## 5 Summary and Conclusions  
We performed 1 h interval radiosonde observations in a central urban area in Seoul, South Korea, to investigate the structures of the wintertime lower troposphere on 17 and 18 January 2017. On 17 January, a CBL with nearly constant potential temperature and water vapor mixing ratio gradually developed in the morning and then abruptly grew within 1 h starting from noon by merging the residual layer. From the afternoon on 17 January, moisture advection with clouds affected the lower troposphere, resulting in the weakening of radiative heating/cooling. Thus, in the lower troposphere, the temperature variation became small, and the layer with the small vertical temperature gradient was maintained on 18 January. The water vapor mixing ratio near the surface increased, and the negative vertical gradient of water vapor mixing ratio in the lower troposphere was maintained on 18 January. These maintained vertical structures of temperature and water vapor mixing ratio implied that the mixing in the lower troposphere weakened. We also investigated the performance of WRF models with the YSU, MYJ, MYNN, and ACM2 PBL parameterization schemes in simulating the structures of the lower troposphere. The overall trends in the air temperature and wind speed in the lower troposphere were generally reproduced well in the four simulations. However, the abrupt development of the layer with nearly constant potential temperature did not appear in the four simulations. The wind speed increase due to air flow in the upper troposphere was also not reproduced well in the simulations. The simulation with the MYJ scheme was the most accurate in reproducing the air temperature and wind speed in the lower troposphere compared with the other schemes. The simulated moisture advection was weaker and decayed earlier than the observed moisture advection. Additionally, the simulated water vapor mixing ratio near the surface was smaller than that at the upper level (\(\sim\)1 km) because the advected moist air was not well-mixed with the pre-existing air near the surface. Small-scale turbulence, which plays a role in mixing air, was not well-reproduced in the four simulations. Among the four schemes investigated, the MYNN scheme performed the best in simulating the CBL development and moisture advection in the observation period. In this study, we found that the changes in the structures of the lower troposphere can quickly occur and be affected by small-scale turbulence. The WRF model with various PBL parameterization schemes does not capture the abrupt changes and mixing of air by small-scale turbulence in the lower troposphere. This is a case study with an entire time span of 2 days to keep a launching interval of 1 h, which is a limitation of this study. Further studies with high-resolution large-eddy simulation (LES) modeling are needed to accurately simulate the structure of the lower troposphere and improve the PBL scheme for multiple cases. Conceptualization, B.-S.H. and J.-H.H. ; formal analysis, B.-S.H. ; validation, B.-S.H. ; investigation, B.-S.H. and J.-H.H. ; data curation, B.-S.H. ; writing--original draft preparation, B.-S.H. ; writing--review and editing, B.-S.H., K.-H.K. and S.-B.P. ; visualization, B.-S.H. ; supervision, K.-H.K. All authors have read and agreed to the published version of the manuscript. This work was funded by the Research Institute of Basic Sciences funded by the National Research Foundation of Korea (NRF-2019R1A6A1A10073437) and funded by the Ministry of Science and ICT (NRF-2020R1C1C1012354). Not applicable. The weather maps and meteorological observation data are available from [[https://data.kma.go.kr](https://data.kma.go.kr)]([https://data.kma.go.kr](https://data.kma.go.kr)) on demand (accessed on 31 March 2017). **Acknowledgments:** The authors would thank the Korea Meteorological Administration for providing the weather maps and meteorological observation data. **Conflicts of Interest:** The authors declare no conflict of interest.