A study of the tidal periodicity of gravity wave energy in the mesosphere observed with MF radar at Poker Flat, Alaska

The interaction between gravity waves and tidal waves has been studied by using observations (e.g., Saskatoon, Canada (Manson et al. 1998), Rothera, Antarctica (Beldon and Mitchell, 2010)), although most of past studies employ datasets with limited length (3 – 4 years at a maximum). In Alaskan region, we have been observing the neutral wind velocity in the mesosphere to lower thermosphere with a MF radar since October 1999, deployed at Poker Flat Research Range of Geophysical Institute, University of Alaska Fairbanks (65°N, 147°W). The long-term wind velocity data at Poker Flat was analyzed for 10 years of 1999 – 2008 to show daily and seasonal behaviors and climatology of gravity waves and tides. The purpose of this study is to improve the understanding of coupling processes of gravity waves and tidal waves from observation and modeling data. First, we extracted these waves from the MF radar observation data. In this study, harmonic analysis was carried out for periods of 48, 24, 12, and 8 hours, which are extracted from the 5 day time series of wind velocity using. Gravity waves are defined as the 1 ~ 12 hour period component of difference between observed wind velocity and these harmonic components. The method is applied to 30-minute-average data to calculate the 5 day running mean amplitude and phase of tidal waves. We made 1- day composite plots of kinetic energy of gravity waves for periods of 1 ~ 4 hours and harmonic components. The results show that the kinetic energy of gravity waves has two peaks in 3 ~ 6 LT and 12 ~ 15 LT respectively, which tend to coincide with the time when easterly wind of the 12 hour component is switched westerly. This feature commonly recognized in April to August. We will discuss more detail of underlying physical processes of the observed gravity wave – tidal wave relation, applying the three dimensional transformed Eulerian mean equations formulated by Kinoshita and Sato (2013a, b) and Sato et al. (2013) to the output data of a gravity wave resolving general circulation model.