Relationship between the Late Spring NAO and Summer Extreme Precipitation Frequency in the Middle and Lower Reaches of the Yangtze River

The relationship between the late spring North Atlantic Oscillation (NAO) and the summer extreme precipitation frequency (EPF) in the middle and lower reaches of the Yangtze River Valley (MLYRV) is examined using an NECP/NCAR reanalysis dataset and daily precipitation data from 74 stations in the MLYRV. The results show a significant negative correlation between the May NAO index and the EPF over the MLYRV in the subsequent summer. In positive EPF index years, the East Asian westerly jet shifts farther southward, and two blocking high positive anomalies appear over the Sea of Okhotsk and the Ural Mountains. These anomalies are favorable to the cold air from the mid-high latitudes invading the Yangtze River Valley (YRV). The moisture convergence and the ascending motion dominate the MLYRV. The above patterns are reversed in negative EPF index years. A wave train pattern that originates from the North Atlantic extends eastward to the Mediterranean and then moves to the Tibetan Plateau and from there to the YRV, which is an important link in the May NAO and the summer extreme precipitation in the MLYRV. The wave train may be aroused by the tripole pattern of the SST, which can explain why the May NAO affects the summer EPF in the MLYRV.

Cause Analysis of Gully Erosion in Yuanmou Basin of Jinshajiang Valley

Some factors （i.e. lithology, topography, climate, the change of population as well as land use during the past 50 years） that could have great influence on the development of gully in the arid-hot basin of Jinshajiang valley were investigated. The results show that the factors leading to the strong gully erosion in this area include： the widely distributed Yuanmou group stratum, which promotes the development of gully erosion; the unique geomorphologic configuration that is prone to rock fall and gully erosion; the strong and time-concentrated rainfall; the arid-humid alternate climate characteristics that prepares the ground for the development of fissures in soils; the arid-hot climate that goes against the growth and recovery of vegetation; and the unreasonable and abusive human activities.

Decadal and Interannual Variability of Persistent Heavy Rainfall Events over the Middle and Lower Reaches of the Yangtze River Valley

This study investigates the relationship between the anomalous atmospheric circulation pattern and summertime persistent heavy rainfall(PHR)over the middle and lower reaches of the Yangtze River valley(MLYRV)on the decadal and interannual timescales.Based on the gridded daily rainfall data of the US Climate Prediction Center,the PHR events on grid-point and the regional PHR events considering both the area of PHR and regionally averaged rainfall intensity are identified over the MLYRV during the summers of 1979–2017.A PHR index(PHRI)is defined,to describe the variability of summertime PHR event number over the MLYRV.The PHRI is then divided into the decadal and interannual components.Further analysis reveals that the decadal PHR events are closely related to anomalous precipitation,intensified vertical motion,and strengthened upper-level divergence over southern China,as well as an anomalous anticyclone over the western Pacific transporting more water vapor from the South China Sea(SCS)to southern China.As for the interannual component,the above pattern still co-exists but over a narrow region around the MLYRV.By choosing the years in which the decadal and interannual components of the PHRI are simultaneously positive(SP)or negative(SN),the features of regional PHR events in SP and SN years are analyzed,respectively.The results show that there are more regional PHR events in SP years with enhanced intensity and larger affected areas compared with those in SN years.Meanwhile,the zonal oscillations of the South Asian high(SAH)and the western Pacific subtropical high(WPSH)during the regional PHR events demonstrate a better regularity in SP years than those in SN years.

拉萨河下游河谷区三叶草引种品比试验研究

针对拉萨河谷的自然气候条件 ,选择了 9种三叶草品种进行集中品比试验 ,观察各种牧草的越冬率、生育期、高生长速率和产草量等生产形状。结果表明 ,拉萨河谷下游河谷区适合引种栽培三叶草 ,三叶草能够顺利越冬 ,能够取得与其它地区相似气候条件下的相当产量。在参试品种中 ,Zgrk880 2红三叶、Beskyd红三叶和Vesna红三叶是适宜于拉萨河谷下游的优良牧草品种 ,在越冬率、产草量、生长期和生长速率方面表现都是优良的品种 ,值得在拉萨河谷下游区推广。

拉萨河下游河谷风沙源分布特征及其成因

根据IKONOS和Qu ick B ird影像解译和实地调查,对拉萨河下游河谷区风沙源分布特征、沙源粒度特征、植被特征以及人类活动的作用进行了探讨。结果表明,受大中小尺度风场的影响,风沙源地沿河谷两侧呈小面积零星分布在多个地貌部位;河流冲积物是最主要的沙源,沙源粒径90%以上分布在0.25 mm以下,以细沙、极细沙和粘粒成分为主,平均含量占60.69%,易于发生风沙活动;风沙活动是影响沙生植被的主导因素,植物种类和盖度能很好反映沙源地的稳定程度;特别在流动沙地和半流动沙地上,植被演替朝着有利于风沙活动发展的方向进行,是风沙活动的主要驱动因素之一,也是风沙活动不断加剧的产物。尽管自然因素是该区域风沙活动的主要成因,人类活动对其发展起到了强化作用。

拉萨河下游春、夏季浮游植物群落特征与水质评价

为了揭示拉萨河下游浮游植物群落特征及其与水质的关系,2015年春季(5月)和夏季(9月),在拉萨河下游设置了9个样点,用25号浮游生物网分两批采集水样36个,对拉萨河下游的主要理化指标及浮游植物的群落组成分布进行了分析,并结合理化指标和生物参数对拉萨河下游春、夏季水质进行了评价。共鉴定出浮游植物264种,其中春季238种,夏季142种。春、夏两季的优势门类依次为硅藻门(73.53%)、绿藻门(17.65%)和蓝藻门(5.46%)。可见,拉萨河下游总体属于硅藻-绿藻-蓝藻型水体类型。两个季节平均多样性指数较高(春季3.49,夏季2.92)、优势度指数较低(春季0.06,夏季0.10)、均匀度指数总体较高(春季0.82,夏季0.79)。表明拉萨河下游浮游植物群落结构比较稳定,受外界干扰较小,水质总体为清洁-寡污型。理化因子指标显示,拉萨河水环境质量总体属于Ⅰ-Ⅲ类水体。浮游植物群落指数和水体理化因子的二元变量相关性分析显示:影响拉萨河下游春季浮游植物多样性的主要理化因子为总磷和总氮,夏季浮游植物环境多样性与水体理化因子间无显著相关性。

拉萨河流域中下游地区水化学及地表水-地下水转化关系研究

为研究拉萨河流域中下游地区水化学及地表水地下水转化关系,利用水化学资料分析了不同水体水化学和稳定同位素特征,并探讨了地表水-地下水之间的转化关系。结果表明:拉萨河流域中下游地区不同水体中主要离子含量均较低,矿化度较低,呈弱碱性,水化学类型均以HCO3·SO4(SO4·HCO3)-Ca·Mg(Mg·Ca)型为主。不同水体的水化学组成主要受岩石风化作用控制,且以碳酸盐岩和硅酸盐岩的溶解风化为主。水化学及同位素分析表明:水样点可分为地表水、地下水以及混合区三个区域,说明研究区地表水和地下水水力联系密切,存在明显的转化关系;地表水、地下水、泉水的主要来源均为大气降水,不同水体均受到不同程度的蒸发作用影响;支流地表水主要受地下水补给,拉萨河中下游地表水和地下水存在地段性互补关系,拉萨河是区域地下水的主要排泄通道。

拉萨河中下游沼泽湿地种子植物区系特征与资源型分析

文章以拉萨河中下游沼泽湿地为研究地点,采用样方法,结合室内分析,对本区域12个沼泽湿地种子植物组成、区系特征、生活型和资源类型进行分析。拉萨河中下游沼泽湿地植物多样性相对丰富,共计录种子植物216种,隶属119属、47科。研究区域沼泽湿地种子植物科的组成中单种科和小型科为种子植物科的主体,而中型科和大型科为优势科。在属的组成上单种属为组成种子植物属的主体,小型属为优势属。研究区域沼泽湿地种子植物生活型以多年生草本植物占优势。种子植物区系分析结果显示,科的地理分布区系类型以世界广布为主,其次为温带性质的地理成分;属的分布区类型上温带性质的分布区类型占据绝对优势。研究区域沼泽湿地种子植物可分为10个资源类型,其中药用植物为主要资源类型。

Extended-Range Forecasts of the Principal 20-30-Day Oscillation of the Circulation over East Asia During the Summer of 2002

Daily 850-hPa meridional wind fields in East Asia from March to September 2002 were used to establish a model of the principal oscillation pattern （POP）. This model was then used to conduct independent extended-range forecasts of the principal temporal and spatial variations in the low-frequency meridional wind field on a time scale of 20-30 days. These variations affect the occurrence of heavy precipitation events in the lower reaches of the Yangtze River valley （LYRV）. The results of 135 forecast experiments during the summer half year show that the predicted and observed anomalies are strongly correlated at a lead time of 20 days （mean correlation greater than 0.50）. This strong correlation indicates that the model is capable of accurately forecasting the low-frequency variations in meridional wind that corresponded to the 3 heavy precipitation events in the LYRV during the summer of 2002. Further forecast experiments based on data from multiple years with significant 20-30-day oscillations show that these prediction modes are effective tools for forecasting the space-time evolution of the low-frequency circulation. These findings offer potential for improving the accuracy of forecasts of heavy precipitation over the LYRV at lead times of 3-4 weeks.