Urban Impacts on
Clouds, Precipitation, and Fog

One cloud that is believed to be influenced by the city is the "Kanpachi Cloud," which appears as a line of cumulus clouds over Kanpachi-dori Avenue in Tokyo. The Kanpachi Cloud appears where sea breezes from Tokyo Bay and Sagami Bay converge, and it is thought to occur spontaneously due to the convergence of sea breezes. 

However, simulation results show that urban areas affect the location and the amount of the Kanpachi Cloud, suggesting that cities can impact the occurrence of clouds.

"Can cities generate or enhance precipitation?" This is an unresolved question that Kusaka Lab has been investigating for some time. One effective method for studying this question is the urban sensitivity experiment (response experiment), which involves comparing numerical simulations with and without urban areas to investigate the impact of cities on precipitation.

In 2014, Kusaka et al. conducted ensemble numerical simulations targeting Tokyo metropolitan area and used the meteorological model WRF (Weather Research and Forecasting) to test whether there was a significant difference between the results with and without urban areas. Their statistical analysis showed that urbanization increases strong summer precipitation over urban areas. They also found that urbanization increases intense precipitation due to modified sea breeze circulation, increased water vapor transport by sea breezes, and enhanced convergence of sea breezes and cold air outflow from precipitation systems over mountains.

The results of the statistical tests and the proposed mechanism by Kusaka et al. (2014) were presented on page 284 of the world-renowned urban climate treatise, "Urban Climates" (Oke et al., 2017)."

Although urban areas have been shown to increase precipitation, they may also decrease the number of fog days in cities. Gu et al. (2019) demonstrated that urbanization reduces the number of fog days in Shanghai through numerical experiments using the WRF model.