一个创新研究——大气数值模式变量的物理分解及其在极端事件预报中的应用

大气温度、湿度、位势高度和风等数值模式变量可以物理分解为纬圈-时间平均的对称部分和时间平均的非对称部分,以及行星尺度瞬变扰动和天气尺度瞬变扰动等四个部分.区域持续性干旱、暴雨、热浪、低温和雨雪冰冻等极端天气事件与前期及同期数值模式中的行星尺度和天气尺度大气扰动系统之间呈现出密切的关系.瞬变扰动天气图可成为预报极端天气事件的新工具.本文在归纳本期9篇原创性文章的基础上,探讨大气变量物理分解后需要进一步研究的理论问题和应用前景.     

北半球大气遥相关型与区域尺度大气扰动

北极涛动(AO)、北大西洋涛动(NAO)和太平洋-北美型(PNA)等北半球大气遥相关型,可以用大气位势高度的物理分解扰动分量解释.结果发现,AO反映的是北极地区行星尺度纬圈平均扰动分量的变化,PNA与持续性天气尺度扰动分量相联系,NAO是行星尺度纬圈平均扰动与天气尺度扰动共同作用的结果.对行星尺度纬圈平均扰动分量和天气尺度扰动分量用旋转经验正交函数(REOF)展开,不但可以证实人们已经命名的区域性大气涛动,还新发现了北极地区的两对偶极涛动、欧亚涛动(EAO)和"大西洋-欧亚型"(AEA)波列.这些涛动连接了相邻地区的异常天气和异常气候.     

西南区域持续性干旱事件的行星尺度和天气尺度扰动信号

以2009-2010年发生在中国西南地区的持续性干旱事件为例,通过干旱和大气变量的物理分解得到了一些干旱事件发生的新认识.气象干旱多为年循环的气候干季与干旱扰动的叠加所致.一次干旱扰动大约为30-50天,而一次持续性干旱事件是由几次干旱扰动组成的.大气高度场和风场中存在三种时间尺度的扰动.一种是年际行星尺度的大气扰动,与ENSO冷暖事件有关,起源于赤道并传播到中高纬度地区需要2-4年.另一种是季节内行星尺度的大气扰动,与来自赤道地区的30-50天振荡有关.此外,大气中还存在天气尺度的扰动.利用行星尺度大气扰动向赤道外传播与天气尺度扰动的叠加,区域持续性干旱事件能够找到前期预报信号.     

如何提高天气预报和气候预测的技巧？

从理论上探讨如何提高天气预报和气候预测的技巧.气候包括以小时为基本单位的昼夜循环、以日为基本单位的年(季节)循环、年代际循环和世纪循环等时间尺度的变化.这些气候变化存在确定的外强迫,是可以被认识和预报的.相对气候昼夜循环和年(季节)循环的偏差是天气尺度扰动.天气尺度的瞬变大气扰动可引发极端天气事件.有技巧的天气预报正是要通过天气尺度大气扰动信号,提前几天甚至十几天,预报出极端天气事件的发生.相对气候年代际和世纪循环的偏差是气候异常,有技巧的气候预测正是要预报出这种异常.距平天气图会大大提高短期和中期—延伸期天气预报的技巧,距平数值预报模式的研制也会加快提高中期—延伸期天气预报和气候预测的技巧.     

中国热浪事件的大气扰动结构及其稳定性分析

利用对流层至平流层大气变量的物理分解,本文发现400 hPa上的天气尺度温度扰动正值和250 hPa上的天气尺度高度扰动正值能够指示地面上的高温和热浪事件.理论和观测分析发现,当扰动静止波中心出现在40°N-45°N时,准静止波波长较长,影响范围大,持续时间久,容易在中国形成区域持续性热浪事件.在江南-华南、长江沿江及河套-华北的区域,干热浪事件静止波的稳定时间在6.5天以上.对流层大气天气尺度扰动波动在垂直方向具有倾斜特征,250 hPa高度上,静止波稳定位置比地面热浪区域中心普遍偏北6~12个纬距.在低纬度(副热带高压带南侧),大气中向西北方向移动的天气尺度扰动,只有到达副高脊线以北的西风带后,才能形成静止波.     

天气尺度扰动流场对区域暴雨的指示能力

观测的大气流场可以物理分解为气候流场、行星尺度纬圈平均扰动流场和天气尺度扰动流场.低层大气的天气尺度扰动流场中的切变线、南方气旋、北方气旋、冷锋槽线、西南涡暖切变线、热带气旋、倒槽切变线、东风波切变线等对区域暴雨具有指示意义.扰动流场中的环流系统更适合天气分析的原理.中国的暴雨带多为扰动辐合线两侧气流对峙的结果.观测流场中,暴雨带出现在低空急流的左侧,是因为气候流场掩盖或削弱了天气尺度扰动流场的作用.对切变线暴雨,天气尺度扰动流场有99%的诊断能力,而原始流场只有66%的指示能力.     

南方持续低温冻雨事件预测的前期信号

1960-2008年冬季期间,中国南方发生了23次低温冰冻(冻雨)天气事件,其中满足站日数大于10的事件有11次.2008年初中国南方发生了一场影响巨大的区域持续性低温冻雨天气事件,2011年初再次发生了类似的区域持续性低温冻雨事件.提前5天预报这类极端事件是国内外大气科学面临的难题.利用去逐日气候变化后的逐日850 hPa温度扰动,可以提前3~10天发现中国南方持续低温雨雪冰冻(冻雨)事件发生的信号.2008年初和2011初,影响中国南方的850 hPa冷空气扰动具有源地和路径相似性,它们都来自北非-中东并绕过青藏高原北侧到达中国南方,在对流层大气中形成"冷-暖-冷"的温度垂直结构.通过对欧洲中期天气预报模式产品中850 hPa温度扰动的相似性分析,成功地提前4~9天实际预报出了2011年初的中国南方低温冻雨天气过程.     

斜压基本气流对东亚夏季风区气旋扰动低频发展影响的数值模拟研究

本文将夏季气候平均的基本气流分解为正压和斜压分量,使用一个线性斜压模式,研究了不同斜压基本气流对热带西北太平洋地区初始气旋性环流扰动低频发展演变的重要作用.其中,控制试验较好地模拟出初始气旋扰动向西北方向传播、在西北太平洋季风槽附近停滞增强、在东亚地区出现经向波列和在南海到海洋大陆地区形成西北—东南向波列等特征.改变斜压分量的敏感性试验结果表明,正压基流不能为西传的初始扰动供给足够的能量;海陆热力差异引起东亚地区的纬向温度梯度和北风垂直切变,是东亚太平洋型经向波列形成和维持的重要因素;当基本气流中的斜压纬向偏差部分线性增大时,扰动的能量会呈e指数迅速增强,提示在气候变化的背景下,基本气流微小的改变可能带来天气或季节内扰动强度的剧烈响应.     

伴随冬季北太平洋副热带海洋锋强度变化的大气扰动异常及对中纬度大气平均场的影响

中纬度海洋热力状况异常影响大气主要通过两种途径:非绝热加热的直接强迫作用和大气瞬变涡旋反馈的间接强迫作用,而后者的作用并没有被很好地认识.为了进一步理解间接强迫作用的物理机制,本文利用观测资料分析和区域大气模式模拟,研究了伴随冬季北太平洋副热带海洋锋强度变化的中纬度大气场异常,特别是对流层中高层不同频率的涡旋扰动活动的异常.实际观测和数值试验结果均表明,当北太平洋副热带海表面温度锋偏强时,其上空的中纬度大气经向温度梯度增强;对应此时的大气斜压性增强,且中纬度大气西风急流整层加速;然而增强的大气斜压性并不对应一致性增强的大气涡旋扰动活动.中纬度大气的涡旋扰动根据其生命周期,进一步划分为高频(2~7天)和低频(10~90天)涡旋扰动.研究结果表明偏强的北太平洋副热带海洋锋对应着增强的中纬度大气高频涡旋扰动和减弱的低频涡旋扰动;其中,中纬度大气高频扰动活动的增强,将有利于削弱中纬度大气经向温度梯度,从而减弱中纬度大气斜压性;而高频扰动对纬向风倾向项的正贡献,有利于中纬度急流中心北侧及下游区域的西风加速,形成中纬度西风相当正压结构的增强;大气低频扰动的减弱,对中纬度大气纬向风倾向项产生负贡献,不利于急流的纬向均匀化,而其热力强迫异常则有利于维持中纬度对流层中层大气的经向温度梯度.     

利用TRMM降水资料对华南和长江流域夏季天气尺度波的对比分析

利用1998~2003年共6 a的TRMM 3B42降水资料, 采用一种新的三维波谱分析方法, 对中国华南地区和长江流域夏季的天气尺度波进行了对比分析. 结果表明: 华南地区存在明显的向西传播的天气尺度波, 波数主要集中在9~21, 频率范围为: -0.22~-0.12 d^-1; 而长江流域主要是向东传播的天气尺度波, 其波数也集中在9~21, 频率范围为: 0.20~0.29 d^-1. 通过对这两种天气尺度波波能的经向和纬向分布、波动随季节的强弱变化的对比分析得出, 夏季华南地区天气尺度波是由西太平洋热带辐合带波动的北移引起的, 而长江流域的天气尺度波则是由西风气流中高原小槽东移所造成的.     

2008年和2012年冬季欧洲气候的差异及成因

2008年冬季（1月和2月）和2012年冬季均发生了较强的拉尼娜事件,但欧洲气候,尤其是西欧在这两年差异较大,2008年异常偏暖,而2012年却出现了极寒事件.诊断表明,大气环流异常是造成气候差异的直接原因.2008年冬季,北大西洋上空大气环流异常呈正位相的北大西洋涛动,有利于欧洲异常偏暖;2012年冬季,北大西洋和欧亚高纬阻塞的长期维持是西欧发生极端严寒的重要原因.通过数值试验,研究了前期海表热状况异常对大气的影响.结果表明：北大西洋海温异常能在一定程度上解释这两年欧洲各自的气候异常;尽管热带海温异常对2012年冬季的北大西洋环流形势和欧洲气候异常起一定的贡献,但不能解释2008年的情形;靠近欧洲的北极海冰异常偏少使得欧洲气候偏冷,对2008年的偏暖气候贡献为负,对2012年则有正贡献.     

Climate Change Projections Based on GCM-Simulated Daily Data

Three-dimensional general circulation models (GCMs) are 'state-of-the-art' tools for projecting possible changes in climate. Scenarios constructed for the Czech Republic are based on daily outputs of the ECHAM-GCM in the central European region. Essential findings, derived from validating, procedures are summarized and changes in variables between the control and perturbed experiments are examined. The resulting findings have been used in selecting the most proper methods of generating climate change projections for assessing possible hydrological and agricultural impacts of climate change in selected exposure units. The following weather variables have been studied: Daily extreme temperatures, daily mean temperature, daily sum of global solar radiation, and daily precipitation amounts. Due to some discrepancies revealed, the temperature series for changed climate conditions (2×CO ₂ ) have been created with the help of temperature differences between the control and perturbed runs, and the precipitation series have been derived from an incremental scenario based on an intercomparison of the GCMs' precipitation performance in the region. Solar radiation simulated by the ECHAM was not available and, therefore, it was generated using regression techniques relating monthly means of daily extreme temperatures and global radiation sums. The scenarios published in the paper consist of monthly means of all temperatures, their standard deviations, and monthly means of solar radiation and precipitation amounts. Daily weather series, the necessary input to impact models, are created (i) by the additive or multiplicative modification of observed weather daily series or (ii) by generating synthetic time series with the help of a weather generator whose parameters have been modified in accord with the suggested climate change scenarios.     

Assessment on variability of extreme climate events for the Upper Thames River basin in Canada

Climate change may affect magnitude and frequency of regional extreme events with possibility of serious impacts on the existing infrastructure systems. This study investigates how the current spatial and temporal variations of extreme events are affected by climate change in the Upper Thames River basin, Ontario, Canada. A weather generator model is implemented to obtain daily time series of three climate variables for two future climate scenarios. The daily time series are disaggregated into hourly to capture characteristics of intense and rapidly changing storms. The maximum annual precipitation events for five short durations, 6‐, 12‐, 24‐, 48‐, and 72‐h durations, at each station are extracted from the generated hourly data. The frequency and seasonality analyses are conducted to investigate the temporal and spatial variability of extreme precipitation events corresponding to each duration. In addition, this study investigates the impacts of increase in temperature using reliability, resilience, and vulnerability. The results indicate that the extreme precipitation events under climate change will occur earlier than in the past. In addition, episodes of extremely high temperature may last longer up to 19·7% than under the no‐change climate scenario. This study points out that the revision of the design storms (e.g. 100‐ or 250‐year return period) is warranted for the west and the south east region of the basin. Copyright © 2011 John Wiley & Sons, Ltd.     

IMAGE: a multivariate multi-site stochastic weather generator for European weather and climate

Capturing the spatial and temporal correlation of multiple variables in a weather generator is challenging. A new massively multi-site, multivariate daily stochastic weather generator called IMAGE is presented here. It models temperature and precipitation variables as latent Gaussian variables with temporal behaviour governed by an auto-regressive model whose residuals and parameters are correlated through resampling of principle component time series of empirical orthogonal function modes. A case study using European climate data demonstrates the model’s ability to reproduce extreme events of temperature and precipitation. The ability to capture the spatial and temporal extent of extremes using a modified Climate Extremes Index is demonstrated. Importantly, the model generates events covering not observed temporal and spatial scales giving new insights for risk management purposes.     

Trends and anomalies in sea-surface temperature,observed over the last 60 years,within the southeastern Bay of Biscay

The Oceanographic Society of Gipuzkoa has recorded daily sea-surface temperature (SST) measurements, since 2nd July 1946, on a (nearly) daily basis. Sixty years of SST measurements (1947–2007) have been considered, in order to analyse the hydrographic trends and anomalies at the southeastern Bay of Biscay. The study reviews initially the consistency and reliability of the time-series; and trends and anomaly patterns. Then, the periodicity of the series; a reference period, for analysing seasonality during the period 2001–2007; and oceano-meteorological coupling within the period 2001–2007, with reference to the baseline period, have been determined. Within this context, a slight cooling trend has been observed for the whole of the time-series, in contrast to the warming over the last three-decadal period. Regarding the periodicity of the series, several cycles have been identified, with periods of about 8, 11 and 18 years; these represent the influence of climate cycles over the (local) SST series. Additionally, seasonal anomaly patterns between 2001 and 2007 have been examined based upon the selected reference period (1980–2002). Several extreme seasonal events have been observed, such as warm summer SST values in 2003 and 2006 and cold winter values in 2005. Such events can be explained by the “deseasonality” phenomenon, observed throughout the study period. In addition to the direct influence of atmospherical parameters, such as air temperature and irradiance on SST, dynamical variables (turbulence and upwelling–downwelling) account also for this coupling. Overall, despite the marginal location and surface character of the time-series, it reveals anomalies that agree with those described for larger zones of the northeastern Atlantic Ocean; hence, it can be characterised as being a reliable and representative long-term SST series.     

Anomalies in the US precipitation extremes and their association with different modes of climate variability

ABSTRACT

In this study, we investigate the temporal oscillations of precipitation extremes in different climate regions of the United States. We apply quantile perturbation analysis to average daily precipitation and, to 1041 weather stations with high-quality data from 1900 to 2016. Moreover, we explore the relationship between the extreme precipitation and different well-known cyclical climate modes. Overall, the analysis of average daily precipitation identifies a drier condition in the middle decades of the twentieth century and, a wetter climate in the early century and recent decades. Moreover, the in situ analysis reveals a significant anomaly, mainly prevalent in the Central and Southern regions of the United States. We applied a finite set of linear regression models with different combinations of cyclical climate modes to inform the variability of anomalies with best performing models. Our results highlight the dominant effect of ENSO and NAO in the wide area of the United States.