Impacts of Urban Expansion on the Diurnal Variations of Summer Monsoon Precipitation Over the South China Coast

This study investigates the possible relationship between changes in monsoonal (May-June) rainfall diurnal variations over the south China coast and urban surface processes using hourly rain gage data during 1981-2014 and semi-idealized numerical simulations. The continuous landward penetration of regional climatic rainband observed prior to urbanization is blocked by the city. The morning precipitation initiated at the coast "jumps" to the downstream area of city in the late afternoon, causing a 2 hr delay of the rainfall peak time. Simulations clearly reproduce this change by turning off/on the urban land use, and demonstrate a direct connection between the change of rainfall diurnal variations with a magnitude-enhancing but slower-penetrating sea breeze as well as the thermodynamic conditions modified by the city. Individual impacts of sensible heating, evapotranspiration, and friction from urban surface are examined using various sensitivity experiments. Urban sensible heat is found to act as the energy source to enhance the sea breeze and induce the asymmetric low-level urban inflow that blocks the penetration of sea breeze and changes the rainfall location. Evapotranspiration and latent heat flux suppressed by the impervious urban surface causes a drier boundary layer and lower CAPE over the city. The "jumping" of the climatic rainband may result from the competing interaction of the dynamic and thermodynamic factors. Urban friction has little impact on the daytime rainfall, but could slow down seaward wind speed and enhance coastal convergence in the early morning if no strong urban heat island is present.