Distribution and evolution of hydrometeors in the stratiform cloud with embedded convection in the autumn ITCZ precipitation in Xisha: Case study

Abstract

In autumn, the clouds over the South China Sea contain more cloud water and cloud ice. Intertropical Convergence Zone sometimes can strengthen and move north, causing heavy precipitation in the northern South China Sea. To reveal the distribution and evolution of hydrometeors in the Intertropical Convergence Zone precipitation clouds, a rainfall process occurred in Xisha and surrounding regions on 16 October 2021 was analyzed by utilizing S-band dual-polarization weather radar data and fuzzy logic algorithm. The classified hydrometeors showed that drizzle, rain, and dry snow were the three most abundant types, while dry crystal was less, indicating deposition and aggregation were more active in the marine environment with sufficient water vapor. The relative content of drizzle and dry snow changed oppositely to that of rain particles, suggesting the coalescence of drizzle and the transformation of dry snow were important processes affecting the formation of rain particles. The precipitation clouds were characterized by stratiform clouds with embedded convections. The strong updraft in the convective clouds transported liquid water upward, thus dry snow could collide with supercooled water and rime to form graupel, then graupel melted to form large raindrops below the 0°C layer. However, compared with continental convective clouds, the riming was weaker. In the stratiform clouds, the ascending motion was weak, no graupel was generated, and the 0°C-layer bright band indicated that dry snow could directly contribute to the small raindrops by melting. This study revealed the evolution of hydrometeors in the Intertropical Convergence Zone precipitation clouds and found that the increase in raindrop size in convective clouds was caused by the combined effects of stronger coalescence and riming.