A Case Study of the Error Growth and Predictability of a Meiyu Frontal Heavy Precipitation Event

The Advanced Regional Eta-coordinate Model (AREM) is used to explore the predictability of a heavy rainfall event along the Meiyu front in China during 3-4 July 2003.Based on the sensitivity of precipitation prediction to initial data sources and initial uncertainties in different variables,the evolution of error growth and the associated mechanism are described and discussed in detail in this paper.The results indicate that the smaller-amplitude initial error presents a faster growth rate and its growth is characterized by a transition from localized growth to widespread expansion error.Such modality of the error growth is closely related to the evolvement of the precipitation episode,and consequcntly remarkable forecast divergence is found near the rainband,indicating that the rainfall area is a sensitive region for error growth.The initial error in the rainband contributes significantly to the forecast divergence,and its amplification and propagation are largely determined by the initial moisture distribution.The moisture condition also affects the error growth on smaller scales and the subsequent upscale error cascade.In addition,the error growth defined by an energy norm reveals that large error energy collocates well with the strong latent heating,implying that the occurrence of precipitation and error growth share the same energy source-the latent heat.This may impose an intrinsic predictability limit on the prediction of heavy precipitation.     

云可分辨模式中误差的增长和传播对定量降水可预报性的影响

Limitations in the predictability of quantitative precipitation forecasting （QPF） that arise from initial errors of small amplitude and scale are investigated by means of real-case high-resolution （cloud-resolving） numerical weather prediction （NWP） integrations. The case considered is the hail and wind disaster that occurred in Sichuan on 8 April 2005. A total of three distinct perturbation methods are used. The results suggest that a tiny initial error in the temperature field can amplify and influence the weather in a large domain, changing the 12-h forecasted rainfall by as much as one-third of the original magnitude. Furthermore, the comparison of the perturbation methods indicates that all of the methods pinpoint the same region （the heavy rainfall areas in the control experiment） as suffering from limitations in predictability. This result reveals the important role of nonlinearity in severe convective events.     

Analysis Study on Perturbation Energy and Predictability of Heavy Precipitation in South China

The AREMv2.3 mesoscale numerical model is used to explore storm processes in South China during the pre-rainy season in 2006 by imposing perturbations on the initial fields of physical variables (temperature, humidity, and wind fields). Sensitivity experiments are performed to examine the impacts of initial uncertainties on precipitation, on the error growth, and on the predictability of mesoscale precipitation in South China. The primary conclusion is that inherent initial condition uncertainties can signi...     

华南暖区暴雨预报失误及可预报性探讨

由于暖区暴雨产生的环境条件复杂和触发机制难以捕捉,数值模式对其预报能力弱,给预报带来困难,经常导致预报失误,是短期预报中的难点。2016年4月19—20日广西出现了一场暖区暴雨天气过程,预报员及数值模式预报出现较大失误。利用业务预报中的数值预报产品、地面中尺度自动气象站观测、常规地面及高空观测、新一代天气雷达及FY-2G卫星探测等资料对此次暖区暴雨预报失误进行剖析。结果表明:中低空急流增强及西南暖低压发展,为越南北部至广西中南部提供了高温、高湿、高能的环境条件,地形性辐合及涡旋触发了对流的发生,中尺度辐合线有效组织了对流的发展,雷达回波具有质心低、降雨效率高等暖云降雨的特征。预报员和数值模式短期时效内对暖区暴雨缺乏预报能力,未能准确把握可触发对流的机制,是预报失败的原因。预报员通过分析上游地区对流云团、地面中尺度辐合线演变及地形作用等触发条件,可以在短时临近时效内对暖区暴雨部分做出定性预报,发布预警信息,弥补短期预报的不足。因此,加强对暖区暴雨形成机理的认识,在预报中做好精细分析,是提高暖区暴雨预报能力的有效途径之一。     

Relationships between the Limit of Predictability and Initial Error in the Uncoupled and Coupled Lorenz Models

In this study, the relationship between the limit of predictability and initial error was investigated using two simple chaotic systems:the Lorenz model, which possesses a single characteristic time scale, and the coupled Lorenz model, which possesses two different characteristic time scales. The limit of predictability is defined here as the time at which the error reaches 95% of its saturation level; nonlinear behaviors of the error growth are therefore involved in the definition of the limit of predictability. Our results show that the logarithmic function performs well in describing the relationship between the limit of predictability and initial error in both models, although the coefficients in the logarithmic function were not constant across the examined range of initial errors. Compared with the Lorenz model, in the coupled Lorenz model-in which the slow dynamics and the fast dynamics interact with each other-there is a more complex relationship between the limit of predictability and initial error. The limit of predictability of the Lorenz model is unbounded as the initial error becomes infinitesimally small; therefore, the limit of predictability of the Lorenz model may be extended by reducing the amplitude of the initial error. In contrast, if there exists a fixed initial error in the fast dynamics of the coupled Lorenz model, the slow dynamics has an intrinsic finite limit of predictability that cannot be extended by reducing the amplitude of the initial error in the slow dynamics, and vice versa. The findings reported here reveal the possible existence of an intrinsic finite limit of predictability in a coupled system that possesses many scales of time or motion.     

Optimal Initial Error Growth in the Prediction of the Kuroshio Large Meander Based on a High-resolution Regional Ocean Model

Based on the high-resolution Regional Ocean Modeling System(ROMS) and the conditional nonlinear optimal perturbation(CNOP) method, this study explored the effects of optimal initial errors on the prediction of the Kuroshio large meander(LM) path, and the growth mechanism of optimal initial errors was revealed. For each LM event, two types of initial error(denoted as CNOP1 and CNOP2) were obtained. Their large amplitudes were found located mainly in the upper 2500 m in the upstream region of the LM, i.e., southeast of Kyushu. Furthermore, we analyzed the patterns and nonlinear evolution of the two types of CNOP. We found CNOP1 tends to strengthen the LM path through southwestward extension. Conversely,CNOP2 has almost the opposite pattern to CNOP1, and it tends to weaken the LM path through northeastward contraction.The growth mechanism of optimal initial errors was clarified through eddy-energetics analysis. The results indicated that energy from the background field is transferred to the error field because of barotropic and baroclinic instabilities. Thus, it is inferred that both barotropic and baroclinic processes play important roles in the growth of CNOP-type optimal initial errors.     

Comparisons of Two Ensemble Mean Methods in Measuring the Average Error Growth and the Predictability

In this paper, taking the Lorenz system as an example, we compare the influences of the arithmetic mean and the geometric mean on measuring the global and local average error growth. The results show that the geometric mean error (GME) has a smoother growth than the arithmetic mean error (AME) for the global average error growth, and the GME is directly related to the maximal Lyapunov exponent, but the AME is not, as already noted by Krishnamurthy in 1993. Besides these, the GME is shown to be more appropriate than the AME in measuring the mean error growth in terms of the probability distribution of errors. The physical meanings of the saturation levels of the AME and the GME are also shown to be different. However, there is no obvious difference between the local average error growth with the arithmetic mean and the geometric mean, indicating that the choices of the AME or the GME have no influence on the measure of local average predictability.     

太平洋海温变化对我国降水可预报性影响的分析

通过讨论我国160个测站的月降水与太平洋海温的关系，研究了太平洋海温对我国月降水可预报性影响的时空分布特征，探讨了利用太平洋海表面温度作我国月降水中长期预报的可行性和局限性。结果表明，海温对月降水的影响存在明显的时空分布特征：从时间上看，利用海温作降水预报在4月和11月全国平均效果较好；从空间上看，海温对降水的影响存在遥相关关系，其贡献在西北地区大于东部地区。     

Progress in the Study of Nonlinear Atmospheric Dynamics and Predictability of Weather and Climate in China (2007--2011)

Recent progress in the study of nonlinear atmospheric dynamics and related predictability of weather and climate in China (2007-2011) are briefly introduced in this article. Major achievements in the study of nonlinear atmospheric dynamics have been classified into two types:(1) progress based on the analysis of solutions of simplified control equations, such as the dynamics of NAO, the optimal precursors for blocking onset, and the behavior of nonlinear waves, and (2) progress based on data analyses, such as the nonlinear analyses of fluctuations and recording-breaking temperature events, the long-range correlation of extreme events, and new methods of detecting abrupt dynamical change. Major achievements in the study of predictability include the following:(1) the application of nonlinear local Lyapunov exponents (NLLE) to weather and climate predictability; (2) the application of condition nonlinear optimal perturbation (CNOP) to the studies of El Nin o-Southern Oscillation (ENSO) predictions, ensemble forecasting, targeted observation, and sensitivity analysis of the ecosystem; and (3) new strategies proposed for predictability studies. The results of these studies have provided greater understanding of the dynamics and nonlinear mechanisms of atmospheric motion, and they represent new ideas for developing numerical models and improving the forecast skill of weather and climate events.     

全球气候背景下厄尔尼诺对中国东部汛期降水的影响

对厄尔尼诺事件进行了定义，并详细讨论了不同气候背景下的厄尔尼诺过程对中国东部地区汛期降水的影响。分析发现：20世纪50年代以来共发生了14次厄尔尼诺事件；在70年代末、80年代初，赤道太平洋的海温有一个明显升高的趋势；不同气候背景下的厄尔尼诺过程中对中国东部汛期降水存在不同的影响，但是对长江中下游及江淮地区降水的影响是一致的，一般偏少。     

夏季热带中太平洋SST异常型与中国东部夏季气候异常的关系

热带中太平洋海温异常型是近年发现的赤道太平洋海温异常分布的新特征,主要表现在热带太平洋海温异常为纬向＂三极型＂分布,其与ENSO的＂偶极型＂分布显著不同。利用1955—2005年HadISST月平均海温资料和中国160站月降水和温度资料,确定了中太平洋海温异常型并研究了这种海温异常型与中国东部同期夏季降水的关系。结果表明,当中太平洋海温为正异常时,我国江淮流域、西南地区的东部整层水汽含量偏少并处于水汽通量辐散区,不利于降水的形成,同时该地区上空垂直温度平流为正异常,利于异常高温的形成;我国华南、西南地区的南部为整层水汽通量辐合区,水汽丰沛,且整层垂直温度平流为负异常,易发生低温多雨的天气气候。     

热带大气季节内振荡对西南地区东部夏季降水的影响及其可能机制

利用1979~2013年6~8月的西南地区东部20个台站日降水量资料、逐日MJO(Madden-Julian Oscillation)指数、全球OLR(Outgoing Longwave Radiation)逐日格点资料以及NCEP/NCAR再分析日资料,采用合成分析和线性回归等方法,对夏季MJO不同位相活动影响西南地区东部夏季降水的原因及其可能机制进行了初步分析。研究表明,MJO与西南地区东部夏季降水之间存在着显著的关系,当MJO处于第4(第6)位相时,由于西太平洋副高位置偏南(偏北)、向西南地区东部的水汽输送偏多(偏少),在异常上升(下沉)气流影响下,西南地区东部夏季降水偏多(偏少)。MJO影响西南地区东部夏季降水的可能原因是:当MJO处于第4位相时,赤道东印度洋地区上空大气释放凝结潜热,其激发东北向传播的异常波动,进而影响东亚环流,使得西南地区东部出现夏季降水偏多的环流形势,西南地区东部夏季降水增多;但在第6位相时,西太平洋地区上空对流释放的凝结潜热,其激发PJ(太平洋-日本)型Rossby波列,出现不利于西南地区东部夏季降水的环流形势,西南地区东部夏季降水偏少。     

东亚夏季风强弱年物理量的逐候变化及其对松嫩流域降水的影响

用施能定义的东亚夏季风指数,分别找出典型夏季风强弱年1961和1980年,分析水平风速、假相当位温、高度等的逐候变化特征与盛夏季节松嫩流域逐候降水量的关系。分析表明:东亚夏季风强弱对松嫩流域盛夏降水量有显著影响。在强夏季风年,较大的南风、较高的假相当位温、较深的东亚大槽使降水持续偏多。同时还发现,无论夏季风强弱,假相当位温高值总是与多降水相联系。     

基于CCSM4.0长期积分试验评估不同辐射强迫对中国干旱半干旱区降水的影响

本文基于气候系统模式CESM4.0长期积分试验,分析评估了工业革命前（1850年）及当前（2000年）两种辐射强迫作用（分别为太阳活动和温室气体）对中国北方干旱半干旱区降水的影响。结果表明,模拟结果与观测之间尽管存在一定的偏差,但仍能再现降水气候态的空间分布以及季节变化特征;两种辐射强迫下的降水长期变化均无明显趋势,但二者的差异却呈现出70~100年的准周期振荡;由人类活动引起的当前辐射强迫作用对降水的多年际变率幅度有一定影响,造成极端强降水事件出现的概率增多,而由太阳活动引起的辐射强迫作用主要对降水多年代际周期具有一定的调制作用。进一步分析表明,两种辐射强迫下中国北方干旱半干旱区降水年多年代际变率的主要模态基本一致,但人类活动引起的辐射强迫作用会影响降水多年代际变率与热带海温异常的相互作用的强度,从而改变降水多年代际变率的幅度。     

黑碳气溶胶直接辐射强迫及其对中国夏季降水影响的模拟研究

利用NCAR的全球大气模式CAM3分析了黑碳气溶胶在大气顶和地表的直接辐射强迫分布及其季节变化,重点讨论了云对黑碳气溶胶直接辐射强迫的影响,以及黑碳气溶胶对中国夏季降水的影响。结果表明：黑碳气溶胶在大气顶和地表的直接辐射强迫分布范围和强度都具有明显的季节变化。有云条件下,黑碳气溶胶在大气顶产生正的直接辐射强迫,全球年平均强迫值为＋0.33 W·m－2;在地表产生负的直接辐射强迫,全球年平均强迫值为－0.56 W·m－2。晴空条件下,黑碳气溶胶在大气顶和地表的全球年平均辐射强迫值分别为＋0.21 和－0.71 W·m－2。云的存在对黑碳气溶胶的辐射强迫产生了很大的影响,使大气顶的正辐射强迫增加,地表的负辐射强迫减小。黑碳气溶胶导致夏季中国北方30°N～45°N之间区域降水明显增加;而中国长江以南地区除了海南和广西的部分城市外,降水明显减少。模拟结果表明,中国夏季近50年来经常发生的南涝北旱并非由黑碳气溶胶引起。     

中国区域降雪量和降雪频率对沙尘天气的影响作用分析

1970—2000年间气象台站降雪量和沙尘天气统计结果显示,在中国冬春季主要积雪覆盖区域,沙尘天气发生频次相对较低,各类沙尘天气基本发生在积雪覆盖率低、年降雪量少的区域。时间序列分析结果进一步显示,年降雪量和沙尘天气之间存在显著的负相关,降雪量的增多对沙尘天气的年发生次数具有明显的抑制作用。同时,年降雪频率也是影响沙尘天气爆发频次的重要因素之一。对于中国西北干旱少雪的地区,尤其体现在新疆北部地区,年降雪频率的增加能够显著地减少各类沙尘天气的发生次数。     

Modeling Study of the Impact of Heterogeneous Reactions on Dust Surfaces on Aerosol Optical Depth and Direct Radiative Forcing over East Asia in Springtime

The spatial distributions and interannual variations of aerosol concentrations,aerosol optical depth(AOD) ,aerosol direct radiative forcings,and their responses to heterogeneous reactions on dust surfaces over East Asia in March 2006-10 were investigated by utilizing a regional coupled climate-chemistry/aerosol model. Anthropogenic aerosol concentrations(inorganic+carbonaceous) were higher in March 2006 and 2008,whereas soil dust reached its highest levels in March 2006 and 2010,resulting in stronger aerosol radiative forcings in these periods.The domain and five-year(2006-10) monthly mean concentrations of anthropogenic and dust aerosols,AOD,and radiative forcings at the surface(SURF) and at the top of the atmosphere(TOA) in March were 2.4μg m-3,13.1μg m-3,0.18,-19.0 W m-2,and-7.4 W m-2,respectively.Heterogeneous reactions led to an increase of total inorganic aerosol concentration;however,the ambient inorganic aerosol concentration decreased,resulting in a smaller AOD and weaker aerosol radiative forcings.In March 2006 and 2010,the changes in ambient inorganic aerosols,AOD,and aerosol radiative forcings were more evident.In terms of the domain and five-year averages,the total inorganic aerosol concentrations increased by 13.7%(0.17μg m-3) due to heterogeneous reactions,but the ambient inorganic aerosol concentrations were reduced by 10.5%(0.13μg m-3) .As a result,the changes in AOD,SURF and TOA radiative forcings were estimated to be-3.9%(-0.007) ,-1.7%(0.34 W m-2) ,and-4.3%(0.34 W m-2) ,respectively,in March over East Asia.     

利用大气环流模式讨论舍入误差对东亚和西北太平洋夏季气候模拟的影响

采用单精度和双精度编译的大气环流模式对东亚以及西北太平洋气候进行集合模拟,分析舍入误差对大气环流模式在东亚和西北太平洋区域夏季气候模拟的影响,结果表明采用双精度编译的模式在850 hPa环流场比单精度模式模拟效果要好。为了进一步分析双精度模式模拟效果优于单精度模式的原因,集中研究了850 hPa纬向风场的模拟情况。结果表明,双精度模式对东亚以及西太平洋850 hPa纬向风场第1模态有着更好的模拟,而且第2模态的时间序列与再分析资料850hPa纬向风场的第2模态时间序列有着更好的相关。这些结果表明,由于舍入误差的影响使得该区域大气内部噪音更强烈,从而使得海温强迫导致的气候变率变弱,进而使得双精度编译的模式对东亚和西北太平洋区域气候的模拟要优于单精度模式。     

A Study of Structure and Mechanism of a Meso-beta-scale Convective Vortex and Associated Heavy Rainfall in the Dabie Mountain Area Part I: Diagnostic Analysis of the Structure

An analysis was conducted on the evolutional process of a mesoscale convective vortex (MCV) and associated heavy rainfall in the Dabie Mountain area on 21-22 June 2008,as well as their structural characteristics in different stages,by using the mesoscale reanalysis data with 3 km and 1 h resolution generated by the Local Analysis and Prediction System (LAPS) in the Southern China Heavy Rainfall Experiment.The results showed that the latent heat released by convection in the midtroposphere was the main energy source for the development of a low-level vortex.There was a positive feedback interaction between the convection and the vortex,and the evolution of the MCV was closely related to the strength of the positive interaction.The most typical characteristics of the thermal structure in different stages were that,there was a relatively thin diabatic heating layer in the midtroposphere in the formative stage;the thickness of diabatic heating layer significantly increased in the mature stage;and it almost disappeared in the decay stage.The characteristics of the dynamic structure were that,in the formative stage,there was no anticyclonic circulation at the high level;in the mature stage,an anticyclonic circulation with strong divergence was formed at the high level;in the decay stage,the anticyclonic circulation was damaged and the high-level atmosphere was in a disordered state of turbulence.Finally,the structural schematics of the MCV in the formative and mature stage were established respectively.     

高原东部凝结潜热及其对北半球500 hPa高度场和我国汛期降水的影响

利用1961—2005年青藏高原东部雨季凝结潜热序列进一步分析其气候特征及其对后期北半球大气环流和中国汛期降水的影响。结果表明,青藏高原东部雨季各月的凝结潜热均有所增加,其年际变化的差异也较大,其中5月最大,6~7月较稳定,极大年份均出现在气候明显变暖的近10年,极小年份多数出现在气候相对较冷的时期。青藏高原东部凝结潜热具有一定的持续影响力,当其潜热增强时,可引起北半球同纬度带的位势高度场偏低,特别是西太平洋副热带高压偏弱,位置偏南,进而使我国长江流域汛期降水偏多,西北区东部、华北、东北区南部及华南降水偏少。