Local Winds

Local Winds (Downslope Winds and Gap Winds)

Local winds are winds that blow only in limited areas and are also known as regional winds in high school textbooks. These winds include "downslope winds," which blow on the leeward slopes of mountains and at their base when air currents cross them, and "gap winds," which blow inside and at the exits of gaps and valleys.

The distinction between the two type of winds may vary among researchers, as actual mountain ranges have valleys and passes, and strong winds may be observed at the exit of valleys as they selectively descend through them. In the case of mountain ranges with valleys, it can be difficult to differentiate between downslope winds and gap winds. However,  with the development of supercomputers and numerical models, the mechanism of downslope winds has recently been reviewed, and some of the traditional concepts in textbooks are changing.

Weak winds caused by thermal factors, such as "sea-land winds" that blow due to the temperature difference between land and sea, and "mountain-valley winds" that blow between plains and mountains, can also be classified as local winds. Since these winds are familiar to us, they have been studied for a long time and their mechanisms are well understood. In recent years, the relationship between cities and sea breezes has been attracting attention.

The students in the Kusaka Laboratory have been researching the local winds that blow in their respective hometowns. This is because local people are familiar with the local winds and know the characteristics of the area, and it is easier to communicate with local people. Below is a partial list of our research to date.

Nishi and Kusaka (2019) : "Karakkaze" blowing in the Kanto region

They proposed a new theory that Karakkaze is a type of downslope winds. However, they observed that it tends to come down from the mountains more easily than general downslope winds, and that this is due to the curvature of the mountain range.

Furthermore, they identified that, contrary to popular belief Karakkaze is not only a type of bora winds, but also foehn-type winds occur as frequently as bora-type winds.

Koyanagi and Kusaka (2019) : "Inami-kaze" blowing in Tonami Plain, Toyama Prefecture

They have shown from long-term data that the strongest local wind in Japan is actually the "Inami-kaze" that blows in the Tonami Plain of Toyama Prefecture, not the three major winds (Yamaji-kaze, Hirodo-kaze, and Kiyokawa-dashi) as previously believed.

They also discovered that the reason why Inami-kaze blows only in a narrow area compared to local winds in the world is because it is accompanied by hydraulic jumps also known as jumping water phenomenon, from Mt.Yaotome.

Asano and Kusaka (2021): "Chokai-oroshi" blowing at the foot of Mt. Chokai in Yamagata Prefecture

They investigated into the cause of the white head (a phenomenon in which rice plants wither) that occurred in Yamagata Prefecture. Through their study, they discovered that the white heads were caused by the local wind "Chokai Oroshi," which blows while detouring Mt. Chokai, locally intensifying the foehn effect. This research was the first study in the world to prove that foehn causes white heads through simulation.

Foehn effect caused by Chokai-oroshi. Potential Temperature (left) and Wind speed (right)

Foehn

Foehn is the name of a local wind that blows in the Alpine region of Europe. They are hot, dry, strong winds blowing on the leeward side of a mountain range, and similar winds have been identified all over the world, including Chinooks (chinook), which blow on the leeward side of the Rocky Mountains. Each has a unique name, such as Chinook, but a foehn-type over-mountain airflow is sometimes referred to as a foehn.

Foehn and foehn phenomena are often confused, but foehn is the name of the wind itself, while foehn phenomena refers to hot, dry, strong wind phenomena caused by over-mountain air currents. In Japan, the term "foehn phenomenon" is often used to refer to a particularly hot phenomenon. In the Alps and Rockies, on the other hand, the image of strong winds seems to be stronger, and much of the research is conducted with a focus on strong winds.

Recently, various types of foehn have been proposed around the world, such as scrambling foehn, and foehn, like downslope wind, is a phenomenon that is constantly evolving. Some of the research that has been done to date is presented below.

Kusaka et al (2021): Foehn event in the Toyama Plain

They classified 198 cases of foehn events that occurred in the Toyama Plain over the past 10 years by type of foehn mechanism. The results showed that about 81% of the foehn events were caused by a mechanical mechanism, with no precipitation on the windward slope of the mountain. Surprisingly, only 1% were pure thermodynamical mechanisms (windward slope of the mountain with precipitation), which are often described in textbooks. The remaining 18% were a combination of mechanical and thermodynamical mechanisms with scrambling involved. 

main foehn types

(Written by: Taketo Nakai and Hirotaka Abe; Edited by: Thisara and Chiho Numata)