Anomalous circulation patterns in association with two types of regional daily precipitation extremes over South China from July to October
Acta Meteorologica Sinica,2019,Vol 77,No. 01
【Abstract】 Using daily precipitation data collected at 753 stations in China and from the Meteorological Information Comprehensive Analysis and Process System (MICAPS), the Northwest Pacific Ocean Tropical Cyclone (TC) best track data (1981–2016) from the Tokyo-Typhoon Center in Japan, and the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis, the statistical characteristics of regional mean daily precipitation extreme (RDPE) events and circulation anomalies in South China were studied. Depending on whether the occurrence of a given RDPE event is affected by TC, the RDPEs are classified into two categories, i.e., TCfree-RDPE and TCaff-RDPE events. The TCaff-RDPE events account for about 42% of the total RDPEs and mainly occur in late August, while the TCfree-RDPE events frequently occur in July. When TCfree-RDPE events occur, South China is controlled by abnormal cyclonic circulations, and the warm and moist airflow from the western Pacific region and the South China Sea merge with the cold air from the north in this region, inducing a long narrow zone of water vapor convergence and significant and strong ascending motion, which are responsible for the occurrence and maintenance of TCfree-RDPE events. Simultaneously, the wave energy propagates from the northeastern side of the Tibetan Plateau and the Hexi Corridor region to South China and converges in this area, which favors the development and maintenance of disturbance over South China. When TCaff-RDPE events occur, the southern part of China is dominated by TC-related strong anomalous cyclonic circulations. The warm, moist air mass is transported into this area from the Bay of Bengal, the western Pacific and the South China Sea, leading to large amounts of latent heat release and strong ascending motion over South China. These results are helpful for better understanding and predicting the occurrence of regional extreme precipitation events in South China.
Projection and uncertainties of extreme precipitation over the Yangtze River valley in the early 21st century
Acta Meteorologica Sinica,2018,Vol 76,No. 01
【Abstract】 Intensity, frequency and duration of extreme precipitation would increase in the future under a warming climate. This phenomenon will be specifically significant in the Yangtze River valley, where precipitation is sensitive to climate change. Due to uncertainties in models results, the accuracy of extreme precipitation projection is still an open issue. In order to better understand possible changes of extreme precipitation over the middle and lower reaches of Yangtze River, this study evaluated the performance of 19 CMIP5 GCMs from the NASA Earth Exchange Global Daily Downscaled Projections (NEX-GDDP) in simulating extreme precipitation and compared the results with observations at 178 stations in Yangtze River valley during 1981–2005. The projection under the RCP4.5 scenario for the early 21st century was then given. The results showed that the downscaling models had excellent performance in simulating extreme precipitation in this region; the spatial correlation coefficients between all models and observations were larger than 0.6 except the R90N, while the correlation coefficients of PRCPTOT and R10 with observations were even higher than 0.95. Extreme precipitation tended to increase during the early 21st century, especially over the western part of the Yangtze River valley. The days of extreme precipitation (R90N) would decrease, and the R95T and PRCPTOT would increase, which indicates that the amount of extreme precipitation was mainly attributed to contributions of the R95T instead of more days of extreme precipitation. The largest uncertainties occurred in southern part of the region while the smallest uncertainties were found in the western part of the region. More attention should be paid to the increase in extreme precipitation in the western part.
Influence of super El Ni?o events on the frequency of spring and summer extreme precipitation over eastern China
Acta Meteorologica Sinica,2018,Vol 76,No. 04
【Abstract】 Features and mechanisms of the probability changes of spring and summer extreme precipitation during super El Niño events are investigated using the 0.5° × 0.5° resolution China daily precipitation data from the National Meteorological Information Center. Through analyzing the water vapor transport and vertical motion characteristics contributed by the super El Niño and its derived mode, the mechanism of super El Niño impacts on extreme precipitation over eastern China are discussed. The results show that during the decaying phase of the super El Niño event, the occurrence probability of spring extreme precipitation increases significantly over the whole eastern China, especially to the north of the Yangtze-Huaihe River Valley. In summer, the probability is almost doubled that in regular years over the Yangtze River Valley, whereas it decreases sharply over southern and northern China. Physical analysis indicates that in spring, the super El Niño itself and the combination mode (C-mode) derived from its nonlinear interaction with the tropical Pacific annual cycle both have significant impacts on the anomalous circulation background, i.e., the strong anti-cyclonic circulation anomaly over the northwestern Pacific leads to abundant moisture convergence and ascending motion over eastern China, which is conducive to the occurrence of the observed extreme precipitation. In the subsequent summer, the super El Niño event has disappeared, but the anomalous Northwest Pacific anti-cyclonic circulation associated with the C-mode still exists, and the favorable condition for extreme precipitation maintains over the Yangtze River Valley. Additionally, in spring and summer, daily meridional wind anomalies in the mid-upper troposphere over eastern China are exceptionally active during the super El Niño events, and the frequent meridional convergence of cold air from north and warm moisture from south could lead to enhanced convective events, which may also contribute to the increased occurrence of extreme precipitation.
A comparative study on frontogenesis characteristics in different phases during an extreme precipitation process
Journal of Natural Disasters,2018,Vol 27,No. 02
【Abstract】 A rainstorm and snowstorm weather occurred in Yunnan from the evening of 8 to 10 January 2015, which was the most extreme precipitation process in January since 1961. Based on NCEP/NCAR reanalysis data, frontogenetic function and characteristics of front secondary circulation in different phases were diagnosed. Furthermore, the production mechanism of strong rain and snow was discussed. It was showed that the water vapor transport from Bay of Bengal to South Yunnan deviating from climatological mean degree reached 3 σ, and center intensity of frontal zone in South Yunnan was –5 × 10 –5K·m −1, namely anomaly cold and warm airflow intersected in Yunnan, which made this strong rain and snow weather. Furthermore, frontogenesis function analysis showed that at the early stage of rainstorm, horizontal convergence in lower layer and ascending motion aggravated, which made precipitation reach peak. Thereafter, horizontal deformation field, especially shear deformation’s increasing made frontogenetic function maintain, but the intensity was obviously weak. For the snowstorm, the first peak formation of snowfall was mainly inclined item function. While the second, they were horizontal deformation and inclined item functions. Among horizontal deformation item, it was mostly growth of elongation deformation. In addition, strongly ascending motion in front secondary circulation’s ascending branch provided favorably dynamic condition to strong rain and snow weather.
Acta Meteorologica Sinica,2017,Vol 75,No. 06
【Abstract】 Based on the daily rainfall data over East Asia for the period of 1971 to 2007, non-linear correlations between different grid points are calculated by event synchronization method, and an extreme rainfall network over East Asia is built. By using complex network method, regional characteristics of extreme rainfall over East Asia are analyzed and a dynamics prediction model is constructed. Spatial distribution of degree shows that spatial synchronization is better in northern inland region than in coastal region. It is found that grid points in different regions have different spatial correlation extents, and the northern inland has larger spatial correlation extent than the coastal region. Furthermore, the results show that the prediction accuracy is higher in coastal region than in northern inland region. This is partly because of the short average link distance among grid points, and partly because of the extreme precipitation intensity. The prediction model constructed from the perspective of space and time continuity has the ability to predict extreme rainfall in East Asia on a certain level, and it has some potential application values in the research of extreme rainfall.
Chinese Journal of Atmospheric Sciences,2016,Vol 40,No. 05
【Abstract】 Based on historical simulation and future projection under the RCP8. 5 scenario by model FGOALS-g2, the authors have analyzed the extreme climate indices and associated potential future changes in the 21 st century over global monsoon region. Results indicate that FGOALS-g2 can reasonably reproduce the spatial pattern of climate state and interannual variability of extreme precipitation indices. However, precipitation is underestimated by FGOALS-g2 in heavy rainfall centers over Asian monsoon region. Due to the overestimation (underestimation) of the frequency of extreme rain (moderate and heavy rain), the extreme precipitation (total precipitation) simulated by FGOALS-g2 is stronger (weaker) than observations. Under the RCP8. 5 scenario, extreme precipitation, total precipitation, and precipitation intensity all tend to increase over global monsoon region. The most significant change occurs over North America (22% and 17% for extreme precipitation and precipitation intensity, respectively) and Australia (37% for precipitation amount). The projected increase in extreme precipitation may be attributed to the increase in precipitable water. However, the projected maximum number of consecutive days with daily precipitation less than 1 mm (hereafter CDD) will increase over land areas within global monsoon region but decrease over ocean areas of global monsoon region. To the end of the 21 st century (2076–2095), projected CDD will decrease (increase) by 30% (40%) over South America (Australia), which is associated with the increase (decrease) in the frequency of rainfall events with daily precipitation less than 1 mm.
Journal of Inorganic Materials,2015,Vol 30,No. 01
【Abstract】 CIGS absorber layers were prepared by sequential sputtering/selenization method. Based on that, CIGS solar cells were fabricated with a structure of glass/Mo/CIGS/CdS/i-ZnO/ZnO:Al/Ni-Al grid. The influences of annealing treatment on the performance of CIGS solar cells were investigated. By optimizing annealing condition, the cell efficiency increased from 4.91% to 14.01%. Further investigation revealed that the post-annealing treatment had two advantages. Firstly, it facilitated the diffusion of Cd ions into CIGS surface to substitute the Cu vacancies. Thus, the surface of CIGS converted from p-type to n-type conduction, leading to the shift of p-n junction from CIGS/CdS interface into the CIGS layer. Therefore, the recombination centers at the p-n junction were greatly reduced. Secondly, most H 2O molecules being adsorbed on the CIGS surface were eliminated by annealing, which improved the uniformity of electrical properties and band-gap of CIGS layer, resulting better performance of CIGS solar cell.