The use of bootstrap confidence intervals for the correlation coefficient in climatology

Summary The importance of defining confidence intervals for sample statistics that are used to estimate characteristics of the parent population(s) is emphasised. Not all sample statistics are unbiased estimators or have normally distributed sampling distributions and so it is not always easy to reflect the reliability of the estimator. In such cases, Efron's bias corrected percentile method, which uses bootstrap samples to estimate the bias and makes no assumptions about the distribution of the sample statistic can be used to define confidence limits for the population parameter. The method is explained and the procedure for calculating the confidence limits is outlined.As an example, bootstrap confidence limits calculated for the maximum correlation between the Southern Oscillation Index and rainfall at South African stations over the period 1935–1983 suggest that the sample correlation is an unreliable measure of the true association. One possible reason for this is that the association is thought to have broken down during the 1940s. However, the reliability of the estimator does not seem to improve when confidence limits are calculated for the 30-year period 1954–1983. It is possible that the width of the confidence interval is an indication of more than one distinct statistical population.     

Improved confidence in climate change projections of precipitation further evaluated using daily statistics from ENSEMBLES models

Probability density functions for daily precipitation data are used as a validation tool comparing station measurements to seven transient regional climate model runs, with a horizontal resolution of 25 km and driven by the SRES A1B scenario forcing, within the ENSEMBLES project. The validation is performed for the control period 1961–1990 for eight predefined European subregions, and a ninth region enclosing all eight subregions, with different climate characteristics. Models that best match the observations are then used for making climate change projections of precipitation distributions during the twenty-first century for each subregion separately. We find, compared to the control period, a distinct decrease in the contribution to the total precipitation for days with moderate precipitation and a distinct increase for days with more intense precipitation. This change in contribution to the total precipitation is found to amplify with time during all of the twenty-first century with an average rate of 1.1% K^?1. Furthermore, the crossover point separating the decreasing from the increasing contributions does not show any significant change with time for any specific subregion. These results are a confirmation and a specification of the results from a previous study using the same station measurements but with a regional climate model ensemble within the PRUDENCE project.     

Improved confidence in regional climate model simulations of precipitation evaluated using drought statistics from the ENSEMBLES models

An ensemble of regional climate model simulations from the European framework project ENSEMBLES is compared with observations of low precipitation events across a number of European regions. We characterize precipitation deficits in terms of two drought indices, the Standardized Precipitation Index and the self-calibrated Palmer Drought Severity Index. Models that robustly describe the observations for the period 1961–2000 in given regions are identified and an assessment of the overall performance of the ensemble is provided. The results show that in general, models capture the most severe drought events and that the ensemble mean model also performs well. Some regions that appear to be more problematic to simulate well are also identified. These are relatively small regions and have rather complex topographical features. The analysis suggests that assessment of future drought occurrence based on climate change experiments in general would appear to be robust. But due to the heterogeneous and often fine-scaled structure of drought occurrence, quantitative results should be used with great care, particularly in regions with complex terrain and limited information about past drought occurrence.     

Percentile-based neighborhood precipitation verification and its application to a landfalling tropical storm case with radar data assimilation

The traditional threat score based on fixed thresholds for precipitation verification is sensitive to intensity forecast bias.In this study, the neighborhood precipitation threat score is modified by defining the thresholds in terms of the percentiles of overall precipitation instead of fixed threshold values. The impact of intensity forecast bias on the calculated threat score is reduced. The method is tested with the forecasts of a tropical storm that re-intensified after making landfall and caused heavy flooding. The forecasts are produced with and without radar data assimilation. The forecast with assimilation of both radial velocity and reflectivity produce precipitation patterns that better match observations but have large positive intensity bias.When using fixed thresholds, the neighborhood threat scores fail to yield high scores for forecasts that have good pattern match with observations, due to large intensity bias. In contrast, the percentile-based neighborhood method yields the highest score for the forecast with the best pattern match and the smallest position error. The percentile-based method also yields scores that are more consistent with object-based verifications, which are less sensitive to intensity bias, demonstrating the potential value of percentile-based verification.     

成都市降水概率预报方法

用实时气象资料与T106数值产品相结合，根据概率统计的方法和原理，采用概率回归估计方法，建立了成都市的夏季降水概率预报方法。此方法可对成都市东、西、北片未来24小时降水作出定量预报。1999年8－9月试用效果较好，可供精力参考应用。     

成都市降水概率预报方法

用实时气象资料与T106数值产品相结合，根据概率 统计的方法 和原理，采用概率回归估计方法，建立了成都市的夏季降水概率预报方法。此方法可对成都 市 东、西、北片未来24小时降水作出定量预报。1999年8～9月试用效果较好，可供业务参考应用。     

锦州地区逐日降水概率分布特征

利用锦州地区的逐日降水量观测资料对逐日降水量的概率分布进行了统计分析,采用最大似然估计法得到Gamma函数分布的形状参数α和尺度参数β,通过Gamma概率分布模拟观测站点逐日降水的概率分布。结果表明：锦州地区逐日降水频率整体趋势先上升后下降,基本呈对称式分布,降水概率有一定的振荡,个别日会出现远超相邻日期的降水频率,7月21日降水频率最高,在不计微量降水的情况下,最低逐日降水概率有多个日期为0。各季降水频率偏低是造成义县地区干旱的原因之一;北镇夏季平均降水频率最低,但其夏季平均降水量却为锦州地区最高,说明北镇可能易出现较大量级降水或易出现极端降水天气。清明期间降水频率在50%以上、高考期间降水频率在80%以上,符合大众日常对特殊日期降水情况的认知;逐日降水频率可以为公众气象服务提供新的思路。凌海、北镇更容易出现极端降水天气;锦州地区日降水出现小雨天气概率最高,暴雨以上降水概率较低,锦州地区各站极少出现大暴雨以上量级降水,对锦州降水量级预报,尤其是暴雨或大暴雨以上降水量级的预报起到一定的指示作用。     

Delaunay三角剖分法在降水量插值中的应用

Delaunay三角剖分方法在空间分析中具有重要地位,文中简要介绍了Delaunay三角网特性和常用的3类算法,并对随机增长法实现过程进行了详细阐述。根据三角分片线性插值原理,求得插值系数,实现对任意点的三角分片线性插值。利用2008年中国2200个观测站的08时24 h降水量资料,对全中国范围及划分的8个区域内相应的0.28125°×0.28125°降水量格点场,使用交叉检验方法,对比分析了三角分片线性插值和反距离权重法的估值准确率。结果表明：在各区域,三角分片线性插值法的均方根误差偏小;在站点较密集的区域,均方根误差、平均绝对误差比较中,三角分片线性插值都有一定的优势;在平均误差对比中,三角分片线性插值优势明显,在全中国范围交叉检验中,三角分片线性插值法对应的年平均误差是0.005 mm,而反距离权重法为-0.107 mm,对其可能的原因进行了分析,证明了Delaunay三角剖分法的合理性。同时,从图形上展示了降水量的Delaunay三角网的三维结构图和三角分片线性插值后的格点场, 在直观上,Delaunay三角剖分后得到降水分布和实况保持一致,并有较好的视觉效果;通过三角分片线性插值得到的格点场降水量分布图,克服了反距离权重法的固有缺陷,使获得的降水量格点场趋于合理,提高了插值精度。最后,探讨了Delaunay三角网在气象领域的应用前景。     

深圳分钟降水数据预处理系统设计与应用

“深圳分区暴雨强度公式推算软件”运行,需要获取其境内所有自动气象站逐年不同历时多个样本最大降水量资料,基于其计算量大、时效性要求高,设计开发了与之配套的“深圳分钟降水数据预处理系统”,并实现与该软件的“无缝衔接”。 从分区原则确定、分钟资料质量控制、不同历时滑动雨量( 强) 计算、不同降水过程自动判断、样本挑选,以及功能、流程、设计技术等方面,对该系统作详细介绍。同时,以深圳本站1961 —2010 年共50 年逐日分钟降水资料为例,采用该软件计算出5、10 、15 、20 、30 、45 、60 、90 、120 min共9 个历时每年8 个最大降水量序列,结果表明,该系统能满足“深圳分区暴雨强度公式推算软件”的要求。     

Response of extreme precipitation to increasing extratropical cyclonic vortex frequency

Since the 2000s, extratropical extremes have been more frequent, which are closely related to anomalies of planetary-scale and synoptic-scale systems. This study focuses on a key synoptic system, the extratropical cyclonic vortex (ECV) over land, to investigate its relations with extreme precipitation. It was found that ECVs have been more active post-2000, which has induced more extreme precipitation, and such variation is projected to persist along with increasing temperature within 1.5°C of global warming. An enhanced quasi-stationary vortex (QSV) primarily contributes to the ECV, rather than inactive synoptic-scale transient eddies (STEs). Inactive STEs respond to a decline in baroclinicity due to the tendency of the homogeneous temperature gradient. However, such conditions are helpful to widening the westerly jet belt, favoring strong dynamic processes of quasi-resonant amplification and interaction of STEs with the quasi-stationary wave, and the result favors an increasing frequency and persistence of QSVs, contributing to extreme precipitation.摘要自21世纪以来, 热带外极端降水频次增加. 随着中高纬度的显著增温, 经向温度梯度减弱导致低层大气斜压性减小, 由此产生的气旋型瞬变涡天气系统等活动减弱. 然而, 热力分布导致西风急流带变宽, 经向环流加大, 有助于行星尺度波动相关的涡旋异常增加, 如东北冷涡, 中亚涡, 东欧-地中海涡, 北美涡等, 进而增加了气旋涡影响范围的极端降水频次. 在未来变暖背景和1.5°C增温的目标内, 热带外气旋涡增强会进一步促进极端降水发生.     

北京地区夏季极端降水变化特征及城市化影响

利用1981-2016年京津冀地区174个国家站逐日降水资料,采用百分位方法和线性倾向估计方法对京津冀地区极端降水的时空分布特征及演变趋势进行了分析。结果表明：（1）对于京津冀地区极端降水空间分布,不同百分位降水阈值表现为一致的分布特征,年平均极端降水量、平均极端降水强度与百分位极端降水阈值分布大体一致,而年平均极端降水日数的分布则与其相反。（2）年平均极端降水量在103.6~259.1 mm之间,年平均极端降水日数在3.0~4.0 d之间,平均极端降水强度在大雨到暴雨之间,极端降水量对总降水量贡献达28%以上。（3）极端降水总站次和极端降水日数年变化趋势一致,7月、8月和10月是极端降水较活跃月份。（4）在36 a期间,年平均极端降水量、年平均极端降水日数、平均极端降水强度以及极端降水量对总降水量贡献的变化趋势分布情况基本一致,呈减少趋势的站点均相对较多,年平均极端降水量增减幅度较大,年平均极端降水日数变化在1 d·（10 a）^-1以内,平均极端降水强度和极端降水量对总降水量贡献减少趋势相对明显。

     

近20年中国气象灾害损失的初步分析

采用全国气象部门收集的县（区）域行政单元灾情普查资料,结合全国气象站点降水观测资料,分析了1984—2008年中国暴雨及其引发的洪涝灾害的时空演变特点及灾害损失情况,揭示了气候变化及人类活动双重作用下中国暴雨洪涝灾害变化趋势和演变特点,以及暴雨洪涝灾害影响的时空差异性。结果表明：近25 a来中国暴雨日数总体上稍有增加,暴雨强度和暴雨天数的空间分布均表现为南方高于北方,东部高于西部的特点,20世纪90年代中后期为中国暴雨高发期。研究时段内,中国暴雨洪涝灾害造成的直接经济损失呈增加趋势,但直接经济损失占当年GDP的比例则呈下降趋势,平均每年经济损失约为573亿元人民币,损失较高的地区主要集中在中国南方地区,县域年平均损失超过2 000万元的县约占15%,其中有34个县超过亿元。受灾人口呈增加趋势,但因灾死亡人口呈下降趋势;暴雨洪涝灾害对农作物受灾面积和绝收面积的影响均呈微弱上升趋势,年平均作物受灾面积近9.00×10⁶ hm²,作物绝收面积为1.27×10⁶ hm²。

     

面向台风暴雨的集合预报成员优选订正技术研究及应用试验

利用欧洲中期天气预报中心和美国国家环境预报中心2017年5—8月逐日降水预报资料及同时段MICAPS降水观测资料,采用频率匹配技术对江淮流域夏季降水预报进行模式偏差订正及改进技术探讨。结果表明：（1）模式对小雨预报偏多,存在大量空报;对大雨及以上降水预报偏少,存在较大漏报。（2）偏差订正通过下调小量级降水、上调大量级降水,使各量级降水的强度和面积偏差均得到一定程度改善,降水量级两端订正效果明显,订正后小雨和暴雨准确率显著提升。（3）偏差订正对暴雨落区预报的改进效果与过程相关,个例分析表明：对于雨带位置预报较准确的大范围梅雨锋暴雨,偏差订正后暴雨TS评分明显提升;对于副高边缘的小范围暴雨以及雨带位置预报失误的梅雨锋暴雨或台风暴雨,偏差订正后暴雨TS评分改善不明显甚至降低。（4）针对上述问题,提出了系数动态调整和模式集成的改进思路,即对于平均雨量5 mm以下的小范围暴雨,适当上调订正系数;平均雨量在15 mm以上的大范围暴雨,适当下调订正系数。模式集成订正可有效提高暴雨TS评分。

     

Microwave Simulations of Precipitation Distribution with Two Radiative Transfer Models

Two microwave radiative transfer models of precipitating cloud are used to simulate the microwave upwelling radiances emerging from precipitating clouds. Comparison of the simulation results shows that significant difference of microwave upwelling radiances exists between these two radiative transfer models. Analysis of these differences in different cloud and precipitation conditions shows that it is complicated but has certain trend for different microwave frequencies. The results may be useful to quantitative rainfall rate retrieval of real precipitating clouds.     

EC-thin资料在低槽冷锋类降水过程人工增雨潜力区识别中的应用

选取8次冀中南地区低槽冷锋类降水过程,基于实况资料、微波辐射计资料、毫米波测云仪资料,利用EC-thin模式资料,分析研究EC-thin资料在人工增雨潜力区识别中的应用。结果表明：低槽冷锋类降水过程的共同特征为中高层30°—50°N、100°—120°E低槽东移,地面冷锋以西北路径或偏西路径南下影响冀中南地区;无论平原还是山区,低云量、气柱总含水量、大气可降水量、850 hPa比湿总体上呈现随降雨量增大而增大的趋势;当冀中南地区受低槽冷锋类天气系统影响时,在平原地区,当低云量、气柱总含水量、大气可降水量、850 hPa比湿预报平均值分别为6成、31 kg·m^-2、37 kg·m^-2、6 g·kg^-1以上时可进行人工增雨作业;在山区,当低云量、气柱总含水量、大气可降水量、850 hPa比湿预报平均值分别为5成、21 kg·m^-2、29 kg·m^-2、4 g·kg^-1以上时可进行人工增雨作业;此外最佳人工增雨阶段为各物理量值达到最大并开始减小阶段。     

Mechanisms of tropical precipitation biases in climate models

Climate Dynamics - We investigate the possible causes for inter-model spread in tropical zonal-mean precipitation pattern, which is divided into hemispherically symmetric and anti-symmetric modes...     

县级区域自动气象站降水资料查询服务系统的开发与应用

为了给县级气象部门开展强降水警报提供方便实用的操作平台,利用区域自动气象站的降水资料,开发了县级区域自动气象站降水警报与服务系统,该系统可对疑误降水数据进行简易甄审,提供了对降水数据的多种统计查询方法,实现了突发性强降水的自动电话语音警报.该系统自动调用Surfer绘制等值线降水色斑图,自动生成服务文档.通过不同的参数设置可适用于其它县级气象部门.