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Jaeyeon Choi, Uran Chung, and Jin I. Yun

Abstract

Gridded temperature data are frequently used to run ecological models at regional scales and are routinely generated by spatially interpolating point observations at synoptic weather stations. If synoptic stations are located in urbanized areas, observed temperature and the interpolated data could be contaminated by the urban heat island effect. Without an appropriate correction, temperature estimates over rural areas or forests might deviate significantly from the actual values. This study was conducted to remove the urban effects embedded in the interpolated surfaces of daily minimum temperature in South Korea, where most weather stations are located in urbanized or industrialized areas. To overcome the spatially discontinuous nature of the population statistics, urban land cover information at a 30 m × 30 m resolution was used along with population data. A population density was calculated by dividing the population of a city by the number of urban pixels falling within the city boundary. Population-density values unique to each city were, in turn, assigned to all the urban pixels. Blocks of 3 × 3 pixels were aggregated to form a “digital population model” (DPM) on a 90 m × 90 m grid spacing. Temperature estimation error from the existing interpolation scheme, which considers both distance and elevation effects, was obtained at 31 synoptic station locations in Korea each month. They were regressed on the population information at the same locations, expressed in DPMs smoothed at the radial extent of 0.5, 1.5, 2.5, 3.5, and 5.0 km. Selected regression equations were added to the widely used distance–altitude interpolation scheme. This new method was used to interpolate monthly normals of daily minimum temperature in South Korea for the 1971–2000 period. Cross validation showed approximately a 30% reduction in the estimation error over all months when compared with those by the best existing method.

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Eun-Young Kwon, Jea-Eun Jung, Uran Chung, Jin I. Yun, and Hee-Seung Park

Abstract

A winter-season warming trend has been observed in eastern Asian countries during the last century. Significant effects on dormancy and the subsequent bud-burst of deciduous fruit trees are expected. However, phenological observations are scant in comparison with long-time climate records in the region. Chill-day accumulation, estimated from daily maximum and minimum temperature, is a reasonable proxy for dormancy depth of temperate-zone fruit trees. A selected chill-day model was parameterized for the Campbell Early grapevine, which is the major cultivar (grown virtually everywhere) in South Korea. To derive model parameters (threshold temperature for chilling and the chilling requirement for breaking dormancy), a controlled-environment experiment using field-sampled twigs of Campbell Early was conducted. The chill-day model to estimate bud-burst dates was adjusted by derived parameters and was applied using 1994–2004 daily temperature data obtained from the automated weather station in the vineyard at the National Horticultural Research Institute. The model gave consistently good performance in predicting bud-burst of Campbell Early (RMSE of 2.5 days). To simulate dormancy depth of Campbell Early at eight locations in South Korea for the last century, the model was applied using data obtained for each location from 1921 to 2004. Calculations showed that the chilling requirement for breaking endodormancy of Campbell Early can be satisfied by mid-January to late February in South Korea, and the date was delayed going either northward or southward from the Daegu–Jeonju line that crosses the middle of South Korea in the east–west direction. Maximum length of the cold tolerant period (the number of days between endodormancy release and the forced dormancy release) showed the same spatial pattern. Dormancy release for 1981–2004 advanced by as much as 15 days relative to that for 1921–50 at all locations except Jeju (located in the southernmost island with a subtropical climate), where an average 15-day delay was predicted. The cold-tolerant period diminished somewhat at six out of eight locations. As a result, bud-burst of Campbell Early in spring was advanced by 6–10 days at most locations, and interannual variation in bud-burst dates increased at all locations. The earlier bud-burst after the 1970s was due to 1) warming in winter that results in earlier dormancy release (Incheon, Mokpo, Gangneung, and Jeonju), 2) warming in early spring that enhances regrowth after breaking dormancy (Busan and Jeju), and 3) a combination of both (Seoul and Daegu).

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