Heavy rainfall features over the Korean peninsula from satellite observations and numerical experiments

Abstract

In order to objectively classify heavy rain types over East Asia during the summer, K-means clustering was applied to the Tropical Rainfall Measuring System (TRMM) Precipitation Radar (PR) reflectivity profiles. Two main types of heavy rainfall were emerged: a well-developed deep convective system that is situated predominantly over mainland China (Type 1, cold type) and a medium-depth rain system that is mostly found in the oceanic region over the western periphery of the North Pacific high (Type 2, warm type). It is noted that Type 1 propagates eastward from mainland China toward the area including Korea and Japan whereas Type 2 expands northward with the progress of summer. Such different temporal evolution appears to bring in the coexistence of two rain types of heavy rainfall over the Korean peninsula. This study further examines the spatio-temporal evolution of cloud systems and thermodynamic/dynamic features associated with heavy rainfall types over the Korean peninsula using geostationary satellites and reanalysis data, respectively. It was revealed that the cold type is characterized by an eastward-moving oval-shaped cloud system, while the warm type is represented by a northeastward-moving broader system. The cold-type heavy rainfall was usually associated with a local cloud system induced by convective instability. In contrast, the large-scale synoptic forcings (i.e., low-level moisture convergence and high-level divergence) under moist-adiabatically near-neutral conditions are thought to facilitate the possibility of warm-type heavy rainfall over the Korean peninsula. Collision and coalescence processes in the lower cloud layer appear to be responsible cloud microphysics for forming heavy rainfall there. In order to examine whether the numerical experiments could provide evidences supporting the hypothesis of causing the warm-type as well as cold-type heavy rainfall, numerical experiments were taken with ideally prescribed thermodynamic conditions. Under the prescribed moist-adiabatically near-neutral conditions, the warm-type simulation results in a lower storm height, earlier onset of precipitation, and heavier precipitation through collision-coalescence process below the melting layer. The lack of upper-level snow and close interaction between super-cooled raindrops and ice particles at the initial stage were also noted in the warm-type experiment. In contrast, the growth of snow and graupel particles and melting process of ice particles appear to be responsible for the cold-type heavy rainfall. In real-case simulations, Double Moment 6-class (WDM6) scheme simulated the most realistic vertical structure of summertime heavy rainfall over the Korean peninsula among eight Weather and Research Forecasting (WRF) microphysics schemes by virtue of the smallest amount of snow and modified warm-rain physics. However, excessive graupel in the WDM6 scheme was thought to be a problem. In addition, a warm-type heavy rain event was reasonably simulated using the WRF model, implying the importance of large-scale environmental setup in the prediction of warm-type heavy rainfall. Therefore, improvement of microphysical parameterization based on observations and a better large-scale environment are thought to be important factors for enhancing the predictability of warm-type heavy rainfall over the Korean peninsula in the humid East Asian summer environment.