Myoung-Seok Suh

Regional Climate Model Intercomparison Project for Asia

Improving the simulation of regional climate change is one of the high-priority areas of climate study because regional information is needed for climate change impact assessments. Such information is especially important for the region covered by the East Asian monsoon where there is high variability in both space and time. To this end, the Regional Climate Model Intercomparison Project (RMIP) for Asia has been established to evaluate and improve regional climate model (RCM) simulations of the monsoon climate. RMIP operates under joint support of the Asia–Pacific Network for Global Change Research (APN), the Global Change System for Analysis, Research and Training (START), the Chinese Academy of Sciences, and several projects of participating nations. The project currently involves 10 research groups from Australia, China, Japan, South Korea, and the United States, as well as scientists from India, Italy, Mongolia, North Korea, and Russia. RMIP has three simulation phases: March 1997–August 1998, which c...

Ten‐year east Asian summer monsoon simulation using a regional climate model (RegCM2)

This paper presents a 10-year (1987–1996) summer climate simulation using a regional climate model. The National Center for Atmospheric Research Regional Climate Model (NCAR RegCM2) was designated over east Asia with a horizontal grid spacing of approximately 50 km. The model was initialized at June 1 and integrated up to the end of August for the 10 years. Initial and boundary conditions were the National Centers for Environmental Prediction/NCAR reanalysis data. The 10-year summer mean biases of the simulated ground temperature and daily precipitation rate over the east Asian land area are about −0.1°C and 1.1 mm/d, respectively. Including the ocean area, the bias of the precipitaion rate is significantly reduced to about 0.05 mm/d. More precipitation is simulated in June over the central part of the model domain. The systematic cold biases of the simulated ground temperature take place over the northern part of the domain. The RegCM2 reproduces fairly well the large-scale features associated with the east Asian summer monsoon system, including the subtropical high over the northwestern Pacific Ocean, the planetary-scale trough over Manchuria, the upper level jet and lower-level southerly or southwesterly flow and the precipitation band extended from south China to Japan. The evolution processes of the onset, break phases and abrupt jumps between two adjacent phases of the east Asian summer monsoon are also well simulated. On the other hand, the intensity and magnitude of the large-scale features are not well reproduced. For example, the subtropical high over the ocean is too intensified, in association with the enhancement of the warm air advection in the lower level to the precipitation area. The maximum baroclinic zone and the planetary-scale trough and thus the monsoon rainbelt are shifted northward by 2°–3° compared to reanalysis data. However, some of the systematic errors are hidden in the anomaly analysis over the 10-year RegCM2 climate period.

Assessment of the RegCM4 over East Asia and future precipitation change adapted to the RCP scenarios

In this study, we investigated spatial and temporal changes in precipitation over the Coordinated Regional Climate Downscaling Experiment (CORDEX) East Asia domain, for present (1986–2005) and future (2031–2050) periods using the Regional Climate Model version 4 (RegCM4). Future meteorology produced by the Hadley Center Global Environmental Model version 2 coupled with the Atmosphere–Ocean (HadGEM2-AO) following global climate change scenarios (Representative Concentration Pathways (RCP) 4.5 and 8.5) was used as meteorological boundary conditions for the RegCM4. Six subregions (South Korea, North China, South China, Japan, Mongolia, and India) in the CORDEX East Asia domain were considered for analysis. The RegCM4 simulated spatial distributions of precipitation over East Asia with a correlation coefficient of 0.7 against Climate Research Unit data. The simulation skills of its temporal variability varied based on geographical regions and seasons, showing relatively poorer performance (underestimation in rainfall amount) in summer than in winter, in general. The future climate simulations by the RegCM4 presented that the East Asian continental regions will be warmer and more humid, leading to increased precipitation amounts, especially in the summer. The summer precipitation amount was projected to increase by about 5%, on average, over the East Asian domain, 5−15% in most subregions, and even higher (44% and 24%) in the South Korean region for the RCP 4.5 and 8.5 scenarios, respectively. It was also expected that heavy rainfall (> 50 mm/d) events may occur more frequently in the future possibly owing to meteorological changes that are favorable to convective heavy precipitation.

High-resolution acoustic characteristics of epicontinental sea deposits, central-eastern Yellow Sea

Abstract Large amounts (45390 line km) of closely spaced (2.2–4.4 km) high-resolution subbottom profiles (Chirp, 2–7 kHz) made it possible to identify detailed echo types and their distribution in an epicontinental shelf environment. On the basis of seafloor morphology, surface bedforms and subbottom acoustic characters, 10 echo types were identified in the uppermost sedimentary sequence of the central-eastern Yellow Sea. Flat seafloor with sharp bottom echoes (echo types 1-1, 1-2 and 1-3a; transgressive sediment sheets or relict sands) is widespread in the offshore area and underlain to the west by an acoustically transparent wedge (echo type 1-3b; highstand muds). Mounded seafloor with either smooth surface or superposed bedforms (echo types 2-1, 2-2 and 2-3; tidal ridges) and flat seafloor with regularly spaced, wavy bedforms (echo type 1-4; large-scale dunes) are dominant in the eastern nearshore area. Large-scale mounds with continuous, inclined internal reflectors (echo type 2-4; giant mud bank) occur in the southeastern nearshore area. Various-scale eroded seafloor (echo types 3-1 and 3-2; channels) and flat seafloor with regularly spaced, wavy bedforms (echo type 1-4; large-scale dunes) are present in the northern part of the sea. The distribution pattern of echo types in the central–eastern Yellow Sea reflects depositional processes and sediment dispersal systems during the Holocene transgression and highstand period: (1) development of tidal ridges and large-scale dunes in response to strong tidal currents and waves in the eastern nearshore area; (2) construction of transgressive to highstand mud bank (Huksan mud belt) by deposition of muds derived from the Keum river in the southeastern nearshore area; (3) active erosion due to intensified currents in the northern part; and (4) highstand mud deposition derived from the Huanghe river on the transgressive sediment sheets in the offshore area.

Evaluation of multiple regional climate models for summer climate extremes over East Asia

In this study, five regional climate models (RCMs) participating in the CORDEX-East Asia project (HadGEM3-RA, RegCM4, SNU-MM5, SNU-WRF, and YSU-RSM) are evaluated in terms of their performances in simulating the climatology of summer extremes in East Asia. Seasonal maxima of daily mean temperature and precipitation are analyzed using the generalized extreme value method. RCMs show systematic bias patterns in both seasonal means and extremes. A cold bias is located along the coast, whereas a warm bias occurs in northern China. Overall, wet bias occurs in East Asia, but with a substantial dry bias centered in South Korea. This dry bias appears to be related to the colder ocean surface around South Korea, positioning the monsoonal front further south compared to observations. Taylor diagram analyses reveal that the models simulate temperature means more accurately compared to extremes because of the higher spatial correlation, whereas precipitation extremes are simulated better than their means because of the higher spatial variability. The latter implies that extreme rainfall events can be captured more accurately by RCMs compared to the driving GCM despite poorer simulation of mean rainfall. Inter-RCM analysis indicates a close relationship between the means and extremes in terms of model skills, but it does not show a clear relationship between temperature and precipitation. Sub-regional analysis largely supports the mean–extreme skill relationship. Analyses of frequency and intensity distributions of daily data for three selected sub-regions suggest that overall shifts of temperature distribution and biases in moderate–heavy precipitations contribute importantly to the seasonal mean biases.

Bayesian Changepoint Analysis of the Annual Maximum of Daily and Subdaily Precipitation over South Korea

Abstract Bayesian changepoint analysis is applied to detect a change point in the 30-year (1976–2005) time series of the area-averaged annual maximum precipitation (A3MP) for the six accumulated time periods (1, 3, 6, 12, 24, and 48 h) over South Korea. Using noninformative priors, Bayesian model selection is performed by posterior probability through the Bayes factor, and the exact Bayes estimators of the parameters and unknown change point for the selected change model are obtained. To investigate the significance of the mean differences in the six A3MP between before and after the change point, posterior probability and 90% highest posterior density credible intervals are examined. The results show that a single change occurred around 1997 in the A3MP without regard to the accumulated time periods over South Korea. This is strongly consistent with the abrupt increases in the intensity and frequency of heavy precipitation after 1997. The A3MP after the change point (1997) significantly increased more th...

Impact of lateral boundary conditions on precipitation and temperature extremes over South Korea in the CORDEX regional climate simulation using RegCM4

In this study, the impact of lateral boundary conditions (LBCs) on the simulation skills of the climate model RegCM4 for extreme climate events (ECEs) over South Korea are investigated using two sets of 20-yr (1989–2008) integration with two LBCs (ERA and R-2). The ECEs were defined by a percentile method and an absolute threshold method. In general, RegCM4 successfully reproduces the spatial distribution and the inter-annual variability (R > 0.76) of ECEs indices (precipitation and temperature extreme 5%) irrespective of LBCs. When driven by ERA, RegCM4 shows better simulation skill for ECEs (PR5%, TX5%, hot days, tropical nights, and wet days) than when driven by R-2. The ERA run simulates more than +7 mm day−1 and +1.1°C for PR5% and TX5% than those by the R-2 run, respectively. Although RegCM4’s simulation skills for the ECEs defined by absolute thresholds are significantly different according to LBCs, RegCM4 captured the interannual variability of frost days, hot days, tropical nights, and dry days well. However, the numbers of hot days and dry days are strongly underestimated and overestimated, respectively, compared to the observations, irrespective of LBCs. The large differences in the simulation skill of RegCM4 for ECEs over South Korea between two simulations can be caused by the differences between two LBCs, ERA and R-2. For the mean climate state, ERA simulates more humidity and warmer temperatures than the R-2, especially in the low-to-mid troposphere, resulting in a warmer and more humid troposphere especially at the central region of the model domain.

Assessment of Surface Urban Heat Islands over Three Megacities in East Asia Using Land Surface Temperature Data Retrieved from COMS

Abstract: Surface urban heat island (SUHI) impacts control the exchange of sensible heat and latent heat between land and atmosphere and can worsen extreme climate events, such as heat waves. This study assessed SUHIs over three megacities (Seoul, Tokyo, Beijing) in East Asia using one-year (April 2011–March 2012) land surface temperature (LST) data retrieved from the Communication, Ocean and Meteorological Satellite (COMS). The spatio-temporal variations of SUHI and the relationship between SUHI and vegetation activity were analyzed using hourly cloud-free LST data. In general, the LST was higher in low latitudes, low altitudes, urban areas and dry regions compared to high latitudes, high altitudes, rural areas and vegetated areas. In particular, the LST over the three megacities was always higher than that in the surrounding rural areas. The SUHI showed a maximum intensity (10–13 °C) at noon during the summer, irrespective of the geographic location of the city, but weak intensities (4–7 °C) were observed during other times and seasons. In general, the SUHI intensity over the three megacities showed strong seasonal (diurnal) variations during the daytime (summer) and weak seasonal (diurnal) variations during the nighttime (other seasons). As a result, the temporal variation pattern of SUHIs was quite different from that of urban heat islands, and the SUHIs showed a distinct maximum at noon of the summer months and weak intensities during the nighttime of all seasons. The patterns of seasonal and diurnal variations of the SUHIs were clearly dependent on the geographic environment of cities. In addition, the intensity of SUHIs showed a strong

Projections of high resolution climate changes for South Korea using multiple-regional climate models based on four RCP scenarios. Part 1: surface air temperature

We projected surface air temperature changes over South Korea during the mid (2026-2050) and late (2076-2100) 21st century against the current climate (1981-2005) using the simulation results from five regional climate models (RCMs) driven by Hadley Centre Global Environmental Model, version 2, coupled with the Atmosphere- Ocean (HadGEM2-AO), and two ensemble methods (equal weighted averaging, weighted averaging based on Taylor’s skill score) under four Representative Concentration Pathways (RCP) scenarios. In general, the five RCM ensembles captured the spatial and seasonal variations, and probability distribution of temperature over South Korea reasonably compared to observation. They particularly showed a good performance in simulating annual temperature range compared to HadGEM2-AO. In future simulation, the temperature over South Korea will increase significantly for all scenarios and seasons. Stronger warming trends are projected in the late 21st century than in the mid-21st century, in particular under RCP8.5. The five RCM ensembles projected that temperature changes for the mid/late 21st century relative to the current climate are +1.54°C/+1.92°C for RCP2.6, +1.68°C/+2.91°C for RCP4.5, +1.17°C/+3.11°C for RCP6.0, and +1.75°C/+4.73°C for RCP8.5. Compared to the temperature projection of HadGEM2-AO, the five RCM ensembles projected smaller increases in temperature for all RCP scenarios and seasons. The inter-RCM spread is proportional to the simulation period (i.e., larger in the late-21st than mid-21st century) and significantly greater (about four times) in winter than summer for all RCP scenarios. Therefore, the modeled predictions of temperature increases during the late 21st century, particularly for winter temperatures, should be used with caution.

Changes of precipitation extremes over South Korea projected by the 5 RCMs under RCP scenarios

The change of extreme precipitation is assessed with the HadGEM2-AO - 5 Regional Climate Models (RCMs) chain, which is a national downscaling project undertaken cooperatively by several South Korean institutes aimed at producing regional climate change projection with fine resolution (12.5 km) around the Korean Peninsula. The downscaling domain, resolution and lateral boundary conditions are held the same among the 5 RCMs to minimize the uncertainties from model configuration. Climatological changes reveal a statistically significant increase in the mid-21st century (2046- 2070; Fut1) and the late-21st century (2076-2100; Fut2) precipitation properties related to extreme precipitation, such as precipitation intensity and average of upper 5 percentile daily precipitation, with respect to the reference period (1981-2005). Changes depending on the intensity categories also present a clear trend of decreasing light rain and increasing heavy rain. In accordance with these results, the change of 1-in-50 year maximum precipitation intensity over South Korea is estimated by the GEV method. The result suggests that the 50-year return value (RV50) will change from -32.69% to 72.7% and from -31.6% to 96.32% in Fut1 and from -31.97% to 86.25% and from -19.45% to 134.88% in Fut2 under representative concentration pathway (RCP) 4.5 and 8.5 scenarios, respectively, at the 90% confidence level. This study suggests that multi-RCMs can be used to reduce uncertainties and assess the future change of extreme precipitation more reliably. Moreover, future projection of the regional climate change contains uncertainties evoked from not only driving GCM but also RCM. Therefore, multi-GCM and multi-RCM studies are expected to provide more robust projection.