Han‐Gyul Jin

Urban breeze is produced by the temperature difference between urban and surrounding rural areas, and its interactions with other atmospheric phenomena are of great interest. In this study, the impacts of background wind on the interactions between urban breeze circulation (UBC) and convective cells are numerically investigated. For this, a large number of idealized ensemble simulations are performed using the Weather Research and Forecasting‐large‐eddy simulation model, where the initial background wind speed varies from 0 to 5 m·s −1 in 0.5 m·s −1 intervals. As the background wind speed increases, the location of the strongest updraft moves from the urban center to the downwind boundary of the urban area. This is associated with different interactions between the UBC and convective cells under different background wind speeds. For no background wind, the UBC is relatively strong and its center is located at the urban center, producing a strong updraft there. Convective cells are advected toward the UBC center and merge with the strong updraft, further intensifying it. For weak background wind, the UBC is relatively weak and its center is located near the downwind urban boundary. Convective cells passing the urban area merge with each other and intensify, and after they pass the downwind urban boundary, the strong vertical wind shear in the right part of the UBC tilts and weakens them. For strong background wind, the UBC is very weak and convective cells are advected mainly by the strong background wind. These changes in the characteristics of convective cells in and around the urban area with background wind speed may lead to the changes in the preferred location of cloud initiation and cloud development.

Bin microphysics schemes prognose the raindrop size distribution (RSD), which can be directly evaluated through comparison with disdrometer observations. This evaluation will provide implications on the reliability of simulated cloud microphysics by bin microphysics schemes. In this study, the RSDs of a precipitation event associated with an extratropical cyclone passing South Korea are simulated using a bin microphysics scheme and compared with those observed by a ground‐based disdrometer. The simulated mean RSD overall agrees with the observation. However, notable overestimations appear in the large‐ (3.3–4.3 mm) and small‐ (0.56–1.88 mm) diameter ranges, which respectively stem from the biases in two different time periods, one dominated by stratiform rain and the other largely involved with convective rain. In the stratiform‐rain‐dominated period, the melting of snow is the largest contributor to RSDs. The overestimation in the large‐diameter range in this period can be associated with overly active ice–ice collection at upper levels, which generates a local maximum in RSD at the diameter of 3.3 mm that is not seen in the observed RSDs. In the convective‐rain‐involved period, the warm‐rain collision–coalescence is the largest contributor to RSDs. The overestimation in the small‐diameter range and underestimation in the large‐diameter range imply that the collisional growth of raindrops is represented to be weaker than that in reality. The findings in this study suggest that the RSDs simulated using a bin microphysics scheme can have some systematic biases associated with misrepresentation of some microphysical processes.