Spatial-Temporal Characteristics of Regional Extreme Low Temperature Events in China during 1960-2009

An objective identification technique is used to detect regional extreme low temperature events （RELTE） in China during 1960-2009. Their spatial-temporal characteristics are analyzed. The results indicate that the lowest temperatures of RELTE, together with the frequency distribution of the geometric latitude center, exhibit a double-peak feature. The RELTE frequently happen near the geometric area of 30°N and 42°N before the mid-1980s, but shifted afterwards to 30°N. During 1960-2009, the frequency~ intensity, and the maximum impacted area of RELTE show overall decreasing trends. Due to the contribution of RELTE, with long duratioh and large spatial range, which account for 10% of the total RELTE, there is a significant turning point in the late 1980s. A change to a much more steady state after the late 1990s is identified. In addition, the integrated indices of RELTE are classified and analyzed.

Distribution Characteristics of Winter Continuous Extreme Low Temperature in China

[Objective] The research aimed to study the distribution characteristics of winter continuous extreme low temperature in China. [Method] Based on the daily minimum temperature data in winter during 1961-2008 in 195 observatories, the continuous extreme low temperature event (cold night) which happened in winter in China and the distribution characteristics of accumulated temperature anomaly in 48 years were analyzed by using the actual probability distribution threshold method. [Result] Besides in Northeast China, Northwest China and Qinghai-Tibet Plateau where the geographical position was northerly, and the altitude was high, the long-time (above 5 d) extreme low temperature event was also easy to happen in the south of Yangtze River, especially in Guizhou, Guangxi, Hunan and so on. The continuous extreme low temperature event in the middle and lower reaches of Yellow River was the least. Started from the metaphase of the 1980s, the frequency trend of continuous extreme low temperature event decreased. But in 1992, 2000 and 2007, the low temperature event which continued above 7 d was more than the average. The accumulated cold in January in 48 years was the strongest. The second one was in February, and the smallest was in December. The accumulated cold in Northeast China and the north of Inner Mongolia was the biggest in December and January. The accumulated cold in the north of Xinjiang was the biggest in February. In the whole winter, the accumulated cold in Qinghai-Tibet Plateau was the smallest. The chilling injury was easier to happen in Guizhou, Guangxi and Hunan than other south areas in December and February. The occurrence probability of chilling injury in most areas of Szechwan Basin, Yunnan and Qinling Mountains in central China was smaller than that in other areas. [Conclusion] The research provided the reference basis for analyzing the influence of chilling injury on the agriculture in China.

Ohmic Contact at Al/TiO_2/n-Ge Interface with TiO_2 Deposited at Extremely Low Temperature

TiO2deposited at extremely low temperature of 120°C by atomic layer deposition is inserted between metal and n-Ge to relieve the Fermi level pinning. X-ray photoelectron spectroscopy and cross-sectional transmission electron microscopy indicate that the lower deposition temperature tends to effectively eliminate the formation of GeOxto reduce the tunneling resistance. Compared with TiO2deposited at higher temperature of 250°C,there are more oxygen vacancies in lower-temperature-deposited TiO2, which will dope TiO2contributing to the lower tunneling resistance. Al/TiO2/n-Ge metal-insulator-semiconductor diodes with 2 nm 120°C deposited TiO2achieves 2496 times of current density at-0.1 V compared with the device without the TiO2interface layer case, and is 8.85 times larger than that with 250°C deposited TiO2. Thus inserting extremely low temperature deposited TiO2to depin the Fermi level for n-Ge may be a better choice.

Study on Temporal-spatial Change Characteristics of the Extremely Low Temperature Climate Event in Winter of China

[ Objective] The research aimed to study temporal-spatial change characteristics of the extremely low temperature climate event in winter of China. [MethodI By using actual probability distribution threshold value method, EOF analysis method and the daily minimum temperature data in winter of 1961 -2008 at 195 observatories of China, temporal-spatial change characteristics of the extremely low temperature climate event in win- ter of China in 48 years were analyzed. [ Result] Threshold value calculated by actual probability distribution was higher than that by traditional method, and increase magnitude in east was bigger than that in west, which could describe climatic change situation in China in 48 years better than traditional method. Cold night number in winter of China did not decrease as latitude declined, and it was more in south and north and was less in Yellow River basin and northwest China. Cold night number was the most in northeast Inner Mongolia. Interannual change of the cold night number presented decline tendency. From the 1960s to the eady 1970s, cold night number consistently increased, and change in middle and late periods of the 1960s was severe. From the early 1970s to the middle 1980s, fluctuation of the cold night number was bigger, and increase of the cold night number was the most in 1976. Cold night number from 1986 to the 21= century continuously declined. Change of the cold night number was the most obvious in east region, Guizhou and north Guangxi. Cold night number in northeast China and north Inner Mongolia presented inverting change relationship with that in Yunnan - Kweichow Plateau and Hexi Corridor. [ Conclusion] The research provided reference for eady warning of the extremely low temoerature event in China.

MULTIMODEL CONSENSUS FORECASTING OF LOW TEMPERATURE AND ICY WEATHER OVER CENTRAL AND SOUTHERN CHINA IN EARLY 2008

Based on the daily mean temperature and 24-h accumulated total precipitation over central and southern China, the features and the possible causes of the extreme weather events with low temperature and icing conditions,which occurred in the southern part of China during early 2008, are investigated in this study. In addition, multimodel consensus forecasting experiments are conducted by using the ensemble forecasts of ECMWF, JMA, NCEP and CMA taken from the TIGGE archives. Results show that more than a third of the stations in the southern part of China were covered by the extremely abundant precipitation with a 50-a return period, and extremely low temperature with a 50-a return period occurred in the Guizhou and western Hunan province as well. For the 24- to 216-h surface temperature forecasts, the bias-removed multimodel ensemble mean with running training period(R-BREM) has the highest forecast skill of all individual models and multimodel consensus techniques. Taking the RMSEs of the ECMWF 96-h forecasts as the criterion, the forecast time of the surface temperature may be prolonged to 192 h over the southeastern coast of China by using the R-BREM technique. For the sprinkle forecasts over central and southern China, the R-BREM technique has the best performance in terms of threat scores(TS) for the 24- to 192-h forecasts except for the 72-h forecasts among all individual models and multimodel consensus techniques. For the moderate rain, the forecast skill of the R-BREM technique is superior to those of individual models and multimodel ensemble mean.

Possible linkage between winter extreme low temperature events over China and synoptic-scale transient wave activity

Based on NECP/NCAR reanalysis data and daily temperature data of 743 stations in China, possible causes of winter extreme low temperature events are explored from the perspective of the synoptic-scale transient wave (STW) activity. Results suggest that there is a close linkage between STW activity and extreme low temperature events. Firstly, case studies are carried out on the years with the most and least frequent extreme low temperature events. In the winter of 1967, two strong and stable STW trains were maintained over the Eurasian continent, and the strong westerly jet provided a good channel for the propagation of STW. Located in the downstream area of those two STW trains, China was significantly influenced by them and experienced frequent extreme low temperature events. Further analysis suggest that the intensity of the upstream transient wave and the areas where the transient waves reached are completely consistent with the intensity of extreme low temperature and the areas where frequent extreme low temperature event happened, respectively. In contrast, Westerly jet in 2006 was weaker and the path of transient wave propagation was shorter and weaker, resulting in the low frequency of extreme temperature. Secondly, in their long term variations, westerly jet is also consistent with the extreme low temperature frequency. The transient wave path changed before and after the 1980s. Further investigation suggests that transient wave intensities in key areas exhibit in-phase changes with the frequency of extreme low temperature events in the periods of 1959-1979 and 1986-2006. Meanwhile, the main features of transient wave activities in high-frequent years and low-frequent years of extreme low temperature events are similar to those of 1967 and 2006, respectively. Results indicate that winter extreme low temperature events in China have a very close relationship with the transient wave activity, implying the propagation and activity of STW are important factors affecting the winter extreme low temperature events in China. This study can also provide a new clue for better understanding the mechanisms of the extreme temperature events.