Min-Hee Lee

Large increase in heavy rainfall associated with tropical cyclone landfalls in Korea after the late 1970s

[1]xa0This study presents a new demonstration of the abrupt increase in the heavy rainfall events (≥100 mm day−1) during August–September in Korea around the late 1970s. The accumulated heavy rainfall averaged for the two months over 12 stations was 57 mm during 1954–77 (ID1); however, it changed to 103 mm during 1978–2005 (ID2). This change is found to be associated with landfalling tropical cyclones (TCs). The most plausible mechanism that accounts for the TC–heavy rainfall relationship is an enhanced TC–upper-tropospheric trough (UTT) interaction, which results from a southward shift of the upper-tropospheric jet in East Asia during ID2. While the intensity and duration of the landfalling TCs in Korea does not appear to exhibit such an interdecadal change based on the data available, the enhanced TC–UTT interaction increases the upper(lower)-tropospheric divergence (convergence) and coherent ascending motion, which strengthen the frontal zone around Korea.

Assessment of the changes in extreme vulnerability over East Asia due to global warming

A number of indices have been employed to describe weather extremes on the basis of climate regimes and public concerns. In this study, we combined these traditional indices into four groups according to whether they relate to warm (Twarm), cold (Tcold), wet (Pwet), or dry (Pdry) extremes. Analysis of the combined indices calculated for the daily temperatures and precipitation at 750 meteorological stations in Korea, China, and Japan for 1960s–2000s shows increasing trends in Twarm and Pdry events and decreasing trends in Tcold events in recent decades, particularly in the northern part of East Asia. A notable regional variation is an increase in the Pwet events in the Korean Peninsula. We applied the same analysis to a 200-year global climate model simulation for 1900–2099 using the National Center for Atmospheric Research-Community Climate System Model 3. During the 20th century, the changes in Twarm and Tcold calculated from the model data are largely consistent with those calculated from the observations, especially in northern East Asia. The model projections for the 21st century indicate statistically significant increasing Twarm and decreasing Tcold trends in extreme events over the region. Results obtained from historical archives and model simulations using our combined weather extreme indices suggest that northern East Asia will be subject to increased warm and dry extremes and the Korea Peninsula will experience more wet extremes.

Technical note on a track-pattern-based model for predicting seasonal tropical cyclone activity over the western North Pacific

Recently, the National Typhoon Center (NTC) at the Korea Meteorological Administration launched a track-pattern-based model that predicts the horizontal distribution of tropical cyclone (TC) track density from June to October. This model is the first approach to target seasonal TC track clusters covering the entire western North Pacific (WNP) basin, and may represent a milestone for seasonal TC forecasting, using a simple statistical method that can be applied at weather operation centers. In this note, we describe the procedure of the track-pattern-based model with brief technical background to provide practical information on the use and operation of the model. The model comprises three major steps. First, long-term data of WNP TC tracks reveal seven climatological track clusters. Second, the TC counts for each cluster are predicted using a hybrid statistical-dynamical method, using the seasonal prediction of large-scale environments. Third, the final forecast map of track density is constructed by merging the spatial probabilities of the seven clusters and applying necessary bias corrections. Although the model is developed to issue the seasonal forecast in mid-May, it can be applied to alternative dates and target seasons following the procedure described in this note. Work continues on establishing an automatic system for this model at the NTC.

The Recent Increase in the Occurrence of a Boreal Summer Teleconnection and Its Relationship with Temperature Extremes

AbstractThis study has investigated the relationship between temperature extremes and a subseasonal hemispheric teleconnection pattern over the Northern Hemisphere during boreal summer. By applying self-organizing map (SOM) analysis to 200-hPa geopotential fields from the European Centre for Medium-Range Weather Forecasts interim reanalysis (ERA-Interim) for the period 1979–2012, a teleconnection pattern is identified that increased dramatically in its occurrence after the late 1990s. This pattern is characterized by a zonal wavenumber-5 pattern with anomalous high pressure cores over eastern Europe, northeastern Asia, the eastern North Pacific, the eastern United States, and Greenland. These high pressure centers coincide with regions of increasingly frequent temperature extremes in recent decades. To investigate the temporal evolution of the identified SOM pattern, time-lagged composites were performed relative to the days in which the 200-hPa geopotential field most closely resembled the SOM pattern. F...

Low‐frequency variability of tropical cyclone‐induced heavy rainfall over East Asia associated with tropical and North Pacific sea surface temperatures

[1]xa0This study investigates the relationship between tropical cyclone (TC)–induced heavy rainfall over East Asia (EA) and large-scale climate variability during June–October for the period of 1961–2005. An empirical orthogonal function analysis is applied to the seasonal-total TC-induced heavy rainfall obtained in meteorological stations over EA. The first leading mode shows a dipole pattern between South China (SC) and Northeast Asia (NEA; i.e., Southeast-East China, Taiwan, and Japan). This dipole pattern is found to be associated with the two modes of sea surface temperature (SST) variations over the Pacific: one in the tropical Pacific, and the other spanning from EA to the North Pacific Ocean. The former is located in the NINO4 region, while the latter is characterized by the North Pacific center of the Pacific Decadal Oscillation (PDO). The dipole mode is generally well explained by the combined NINO4 and PDO impacts on TC tracks. During positive NINO4, cyclonic steering flows appear over inshore Southeast China, which increases recurving TCs. Meanwhile, the midlatitude North Pacific SST warming during negative PDO is overlaid by the barotropic anticyclone. The anomalous steering easterlies along 20°–40°N related to the anticyclone increase TC occurrence toward Southeast-East China and Taiwan. Furthermore, the precipitable water greatly increases in the midlatitude ocean during negative PDO years, which may help to enhance the rainfall amount while TCs approach Japan. To sum up, in a climatological sense, the first mode of TC-induced heavy rainfall over EA can be interpreted by the combined variations of negative (positive) PDO with positive (negative) NINO4.