Statistical Study of Concentric Gravity Wave in the Lower Thermosphere by using the ISS-IMAP/VISI Data of 2013
Friday, 19 December 2014
Concentric Gravity Waves (CGWs) have been intensively studied in the past few years because of its unique characteristic that shows the direct coupling between lower and upper atmosphere. However, the past studies were mostly based on a single event and gave only limited information. Therefore, a statistical study on global distribution of the CGWs is needed to get further comprehensive understanding on the generation, propagation and the dissipation of the waves. To address this issue, a space-based observation is preferable since it covers wider area. Until recently, IMAP/VISI has been the only space-based instrument that capable of imaging GWs above the troposphere in the nadir direction. We analyzed the CGWs events from IMAP/VISI data of 2013. We found total 172 CGWs events in the O2 (762 nm) airglow emissions (~95km) out of 4853 data paths. The monthly occurrence probability of the CGWs shows a clear seasonal dependence with peaks around March-April and August-September. The weak background winds in the middle atmosphere during the March and September equinox are likely responsible for the seasonal dependence. We determined the source of CGWs by estimating the center of the circular pattern. In most events, the sources (identified by meteorological satellite) were found to be convective activity (convective plum, tropical storm and typhoon). We found that the convective plum generated small-scale CGWs with the λh < 100 km. In the other hand, more intense convective activity such as typhoon generated small-scale as well as large-scale CGWs. We also found that the small-scale waves expanded from the center up to several hundred km, while the large scale expanded up to thousands of km. Generally, the concentric pattern appeared as arc like shape instead of full circle. It indicates that the background wind filter allows the wave to propagate in a particular direction and filter out the other directions. The data from March to December showed that in Japan and central Asia sector, the circular arc appeared mostly in the northeastward direction, while in Australia and south pacific sector dominated by the southeastward direction. The other sectors are still under the investigation. The detailed analysis of the background wind condition based on the GAIA model will be used to conduct further analysis in this study.