The present study aims to analyze climate change and trend of extreme temperature events occurred over the Republic of Korea. The observation data used are daily average, maximum, and minimum temperature from 6 weather stations for the period of 1912-2020. Seven extreme indices regarding frequency and extreme value of temperature are calculated in seasonal and annual time range. In addition, hot extremes and their changes by four physical terms that include information on the annual mean temperature, the amplitude of the annual cycle, the diurnal temperature range and the local temperature anomaly on the day of the extreme are analyzed. The climatology for the analysis is updated to the new normal year of 1991-2020. Consistent with the previous findings, statistically significant change was detected in the indices of annual lowest daily minimum temperature, annual extreme temperature range, frequency of daily minimum temperature below -12℃ and 10%ile(TN10p) during winter. Due to the gradual decrease of the occurrence days regarding the extreme minimum temperature during winter, the frequency calculated by the relative threshold in extreme high temperature during summer prevail since 1990s. Indices related with extreme high temperature had larger low-frequency variability than significant climate change during the analysis period. However, the assessment of hot extremes according to the terms describing mean, variability and tails during the new normal year of 1991-2020, significant increasing trend was detected not only in the annual mean and the amplitude of the seasonal cycle, but also in the daily hot extreme anomaly.

Agreement in the vertical profiles of the temperature trends from radiosonde observation (HadAT) and four kinds of reanalysis dataset (ERA40, ERA-I, NCEP-DOE, and 20CR) are examined for the period of 1979-2000. There are noticeable spread among reanalysis and observation datasets in the temperature trend depending on region and vertical level. East Asia shows large discrepancy among datasets, while Europe shows relatively good agreement. Generally, biases in temperature trends are larger in the upper troposphere (above 300 hPa) than in the lower and middle troposphere. Comprehensive comparison of the long-term temperature trends among reanalyses is made for horizontal distributions with height, latitude-pressure cross-sectional distributions, zonally-averaged meridional distributions with height, and area-averaged vertical profiles in both DJF and JJA. Consequently, we find that the degree of agreement among reanalyses significantly varies with vertical level, region, and season. The highest discrepancy is found over southern high-latitudes and in the upper troposphere over southern tropics. In the tropical upper troposphere above 200 hPa, observation (HadAT) shows cooling trend increases with height, but three reanalyses show warming trends except NCEP-DOE reanalysis in which cooling trend is overestimated. In conclusion, discrepancies in the vertical profiles of long-term temperature trends among four kinds of reanalysis datasets are quite large, and then a scrupulous approach should be needed when reanalysis dataset is used for climate change study.

본 연구에서는 주위환경이 상이한 6개(부산, 추풍령, 대구, 대관령, 강릉, 서울) 지점에서의 최근 30년 간의 일 최고·최저기온과 일교차 자료를 이용하여 기온의 주-월내 변동성과 변화경향을 변동계수(CV)를 이용하여 분석하였다. 일 최고·최저기온과 일교차의 주-월내 변동성은 대관령 등 내륙의 비도시 지역에서 크고, 서울과 부산 등 대도시와 해안지역에서 작다. 계절적으로는 봄과 가을에 크고 여름에 작은 뚜렷한 계절변동을 보인다. 최저기온에 비해 최고기온 주-월내 변동성의 지리적 편차가 크며 일교차의 주-월내 변동성과지역편차는 봄과 가을에 크고, 여름과 겨울에 작아 쌍봉형 형태의 계절변동 패턴을 보인다. 일 최고·최저기온 주-월내 변동성은 내륙의 비도시 지역에서 증가 또는 약한 감소경향을 보이고 그 외 지역에서는 감소경향이 크게 나타나고 있다. 계절적으로는 겨울에 일 최고·최저기온의 주-월내 변동성의 감소경향이 크게 나타 나고 있다. 일교차의 주-월내 변동성은 추풍령을 제외한 전 지역에서 계절에 관계없이 감소하고 있으며 특히 대구와 대관령에서 크게 감소하고 있다.