To analyze the difference in the microphysical development characteristics of orographic rainfall, several Parsivel disdrometers were installed along the windward and leeward slope of a mountain. There were differences in the raindrop size distribution according to the difference in height and distance from the center of the mountain. In low-altitude coastal areas and adjacent areas, the number concentration of raindrops smaller than 1 mm was relatively lower than in mountainous areas, and the rain rate increased with the growth in the size of the raindrops. On the other hand, a higher rain rate was observed as the number concentration of raindrops smaller than 1 mm increased in the hillside area. The increase in the number concentration of small raindrops was evident at the LCL (lifting condensation level) altitude. The main factors affecting the increase in the rain rate on the windward and leeward slopes were the concentration of raindrops and the growth of raindrops, which showed regional differences. As a result of a PCA (principal component analysis), it was found that raindrop development by vapor deposition and weak convection were the main rainfall development characteristics on the windward and leeward slopes, respectively. The difference in regional precipitation development characteristics in mountainous areas affects the parameters of the rainfall estimation relational expression. This means that the rainfall relation calculated through the disdrometer observation data observed in a specific mountainous area can cause spatial and quantitative errors.

The precipitation systems that pass over mountains develop rapidly due to the forcible ascent caused by the topography, and spatial rainfall distribution differences occur due to the local development of the system because of the topography. In order to reduce the damage caused by orographic rainfall, it is essential to provide rainfall field data with high spatial rainfall accuracy. In this study, the rainfall estimation relationship was calculated using drop size distribution data obtained from 10 Parsivel disdrometers that were installed along the long axis of Mt. Halla (oriented west–east; height: 1950 m; width: 78 km; length: 35 km) on Jeju Island, South Korea. An ensemble rainfall estimation relationship was obtained using the HSA (harmony search algorithm). Through the linear combination of the rainfall estimation relationships determined by the HSA, the weight values of each relationship for each rainfall intensity were optimized. The relationships considering K_DP, such as R(K_DP) and R(Z_DR, K_DP), had higher weight values at rain rates that were more than 10 mm h⁻¹. Otherwise, the R(Z_H) and R(Z_H, Z_DR) weights, not considering K_DP, were predominant at rain rates weaker than 5 mm h⁻¹. The ensemble rainfall estimation method was more accurate than the rainfall that was estimated through an independent relationship. To generate the rain field that reflected the differences in the rainfall distribution according to terrain altitude and location, the spatial correction value was calculated by comparing the rainfall obtained from the dual-polarization radar and AWS observations. The distribution of Mt. Halla’s rainfall correction values showed a sharp difference according to the changes in the topographical elevation. As a result, it was possible to calculate the optimal rain field for the orographic rainfall through the ensemble of rainfall relationships and the spatial rainfall correction process. Using the proposed methodology, it is possible to create a rain field that reflects the regional developmental characteristics of precipitation.

To investigate the variation in aerosol particles removed by rainfall, we determined the relationship between aerosols and raindrops observed on Jeju Island, Korea, during a heavy rainfall period (Changma) from 1–15 July 2012. Various instruments, including an aerodynamic particle sizer, rain gauge, and disdrometer, were installed at the Gosan meteorological observation site on Jeju Island. During the Changma period, precipitation between 0.7 and 25.4 mm per day was recorded, and large variations in aerosol and raindrop size distributions were observed. Aerosol removal by precipitation was indicated, and its scavenging effect was confirmed from the results. Three major parameters (Brownian diffusion, interception, and impaction) described the collision efficiency based on aerosol and raindrop size distributions. The variations in the scavenging coefficient and below-cloud scavenging rate produced similar results for the accumulated rain amount. Therefore, these field observations explained the relationship between aerosol and raindrop size distributions.

To understand the microphysical characteristics of rainfall in four different climatological regions (called BOS, BUS, CPO, and JIN) in South Korea, DSDs and their variables, including the mass-weighted mean diameter (Dm) and normalized number concentration (logNw), were examined. To examine the characteristics of DSDs at four sites with different climatology and topography, data measured from Parsivel disdrometer and wind direction from Automatic Weather System (AWS) during rainy seasons from June to August for three years (2018 to 2020) were analyzed. The DSDs variables were calculated using Gamma distribution model. In the coastal area, larger raindrops with a lower number concentration occurred, whereas smaller raindrops with a higher number concentration dominated in the middle land and mountain region. The mountain area of CPO and middle land area of JIN had a larger contribution to the rain rate than that of the coastal area of BOS and JIN in the range of the smallest diameter. The contribution of the drop size to the total number concentration at the CPO and JIN sites was larger (smaller) than that at BOS and BUS in the smallest (larger) diameter. The average shape and slope parameter of gamma model were higher values at the mountain area than at other sites for both rain types, Z-R relation and polarimetric variables were also shown different values at the four studied sites. The intercept coefficient of Z-R relation showed higher values in the mountain area and middle land area than the coastal area. The slope values of Z-R relation were the smallest in the mountain area. The polarimetric variables of Z_H and Z_DR were shown highest (lowest) value at the coastal region of BOS (mountain area of CPO) site for both rain types. The Dm-rose, which shows the Dm distributions with the wind direction, was used in this study. In the coastal area (mountain and middle land area), the dominant wind was east–southeast (east) direction. The ratio of the smaller diameter to the middle size at BOS was much smaller than that at CPO. In the analysis of the hourly distribution of the Dm and logNw, there were two and four peaks of Dm at BUS and BOS, respectively. There was one peak of the Dm at the CPO and JIN sites. The time variation of the Dm was much higher than that of the logNw.

This study investigated the microphysical attributes of orographic precipitation based on the Froude number (Fr). Orographic precipitation types were classified according to the background airflow movement condition, either around (Fr² < 0.1, F₁) or over (0.1 ≤ Fr² < 1, F₂) the mountain. Raindrop size distribution (DSD) was measured for 27 stratiform precipitation cases during a two-year Changma (summer monsoon) season using two S-band single-pol weather radars and four PARSIVEL disdrometers installed along the major axis of Jeju Island—a single bell-shaped mountainous region (Mt. Halla) in South Korea. The present study revealed that the orographic effect influenced by regional differences in precipitation characteristics during F₁ condition but these features were weakened during F₂ condition. The leeward highlands exhibited the highest regional variation in precipitation characteristics: the smallest mass-weighted mean diameter (D_m) was 0.84 mm during F₁ but it became similar to other regions during F₂ (1.04 mm). The effects of orographic processes on precipitation and airflow conditions were apparent in the radar reflectivity (Z)–rainfall rate (R) relationship. Under F₁ conditions, the radar-based estimated ground rainfall amount (R_A) on the leeward highlands was significantly underestimated (−28.2%) by the Z–R relationship optimized for the windward lowlands, where precipitation is negligibly affected by orographic effects. However, this regional bias in R_A estimation caused by orographic effects was reduced (−4.2%) under F₂ conditions.