Day-Night and Land-Sea Contrasts of Upscale Convective Growths at Coastal South China

Upscale convective growth (UCG) is the key stage for heavy rainfall production within the life cycle of a mesoscale convective system (MCS). The precise prediction of heavy rainfall hinges upon a comprehensive understanding and accurate representation of UCG dynamics. This study investigates the characteristics of pre-summer UCG at coastal South China (CSC), and uncovers noteworthy diurnal and land-sea disparities in UCG at CSC. Convective cells initiated over land tend to grow upscale during the daytime, whereas those triggered over sea surface are more likely to develop at night. Compared with convection initiation, which is closely associated with boundary-layer convergence, UCG relies upon large-scale deep vertical motion in the atmosphere. Environmental dynamic conditions play a dominant role in promoting the UCGs at CSC. The key environmental factors include low-level jet, low-level vortex/shearline, mid-level westerly jet, and vertical wind shear. Nocturnal convective cells triggered over the sea surface demonstrate the greatest likelihoods of upscale growth due to the lower prerequisites for additional environmental conditions for their upscale developments. The MCSs stemming from these convective cells generate the lightest rainfall among all MCSs initiated at CSC. Conversely, daytime convective cells initiated over sea surface and nocturnal convections initiated over land, particularly the former, necessitate exceptionally favorable environmental conditions for their developments, and thus exhibit low probability to grow upscale. The MCSs developed from daytime convective cells initiated over sea surface produce the most intensive rainfall in CSC.