Upscale Convective Growth Prevailing on the Monsoon Coast with Changing Atmospheric Conditions and Local Forcings

Active atmospheric convection on the monsoon coast is crucial for the Earth’s climate system. In particular, the upscale convective growth (UCG) from ordinary isolated convection to organized convective system is a key process causing severe weather, but its activities on the monsoon coast are less understood because of the lack of fine-resolution datasets. For the first time, we present the climatology of UCG on a typical monsoon coast using kilometer-mesh radar data from southern China. The UCG undergoes pronounced subseasonal and diurnal variations in the early-summer rainy season. The subseasonal UCG increase is attributed to the onshore flows shifting from easterlies in April to monsoon southwesterlies in June. UCG becomes vigorous following summer monsoon onset, with hotspots near windward coastal mountains. Daytime UCG first peaks near noontime along coastal land, where onshore flows are destabilized by boundary-layer heating and mountains. Afternoon inland peaks and off-coast minimums are recognized due to land-sea thermal contrast and sea-breeze circulation. Nighttime UCG is revived at the coast by nocturnally-enhanced southerlies, followed by offshore activity as the convergence of land-breeze northerlies shifts seaward. The UCG thus responds strongly to changing atmospheric conditions, land heating/cooling, and thermally-driven local circulations. Our results may help clarify the predictability of monsoon coastal convection.