西南地区气候季节划分及特征分析

本文运用聚类分析方法对西南地区进行分区讨论,研究了各区域的气候季节及其变化特征。我国西南地区5个区域夏季和秋季的分配基本一致,而冬春两季差异明显;而降水场和温度场四季划分也有一定相关和相异性,均按照云南区-川西高原区-贵州区-四川盆地区-川东区的顺序逐级递减。     

西南地区夏半年降水的多时空尺度特征

根据国家气象中心提供的西南地区夏半年月降水资料,利用旋转经验正交函数(REOF)展开方法,将西南地区划分为川西北、川渝区、云川区和贵州区4个区域,并在此基础上,利用多项式拟合趋势线、小波变换等方法,对各区域降水的变化特征进行了分析。结果表明,西南地区各区域降水的长期变化趋势不尽相同,川西北没有显著的趋势变化,川渝区表现为缓慢上升的趋势,而云川区和贵州区则均表现为先下降后上升的趋势。西南地区各区域降水的周期特征也不尽相同,较短时间尺度的周期变化在西南地区各区域普遍存在;而较长时间尺度的周期,各个区域则有较大的差别,不仅表现在长时间尺度周期的显著性上,还表现在具有相同时间尺度周期,不同区域的干湿期配置等方面。     

基于CRA空间检验技术的西南地区东部强降水EC模式预报误差分析

CRA(contiguous rain area)空间检验技术是将连续雨区作为目标进行检验。通过设定降水量阈值,识别、分离及平移降水目标,将预报偏差分解为落区、强度和形态误差,该方法可避免传统TS评分的双惩罚效应。利用CRA空间检验技术对2011—2014年5—9月西南地区东部EC细网格模式36 h预报时效119个降水目标的预报误差进行分析,并按照环流形势和影响系统对强降水个例进行分型,分为西南地区东部低涡切变型、西南地区东部-江淮切变型、南风型,分别对上述三类不同类型强降水个例的落区和强度误差进行了对比。得到如下结论:西南地区东部降水预报形态误差占比最大,为60%左右,其次是落区误差,为30%左右,强度误差最小,约为10%;落区平均偏西0.7°,经向偏差不明显;模式对于水平尺度较小的降水目标漏报可能性较大,而对于天气尺度降水目标模式预报面积偏大,总降水量偏大,雨强偏小;西南地区东部低涡切变型和西南地区东部-江淮切变型降水强度预报误差类似,模式预报雨区面积均偏大,降水尺度越大,偏大的概率越大,实况平均降水强度越强,模式预报强度越偏弱;南风型预报强降水面积和平均强度均偏弱,出现漏报的概率较大;而对于最大降水量,三种类型模式预报的最强降水均较实况偏弱;西南地区东部低涡切变型模式预报落区偏西,江淮至西南地区东部切变型模式预报落区偏西偏北,南风型模式预报落区偏西偏南。     

西南地区不同地质灾害影响区的降水阈值研究

根据1961~2010年西南地区(四川省、云南省、贵州省、重庆市)重大地质灾害资料和全国重大地质灾害年鉴资料统计不同地质灾害(滑坡、坍塌、泥石流)事例,确定其影响区域。利用西南地区77个气象观测站逐日降水资料,根据降水阈值的定义和计算方法,运用线性回归法研究西南地区各灾害影响区的降水阈值。结果表明:我国西南地区是地质灾害多发的地区;1961~2010年西南地区重大滑坡、泥石流灾害具有密集成群、成片或成带的规律,有明显的稀疏区和密集区;且主要发生在6~9月,其中7月份发生最多,与降水的时空分布具有很好的一致性,降水是诱发地质灾害的主要因素。西南地区不同区域内诱发重大滑坡、泥石流灾害的降水阈值存在差异。西南地区的重大滑坡灾害主要是持续性强降雨型的。      

中国西南复杂地形区降水观测年际变化代表性问题初步分析

在中国西南地区,受到复杂地形、台站分布等因素的影响,台站的代表性问题较为突出,文章主要利用降水的年际变率来研究该地区降水资料的代表性问题。首先以大理国家气候观象台为例,定量评估单站的代表性区域,发现受周边南北走向高大山地的影响,大理站降水的代表性区域主要集中在以该站为中心南-北向的狭长带状范围内,面积大致5000 km~2。在对单站代表性有一定认识的基础上,结合台站资料及APHRODITE资料开展西南地区的站网覆盖度分析,结果表明台站的覆盖度在台站绝对密度较高及地形平坦的地区呈现高值,覆盖度可达40个站点以上,而在高原主体上,台站的覆盖度低,观测空白区占整个西南地区的15.90%,主要分布于高原主体北侧及西南侧。考虑到降水年际变率的空间差异对台站的代表性有重要影响,最后基于CMORPH卫星资料针对西南地区降水年际变率的空间差异进行研究,发现降水在盆地和高原河谷地区空间差异较小而山脉盆地的过渡区空间差异较大,建议在降水变率空间差异较大的观测空白区设立更多站点。     

西南地区春季降水时空变化特征

根据1961~2012年西南地区(四川、重庆、贵州、云南)116个气象台站逐月降水资料,利用REOF、Morlet小波分析、Mann-Kendall突变检验等方法分析了西南地区春季降水的年代际变化、空间分布特征、周期变化、突变年份等。结果表明,近52 a来,西南地区春季降水有明显的地域性,从20世纪60年代到21世纪初,几乎每个年代降水都呈现东西部反相趋势。据此把西南地区春季降水进行REOF分区,可分为4个区域:Ⅰ区(云南地区和四川西南部)和Ⅳ区(川西高原和攀西高原部分地区)春季降水呈增加趋势,Ⅱ区(贵州地区和四川东南部个别地区)和Ⅲ区(四川东北部和重庆地区)呈减少趋势。4个区域春季降水都是长周期中包含着短周期,24~28 a的周期和10 a以下的周期较为普遍。4个区域春季降水突变都普遍发生在1980年代后,降水变化显著时期基本集中在21世纪初期。     

我国西南地区东部夏季降水的时空特征

利用1959—2006年西南地区东部20个测站和中国740个测站的逐日降水量资料,采用EOF、Mann-Kendal、MESA(最大熵谱分析)、Morlet小波、线性相关等方法,分析了西南地区东部夏季降水的时空演变特征。结果表明:西南地区东部夏季降水量大体分为3种类型,即全区域一致型、南北相反型和东西相反型,其中全区域一致型是西南地区东部夏季降水最主要的分布型,出现频率达到60%以上。在整个时间域内,西南地区东部夏季降水略有增加的趋势;在20世纪60年代和70年代总体偏旱,而在80年代前期总体偏涝,80年代中后期开始波动加剧,旱涝交替发生,但总体相对偏涝;21世纪初以来,西南地区东部总体偏旱;1959年以来,1998,1980和1993年是降水偏多最明显的年份,而2006年和1972年降水偏少最明显。西南地区东部夏季降水的年际及年代际变化特征均较明显,存在2～3年、15年左右的显著周期。西南地区东部与宜宾以下的整个长江流域和西藏东部及川西高原的降水呈显著正相关,而与华南及东南沿海地区、华北中部和四川盆地西部地区呈显著负相关。     

数值模式预报性能的地域性特点初步分析

根据地理地貌,将全国分成东北、西北、东南、西南四个区域,对T639、T213、ECMWF、日本和德国数值预报模式2008年5—9月的预报分区域进行了客观检验分析。结果表明:模式的降水预报能力与地理的关系似乎比与地形的关系更为紧密一些;2008年,各模式在西南地区的降水预报能力随着降水级别的增加而减弱,到暴雨级别,西南区已成为各模式Ts评分最低区域;降水级别预报正确率高值区域在北方两区,正确率最低值出现在四川盆地;模式降水量预报平均误差显示,大误差并不出现在高原主体,而基本都与大的山脉对应,且都为负误差,但四川盆地除外,仍然是大误差所在地;2 m温度预报平均误差的量级是随海拔高度的增加而增加,误差由正转为负;各模式高空要素预报最大误差大都出现在西南区,其次是西北区。     

GRAPES_GFS在西南地区的预报稳定性 及其误差与地形的关系

根据地形特征,将西南地区划分为高原区、边坡区和盆地区,引入统计学"不稳定度"定量描述模式预报稳定性,对2016年6月—2017年9月全球中期天气预报(GRAPES_GFS)和欧洲中期天气预报中心(EC)在西南地区的高层形势场、主要的天气影响系统和地面要素预报性能进行了主客观检验,一定程度揭示了GRAPES_GFS和EC在西南地区的预报稳定性、地形的影响以及二模式预报性能的异同。结果显示:GRAPES_GFS高空高度场、温度场预报不稳定度分布呈北高南低型,相对湿度、风速预报不稳定度大值区在高原边缘;各要素预报不稳定度季节性周期最为显著,其位相和振幅因要素不同而有所不同;地形主要影响温度和风向预报误差值,但对相对湿度和风速预报的影响则体现在误差随时效的增长速率差异上;"漏报"是模式对西南地区天气系统的主要预报误差源,"低报"则是模式对西南地区2 m温度预报误差的最大来源;模式对西南地区降水落区预报有效率大约为50%,但强度预报通常偏低。EC与GRAPES_GFS的误差特征没有本质区别,但EC误差更小,稳定性更高。     

青藏高原东部和西南地区低温冰冻雨雪事件的时空变化特征

利用国家气象信息中心1961-2020年青藏高原东部和我国西南地区157个站点逐日最低气温和降水量数据,定义了一个低温冰冻雨雪事件的标准,采用Mann-Kendall突变检验、滑动t检验和EOF等方法分析了两个地区低温冰冻雨雪事件的气候特征和时空变化特征。结果表明：（1）青藏高原东部和西南地区低温冰冻雨雪发生的频次及其降水量的年变化的气候特征有明显的不同。青藏高原呈现出明显的双周期年变化,初春3月达最大值,秋季10月达第二峰值;西南地区则是U型的单周期年变化,在冬季1月最多。（2）高原上发生频次最多的区域在青海三江源和藏北怒江源地区,西南地区发生频次最多的区域在贵州的西部。（3）EOF分解得到的前3个模态表明不同季节低温冰冻雨雪频次的时空间变化有明显的不同。青藏高原春季和秋季第一空间分布模态呈全区一致变化,时间系数具有明显年代际振荡特征,春季在20世纪70年代末到2000年发生的频次较多,经历了“偏少-偏多-偏少”的演变过程;秋季在20世纪60～70年代低温冰冻雨雪频次偏多,在1981年前后发生年代际突变,之后发生的频次偏少;冬季第一空间分布模态以西南地区冬季变化为主,表明西南地区冬季...     

A cloud-resolving modeling study of short-term surface rainfall processes

The short-term tropical surface rainfall processes in rainfall regions (raining stratiform and convective regions) and rainfall-free regions (non-raining stratiform and clear-sky regions) are investigated based on the hourly data from a two-dimensional cloud-resolving model simulation. The model is integrated over a 21-day period with imposed zonally uniform vertical velocity, zonal wind, horizontal temperature and vapor advection, and sea surface temperature from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). The analysis of the model domain-mean surface rainfall budget reveals that surface rainfall is mainly associated with water vapor convergence and local atmospheric drying. The mean surface rainfall lags the mean water vapor convergence by 3?h. The convective?Cstratiform rainfall separation analysis shows that convective rainfall is associated with water vapor convergence, whereas stratiform rainfall is related to the local atmospheric drying and hydrometeor loss/convergence. The transport of water vapor from rainfall-free regions to rainfall regions creates the main water vapor source for rainfall while it balances local atmospheric drying in rainfall-free regions. Surface evaporation plays a minor role in short-term surface rainfall processes.     

A Hybrid Dynamical-Statistical Approach for Predicting Winter Precipitation over Eastern China

Correlation analysis revealed that winter precipitation in six regions of eastern China is closely related not only to preceding climate signals but also to synchronous atmospheric general circulation fields.It is therefore necessary to use a method that combines both dynamical and statistical predictions of winter precipitation over eastern China(herein after called the hybrid approach).In this connection,seasonal real-time prediction models for winter precipitation were established for the six regions.The models use both the preceding observations and synchronous numerical predictions through a multivariate linear regression analysis.To improve the prediction accuracy,the systematic error between the original regression model result and the corresponding observation was corrected.Cross-validation analysis and real-time prediction experiments indicate that the prediction models using the hybrid approach can reliably predict the trend,sign,and interannual variation of regionally averaged winter precipitation in the six regions of concern.Averaged over the six target regions,the anomaly correlation coefficient and the rate with the same sign of anomaly between the cross-validation analysis and observation during 1982-2008 are 0.69 and 78%,respectively.This indicates that the hybrid prediction approach adopted in this study is applicable in operational practice.     

ON THE KEY REGIONS OF 500 hPa GEOPOTENTIAL HEIGHTS OVER NORTHERN HEMISPHERE IN WINTER

Variance analysis, correlation analysis and regression analysis methods are applied to analyze the variation of circulation at 500 hPa. In winter, there are three regions (180°E – 150°W, 45°N – 60°N, 70°W – 100 °W，45°N – 75°N, 60°E – 100°E, 65°N – 80°N) whose variations are strong. Those regions are the key regions in which atmospheric circulation can change. Those regions are correlated to some teleconnections and can present a part of variations of 500 hPa to some degree. The linear contemporary correlation between those regions and the height at 500 hPa is significant. Those regions can account for 88 % of variations of concurrent height at 500 hPa. Those regions can present and forecast some variations to some degree in March and April. The longer the time interval, the worse the forecast effect will be. The interannual variations of Q1, Q2 and the SST are weak in the western Pacific.     

中国东部冬季降水的动力结合统计预测方法研究

针对中国东部6个气候关键区,首先,通过相关分析指出,冬季降水既与前期气候因子有关,又受同期大气环流的影响。因此,有必要采用动力与统计相结合的方法进行气候预测研究。然后,从实时预测的角度出发,综合考虑前期预测因子的观测信息和具有数值可预测性的同期气候因子的数值模式结果,使用多元线性回归分析方法就各区域平均冬季降水逐一建立了短期气候预测模型,并在预测模型中考虑了模型结果中系统误差的订正。交叉检验分析结果表明,所建立的各区域预测模型普遍具有较好的预测效果,预测优势主要表现在对冬季降水的变化趋势、年际变化、以及异常符号的预测准确率上。就6个区域平均而言,1982—2008年交叉检验结果与实况间的相关系数和距平同号率分别为0.69和78%,表明该预测思想具有可行性。     

热带气旋的内、外区结构──Ⅰ.正压模式

将热带气旋分为内外两区，两区的空间、时间及物理量有不同的尺度，应用尺度分析和摄动法到热带气旋的两个区域，分别在正压模式和斜压模式中求得了这两个区域的控制方程，在两个模式中，还解析地求得了两个区域的流场和气压场，得到的结果与实际热带气旋的结构较为接近。研究表明，热带气旋内区和外区有不同的控制方程，内区受旋转风和一个演化方程制约，而外区受梯度风和另一个演化方程制约。     

A climatological perspective of water vapor at the UTLS region over different global monsoon regions: observations inferred from the Aura-MLS and reanalysis data

The Aura-MLS observations of eight years from 2004 to 2011 have been utilized to understand the hydration and the dehydration mechanism over the northern and the southern hemispheric monsoon (NH and SH) regions. The monsoon regions considered are the Asian Summer Monsoon, East Asian Summer Monsoon, Arizona Monsoon (AM), North African Monsoon, South American Monsoon and the Australian Monsoon. The annual cycle of water vapor as expected shows maxima over the NH during June–August and during December–February over the SH. The time taken by the air parcels over the NH monsoon regions is found to be different compared to that over the SH monsoon regions. The analysis shows the concentration of water vapor in the upper troposphere and the lower stratosphere (UTLS) has not changed over these eight years in both the hemispheres during their respective monsoon seasons. The present analysis show different processes viz., direct overshooting convection, horizontal advection, temperature and cirrus clouds in influencing the distribution of water vapor to the UTLS over these different monsoon regions. Analysis of the UTLS water vapor with temperature and ice water content shows that the AM is hydrating the stratosphere compared to all the other monsoon regions where the water vapor is getting dehydrated. Thus it is envisaged that the present results will have important implications in understanding the exchange processes across the tropopause over the different monsoon regions and its role in stratosphere chemistry.     

An Analysis of Thermally-Related Surface Rainfall Budgets Associated with Convective and Stratiform Rainfall

Both water vapor and heat processes play key roles in producing surface rainfall.While the water vapor effects of sea surface temperature and cloud radiative and microphysical processes on surface rainfall have been investigated in previous studies,the thermal effects on rainfall are analyzed in this study using a series of two-dimensional equilibrium cloud-resolving model experiments forced by zonally-uniform,constant,large-scale zonal wind and zero large-scale vertical velocity.The analysis of thermally-related surface rainfall budget reveals that the model domain mean surface rain rate is primarily associated with the mean infrared cooling rate.Convective rainfall and transport of hydrometeor concentration from convective regions to raining stratiform regions corresponds to the heat divergence over convective regions,whereas stratiform rainfall corresponds to the transport of hydrometeor concentration from convective regions and heat divergence over raining stratiform regions.The heat divergence over convective regions is mainly balanced by the heat convergence over rainfall-free regions,which is,in turn,offset by the radiative cooling over rainfall-free regions.The sensitivity experiments of rainfall to the effects of sea surface temperature and cloud radiative and microphysical processes show that the sea surface temperature and cloud processes affect convective rainfall through the changes in infrared cooling rate over rainfall-free regions and transport rate of heat from convective regions to rainfall-free regions.     

DOMINANT PHYSICAL PROCESSES ASSOCIATED WITH PHASE DIFFERENCES BETWEEN SURFACE RAINFALL AND CONVECTIVE AVAILABLE POTENTIAL ENERGY

A lag correlation analysis is conducted with a 21-day TOGA COARE cloud-resolving model simulation data to identify the phase relation between surface rainfall and convective available potential energy (CAPE) and associated physical processes. The analysis shows that the maximum negative lag correlations between the model domain mean CAPE and rainfall occurs around lag hour 6. The minimum mean CAPE lags mean and convective rainfall through the vapor condensation and depositions, water vapor convergence, and heat divergence whereas it lags stratiform rainfall via the transport of hydrometeor concentration from convective regions to raining stratiform regions, vapor condensation and depositions, water vapor storage, and heat divergence over raining stratiform regions.