Potential predictability of dry and wet periods: Sicily and Elbe-Basin (Germany)

Summary The purpose of this paper is to evaluate a viable tool for the potential predictability of dry and wet spells. We select two regions in Europe that have distinct precipitation regimes: Sicily and Elbe basin (Germany). The analysis of dryness and wetness in these regions from 1951 to 2000 is based on the Standardized Precipitation Index (SPI) computed on a long-time scale (two years) and the evaluation of their time-space variability is carried out using Principal Component Analysis. Results suggest that periodicities ranging from 3.4 to 12 years characterise the SPI signals in both regions and essentially drive the main dry and wet occurrences. In Sicily, at variance with the Elbe basin, superimposed to this variability there is also a clearly detectable linear trend that is perhaps related to long-term periodicity. Moreover, the shift in phase found between the common periods implies that often on the longer time scale if the Elbe region has dry conditions, Sicily is wet and viceversa. The reconstruction of the SPI time series by considering the periodicity that greatly contribute to the total power spectrum variance gives good results and provides good opportunities for predictability.     

Spatial and temporal variability of precipitation indices during 1961–2010 in Hunan Province,central south China

Based on daily precipitation records at 75 meteorological stations in Hunan Province, central south China, the spatial and temporal variability of precipitation indices is analyzed during 1961–2010. For precipitation extremes, most of precipitation indices suggest that both the amount and the intensity of extreme precipitation are increasing, especially the mean precipitation amount on a wet day, showing a significant positive trend. Meanwhile, both of the monthly rainfall heterogeneity and the contribution of the days with the greatest rainfall show an upward trend. When it comes to rainfall erosivity, most of this province is characterized by high values of annual rainfall erosivity. Although the directions of trends in annual rainfall erosivity at most stations are upward, only 6 of the 75 stations have significant trends. Furthermore, the spatial and temporal variation of dryness/wetness has been assessed by the standardized precipitation index (SPI). The principal component analysis (PCA) was applied to the SPI series computed on 24-month time scales. The results demonstrated a noticeable spatial variability with three subregions characterized by different trends: a remarkable wet tendency prevails in the central and southern areas, while the northern areas are dominated by a remarkable dry tendency.     

Risk assessment of precipitation extremes in northern Xinjiang,China

This study was conducted using daily precipitation records gathered at 37 meteorological stations in northern Xinjiang, China, from 1961 to 2010. We used the extreme value theory model, generalized extreme value (GEV) and generalized Pareto distribution (GPD), statistical distribution function to fit outputs of precipitation extremes with different return periods to estimate risks of precipitation extremes and diagnose aridity–humidity environmental variation and corresponding spatial patterns in northern Xinjiang. Spatiotemporal patterns of daily maximum precipitation showed that aridity–humidity conditions of northern Xinjiang could be well represented by the return periods of the precipitation data. Indices of daily maximum precipitation were effective in the prediction of floods in the study area. By analyzing future projections of daily maximum precipitation (2, 5, 10, 30, 50, and 100 years), we conclude that the flood risk will gradually increase in northern Xinjiang. GEV extreme value modeling yielded the best results, proving to be extremely valuable. Through example analysis for extreme precipitation models, the GEV statistical model was superior in terms of favorable analog extreme precipitation. The GPD model calculation results reflect annual precipitation. For most of the estimated sites’ 2 and 5-year T for precipitation levels, GPD results were slightly greater than GEV results. The study found that extreme precipitation reaching a certain limit value level will cause a flood disaster. Therefore, predicting future extreme precipitation may aid warnings of flood disaster. A suitable policy concerning effective water resource management is thus urgently required.     

FUTURE CHANGE OF PRECIPITATION EXTREMES OVER THE PEARL RIVER BASIN FROM REGIONAL CLIMATE MODELS

Based on RegCM4, a climate model system, we simulated the distribution of the present climate (1961-1990) and the future climate (2010-2099), under emission scenarios of RCPs over the whole Pearl River Basin. From the climate parameters, a set of mean precipitation, wet day frequency, and mean wet day intensity and several precipitation percentiles are used to assess the expected changes in daily precipitation characteristics for the 21st century. Meanwhile the return values of precipitation intensity with an average return of 5, 10, 20, and 50 years are also used to assess the expected changes in precipitation extremes events in this study. The structure of the change across the precipitation distribution is very coherent between RCP4.5 and RCP8.5. The annual, spring and winter average precipitation decreases while the summer and autumn average precipitation increases. The basic diagnostics of precipitation show that the frequency of precipitation is projected to decrease but the intensity is projected to increase. The wet day percentiles (q90 and q95) also increase, indicating that precipitation extremes intensity will increase in the future. Meanwhile, the 5-year return value tends to increase by 30%-45% in the basins of Liujiang River, Red Water River, Guihe River and Pearl River Delta region, where the 5-year return value of future climate corresponds to the 8- to 10-year return value of the present climate, and the 50-year return value corresponds to the 100-year return value of the present climate over the Pearl River Delta region in the 2080s under RCP8.5, which indicates that the warming environment will give rise to changes in the intensity and frequency of extreme precipitation events.     

Reconstructed Temperature And Precipitation On A Millennial Timescale From Tree-Rings In The Southern Colorado Plateau,U.S.A.

Two independent calibrated and verified climate reconstructions from ecologically contrasting tree-ring sites in the southern Colorado Plateau, U.S.A. reveal decadal-scale climatic trends during the past two millennia. Combining precisely dated annual mean-maximum temperature and October through July precipitation reconstructions yields an unparalleled record of climatic variability. The approach allows for the identification of thirty extreme wet periods and thirty-five extreme dry periods in the 1,425-year precipitation reconstruction and 30 extreme cool periods and 26 extreme warm periods in 2,262-year temperature reconstruction. In addition, the reconstructions were integrated to identify intervals when conditions were extreme in both climatic variables (cool/dry, cool/wet, warm/dry, warm/wet). Noteworthy in the reconstructions are the post-1976 warm/wet period, unprecedented in the 1,425-year record both in amplitude and duration, anomalous and prolonged late 20th century warmth, that while never exceeded, was nearly equaled in magnitude for brief intervals in the past, and substantial decadal-scale variability within the Medieval Warm Period and Little Ice Age intervals.     

Recent changes in precipitation extremes in the Heihe River basin,Northwest China

Changes in rainfall extremes pose a serious and additional threat to water resources planning and management, natural and artificial oasis stability, and sustainable development in the fragile ecosystems of arid inland river basins. In this study, the trend and temporal variation of extreme precipitation are analyzed using daily precipitation datasets at 11 stations over the arid inland Heihe River basin in Northwest China from 1960 to 2011. Eight indices of extreme precipitation are studied. The results show statistically significant and large-magnitude increasing and decreasing trends for most indices, primarily in the Qilian Mountains and eastern Hexi Corridor. More frequent and intense rainfall extremes have occurred in the southern part of the desert area than in the northern portion. In general, the temporal variation in precipitation extremes has changed throughout the basin. Wet day precipitation and heavy precipitation days show statistically significant linear increasing trends and step changes in the Qilian Mountains and Hexi Corridor. Consecutive dry days have decreased obviously in the region in most years after approximately the late 1980s, but meanwhile very long dry spells have increased, especially in the Hexi Corridor. The probability density function indicates that very long wet spells have increased in the Qilian Mountains. The East Asian summer monsoon index and western Pacific subtropical high intensity index possess strong and significant negative and positive correlations with rainfall extremes, respectively. Changes in land surface characteristics and the increase in water vapor in the wet season have also contributed to the changes in precipitation extremes over the river basin.     

菏泽市气候极值的统计分布和再现期研究

科学合理地估算某地区的气候极值再现期是气象科技服务的重要内容,对当地经济建设具有重要意义。对菏泽市35种气候极值,分别都用皮尔逊Ⅲ型（Pearson Ⅲ）、对数正态（log normal）、耿贝尔（Gumbel）、韦伯（Weibull）分布函数进行拟合,用χ2检验选出最优者。使用选出的最优分布函数,用多个再现期算出再现期极值,列成表格以备气象服务中使用。通过最优分布函数的选取,对哪个类型的气候要素较多服从于哪个分布函数问题进行了研究。研究结果发现极端气温类型的极值和日数类型的极值较多服从于皮尔逊Ⅲ型分布,降水和其他类型的极值较多服从于耿贝尔分布。简要介绍了计算中使用Excel的技巧。     

Changes in Mean and Extreme Temperature and Precipitation over the Arid Region of Northwestern China:Observation and Projection

This paper reports a comprehensive study on the observed and projected spatiotemporal changes in mean and extreme climate over the arid region of northwestern China, based on gridded observation data and CMIP5 simulations under the RCP4.5 and RCP8.5 scenarios. The observational results reveal an increase in annual mean temperature since 1961, largely attributable to the increase in minimum temperature. The annual mean precipitation also exhibits a significant increasing tendency. The precipitation amount in the most recent decade was greater than in any preceding decade since 1961. Seasonally,the greatest increase in temperature and precipitation appears in winter and in summer, respectively. Widespread significant changes in temperature-related extremes are consistent with warming, with decreases in cold extremes and increases in warm extremes. The warming of the coldest night is greater than that of the warmest day, and changes in cold and warm nights are more evident than for cold and warm days. Extreme precipitation and wet days exhibit an increasing trend, and the maximum number of consecutive dry days shows a tendency toward shorter duration. Multi-model ensemble mean projections indicate an overall continual increase in temperature and precipitation during the 21 st century. Decreases in cold extremes, increases in warm extremes, intensification of extreme precipitation, increases in wet days, and decreases in consecutive dry days, are expected under both emissions scenarios, with larger changes corresponding to stronger radiative forcing.     

Projections of temperature and precipitation extremes in the North Western Mediterranean Basin by dynamical downscaling of climate scenarios at high resolution (1971–2050)

The North Western Mediterranean basin (NWMB) is characterised by a highly complex topography and an important variability of temperature and precipitation patterns. Downscaling techniques are required to capture these features, identify the most vulnerable areas to extreme changes and help decision makers to design strategies of mitigation and adaptation to climate change. A Regional Climate Model, WRF-ARW, is used to downscale the IPCC-AR4 ECHAM5/MPI-OM General Circulation Model results with high resolution (10 km), considering three different emissions scenarios (B1, A1B and A2) for 2001–2050. Model skills to reproduce observed extremes are assessed for a control period, 1971–2000, using the ERA40 reanalysis to drive the WRF-ARW simulations. A representative set of indices for temperature and precipitation extremes is projected. The modelling system correctly reproduces amplitude and frequency of extremes and provides a high degree of detail on variability over neighbouring areas. However, it tends to overestimate the persistence of wet events and consequently slightly underestimate the length of dry periods. Drier and hotter conditions are generally projected for the NWMB, with significant increases in the duration of droughts and the occurrence of heavy precipitation events. The projected increase in the number of tropical nights and extreme temperatures could have a negative effect on human health and comfort conditions. Simulations allow defining specifically vulnerable areas, such as the Ebro Valley or the Pyrenees, and foreseeing impacts on socio-economic activities in the region.     

Assessment of climate extremes in the Eastern Mediterranean

Summary Several seasonal and annual climate extreme indices have been calculated and their trends (over 1958 to 2000) analysed to identify possible changes in temperature- and precipitation-related climate extremes over the eastern Mediterranean region. The most significant temperature trends were revealed for summer, where both minimum and maximum temperature extremes show statistically significant warming trends. Increasing trends were also identified for an index of heatwave duration. Negative trends were found for the frequency of cold nights in winter and especially in summer. Precipitation indices highlighted more regional contrasts. The western part of the study region, which comprises the central Mediterranean and is represented by Italian stations, shows significant positive trends towards intense rainfall events and greater amounts of precipitation. In contrast, the eastern half showed negative trends in all precipitation indices indicating drier conditions in recent times. Significant positive trends were revealed for the index of maximum number of consecutive dry days, especially for stations in southern regions, particularly on the islands.Current affiliation: National Observatory of Athens, Athens, Greece.     

大兴安岭气候干湿变化及对森林火灾的影响

明确气候变化背景下大兴安岭林区气候干湿状况特征,揭示其对森林火灾的影响,可为该区域森林火灾管理和森林资源保护提供科学依据。基于大兴安岭林区1974—2016年标准化降水指数（SPI）,采用统计分析和对比分析方法,系统研究不同干湿情景对森林火灾发生次数及过火面积的影响,并讨论不同等级干旱对其影响的异同性。结果表明:1974—2016年,年、季尺度上大兴安岭林区气候均呈湿润化趋势。森林火灾发生次数多（少）和过火面积大（小）与气候的干湿状况（等级）基本一致,但森林火灾的发生次数与气候干湿状况相关更为密切。年尺度上,SPI与火灾次数呈负相关,与过火面积的自然对数则呈较弱的负相关;季尺度上,各季节SPI与对应的林火次数和过火面积自然对数均呈显著的负相关,但与过火面积的相关程度差异较大,以春季相关最为显著,秋季次之,夏季则相对较弱;不同季节SPI与年林火次数和过火面积自然对数呈负相关,前一年冬季SPI对当年火灾次数的贡献最大。可见,气候干湿状况对森林火灾的影响存在明显的滞后效应。SPI不仅能较好地反映区域气候的干湿状况,亦能较好地指示森林火灾发生的可能性及发生火灾的过火面积的相对变化情况,可为森林火灾预测和管理提供科学依据。     

Rainfall characterisation by application of standardised precipitation index (SPI) in Peninsular Malaysia

The interpretations of trend behaviour for dry and wet events are analysed in order to verify the dryness and wetness episodes. The fitting distribution of rainfall is computed to classify the dry and wet events by applying the standardised precipitation index (SPI). The rainfall amount for each station is categorised into seven categories, namely extremely wet, severely wet, moderately wet, near normal, moderately dry, severely dry and extremely dry. The computation of the SPI is based on the monsoon periods, which include the northeast monsoon, southwest monsoon and inter-monsoon. The trends of the dry and wet periods were then detected using the Mann–Kendall trend test and the results indicate that the major parts of Peninsular Malaysia are characterised by increasing droughts rather than wet events. The annual trends of drought and wet events of the randomly selected stations from each region also yield similar results. Hence, the northwest and southwest regions are predicted to have a higher probability of drought occurrence during a dry event and not much rain during the wet event. The east and west regions, on the other hand, are going through a significant upward trend that implies lower rainfall during the drought episodes and heavy rainfall during the wet events.     

Estimation of the monthly precipitation predictability limit in China using the nonlinear local Lyapunov exponent

By using the nonlinear local Lyapunov exponent and nonlinear error growth dynamics, the predictability limit of monthly precipitation is quantitatively estimated based on daily observations collected from approximately 500 stations in China for the period 1960–2012. As daily precipitation data are not continuous in space and time, a transformation is first applied and a monthly standardized precipitation index (SPI) with Gaussian distribution is constructed. The monthly SPI predictability limit (MSPL) is quantitatively calculated for SPI dry, wet, and neutral phases. The results show that the annual mean MSPL varies regionally for both wet and dry phases: the MSPL in the wet (dry) phase is relatively higher (lower) in southern China than in other regions. Further, the pattern of the MSPL for the wet phase is almost opposite to that for the dry phase in both autumn and winter. The MSPL in the dry phase is higher in winter and lower in spring and autumn in southern China, while the MSPL values in the wet phase are higher in summer and winter than those in spring and autumn in southern China. The spatial distribution of the MSPL resembles that of the prediction skill of monthly precipitation from a dynamic extended-range forecast system.     

极值统计理论的进展及其在气候变化研究中的应用

着重论述极值统计分布在极端天气气候事件和重大工程设计中的重要意义,综述该领域国内外研究进展。例如,基于超门限峰值法（POT）的广义帕累托分布（GPD)和基于单元极大值法（BM）的广义极值分布（GEV）及其参数间的理论关系;采用极值分布模型与多状态一阶Markov链相结合构建降尺度模型模拟局地极端降水事件,推算一定重现期的极端降水量的分位数;探讨极值分布模型分位数估计误差问题,多维极值分布理论及其应用等问题。     

Potential future changes in the characteristics of daily precipitation in Europe simulated by the HIRHAM regional climate model

In this study the potential future changes in various aspects of daily precipitation events over Europe as a consequence of the anticipated future increase in the atmospheric greenhouse gas concentrations are investigated. This is done by comparing two 3-member ensembles of simulations with the HIRHAM regional climate model for the period 1961–1990 and 2071–2100, respectively. Daily precipitation events are characterized by their frequency and intensity, and heavy precipitation events are described via 30-year return levels of daily precipitation. Further, extended periods with and without rainfall (wet and dry spells) are studied, considering their frequency and length as well as the average and extreme amounts of precipitation accumulated during wet spells, the latter again described via 30-year return levels. The simulations show marked changes in the characteristics of daily precipitation in Europe due to the anticipated greenhouse warming. In winter, for instance, the frequency of wet days is enhanced over most of the European continent except for the region on the Norwegian west coast and the Mediterranean region. The changes in the intensity and the 30-year return level of daily precipitation are characterized by a similar pattern except for central Europe with a tendency of decreased 30-year return levels and increased precipitation intensity. In summer, on the other hand, the frequency of wet days is decreased over most of Europe except for northern Scandinavia and the Baltic Sea region. In contrast, the precipitation intensity and the 30-year return level of daily precipitation are increased over entire Scandinavia, central and eastern Europe. The changes in the 30-year return level of daily precipitation are generally stronger than the corresponding changes in the precipitation intensity but can have opposite signs in some regions. Also the distribution of wet days is changed in the future. During summer, for instance, both the frequency and the length of dry spells are substantially increased over most of the European continent except for the Iberian Peninsula. The frequency and the length of wet spells, on the other hand, are generally reduced during summer and increased during winter, again, with the exception of the Iberian Peninsula. The future changes in the frequency of wet days in winter are related to a change in the large-scale flow over the North Atlantic and a corresponding shift of the North Atlantic storm track. The reduction in the frequency of wet days in summer is related to a northward extension of the dry subtropical region in the future, with a reduction of the convective activity because of the large-scale sinking motion in the downward branch of the Hadley cell. Because the atmosphere contains more moisture in the warmer future climate, the amount of precipitation associated with individual low-pressure systems or with individual convective events is increased, leading to a general increase in the intensity of individual precipitation events. Only in regions, where all the moisture evaporates from the ground already in spring, the intensity of precipitation events is reduced in summer.     

Trends of precipitation extreme indices over a subtropical semi-arid area using PERSIANN-CDR

In this study, satellite-based daily precipitation estimation data from precipitation estimation from remotely sensed information using artificial neural networks (PERSIANN)-climate data record (CDR) are being evaluated in Iran. This dataset (0.25°, daily), which covers over three decades of continuous observation beginning in 1983, is evaluated using rain-gauge data for the period of 1998–2007. In addition to categorical statistics and mean annual amount and number of rainy days, ten standard extreme indices were calculated to observe the behavior of daily extremes. The results show that PERSIANN-CDR exhibits reasonable performance associated with the probability of detection and false-alarm ratio, but it overestimates precipitation in the area. Although PERSIANN-CDR mostly underestimates extreme indices, it shows relatively high correlations (between 0.6316–0.7797) for intensity indices. PERSIANN-CDR data are also used to calculate the trend in annual amounts of precipitation, the number of rainy days, and precipitation extremes over Iran covering the period of 1983–2012. Our analysis shows that, although annual precipitation decreased in the western and eastern regions of Iran, the annual number of rainy days increased in the northern and northwestern areas. Statistically significant negative trends are identified in the 90th percentile daily precipitation, as well as the mean daily precipitation from wet days in the northern part of the study area. The positive trends of the maximum annual number of consecutive dry days in the eastern regions indicate that the dry periods became longer in these arid areas.     

广义极值分布理论在重现期计算的应用

在气候统计学上,常用Weibull、Gumbel、Frechet统计分布函数对极端气候要素的分布进行拟合,广义极值分布理论综合了以上三种极值分布模型,在气候分析中得到了广泛应用。以南昌市年汛期日最大降水量为例,利用广义极值分布理论对其分布进行拟合,并对重现值及其置信区间进行计算,为气候要素极值的统计分析提供了一种新的手段。     

Spatio-temporal variability of dry and wet periods in eastern China

Summary An analysis, based on rain gauge observations, of the time-space variability of dry and wet periods during the last fifty years in eastern China is presented. The Standardized Precipitation Index (SPI) is used to assess the climatic conditions of the area, and principal component analysis (PCA) is applied to capture the pattern of co-variability of the index at different stations. Results suggest that the northern part of eastern China is experiencing dry conditions more frequently from the 1970s onwards indicated by a negative trend in the SPI time series. Long-term fluctuations characterize the SPI signal and contribute to the power spectrum variance at periods ranging from interdecadal to interannual time scales, that is respectively, 24 years and from 16 to 4–3.7 years. These periodic components provide a useful resource for long-term predictability of dry and wet periods in eastern China.     

Variability and trends in daily precipitation extremes on the northern and southern slopes of the central Himalaya

This study focuses on the precipitation extremes recorded on the northern and southern slopes of the central Himalaya, especially those documented at higher altitudes. Daily precipitation data recorded over a 35-year period at nine meteorological stations in the region were studied. We used the precipitation extreme indices delineated by the Expert Team on Climate Change Detection and Indices (ETCCDI). The spatial and temporal variations in these precipitation extremes were calculated. When regional patterns were investigated to detect any anomalies, only 1 of the 10 precipitation extreme indices from the southern slopes of the central Himalaya showed a statistically significant trend; none from the northern slopes of the central Himalaya highlighted a statistically significant trend. On the southern slopes, all indices increased, apart from the maximum 1-day precipitation (RX1) and simple daily precipitation intensity (SDII) indices. Indices such as the consecutive dry days (CDDs) and RX1 indices exhibited similar increases on both the northern and southern slopes of the central Himalaya. These results suggest that increases in precipitation have been accompanied by an increasing frequency of extremes over the southern central Himalaya. Nonetheless, no relation could be established between the precipitation extreme indices and circulation indices for higher altitudes.

     

Synoptic aspects of wet and dry conditions in central Australia: observations and GCM simulations for 1 × CO2 and 2 × CO2 conditions

Synoptic climatological patterns that produce anomalous wet conditions in central Australia during the period from September to April have been studied. The analysis was done by using observed daily rainfall data at a number of stations, wind and mean sea level pressure from the European Centre for Medium Range Weather Forecasts (ECMWF), Tropical Ocean and Global Atmosphere (TOGA) data from 1985 to 1991, and the CSIRO 9-level (CSIR09) global climate model (GCM) simulated data for 1 × CO₂ and 2 × CO₂ experiments. On the basis of rainfall values above 99.5 percentile in observed and simulated data, wet days have been selected to study the synoptic-scale weather systems that produce anomalous wet events in central Australia. As the vast majority of days in central Australia are dry, the same number of days with no rainfall for both observed and simulated conditions have been selected randomly. The observed synoptic climatological patterns have been compared with the results of the control simulation of CSIRO9. A comparison between CSIRO9 simulated synoptic patterns and observed synoptic patterns reveals that the model fairly well captures the synoptic climatological characteristics which produce anomalous wet and contrasting dry weather conditions during the period from September to April. Under enhanced greenhouse experiments, the main features of the synoptic patterns are intensified both for wet and dry conditions, which result in an increase in extreme weather conditions, an increase in rainfall intensity, a spatial expansion of the heavy rainfall region during wet days, and an expansion of the dry area during dry days. During anomalous wet conditions, the low pressure area is intensified, monsoonal winds and southeasterlies are strengthened and strong wind shear over tropical Australia is simulated. During this condition, the monsoon shear line moves poleward particularly over the Northern Territory. In contrast, during dry conditions, the anticyclonic circulation over the continent is strengthened.