A Study on Mesoscale Convective Systems over East Asia using a Convection-Permitting Model

Abstract

This doctoral thesis investigates the characteristics, long-term variability, and future changes of mesoscale convective systems (MCSs) over East Asia using satellite-based observation data and convection-permitting regional climate simulations. MCSs are a primary driver of warm-season heavy rainfall in the East Asian summer monsoon (EASM) region, yet their frequency, structure, and rainfall contribution remain challenging to represent in climate models, particularly for smaller and more localized meso-β scale systems that are closely tied to short-duration extremes. To address this challenge, the dissertation develops a consistent analysis framework that applies an MCS tracking algorithm (PyFLEXTRKR) to both satellite products and Weather Research and Forecasting (WRF) model output, enabling direct model– observation comparisons of MCS morphology, lifecycle, and precipitation contribution across meso-α and meso-β MCSs (αMCSs and βMCSs). Using multi-decadal diagnostics, the dissertation first quantifies the climatological characteristics and variability of East Asian MCSs and evaluates the ability of a convection-permitting model (CPM) to reproduce observed spatial distributions, seasonal contributions, and characteristics. The CPM captures key monsoon precipitation structures and broad interannual variability, supporting its use for MCS diagnostics, but systematic biases persist in MCS-type classification due to fragmentation of cold cloud systems, with βMCS statistics exhibiting the strongest sensitivity. The dissertation then advances CPM credibility through targeted process evaluation and model optimization. Resolution and physics sensitivity experiments identify where additional computational complexity yields diminishing returns, while a comprehensive microphysics assessment demonstrates that the Unified Forecast System (UFS) Double–Moment microphysics scheme (UDM) provides a distinctly improved representation of MCS-type identification. Particularly, UDM substantially mitigates the unnecessary increase in fragmented βMCSs and better reproduces the observed separation between α- and β-MCSs. Component-wise sensitivity experiments attribute a key role to updated UDM process treatments, especially in-cloud microphysics, which regulates cloud–precipitation structure and storm-scale organization. This yields a physically tested CPM configuration suitable for robust climate applications. Using a CPM with a valid configuration, pseudo-global-warming (PGW) experiments are conducted by incorporating climate change increments derived from Coupled Model Intercomparison Project Phase 6 (CMIP6) Shared Socioeconomic Pathway (SSP) scenarios into reanalysis data for weak, normal, and strong EASM cases to assess future changes in MCS activity and precipitation. The projections indicate modest changes in seasonal-mean total precipitation but a pronounced reorganization of convective rainfall: with stronger intensity, αMCS occurrence decreases whereas βMCS occurrence increases, leading to an increased fraction of seasonal precipitation attributable to MCSs, particularly βMCSs. The precipitation intensity shifts toward fewer light-precipitation occurrences and higher probabilities of extreme rainfall. MCS- centered composites at mature time reveal strengthened convective cores and structural tendencies consistent with more core-dominated systems and reduced stratiform components. These changes align with increased lower-tropospheric equivalent potential temperature and a weakened upper-level jet, suggesting that enhanced mid-low-level instability promotes more frequent, intense, and shorter-lived βMCSs even under reduced synoptic dynamical support. Overall, this dissertation provides an end-to-end pathway from observation-constrained evaluation to process-informed model improvement and future projection for understanding East Asian MCS behavior. The UDM-based CPM configuration improves storm morphology and α- and β-MCSs classification by reducing MCS fragmentation and strengthening the physical consistency of cloud–precipitation structure. With this improved CPM configuration, the PGW projections show that future hydroclimate change over East Asia is expressed primarily through more frequent and more intense βMCSs with a stronger contribution, even when seasonal-mean precipitation changes remain modest.