Yumi Cha

Interdecadal variation of Korea affecting TC activity in early 1980s

By performing a statistical change-point analysis of activities of the tropical cyclones (TCs) that have affected Korea (K-TCs), it was found that there was a significant change between 1983 and 1984. During the period of 1984–2004 (P2), more TCs migrated toward the west, recurved in the southwest, and affected Korea, compared to the period of 1965–1983 (P1). These changes for P2 were related to the southwestward expansion of the subtropical western Pacific high (SWPH) and, simultaneously, elongation of its elliptical shape toward Korea. Because of these changes, the central pressure and lifetime of K-TC during P2 were deeper and longer, respectively, than figures for P1. This stronger K-TC intensity for P2 was related to the more southwestward genesis due to the southwestward expansion of the SWPH. The weaker vertical wind shear environment during P2 was more favorable for K-TC to maintain a strong intensity in the mid-latitudes of East Asia.

Relationship between the Maximum Wind Speed and the Minimum SeaLevel Pressure for Tropical Cyclones in the Western North Pacific

The wind-pressure relationships for tropical cyclones (TCs) in the western North Pacific are re-examined based on the aircraft data, China Meteorological Administration (CMA) TC best track data and daily NCEP reanalysis data during 1949-1987. Minimum Sea Level Pressure (MSLP) is estimated from aircraft reconnaissance, and maximum wind speeds are adjusted from the maximum wind speed at a flight level observed by Doppler radar. Using these datasets (1,766 cases), the effects of latitude, environmental pressure and intensification trend on the wind-pressure relationships was examined. Results show that adding these factors into equations that estimate the minimum pressure given a fix maximum wind speed with greater accuracy than current methodologies. The environmental pressure and latitude produce the greatest improvements, with small additional improvements form development rate and translation speed. This new wind-pressure relationship can be used easily both operationally and in postanalysis.

Latitudinal Change of Tropical Cyclone Maximum Intensity in the Western North Pacific

This study obtained the latitude where tropical cyclones (TCs) show maximum intensity and applied statistical change-point analysis on the time series data of the average annual values. The analysis results found that the latitude of the TC maximum intensity increased from 1999. To investigate the reason behind this phenomenon, the difference of the average latitude between 1999 and 2013 and the average between 1977 and 1998 was analyzed. In a difference of 500 hPa streamline between the two periods, anomalous anticyclonic circulations were strong in 30°–50°N, while anomalous monsoon trough was located in the north of South China Sea. This anomalous monsoon trough was extended eastward to 145°E. Middle-latitude region in East Asia is affected by the anomalous southeasterlies due to these anomalous anticyclonic circulations and anomalous monsoon trough. These anomalous southeasterlies play a role of anomalous steering flows that make the TCs heading toward region in East Asia middle latitude. As a result, TCs during 1999–2013 had higher latitude of the maximum intensity compared to the TCs during 1977–1998.

Climatological Features of Korea-Landfalling Tropical Cyclones

The present study analyzed the interdecadal variation by applying the statistical change-point analysis to the frequency of the tropical cyclone (TC) that landed in the Korean Peninsula (KP) for the recent 54 years (1951 to 2004) and performed cluster classification of the Korea-landfall TC tracks using a Fuzzy Clustering Method (FCM). First, in the interdecadal variation analysis, frequency of TC that landed in the KP was largely categorized into three periods: high frequency period from 1951 to 1965, low frequency period from 1966 to 1985, and high frequency period from 1986 to 2004. The cluster analysis result of the Korea-landfall TC tracks produced the optimum number of clusters as four. In more detail, Cluster A refers to a pattern of landing in the southern coast in the KP starting from East China Sea followed by heading north while Cluster B refers to a pattern of landing in the west coast of the Korean Peninsula, also starting from East China Sea followed by heading north. Cluster C refers to a pattern of landing in the southern region of the west coast in the KP moving from mainland China while Cluster D refers to a pattern of landing in the mid-north region of the west coast in the Korean Peninsula, also moving from mainland China.

Prediction Experiment of Regional Drought over Korea Using the Similarity of Spatiotemporal Patterns of Past Droughts

This study investigated a drought prediction method on the basis of similarity of spatiotemporal patterns of past droughts in Korea. The method was implemented in the following steps: First, drought areas in Korea were divided into four drought subregions by means of hierarchical clustering analysis. Second, spatiotemporal drought statistics for each subregion for the period from 1926 to 2008 were established. Temporal statistics involve the drought onset, end dates, duration, and regional drought intensities, measured by the Effective Drought Index (EDI). These statistics were collected over the four subregions, and spatial patterns of drought were examined. Third, the analogous drought events that had spatiotemporal patterns similar to those of the current or subject drought were selected. Fourth, the progress of the subject drought and the selected drought were compared. Finally, the progress pattern of the subject drought was predicted on the basis of the hypothesis that it will progress in a way similar to the selected analogous case. We applied this predicted method to several previous drought cases and evaluated the prediction efficiency. The results showed that this method was efficient in predicting droughts for about 1 year.

Evaluation of Future East Asia Drought Using Multi-Model Ensemble

We analyzed the changes in precipitation and drought climatology over East Asia by global warming using the daily precipitation data from 14 coupled atmosphere-ocean general circulation model simulations under the Special Report on Emission Scenarios (SRES) A1B scenario at the end of the twenty-first century. The models were consistent in predicting an increase in the mean precipitation over East Asia. However, the increase was less significant in Southeast Asia, and was accompanied by even larger increase in precipitation variability. This predicted precipitation climatology was translated into a change in drought climatology using the effective drought index (EDI). According to the increased precipitation, East Asia tends to be wetter with a decreased frequency and duration of drought. However, because of the enhanced precipitation variability, extreme droughts are predicted to be more frequent, especially over Southeast Asia.

Reverse relationship between drought of mid-latitudes in East Asia and Northwest Pacific tropical cyclone genesis frequency in summer

This study found that there is a significant negative correlation between summer drought in Korea, China and Japan and the frequency of tropical cyclone (TC) in the subtropical western North Pacific (SWNP) using effective drought index (EDI). The frequency of TCs that affect Korea is low (high) in a year of summer drought (non-drought). As a case study, in 1994 when there is extremely severe summer drought in Korea, there was high frequency of TCs while in 2003 when there was least severe summer drought, the frequency of TCs is the lowest. Changes in the anomalous secondary circulation, namely anomalous upward (downward) flow in the SWNP and anomalous downward (upward) flow in the mid-latitudes of East Asia, are one of the causes of drought (non-drought).

Synoptic Climatological Characteristics of the Asian Dust Frequency in Seoul during Spring

A statistical change-point analysis examined the existence of climate regime shift in the time series of the Asian dust frequency in Seoul during spring. As a result, the Asian dust frequency in Seoul during spring has sharply increased since 1993. To investigate the cause of the increasing Asian dust frequency in Seoul during spring, therefore the averages during the period of 1993 to 2011 and the differences in large-scale environment during the period of 1974 to 1992 were analyzed. According to the analysis results for 850 hPa, 500 hPa, and 200 hPa stream flows, northwesterly anomaly was formed from the Lake Baikal to the Korean Peninsula due to the intensification of anomalous anticyclonic circulation in Northern China. This northwesterly anomaly has become a major circulation that moves the sand particles from Northern China to Seoul.