Numerical study on cloud lines over an urban street in Tokyo

'Kanpachi Street Cloud (KSC)' is an unusual small-scale cumulus cloud line visible during calm summer conditions over a major street in the Tokyo metropolitan area. In order to understand the mechanism leading to the formation of this cloud line, numerical simulations have been performed using the Regional Atmospheric Modelling System. The general characteristics of the simulated KSC agree well with observations. On a large-scale view, the KSC can be characterized as a cumulus cloud line generated at the convergence line of two sea breezes on the western side of Tokyo Bay, while on the microscale view, it resembles Benard-type thermal convection modified longitudinally by wind shear. The location of the convergence line leading to the KSC formation coincides with the Kanpachi street location, although the street itself does not make any direct effect on the KSC formation. Additional numerical experiments were performed to identify causes of the KSC formation in accordance with urbanization, by changing anthropogenic heat impact, land-cover and grid resolution. They confirmed that the formation of the KSC requires two meteorological processes at the same time: (1) the convergence of two sea breezes - the local south-eastern sea breeze from Tokyo Bay and an extended southern sea breeze from the Pacific Ocean, respectively; (2) Forcing due to an urban heat island. It is shown that urbanization could change the intensity and position of the KSC through enhancement of local upward motions and changes in the near-surface horizontal pressure gradient between urban and sub-urban areas. Further, fine horizontal grid resolution is needed to be able to resolve these local thermal convection issues.