A study of the modulation of semidiurnal tide associated with mesospheric gravity waves observed with Poker Flat MF radar

Abstract:

The neutral wind velocity data from mesosphere to lower thermosphere observed by MF radars at Poker Flat in Alaska has been observed since the late 1990s. The long-term wind velocity data at Poker Flat has been analyzed for 10 years of 1999 – 2008 to understand the coupling processes between mesospheric gravity waves and tidal waves in more detail. Climatological 1-day composite plots of semidiurnal components of zonal wind and GW-KE showed that the maximum of GW-KE occurs at Poker Flat when zonal wind is westward in winter. It was confirmed that the phases between semidiurnal components of zonal wind and GW-KE are locked for more than 10 days in several years. The present study focuses on the phase loch events and examines the modulation of background wind including tides due to gravity wave drags under an assumption that gravity waves dissipate and/or break when the GW-KE is large.

As with previous studies (AGU2014, IUGG2015), observed wind velocities having the 1~4 hour period components are analyzed as short-period gravity waves and those having harmonic components with periods of 24, 12, and 8 hours are calculated every 30 minutes. The amplitude modulations of 12 hour components of zonal wind and GW-KE show that the time derivative of zonal wind is negative after the GW-KE has large amplitude during the phase lock events in winter of 1999, 2000, and 2004. We plan to discuss more detail of underlying the physical processes, focusing on the local modulation of tidal waves associated with gravity wave drags using not only observation data but also reanalysis and modeling data.