Numerical Experiments on Fair-Weather Clouds Forming over the Urban Area in Northern Tokyo

On clear, calm summer days, cloud lines are often observed above the expanding urban area along the railroads from Tokyo. The cloud lines were simulated using a numerical model with a simplified urban surface parameterization. The horizontal distributions of the simulated clouds agree well with the observed clouds in satellite images. Some sensitivity tests indicate that the urban thermal effect enhances cloud formation but suppresses clouds in the adjacent rural area. The simulated clouds are consistent with the observations when the urban maximum sensible heat flux is substantially larger than the rural one. With a reduction of the thermal contrast, cloud suppression in the rural area is gradually weakened. The mechanisms of the cloud contrast are as follows: 1) Thermals form in the mixed layer above the land surface. 2) Because of the larger sensible heat flux, the mixed layer tends to be higher in the urban area; thus, thermals reach the lifting condensation level more easily than they do in the rural area. 3) The wide compensating downdrafts of the strong urban thermals cover the entire rural area and suppress thermals and clouds there.