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 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.