Orographic Cloud & Precipitation

Clouds and precipitation areas are often formed or intensified by terrain features such as mountains. These terrain-induced clouds and precipitation areas can lead to localized heavy rain or snowfall, or in some cases, suppress precipitation altogether. At the Kusaka Laboratory, we are working to uncover the mechanisms behind these climatological phenomena.

Local Front

Local fronts can generate cumulonimbus clouds or increase precipitation due to sudden changes in wind and temperature. Kusaka Laboratory studies local fronts in the Kanto region, which are generated by low pressure systems over the Sea of Japan and along the southern coast of Japan. Local fronts in the Kanto region are several hundred kilometers long and several hundred meters thick. They are also said to be generated when warm air from the ocean slides up into the cold air layer generated inland of the Kanto region. Kusaka and Kitahata (2009) investigated the relationship between precipitation patterns observed when cold fronts pass through the Japanese archipelago and local fronts.

Rain and Snow Shadow Effects

    On the leeward side of mountain ranges, precipitation tends to be reduced due to a phenomenon known as the "rain shadow" or "snow shadow" effect. In winter, for example, Japan’s Sea of Japan side experiences heavy snowfall, while the Pacific side sees little to no snow—this is a result of that effect. At the Kusaka Laboratory, we studied a localized snow shadow phenomenon known as the “Sado Island Block,” which occurs around Sado Island, a relatively small island even by global standards.

Sado Island Block

    Although Niigata City is located in one of the world’s heaviest snowfall regions, it receives less snow compared to other areas along the Sea of Japan coast. One proposed explanation is that Sado Island blocks snow clouds, leading to reduced snowfall in the region. This snow shadow effect caused by Sado Island is widely believed among weather forecasters and local residents, who refer to it as the “Sado Block” or “Sado Guard.”

At the Kusaka Laboratory, we sought to answer questions like, “Does the Sado Block really exist?” and “What is the mechanism behind the snow shadow caused by Sado Island?” Using statistical analyses of weather radar data and numerical experiments with meteorological models, we found that the Sado Block occurs on about 80% of days when monsoon winds are blowing. Furthermore, the numerical experiments revealed that Sado Island’s snow shadow effect can extend up to 150 km downwind (Kusaka et al., 2023).

    This research garnered significant attention among researchers and meteorologists and was also featured in local news and newspapers in Niigata Prefecture.

Cold-Air Damming

    Cold-Air Damming is an atmospheric phenomenon in which cold air becomes trapped by a mountain range, leading to the formation of a substantial cold air layer that covers the plains on the eastern side of the mountains. This phenomenon occasionally occurs in the Kanto region as well (Sengoku et al., 2024), and has been identified as a factor contributing to extreme weather events such as snowfall in the Tokyo metropolitan area or record-breaking rainfall and strong winds during typhoon approaches.

    We are working to clarify the nature of this phenomenon through independent upper-air observations and analyses based on numerical simulations.

Convergence Lines

We have been studying convergence lines that occur at the leeward side of mountains, focusing on banded cumulus sequences that outbreak on the Pacific side of Japan. The most famous convergence line is called "Japan Sea Cold Air Cluster Convergence Zone (JPCZ)", but there is also the "Boso front", which occurs downwind of the Chubu mountains, and convergence lines that outbreak leeward of the Kii and Shikoku mountains. Even the convergence lines that outbreak on the Pacific side are of interest because they produce rainfall, snowfall, tornadoes, and other wind gusts. We are currently studying the different locations of convergence lines and the mechanism of their movement, as well as the formation mechanism of banded cumulus sequences.

(Written by: Nobuyasu Suzuki)