Observed decrease of summer sea-land breeze in Shanghai from 1994 to 2014 and its association with urbanization

Sea-land breeze (SLB) is widely known as a common wind circulation in coastal cities, which plays an important role to heat transport and pollution diffusion. Using 21-year observation data, a continuous decrease of SLB is discovered in a Chinese metropolis city - Shanghai. In general, the thermodynamic difference between sea and land accounts for the magnitude of SLB. However, other factors associated with urbanization could also affect the SLB significantly, such as anthropogenic aerosol loading, surface roughness, heat release, and so on. This study statistically analyzes the influence of these factors on SLB under the background of global warming. As expected, the decrease of SLB is a combined effect of global warming and great urbanization in Shanghai. The latter, which affects the circulation in both thermodynamic and dynamic ways, is consisted of three parts: temperature increase with urbanization, change of atmospheric radiative forcing, and variation of the underlying surface. At night, the urbanization effect and global warming effect play the similar role by decreasing the temperature gap between sea and land, which would cause the decrease of land wind. At day, decreasing downwelling solar radiation partially offsets the temperature increase caused by both global warming and urbanization although the increasing trend of temperature remains. Different from the expected increasing trend of sea breeze based on the increasing temperature in Shanghai, sea breeze demonstrates a decreasing trend, which could be caused by the dynamic effect from increased surface roughness in Shanghai along with the cooling contribution from radiation-influential factors such as anthropogenic aerosols.