全球温度变化对我国降水的影响

该文计算了１９５１－１９９１年我国１６０个站年降水量与全球气温的交叉谱。得到在１０－３０年周期上我国降水量与全球温度变化的正、负相关区可分为界线分明的几个区，其中最明显的分界线位于我国半干旱区的中轴附近。在此线以北和以西的西北、内蒙和东北北部，降水量与全球温度呈同步演变趋势；在此线以南和以东的大部分地区，降水量与全球温度变化趋势相反。上述分界线恰好是我国盛夏夏季风的平均北界，表明我国降水分布对全球温度变化的响应与东亚夏季风的强弱变化有关。      

距平植被指数在1992年特大干旱监测中的应用

本文重点阐述ＮＯＡＡ极轨气象卫星距平植被指数的处理技术及算法，以及在１９９２年干旱监测中的应用。距平植被指数是以归一化植被指数（ＮＤＶＩ）多年旬、月平均值作为背景，然后用当年旬、月的ＮＤＶＩ值减去背景值。植被指数的距平值不仅反映了植被年际间的变化，而且也指示了天气对植被的影响。用这个量监测农作物是否遭到旱灾威胁比只用ＮＤＶＩ的瞬时值优越。研究结果表明当月的距平植被指数与当月降水量距平百分率相一致。     

自检校平差中过度参数化的克服

带附加参数的自检校平差是当今解析空中三角测量中补偿系统误差的最有效方法。可惜的是,当采用一个十分庞大的附加参数组时,在某些几何条件下将导致很坏的法方程系状态,从而使测求的平差结果很差。本文研究和试验了几种克服过度参数化的方法。它们不是用统计分析方法来消除某些附加参数的,因为这会引起实际计算的困难。本试验研究表明: ——如果采用飞行方向交叉的双区域,用自由附加参数平差,附加参数的可测定性和系统的可靠性均是可接受的; ——如果附加参数作为带权观测值,而且按信噪比确定其权,在许多情况下过度参数化将是可避免的。——如果所有的未知数按极大验后估计原理或岭估计原理处理成带权观测值,並给予一个小权(例如P=0.005～0.001),则在所试验的任何情况下均可克服过度参数化。     

Climatology Comparison Studies of Precipitations Between GPCP and Rain Gauges in China

The Global Precipitation Climatology Project (GPCP) monthly rainfall data and the rainfall records observed by 740 rain gauges in the mainland of China are used to analyze similarities and differences of the precipitation in China in the period from January 1980 to December 2000. Results expose significantly consistent rainfall distributions between the both data in multi-year mean, multi-year seasonal mean, and multi-year monthly mean. Departures of monthly rainfall for each dataset also show a high correlation with an over 0.8 correlation coefficient. Analysis indicates small differences of both datasets during autumn, winter, and spring, but relative large ones in summer. Generally, the GPCP has trend of overestimating the rainfall rate. Based on above good relationship of both datasets, the GPCP data are used to represent distributions and variations of precipitation in the Tibetan Plateau and Northwest China. Results indicate positive departures of precipitation in summer in the west part of Tibetan Plateau in the 1980s and negative departures after the 1980s. For the west part of Northwest China, analysis illustrates precipitation decreases a little, but no clear variation tendency.     

用动态时间序列周期分析预测模型作郑州汛期降水预报

动态时间序列周期分析预测模型是将多层递阶方法与逐步回归周期分析的基本原理相结合,可以有效地选取时间序列的各个隐含周期。利用所选取的隐含周期,可作更长时间的预测。本文以郑州汛期降水为样本,对该预测模型进行了应用及讨论。     

Assessing desertification

It is widely recognized that desertification is a serious threat to arid and semiarid environments—which cover 40% of the global land surface and are populated by approximately 1 billion humans. Given the potential relevance of this problem, it is surprising that there is no consensus on the proper way to assess the desertification status of a piece of land. During the last 70 years, conflicting definitions have produced both different assessment methodologies and divergent estimates. Contrary to conceptual issues on desertification, assessment methodologies have not been reviewed comprehensively. Here, we critically review the most common methodologies to assess desertification, and describe their principal consequences on scientific and social arenas.We show that desertification assessment has shifted from simple appraisals of the interannual movement of desert boundaries to complex multivariate field surveys, to practical methodologies based on indicators of ecosystem functioning, such as rain use efficiency. Although often regarded as an evidence of stagnation and failure, these methodologies reflect the progress that desertification ecology has experienced. Future challenges for properly assessing desertification are (1) the lack of reference situations against which actual desertification could be compared, and (2) the difficulties that appear when desertification operates through structural rather than functional ecosystem changes.The coexistence of conflicting definitions and divergent estimates negatively affects societal perception, leading to scepticism and, ultimately, to a delay of eventual solutions. Societies must recognize the progress desertification ecology has made, leave behind concepts that no longer represent current knowledge, grasp the opportunity to better assess the extent and intensity of the problem, and, for the time being, realize that assessing desertification is an unsolved issue.     

Holocene variations in the Asian monsoon inferred from the geochemistry of lake sediments in central Tibet

We present a record of monsoon variations for the early and middle Holocene that is inferred from the geochemistry of sediment cores from Ahung Co, a lake in central Tibet. The resolution of this record is better than 50 yr and the age model is derived from radiocarbon ages of terrestrial charcoal, which eliminates errors associated with the lake hard-water effect. We made down-core geochemical measurements of % carbonate, % organic carbon, C/N and δ¹³C of bulk organic matter, δ¹³C and δ¹⁸O of carbonate, and % dolomite. Proxy calibration and modern water-balance reconstruction show that these are proxies for lake depth and the amount of monsoon precipitation. We find that lake level and monsoon precipitation have been decreasing at Ahung Co since the early Holocene (∼7500 cal yr B.P.). Superimposed on this trend are rapid declines in monsoon rainfall at 7000-7500 and 4700 cal yr B.P. and seven century-scale wet-dry oscillations. The cores do not contain sediment from the last ∼4000 yr. Surface sediments from the lake accumulated during the 20th century, however. From this, we argue that lake levels have risen again recently following a late Holocene dry period.     

上海区域数值预报模式集合预报系统的建立与试验

以目前运行的上海区域业务数值预报模式为基础,从预报模式的不确定性出发构造8个预报成员,建立了上海区域数值预报模式集合预报系统的初步模型,并对2004年夏季进行了逐日48 h预报试验。结果表明:集合平均对华东地区城市降水、温度、海平面气压等气象要素的总体预报能力与分辨率高3倍的业务模式相当,其中对雨量较大降水、最低温度、海平面气压(0~24 h)的预报效果好于业务模式;集合预报还能提供客观化、定量化的降水概率预报,对降水的发生、尤其是特大降水的发生有着很好的提示作用。     

台风降水云区中单站强降水诊断分析和预报

实际工作表明,在台风降水云区中,有的站点有大暴雨、有的站点无大暴雨发生。这给单站预报带来较大难度。为了提高降水云区中单站大暴雨预报准确率,本文利用T213客观分析场和预报产品,采用多种物理量综合诊断分析方法,对登陆台风“云娜”在西进途中多站点进行水汽来源、不稳定层结的维持以及次生中尺度辐合等方面的研究,探讨单站大暴雨发生、发展机制和不发生大暴雨的成因。结果表明:单站有大暴雨发生的主要成因是有次级环流出现,如:站点上空∑θse(5 7 8)≥230℃。站点200 hPa为明显的负温度平流,850 hPa为明显的正温度平流。站点高低层均为正涡度中心,且低层的正值大于高层的正值。高低层散度都为负值中心;或高层为正值中心、低层为负值中心。站点垂直运动中心速度值要达-140×10-3hPa/s以下。站点需要有源源不断的充足水汽的供给和辐合。台风中心西北部水汽通量大值区靠近台风中心西部干舌梯度最大处以及风向和风速辐合最大处。在台风降水云区中,当站点满足上述各要素时大暴雨易发生。     

东北地区月平均大气环流型与哈尔滨-气候关系的初步研究

利用基于英国Lamb(1950年)发展的大气环流分型方法的Jenkinson(1977年)法对东北地区1951—2002年的月平均海平面气压场(MSLP)进行环流分型。由月平均海平面气压场算出6个环流指数,并由此划分出27种环流类型,分析了其中出现频率最高的5种主要环流类型(N,NW,C,CSW,SW)在不同时间尺度下的变化规律及它们与哈尔滨月平均温度的关系,利用逐步回归方法得到了温度距平的拟合曲线。给出了各种环流类型的月平均降水量和与哈尔滨降水密切相关的C,CSW和SW 3种环流类型对应的平均海平面气压合成图。结果表明:哈尔滨冬季以N,NW型为主,夏季以C,CSW和SW型为主。出现N和NW型时气温偏低,降水偏少;而出现C,CSW和SW型时气温较高,降水偏多。用6个环流指数中的地转风V和大尺度平均温度t可以建立其与温度距平之间的一个统计模式,利用此模式,能解释哈尔滨1951—2002年温度变化方差的77.3%。C,CSW和SW 3种环流类型为哈尔滨的主要降水类型,C型与哈尔滨总降水的相关关系很好,并且近20年来哈尔滨主要以C型降水为主。这种研究大尺度大气环流与区域气候变量—温度、降水之间关系的方法是一种统计降尺度(statistical downscaling)方法,可以用于区域气候预测。