## 4 Results  
### Observed Structures of Lower Troposphere  
The Hovmoller diagrams of potential temperature, wind speed, wind direction, and water vapor mixing ratio from the radiosonde observation are shown in Figure 4 to illustrate temporal changes in the overall characteristics of the lower troposphere. From \(\sim\)1900 LST on 17 January, the water vapor mixing ratio below 2 km from the ground abruptly increased and the dominant wind direction near the surface tended to change to westerly or northwesterly. These changes in water vapor mixing ratio and wind direction showed that the moisture advection by the westerly or northwesterly winds, illustrated in the synoptic weather patterns in Figure 1b,d, began to affect Seoul at \(\sim\)1900 LST on 17 January. Because the moisture advection also caused cloudy weather in Seoul, the magnitude of the diurnal variation of the near-surface air temperature on 18 January was smaller than that on 17 January. From 0000 LST on 18 January, wind speed above a height of 1.2 km considerably increased. The strengthened air flow in the upper troposphere was related to the increase in wind speed above a height of 1.2 km.  
To investigate the changes in the lower troposphere structure in detail, the vertical profiles of potential temperature and water vapor mixing ratio are shown in Figure 5. From 1000 LST to 1200 LST on 17 January, the potential temperature near the surface gradually increased, and the layer with a nearly constant potential temperature, i.e., the CBL, developed (Figure 5a). At 1300 LST on 17 January, the height of the CBL abruptly increased compared with that at 1200 LST on 17 January. A residual layer with a small vertical temperature gradient at a height of 0.3-0.9 km, appearing in the morning of January 17 th, became a part of the CBL from 1200 LST to 1300 LST on 17 January. The abrupt increase in the CBL height was also previously observed [11]. At 1500 LST on 17 January, the CBL further developed. These characteristics of the CBL development are also reflected in the vertical profiles of water vapor mixing ratio in Figure 5b. Below 1 km, the water vapor mixing ratio, which generally decreased with height at 1200 LST, abruptly became almost constant at 1300 LST, implying a conversion process from the residual layer to the CBL.  
Figure 3: Domain configuration of the WRF model and terrain height.  
Figure 4: Hovmöller diagrams of (**a**) potential temperature, (**b**) wind speed, (**c**) wind direction, and (**d**) water vapor mixing ratio.  
From 1600 LST on 17 January to 0400 LST on 18 January, the cloudy weather induced by the moisture advection weakened radiative cooling and decreased potential temperature in the lower troposphere (Figure 5c). Additionally, the thick layer with small vertical temperature gradient was generally maintained. The vertical profiles of the water vapor mixing ratio more directly exhibit the effects of the moisture advection. At 2000 LST on 17 January, the water vapor mixing ratio below 2.2 km increased compared with that at 1600 LST on 17 January because the lower-level moisture advection began to affect the lower troposphere at \(\sim\)1900 LST on 17 January (Figure 5d). Additionally, we observed the negative vertical gradient of water vapor mixing ratio in the lower troposphere after 2000 LST on 17 January. This negative vertical gradient of water vapor mixing ratio in the lower troposphere was still maintained until the daytime on 18 January, implying that water vapor was not well-mixed in the lower troposphere on 18 January because the radiative heating was weak under the cloudy weather.  
Figure 5: Vertical profiles of the (**a**,**c**) potential temperature and (**b**,**d**) water vapor mixing ratio at (**a**,**b**) 1000, 1200, 1300, and 1500 LST on 17 January 2017, and (**c**,**d**) 1600 and 2000 LST on 17 January 2017 and 0000 and 0400 LST on 18 January 2017.