Evaluation of 2-m temperature and precipitation products of the Climate Forecast System version 2 over Iran

Using a continuous multi-decadal simulations over the period 1981–2010, subseasonal to seasonal simulations of the Climate Forecast System version 2 (CFSv2) over Iran against the Climatic Research Unit (CRU) dataset are evaluated. CFSv2 shows cold biases over northern hillsides of the Alborz Mountains with the Mediterranean climate and warm biases over northern regions of the Persian Gulf and the Oman Sea with a dry climate. Magnitude of the model bias for 2-m temperature over different regions of Iran varies by season, with the least bias in temperate seasons of spring and autumn, and the largest bias in summer. The model bias decreases as temporal averaging period increases from seasonal to annual. The forecast generally produces dry and wet biases over dry and wet regions of Iran, respectively. In general, 2-m temperature over Iran is better captured than precipitation, but the prediction skill of precipitation is generally high over western Iran. Averaged over Iran, observations indicated that 2-m temperature has been gradually increasing during the studied period, with a rate of approximately 0.5 °C per decade, and the upward trend is well simulated by CFSv2. Averaged over Iran, both observations and simulation results indicated that precipitation has been decreasing in spring, with averaged decreasing trends of 0.8 mm (observed) and 1.7 mm (simulated) per season each year during the period 1981–2010. Observations indicated that the maximum increasing trend of 2-m temperature has occurred over western Iran (nearly 0.7 °C per decade), while the maximum decreasing trend of annual precipitation has occurred over western and parts of southern Iran (nearly 45 to 50 mm per decade).     

Winter precipitation variability and large-scale circulation patterns in Romania

Summary The spatial and temporal variability of winter precipitation and its links to the large-scale atmospheric circulation patterns in Romania are examined. The data set is composed of observed rainfall at 30 meteorological stations during the 1961–1996 period. The large-scale field is represented by the observed geopotential height at 500 hPa (Z500) over the same period, covering the latitudinal belt between 20° N–90° N (resolution 2.5°×2.5°).The Standard Normal Homogeneity Test (SNHT) is applied to detect inhomogeneities in the data, and the Mann-Kendall and Pettitt non-parametric tests are used in order to identify trends and change points in the winter precipitation time series. The empirical orthogonal functions (EOF) technique is used for data reduction in order to highlight the basic patterns of rainfall variability in Romania. The covariance map between precipitation EOF time series (PCs) and the Z500 field, as well as the correlation coefficients between the PCs and circulation indices are calculated in order to identify the influence of large-scale circulation patterns on winter precipitation in Romania.A significant decreasing trend is identified in winter precipitation with a downward shift in winter 1969/1970, most significant from a statistical point of view in the extra-Carpathian region. This change seems to be real since the SNHT test does not reveal any inhomogeneity during the period tested. Significant relationships are found between winter precipitation variability in Romania and the large-scale circulation pattern, such as the North Atlantic Oscillation and the blocking phenomenon in the Atlantic-European sector. The positive phase of the NAO and the reduction in blocking activity could be one of the causes of the decrease in winter precipitation in Romania.     

A 1951–80 global land precipitation climatology for the evaluation of general circulation models

Previous evaluations of model precipitation fields have suffered from two weaknesses; they have used only mean observed climatologies which have prevented an explicit evaluation of interannual variability, and they have generally failed to quantify the significance of differences between model and observed fields. To rectify these weaknesses, a global precipitation climatology is required which is designed with model evaluation in mind. This paper describes such a climatology representative of the period 1951–80. The climatology is based on historical gauge-precipitation measurements from over 2500 land-based station time series representing over 28% of the Earth's surface. It is necessarily biased towards terrestrial areas. The climatology (CRU5180) is derived from month-by-month gridbox precipitation estimates at 5° resolution. Although other global precipitation climatologies exist, this is the first one to have used a consistent reference period for each station, and to include the details of interannual variability. Fields of mean seasonal and annual precipitation and mean temporal variability are presented, and the variability of global-mean precipitation over 1951–80 assessed. The resulting mean monthly global precipitation fields are compared briefly with two other observed climatologies used for model evaluation, those prepared by Jaeger and Legates and Willmott. The global and hemispheric means, mean seasonal cycles, and spatial patterns of the three cimatologies are compared. Although based on a smaller set of stations than Legates and Willmott, the CRU5180 precipitation estimates agree closely with their uncorrected climatology.     

Wintertime variability of Mediterranean precipitation and its links with large-scale circulation anomalies

A standard principal component analysis has been performed over the Mediterranean and over the larger European region on monthly precipitation anomalies for the winters between 1979 and 1995. The main centres of action of the associated EOFs are very similar for the two regions and the two sets of PCs are highly correlated with each other. Focusing on the Mediterranean region, the same analysis has been performed using 500?hPa geopotential height monthly anomalies taken from the operational NCEP analysis. Comparing the two sets of PCs associated with upper-air and surface data, a strong correlation has been found suggesting the presence of a two-way link between regional precipitation patterns and large-scale circulation anomalies. For both fields, the largest fraction of variance is explained by the North Atlantic Oscillation, while smaller but still substantial fractions are explained by other known patterns of large-scale variability such as the Eastern Atlantic pattern and the Euro-Atlantic blocking. No detectable connection has been found between Mediterranean precipitation patterns and El Niño SST anomalies during winter. With respect to temporal variability, significant trends have been found over most European areas during the winters considered. The associated pattern is characterised by a substantial increase of precipitation over western Scandinavia and a general decrease over southern Europe. This result is confirmed by analysing data from stations located in northern Italy.     

An analysis of precipitation climatology in Jordan

Summary ?One of the most important features in analysing the climatology of any region is to study the precipitation and its periodicity of different harmonics in order to study the behavior of the observed data. In this study the amplitude of frequencies, phase angle and basic statistical parameters are calculated in order to depict spatial characteristics of precipitation over Jordan. Precipitation records of 17 stations were chosen according to climatic regions of Jordan. The first and second harmonic analyses explain more than 90% of the precipitation variation in Jordan effectively. The amplitudes of the first and second harmonic were calculated in order to describe the climatic regions in the country. The maximum amplitudes were found in the northern mountainous region. The phase angle representing the time of maximum rainfall is also used in the form of a contour chart. It is found that Jordan has its main rainfall season in winter with maximum around January. The coefficient of variation shows the high variability of rainfall of the country. Received February 4, 2002; revised August 1, 2002; accepted August 6, 2002     

European winter precipitation extremes and large-scale circulation: a coupled model and its scenarios

Summary The physical coupling between the occurrence of winter heavy precipitation in Europe and the surface large-scale circulation is studied by isolating their coupled modes with a singular value decomposition technique. The leading mode is a clear manifestation of the North Atlantic Oscillation forcing. The second mode reflects the influence of a centre-of-action in the pressure field westward of the British Isles. The Hadley Centre Coupled Model (3^rd generation) is skilful in reproducing these two modes and an eastward extension of the North Atlantic Oscillation towards the Mediterranean Basin is projected under two future climatic scenarios. This extension yields an increase in the North Atlantic Oscillation forcing over the occurrence of heavy precipitation in several regions of Southern Europe, which is corroborated by the changes in the coupling of the daily precipitation. A combination of the first six coupled modes of the daily precipitation revealed that its amounts in some parts of Western Europe and the Mediterranean are effectively governed by the large-scale circulation. The model is still reasonably skilful in reproducing this large-scale coupling. The projected modifications, both in the strength and in the patterns of the coupled modes, explain important fractions of the projected changes in variance, which ultimately have implications in the occurrence of heavy precipitation in several European areas. Therefore, the ability of a model in reproducing the large-scale forcing over the daily precipitation is important for the reliability of its projections of the occurrence of heavy precipitation in Europe.     

Five hundred years of gridded high-resolution precipitation reconstructions over Europe and the connection to large-scale circulation

We present seasonal precipitation reconstructions for European land areas (30°W to 40°E/30–71°N; given on a 0.5°×0.5° resolved grid) covering the period 1500–1900 together with gridded reanalysis from 1901 to 2000 (Mitchell and Jones 2005). Principal component regression techniques were applied to develop this dataset. A large variety of long instrumental precipitation series, precipitation indices based on documentary evidence and natural proxies (tree-ring chronologies, ice cores, corals and a speleothem) that are sensitive to precipitation signals were used as predictors. Transfer functions were derived over the 1901–1983 calibration period and applied to 1500–1900 in order to reconstruct the large-scale precipitation fields over Europe. The performance (quality estimation based on unresolved variance within the calibration period) of the reconstructions varies over centuries, seasons and space. Highest reconstructive skill was found for winter over central Europe and the Iberian Peninsula. Precipitation variability over the last half millennium reveals both large interannual and decadal fluctuations. Applying running correlations, we found major non-stationarities in the relation between large-scale circulation and regional precipitation. For several periods during the last 500 years, we identified key atmospheric modes for southern Spain/northern Morocco and central Europe as representations of two precipitation regimes. Using scaled composite analysis, we show that precipitation extremes over central Europe and southern Spain are linked to distinct pressure patterns. Due to its high spatial and temporal resolution, this dataset allows detailed studies of regional precipitation variability for all seasons, impact studies on different time and space scales, comparisons with high-resolution climate models as well as analysis of connections with regional temperature reconstructions. Electronic Supplementary Material Supplementary material is available for this article at and is accessible for authorized users.     

Long-term precipitation variability in Morocco and the link to the large-scale circulation in recent and future climates

Summary ?Monthly precipitation data from the Global Historical Climatology Network for 42 stations in Morocco and its vicinity are investigated with respect to baroclinicity, storm track and cyclone activity, moisture transports, North Atlantic Oscillation (NAO) variations, and different circulation types by means of correlation and composite studies. The results are related to a climate change scenario from an ECHAM4/OPYC3 transient greenhouse gas only (GHG) simulation. Precipitation in northwestern Morocco shows a clear link to the baroclinic activity over the North Atlantic during boreal winter (DJF). In large precipitation months the North Atlantic storm track is shifted southward, more westerly and northwesterly circulation situations occur and moisture transports from the Atlantic are enhanced. The occurrence of local cyclones and upper-level troughs is more frequent than in low precipitation months. The negative correlation to the NAO is relatively strong, especially with Gibraltar as a southern pole (−0.71). The northward shift of the storm track and eastward shift of the Azores High predicted by the ECHAM model for increasing GHG concentrations would therefore be associated with decreasing precipitation and potentially serious impacts for the future water supply for parts of Morocco. In the region south of the Atlas mountains, moisture transports from the Atlantic along the southern flank of the Atlas Mountains associated with cyclones west of Morocco and the Iberian Peninsula can be identified as a decisive factor for precipitation. Northeastern Morocco and Northwestern Algeria, however, is rather dominated by the influence of cyclones over the Western Mediterranean that are associated with a strong northwesterly moisture transport. As both regions appear to be less dependent on the North Atlantic storm track and more on local processes, a straight forward interpretation of the large-scale changes predicted by the ECHAM4/OPYC3 cannot be done without the application of down-scaling methods in the future. Received July 19, 2001; revised May 31, 2002     

On the climatology of persistent circulation anomalies in the atmosphere and in a general circulation model

Abstract

The climatologies of upper‐air persistent circulation anomalies found in observations of the Northern Hemisphere and in a General Circulation Model (GCM) integration are compared with each other and with those found in previous studies. The model simulation is that of the Canadian Climate Centre GCM run at resolution T20. The objective criteria that define the persistent events differ from those of some earlier investigations in that the anomalies are not required to be as nearly stationary. It is found that the GCM generates persistent circulation anomalies downstream of the synoptic‐scale storm tracks, in very nearly the correct geographical locations, but that the frequency of occurrence is too low in the model. A kinetic energy and streamfunction variance analysis is presented for both dataseis to clarify the differences between the observed and simulated distributions of circulation anomalies. It is evident that, apart from the mean annual cycle, the middle‐latitude transient eddies of the model are too weak.     

The synoptic climatology of monthly mean surface temperature in Asia in relation to the 700 hPa circulation

The synoptic climatology of monthly mean surface air temperature anomaly (MMATA)at 47 surface stations in Asia is assessed by two types of maps. The first consists of simple linear correlation coefficients between the MMATA at a station and the monthly mean 700 hPa height anomaly (MMHA) at 134 grid points in the Northern Hemisphere. In winter, these fields generally contain two equally important centers: a center of maximum positive correlation approximately 1300 km from the reference station and a negative center about 2800 km to its northwest. In summer, the positive center is located an average of 800 km from the reference station and strongly dominates the MMATA, while the negative center is no longer as important as in winter. The second kind of map is a special type of anomaly composite, constructed by multiplying the correlation coefficient at each grid point by the standard deviation of MMHA at the same point. This map shows the optimum distribution of height anomalies for abnormally warm (or cold) weather for winter and summer in each of several climate-Logically similar regions into which Asia is subdivided.     

An analysis of precipitation climatology over Indian urban agglomeration

Theoretical and Applied Climatology - While urban areas in India are rapidly expanding, the analysis of how the precipitation regimes are changing is very limited. In the present study, an attempt...     

中国南方地区盛夏高温类型及其对应的大尺度环流和海温异常

利用中国2374个站点的日最高气温资料、NCEP/NCAR大气环流资料以及NOAA海表温度资料,分析了中国南方盛夏高温的主要类型,比较了各类型高温对应的大尺度环流以及与海表温度异常信号的联系。结果表明:中国南方地区年高温日数有明显的增多趋势,利用聚类分析将中国南方盛夏高温分为江淮型、华南型和华中型3类。江淮型高温中心区域位于江淮地区,该类型高温空间范围大、发生频次高,典型的环流系统为高、低空呈现异常反气旋,西太平洋副热带高压(西太副高)偏强偏西,南北位置略偏北。该类型高温是西太副高直接控制下的高温类型,与前冬到夏季中部型厄尔尼诺衰减和春季赤道大西洋地区海温偏高有关。华南型高温中心区域在江南南部到华南地区,典型的环流系统是东亚副热带急流的位置偏南,西太副高偏强、偏西和脊线偏南,华南型高温也是西太副高直接控制下的高温,且伴随着西南季风的减弱,干热特征明显,该类型高温与东部型厄尔尼诺衰减及其与之相联系的印度洋"电容器"效应密切相关。华中型高温主要位于湖北和湖南两省,对应的环流型为西太副高偏弱、偏东,在中高纬度与北大西洋-欧亚遥相关型类似,是大陆高压控制下的高温,使得水汽条件比另两类高温好。北...     

大尺度环流的年代际变化对初夏华南持续性暴雨的影响

利用1961-2010年中国逐日降水数据和NCEP/NCAR逐日再分析资料集,研究了大尺度环流的年代际变化对不同频发年代华南持续性暴雨的影响。结果表明,虽然过去50年里华南持续性暴雨的发生存在两个频发时段(20世纪60年代中期到70年代中期、20世纪90年代初到2010年),其典型环流配置都表现为中高纬度冷空气和低纬度充沛水汽的配合,但不同频发时段的大尺度环流配置明显不同,这种配置使得两个频发时段的暴雨发生特点有所不同,最近20年的暴雨发生次数更多,强度更强,持续时间更长。在频发的1964-1976年,高纬度的冷空气主要来自西西伯利亚上空深厚的低槽前部,而此时南亚高压偏弱,西太平洋副热带高压偏弱、偏东,低纬度的水汽主要来自孟加拉湾印缅低压槽前,可降水量偏小,但垂直速度较大,水汽辐合较旺盛,华南上空大气环流较不稳定;在频发的1991-2010年,鄂霍次克海上空附近的高压脊活跃,高纬度冷空气主要来自高压后部,低纬度西太平洋副热带高压偏强,位置偏西,其西北侧的水汽源源不断输送到华南上空,故可降水量增多,伴随着强大的南亚高压提供的良好的高层辐散条件,华南上空垂直速度增大,水汽辐合明显,强有利的环流配置导致1991-2010年华南持续性暴雨强度更大、范围更广。因此,大气环流的年代际转型使得华南持续性暴雨发生了巨大改变,故在未来做预测时应充分考虑不同年代际环流背景场对华南持续性暴雨的重要影响。     

Inter-annual temperature and precipitation variations over the Litani Basin in response to atmospheric circulation patterns

This study examines the sensitivity of a mid-size basin’s temperature and precipitation response to different global and regional climate circulation patterns. The implication of the North Atlantic Oscillation (NAO), El Ni?o Southern Oscillation (ENSO), Indian Monsoon and ten other teleconnection patterns of the Northern Hemisphere are investigated. A methodology to generate a basin-scale, long-term monthly surface temperature and precipitation time series has been established using different statistical tests. The Litani River Basin is the focus of this study. It is located in Lebanon, east of the Mediterranean Basin, which is known to have diverse geophysical and environmental characteristics. It was selected to explore the influence of the diverse physical and topographical features on its hydroclimatological response to global and regional climate patterns. We also examine the opportunity of conducting related studies in areas with limited long-term measured climate and/or hydrological data. Litani's monthly precipitation and temperature data have been collected and statistically extrapolated using remotely sensed data products from satellites and as well as in situ gauges. Correlations between 13 different teleconnection indices and the basin’s precipitation and temperature series are investigated. The study shows that some of the annual and seasonal temperature and precipitation variance can be partially associated with many atmospheric circulation patterns. This would give the opportunity to relate the natural climate variability with the watershed’s hydroclimatology performance and thus differentiate it from other anthropogenic induced climate change outcomes.     

Seasonal climatology of cut-off lows and associated precipitation patterns over Northeast China

Cut-off lows (COLs) are significantly associated with many convective events and anomalous regional climate in the Northeast China. By using 49-year NCEP/NCAR reanalysis data, COL events are detected and tracked by an objective and automatic method based on synoptic concept model. Based on this dataset and daily rain-gauge records in Northeast China during 1979–2005, seasonal climatology of COLs and associated precipitation patterns over Northeast China are investigated. Most COLs have a short lifetime of less than a week and have a spatial size ranging from 500 to 1,000 km, with slight seasonal differences. Temporal variation of COL occurrence exhibits a seasonal cycle, with a peak in summer, and considerable interannual variability. The COLs tend to occur more frequently over the northern Northeast China Plain, and the center for maximum frequency shows a zonal oscillation, with an extension to continent in summer and a shift to western North Pacific coast in winter. Most COLs form to the east of Lake Baikal and decay over the western North Pacific coast. COLs are apt to move along east or southeast passages around the year, and tracks are relatively more complicated in warm seasons. About a quarter of annual mean precipitation over Northeast China is associated with COLs. Moreover, COL-associated precipitation contributes greatly to total precipitation in northern and northwestern parts of Northeast China, and the ratios of COL contribution are stronger during spring and autumn than in summer.     

The relation between circulation intensity in the temperate latitude cyclone and air temperature and precipitation anomalies

The relationship between air temperature, precipitation, and circulation intensity in extratropical cyclones at subsequent stages of their evolution is studied. The cyclones observed in the Atlantic and Pacific sectors in winters from 1991 to 2000 are considered. The NCEP/NCAR reanalysis data, GPCP database, and cyclone trajectory archive of the Shirshov Institute of Oceanology, Russian Academy of Sciences, are used. It is demonstrated that there are significant connections between circulation intensity, air temperature, and precipitation in cyclones. The intensity of circulation along the cyclone contour reflects both the precipitation fall in the cyclone center and a synoptic structure of frontal precipitation. It was concluded that speed circulation along the cyclone contour can be recommended as a characteristic of cyclone intensity together with the currently used pressure in the cyclone center.