Soon-Hwan Lee

This study examined the effects of high-rise building height distributions and surface radiative heating on near-surface wind fields and turbulence characteristics in an idealized urban environment using the PALM large-eddy simulation model. Four high-rise building arrays with different height distributions were designed, and simulations were conducted under radiative and non-radiative conditions to assess building-induced mechanical effects and the thermal effects associated with radiative heating.,Under non-radiative conditions, mechanically driven flow structures, such as downdrafts in front of buildings, wakes, and channelized flows, dominated the wind distribution near the surface. Specifically, the strength of the channelized flows and the mean wind speed varied substantially with the distribution of upwind building heights. Conversely, under radiative conditions, daytime radiative heating enhanced turbulence intensity and vertical mixing, thereby weakening low-wind-speed regions within the building array and increasing the mean wind speed. However, mechanically induced flow structures, including wakes and channelized flows, did not disappear under radiative heating, and the differences in wind structures associated with building height distributions were largely maintained.,Analysis of the turbulence kinetic energy production terms indicated that shear production exceeded buoyancy production by more than one order of magnitude in all cases, confirming that turbulence within the urban canopy was primarily generated by mechanical shear, with buoyancy acting as a secondary contributor during daytime conditions.

In urban environments, wind is a key meteorological factor that strongly affects the lives of urban residents, including the dispersion of air pollutants and heat as well as thermal comfort at pedestrian level. The distribution of building heights is a major determinant of the surrounding wind field patterns. Solar radiation is also known to exert a substantial influence on the wind field through surface heating and shadow induced thermal contrasts. Therefore, as a fundamental study for urban modeling aimed at predicting urban microclimates, this work quantitatively analyzes how building height variation and radiative heat transfer affect the urban wind environment. To simulate the urban wind environment, we use the PALM large eddy simulation model under idealized urban conditions to examine differences in the wind field around buildings associated with changes in the height distribution of high rise buildings. Under conditions without the radiation module, we compare experiments with different high rise building heights to identify differences in wind distribution at pedestrian level. With the radiation module activated for the same building configurations, changes in turbulent kinetic energy driven by radiation are found. In future work, we plan to extend these findings to simulations of real urban environments. The results obtained under such idealized conditions are expected to provide a basic reference for interpreting physical processes and validating model results in simulations of complex real world urban settings.

Since fog is an important weather phenomenon affecting the traffic safety, accurate fog forecasting should be attained to minimize meteorological disasters. Most fog forecasts determine only the presence or absence of fog based on less visibility than 1 km, which is known as the visibility diagnostic method. During this process, fog could be predicted by the visibility calculated in the numerical weather prediction (NWP) model using the cloud liquid water content (LWC) near the surface. In this study, we investigated to increase the accuracy of fog forecast by optimizing the reconstruction of moisture distribution method, which can simulate the intensity of fog as well as the presence or absence of fog. The performances of the fog simulations were examined by modifying the relative humidity threshold at a height of 2 m and the stability parameters which affect turbulence and also one of the important criteria for fog occurrence. When we applied the optimize parameters to fog prediction in the winter seasons, the probability of detection (POD) has been increased significantly from 0.21 to 0.54. These improvements were attributed to the corrected relative humidity threshold and the stability parameters. Although the false alarm rate (FAR) remained almost unchanged, the critical success index (CSI) has been improved slightly lesser than those of the POD. When we analyzed the life cycle of fog, it takes time for the NWP model to simulate water droplets in the fog-developing stage. Therefore, the accuracy of the fog simulation is intimately related to the reconstruction of moisture distribution. The NWP model, however, showed a better performance in the process of fog dissipation than the reconstruction of moisture distribution method that was sensitive to temperature and turbulence. In conclusion, the reconstruction of moisture distribution led to a considerable improvement of the fog prediction in the generation and development stage since we used the optimized humidity threshold. It is also expected that accurate fog prediction could be achieved in the future by considering the aerosol effects, which is another importance factor for the fog generation.

Due to rapid urbanization and population growth, high aerosol pollution in the Korean Peninsula has become a major environmental concern. High concentrations of PM2.5 in atmosphere is not only highly hazardous to humans, but also contributes significantly to visibility degradation and climate change. In study, K-means clustering analysis was performed to classify major synoptic patterns on the Korean Peninsula during the seasonal PM2.5 management from 2015 to 2019. Also, we analyzed synoptic patterns according to the air quality on each cluster by dividing the PM2.5 pollution standard into four levels to identify the differences in the occurrence of PM2.5 concentrations in the similar meteorological environment. As a result, the synoptic patterns were classified into five clusters (C1~C5). The clusters (C1, C3, C4) with pressure gradient from east to west showed differences of PM2.5 concentrations in Seoul as the pressure gradient between east and west changed. The clusters (C2, C5) with pressure gradients from south to north had different PM2.5 concentrations in Seoul depending on the location and intensity of high pressure located in southeast of the Korean Peninsula and the intensity and location of high- and low-pressure systems located in the North Pacific and Kamchatka Peninsula, respectively. This study confirmed that air quality can vary depending on the location and strength of high- and low-pressure systems in the similar synoptic meteorological environment.