechanism of downwelling intensification differs below z= ~50 km and above z= ~60 km. The downwelling intensification below z= ~50 km is attributable to the latitudinal expansion of the Hadley cell and that above z=~60 km is to the considerably strong residual mean meridional flow v* reaching the winter polar region.
The wave forcing due to resolved waves (RWs) in EMARS can be estimated directly as the Eliassen–Palm flux divergence. However, the wave forcing associated with unresolved waves and processes (UWs) including subgrid-scale GWs to the residual mean circulation may not be negligible. Therefore, the UW contribution is indirectly estimated by applying the method devised originally for a study of Earth’s middle atmosphere by Sato and Hirano (2019). As a result, during the GDS in MY28, the structure of the residual mean circulation is largely explained by the UW forcing. In particular, in the winter poler region, the contribution of UWs to the intensified downwelling is much larger than that of RWs.
By comparing the residual mean circulation during the GDS with the climatology, changes in the general circulation in MY28 associated with the GDS event were quantitatively investigated. It is shown that during the GDS in MY28, the temperature enhancement is observed with its maximum value of 68 K at z=50 km around the north pole and that the downward branch of the circulation is strengthened in the winter polar region. The mechanism of downwelling intensification differs below z= ~50 km and above z= ~60 km. The downwelling intensification below z= ~50 km is attributable to the latitudinal expansion of the Hadley cell and that above z=~60 km is to the considerably strong residual mean meridional flow v* reaching the winter polar region.
The wave forcing due to resolved waves (RWs) in EMARS can be estimated directly as the Eliassen–Palm flux divergence. However, the wave forcing associated with unresolved waves and processes (UWs) including subgrid-scale GWs to the residual mean circulation may not be negligible. Therefore, the UW contribution is indirectly estimated by applying the method devised originally for a study of Earth’s middle atmosphere by Sato and Hirano (2019). As a result, during the GDS in MY28, the structure of the residual mean circulation is largely explained by the UW forcing. In particular, in the winter poler region, the contribution of UWs to the intensified downwelling is much larger than that of RWs.