An investigation has been carried out using rainfall observation data, an analysis and forecast data by National Centers for Environmental Prediction (NCEP) Climate Forecast System Reanalysis (CFSR) to explain the environment and processes that lead to heavy rainfall in the early morning over the Korean peninsula during episodes of cloud clusters associated with mesoscale troughs (CCMTs). For this study, nine episodes with a maximum hourly rainfall amount in the early morning (i.e., 0300-0900 LST) are selected from seventeen heavy-rainfall episodes associated with CCMTs during 2001-2011. Case studies on two episodes have revealed that, for both episodes, 1) a low-level trough develops over eastern China and its coastal area during day time; 2) the strong southwesterly band (SWB; an area with wind speeds > 12.5 m s −1) on the pressure level of 925 hPa over the East China Sea, which is located southeast of the trough, strengthens and expands at night time toward the southwestern Korean peninsula; 3) the SWB supplies a large amount of moisture and increases convective instability over the southwestern Korean peninsula with a convection trigger mechanism (i.e., strong horizontal convergence); and 4) heavy rainfall occurs in the early morning over the southwestern Korean peninsula, where the exit region of the SWB is located. A mechanism for the SWB growth is presented. Furthermore, generality of the major results from the two case studies is verified using the results obtained for the composite fields of the nine CCMT episodes. Keywords diurnal variation in rainfall amount; heavy rainfall; cloud cluster; strong southwesterly band Citation Shin, U., T.-Y. Lee, and S.-H. Park, 2019: Environment and processes for heavy rainfall in the early morning over the Korean peninsula during episodes of cloud clusters associated with mesoscale troughs.

An investigation was conducted to describe the locations of initial occurrence, evolution and structure of meso-α-scale lows (MLs) associated with cloud clusters and heavy rainfall over the Korean Peninsula using observation and reanalysis data. We selected 29 heavy rainfall events associated with MLs during the 10-year period of 2001-2010. The locations of initial ML occurrence are widely spread from the eastern flank of the Tibetan Plateau to the Yellow Sea. These locations are grouped into 3 regions: 1) the eastern flank of the Tibetan Plateau (R1, 6 cases), 2) central and eastern China (R2, 16 cases), and 3) the Yellow Sea (R3, 7 cases). Initial MLs tend to occur within a deep trough over the eastern flank of the Tibetan Plateau (R1 cases) or a long trough extended northeastward from the southeast of the Tibetan Plateau along the northwestern rim of the western Pacific subtropical high (a majority of R2 cases and two R3 cases). Horizontal temperature gradients are weak over the areas of initial MLs. Meso-α-scale lows tend to develop in the lower troposphere (e.g., below 700 hPa) in an environment of existing cyclonic vorticity. And they are accompanied by anticyclonic vorticity in the upper troposphere. The speed of ML movement varies with each case and with the location of ML. The average speeds of ML movement are 32.4, 37.2 and 40.4 km h −1 for the R1, R2 and R3 groups, respectively. Meso-α-scale lows are shown to have a warm-core structure in general. They are tilted toward various directions with a majority of them (14 of 29 MLs) tilting northward with height.

This study examines atmospheric structures causing convective development in the events of cloud cluster (CC) over the Korean peninsula using the analysis and forecast data of National Centers for Environmental Prediction (NCEP) climate forecast system reanalysis (CFSR) and observation data. Two CC types-CCs associated with meso-α-scale lows (CCMLs) and mesoscale troughs (CCMTs)-were investigated. The common atmospheric structure for convective development in CC events is comprised of i) a strong southwesterly band (SWB; a region with southwesterly wind speeds >12.5 m s −1) in the lower troposphere upstream of CCs with a mesoscale convergence zone in its exit area, ii) a layer of high-θ e air in the lower troposphere near the surface extending from the southwest to SWB exit, iii) elevated height of maximum θ e in the lower troposphere near and over the convergence zone, above which a convectively unstable layer exists. Generality of the above-described structure has been demonstrated via examination of composite fields. SWB plays a major role in producing the structure for convective development in CC events over the Korean peninsula mainly through i) advection of high-θ e air from the southwest, and ii) significant horizontal convergence in the exit area, which can facilitate convection initiation. The two types of CC show notable differences in atmospheric structure across the boundary between high-θ e air from the southwest and low-θ e air in the northeast and in the mode of high-θ e air transport to the region of convective development. The boundary is generally tilted northeastward with height for CCML cases, whereas it is nearly vertical for the majority of CCMT cases. This study indicates that, despite the abovementioned differences, convective developments in both CC types can be considered as elevated convection that occurs as air parcels in an elevated layer of convective instability are lifted by upward motion in the convergence zone. For both types of CC, differential θ e advection plays the key role for the occurrence of elevated layer of convective instability. And θ e front in CCML events indicates the presence of elevated convective instability above it and the possibility of elevated convection provided that a lifting mechanism is available.

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Currently, significant efforts are being made to enhance the performance of the National Institute of Environmental Research (NIER) operational model. However, the model performance concerning Aerosol Optical Depth (AOD) estimation remains uninvestigated. In this study, three different estimation methods for AOD were implemented using the NIER operational model and validated with satellite and ground observations. In the widely used Interagency Monitoring of Protected Visual Environments (IMPROVE) method, AOD exponentially increases with relative humidity owing to a hygroscopic growth factor. However, alternative methods show better performance, since AOD estimation considers the size dependency of aerosol particles and is not sensitive to high relative humidity, which reduces the high AOD in areas with large cloud fractions. Although some R values are significantly low, especially for a single observational comparison and small numerical domain analysis, one of the alternative estimation methods achieves the best performance for diagnosing AOD in the East Asia region.