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Woogyung Kim, Jhoon Kim, Sang Seo Park, and Hi-Ku Cho

Abstract

The total ozone (O3) and aerosol optical depth (AOD) at 320 nm have been observed from the ultraviolet (UV) measurements made at Yonsei University in Seoul, South Korea, with Dobson and Brewer spectrophotometers, respectively, during 2004–10. The daily datasets are analyzed to show the sensitivities of UV radiation to changes in O3, AOD, and cloud cover (CC) together with global solar radiation (GS), including the long-term characteristics of surface UV irradiance in Seoul. The UV sensitivities show that 1% increases of O3 and AOD relative to their reference values under all- and clear-sky conditions similarly manifest as 1–1.2% and 0.2% decreases of both daily erythemal UV (EUV) and total UV (TUV) irradiance at the ground level except for TUV sensitivity to O3 (~0.3%). Those UV sensitivities to CC and GS changes are associated with a 0.12% decrease and 0.7% increase, respectively, in fractional UV changes. The trends show that the positive trends of O3 (+7.2% decade−1), AOD (+22.4% decade−1), and CC (+52.4% decade−1) induce negative trends in EUV (−8.4% decade−1) and TUV (−2.5% decade−1), in both UV (−4.7% decade−1), and in EUV (−6.3% decade−1) and TUV (−6.8% decade−1), respectively. On the basis of the multiple linear regression analyses, it is found that UV sensitivity to O3 is relatively high in the forcing factors, but the contributions of the UV forcing factors to the daily variability and the range of UV disturbances due to the variability of the forcing factors are affected more by AOD than by O3 and CC in both UV fractional changes.

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Youngsin Chun, Hi-Ku Cho, Hyo-Sang Chung, and Meehye Lee

The observation of dust events in Korea must have been important through its long history because of its geographical and meteorological setting. Descriptions about dust events were well documented in historical archives, such as Samguk sagi (57 BC–AD 938), Goryeo sa (918–1392), Joseon wangjosillok (1392–1853), and Munhuenbigo (~1776). In this study, records of Asian dust events were compiled from the above historical archives, covering the period of the second to the eighteenth century. These historical records were investigated along with the recent data (1915–2005) of dust event days in Seoul, Korea. The first record was made in AD 174 in Silla during the period of the Three Kingdoms. A dust event, now called hwangsa, was commonly written down as Woo-Tou or Tou-Woo standing for “dustfall” in the historical archives. Asian dust events took place most frequently during spring from March to May and there was almost no occurrence in summer. The main seasonal feature of the historical dust events was found to be in good agreement with that of the last 90 yr. The result suggests that the past seasonal mechanism of the dust event occurrence and transport in northeast Asia is not significantly different from the present.

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Hi Ku Cho, Jhoon Kim, Yeonjin Jung, Yun Gon Lee, and Bang Yong Lee

Abstract

Effects of cloud, air temperature, and specific humidity on downward longwave irradiance and their long-term variabilities are examined by analyzing the measurements made at the King Sejong Station in the Antarctic Peninsula during the period of 1996–2006.

It has been shown that the downward longwave irradiance (DLR) is significantly correlated with three variables: air temperature, specific humidity, and cloudiness. Based on the relationship of the three variables with DLR, a multiple linear regression model has been developed in order to evaluate the relative contribution of each of the variables to the variation of DLR. The three variables together explained 75% of all the variance in daily mean DLR. The respective contribution from specific humidity and cloudiness to the variation of DLR was 46% and 23%; thus most of the DLR variability can be explained by the variations in the two variables. The annual mean of longwave cloud forcing shows 52 W m−2 with no remarkable seasonal cycle. It is also noted that the overcast cloud effect gives an increase by 65 W m−2 with respect to clear-sky flux throughout the year. It is suggested that the multiple regression model can be used to estimate the radiative forcings of variables influencing the DLR variability.

A highly significant decrease in DLR with an average of −1.52 W m−2 yr−1 (−0.54% yr−1) is found in an analysis from the time series of the deseasonalized monthly mean values. Accordingly, the atmospheric flux emissivity, air temperature, and specific humidity have also decreased in their time series, while the cloudiness has increased insignificantly. Consequently, it may be concluded that the recent decrease in DLR is mainly attributed to the net cooling effect due to the decrease in air temperature and specific humidity, which overwhelm the slight warming effect in cloudiness. Analysis of mean monthly trends for individual months shows that, as for the annual data, the variations in DLR are mostly associated with those of air temperature, specific humidity, and cloudiness.

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