A study of long-term variation in momentum flux in the mesosphere and lower thermosphere, using meteor radar observation data.
|Dennis M. Riggin (GATS Inc.)
Clara Yatini （インドネシア航空宇宙庁（LAPAN））
Rizal Suryana （インドネシア航空宇宙庁（LAPAN））
Background and purpose of this study
The part of the Earth’s atmosphere at a height range of 60–150 km is termed the mesosphere and lower thermosphere (MLT), and is affected by both solar activity and atmospheric disturbances originating from the lower atmosphere. Various greenhouse gases (CO2, CH4, etc.) and atmospheric trace species discharged near the ground by human activity are globally transported into all the atmospheric regions by atmospheric general circulation, which extends to ~100 km. The atmospheric general circulation is driven by both solar radiative heating and atmospheric waves propagating upward from the lower atmosphere. Therefore, in order to understand the physical process by which atmospheric waves accelerate and decelerate the atmospheric general circulation, it is essential to investigate the characteristics of long-term variation in the momentum flux in the MLT associated with atmospheric waves. This physical parameter has previously been measured using only large atmospheric radars such as the MU radar. Because large atmospheric radars have been installed at only a few observatories worldwide, the global distribution of the momentum flux in the MLT, and its long-term variation, remain unknown.
Recently, a new method for the measurement of momentum flux using meteor radar data was proposed by Hocking . In this study, we clarify characteristics of the long-term variation in momentum flux in the MLT using long-term observation data from meteor radars operating in Indonesia since 1992. The results of the long-term analysis in the present study are expected to contribute to understanding the long-term trend of the Earth’s atmospheric environment.
Research plan and method
Two meteor radars with the same observation system have been installed at Biak (May 31, 2011– ) and Koto Tabang (November 15, 2002– ) in Indonesia, 4000 km apart on the equator, and the long-term observation data from these are stored in the RISH database. These meteor radars have been operated in an international collaboration between LAPAN in Indonesia and RISH at Kyoto University. In this study, we compare the long-term variation in the zonal and meridional momentum fluxes (u′w′ and v′w′) in the MLT, derived from radial wind velocity obtained from the two meteor radars, and investigate the similarities and differences in seasonal variation and long-term trend between Biak and Koto Tabang. For example, in the equatorial MLT region, semi-annual and annual oscillations are dominant in the zonal and meridional wind. These wind oscillations are strongly affected by atmospheric waves originating from the lower atmosphere. These wind disturbances are observed on a global scale, and show similar behavior within a few thousand kilometers in a horizontal direction. Therefore, it is expected that the same semi-annual cycle appears in u′w′ at Biak and Koto Tabang. Moreover, by subtracting the average amplitude of the semi-annual cycles, we can investigate characteristics of the long-term variation in the amplitude of these oscillations.
Detailed research plans for the present study are as follows:
- We will compare the observation results obtained from two meteor radars, at Biak and Koto Tabang in Indonesia, and investigate the similarities and differences in seasonal variation and long-term trend in the zonal and meridional momentum fluxes between the two radar sites.
- In order to analyze recent observation data taken by these meteor radars, we will ask LAPAN’s staff to correct the radars and maintain radar operation.
- Using the long-term observation data obtained from the meteor radar at Koto Tabang since November 15, 2002, we will clarify climatological features of u′w′ and v′w′ in the equatorial MLT region, and investigate their relationship with the background wind.
- We will report new results obtained in this study at an international workshop/meeting, and summarize them as a paper to be submitted to an international research journal.