Relationship between frequency of excitation of mountain waves, topography and a dominant wind

Abstract:

Imaging observation of OH airglow had been conducted at Meiji University, Japan (35.613 °N, 139.549 °E) from Dec. 2015 to Dec. 2019. Mountainous area including Mt. Fuji locates in west side of the imager, and westerly winds predominate in the lower atmosphere throughout the year. It has been expected that many mountain waves (MWs) are excited in lee side and propagate to upper atmosphere. However, during the 4 years, only 8 events were identified as possible MWs. On the other hand, Criddle et al. [2011] reports 68 cases of MWs (ground phase velocity <5 m/s) which is thought to be excited by the Andes. They detected MWs by airglow imaging observation from Aug. 2009 to Aug. 2010. Comparing with this event rate (68 cases/year), the number of events detected in our study (8 cases/4 years) is very small. There are two possible reasons why the number was small: (1) The frequency of MWs excitation is small in the lower layer, and/or (2) MWs do not easily propagate to the upper mesosphere. This study verified the possibility of the former case. When the over-mountain airflow exists, wavy clouds are often generated on the lee side. Since over-mountain airflow is essential for the excitation of MWs, a frequency of wavy clouds can be considered to be a measure of the occurrence of MWs. The frequency and spatial distribution of MWs over Japan were investigated by detecting the wavy clouds from color images taken by Himawari-8 meteorological satellite for one year in 2018. The clouds were detected on more than 70 days a year around the Ou Mountains in Tohoku region, but just 20 days a year around Mt. Fuji. It is suggested that few MWs are excited around Mt. Fuji. The differences between these two regions were examined focusing on the relationship between topography and horizontal wind in lower atmosphere. As a result, Tohoku region has a simple topography with the Ou Mountains running north to south, and the westerly wind blows almost perpendicular to the ridgeline. In contrast, the vast mountainous area extends upwind of Mt. Fuji is a very complex terrain. In altitudes from 100 to 1000 m, the wind was blowing in various directions due to obstacle effects by mountains. From these results, we considered a general condition for frequent excitation of MWs in mountain area. We suggest "mountain wave hotspots", which is thought to be the frequent excitement of MWs in the world.