Improvement of the scale-adaptive convective parameterization schemes in the simulation of heavy rainfall events over South Korea

Abstract

Recent advances in high-resolution numerical modeling have led to the emergence of gray- zone issue, raising questions about the need for cumulus parameterization scheme (CPS) use. The study of the necessity and role of CPS in gray zone has not yet been clearly established. This paper provides new insights into the field, exploring obscure areas that have not been revealed. The first part of thesis (Chapter 3) focuses on the necessity of a CPS in the gray zone by evaluating the impact of non-scale-adaptive and scale-adaptive cumulus parameterization on precipitation predictability over the Korean Peninsula. The results indicate that WRF simulations with explicitly resolved convection in the gray zone tend to overestimate and mispredict precipitation locations due to inadequate induction and reduction of convection and atmospheric instability. Non- scale-adaptive CPS exaggerates convection and distorts the synoptic field, leading to incorrect high- resolution precipitation simulations. Conversely, scale-adaptive CPS improves heavy rainfall simulations by mitigating atmospheric instability in the gray zone, reducing CPS roles, and enhancing MPS roles with finer grid spacing. The findings indicate that using a scale-adaptive CPS and managing convective processes are crucial in controlling the CPS and microphysics parameterization scheme (MPS) in the gray zone. In the second part of thesis (Chapter 4), the performance of the CPSs were evaluated for 135 heavy HREs over the Korean Peninsula from 2011 to 2020, investigating to generalize CPS performance in the gray zone. The scale-adaptive CPS had overall improved performance compared to the non-scale-adaptive CPS, but the forecast accuracy varied with HRE characteristics. For HREs under synoptic-scale conditions with strong winds and large-scale water vapor transport, both CPSs performed similarly due to the cloud microphysics scheme’s ability to resolve most precipitation. However, for localized and meso-scale HREs with weak environmental conditions, the scale-adaptive CPS improved overall precipitation simulations by increasing grid-scale precipitation and reducing subgrid-scale overestimation seen in the non-scale-adaptive CPS. Finally, the concluding chapter (Chapter 5) investigates the predictive performance of the KSAS scheme adopted by the Korean Integrated Model (KIM) in Korea, suggests improvements through scale-adaptive parameter adjustment, and outlines the potential future direction of development of CPS for KIM. The findings presented in this dissertation contribute significantly to the field of CPS research and are expected to provide a foundational basis for establishing the direction of future development of scale-adaptive CPS by offering insights into the role of CPS and the balance between CPS and MPS in the gray zone, which occurs during the transition to high resolution when using CPS.