Dominance of Shortwave Radiative Heating in the Sea-Land Breeze Amplitude and its Impacts on Atmospheric Visibility in Tokyo, Japan

This study uses 21-year observations to analyze the characteristics of sea-land breeze (SLB) circulation and its potential influencing factors in the metropolis of Tokyo. Unlike in other megacities, the magnitude of the SLB in Tokyo does not show a decreasing trend over the years. It is found that the diurnal variation magnitude of the SLB fits an approximately sinusoidal function. The diurnal variation amplitude mainly depends on shortwave radiation that reaches the ground. Stronger radiation is often accompanied by SLB circulation with greater energy, which helps transport more air pollution in the urban area of Tokyo. Correspondingly, atmospheric visibility shows good correlation with in situ shortwave radiation. Generally, strong radiation heats the ground surface, which causes larger land-sea temperature differences and thus greater SLB circulation amplitudes, which is favorable for pollution dispersion and better atmospheric visibility. The presence of less aerosols and the associated better atmospheric visibility weaken the umbrella effect on solar radiation, which allows more solar radiation to reach the ground. This positive feedback could further strengthen the connection between solar radiation, sea-land breezes, and atmospheric visibility. Increasing solar radiation and atmospheric visibility even produces stronger SLB magnitudes at night. Moreover, it is noteworthy that urban heat island circulation plays a similar role to that of SLB circulation, which also makes the variation of in situ radiation and visibility synchronous.