Observed and projected changes in temperature and precipitation extremes based on CORDEX data over Iran

Theoretical and Applied Climatology - Due to increasing greenhouse gases, Iran is experiencing changes in patterns and trends of extreme climate events. Future climate extremes are one of the...     

Nonparametric kernel estimation of annual precipitation over Iran

In this paper, annual precipitation data sets from five old rain gauge stations (Bushehr, Isfahan, Meshed, Tehran, and Jask) in Iran were fitted to nonparametric kernel function by using rectangular, triangular, and Gaussian or normal as kernel functions. The smoothing parameter was calculated by four methods including rule of thumb, Adamowski criterion, least squares cross-validation, and Sheater and Jones plug-in. The Adamowski criterion showed a better performance compared to other methods due to goodness of fit tests. The results of these proposed nonparametric methods will be then compared to the results of the parametric density functions including normal, two and three parameter log-normal, two parameter gamma, Pearson and log-Pearson type 3, Gumbel or extreme value type 1 and also Fourier series method which were applied by a previous study for the same stations. It was concluded that the annual precipitation data were fitted to nonparametric methods better than parametric methods.     

Daily precipitation forecast of ECMWF verified over Iran

In this paper, the performance of the Centre for Medium Range Weather Forecast (ECMWF) model (t?+?27 h to t?+?51 h) in predicting precipitation is discussed. This model is the first, which has been verified over Iran. The spatial resolution of the model is 0.351° and the 24-h forecasts are compared with daily observations. The study concentrates on year 2001 and the precipitation measurements were collected from the data of 2,048 rain gauges in Iran. The accuracy of four different interpolation methods (nearest neighborhood, inverse distance, kriging, and upscaling) was investigated. Using cross-validation, the inverse distance method (IDM) with minimum mean error was applied. Verification results are given in terms of difference fields (mean error?=?0.46 mm/day), rank–order correlation coefficients (0.70), as well as accuracy scores (false alarm ratio?=?0.50 and probability of detection?=?0.60) and skill scores (true skill statistics [TSS]?=?0.45) in year 2001. The position of the rain band was only partly captured by the ECMWF model; however, the position of maximum precipitations agrees with the observations well. The results show that the high values of TSS are associated with the large amounts of precipitation (over 25 mm). Slight to moderate precipitation events have been underforecasted by the model (bias?<?1) and it leads to a small value of TSS for these thresholds (5–25 mm/day). The ECMWF model has better performance in high and mountainous regions than over flat terrain and in deserts. Comparing TSS over the Alborz and the Zagros Mountains, it is obvious that the ability of the model to predict the convective precipitation events needs some improvement. The amount of daily precipitation has been also slightly overestimated over Iran. From the beginning of January up to 21 March 2001, the ECMWF time series indicates an obvious phase shift of 1 day, although in other months, no phase shift is noticed.     

Spatiotemporal analysis of precipitation and temperature concentration using PCI and TCI: a case study of Khuzestan Province,Iran

Theoretical and Applied Climatology - Temperature and precipitation are the basic elements of climate, and their variation can change the water demands of different uses. In this study, the trend...     

UKGrsHP: a UK high-resolution gauge–radar–satellite merged hourly precipitation analysis dataset

Climate Dynamics - There is an urgent need for high-quality and high-spatial-resolution hourly precipitation products around the globe, including the UK. Although hourly precipitation products...     

中国典型地区夏季气溶胶时空分布及其对云和降水的影响

利用2006—2016年夏季中分辨率成像光谱仪（Moderate-resolution Imaging Spectroradiometer,MODIS）气溶胶和云资料以及热带降水测量计划（Tropical Rainfall Measuring Mission,TRMM）降水数据,分析了中国8个典型地区气溶胶、云和降水的时空分布特征,探讨了气溶胶与云和降水的相互关系。结果表明：中国8个典型地区夏季平均气溶胶光学厚度（Aerosol Optical Depth,AOD）、云光学厚度（Cloud Optical Depth,COD）、云水路径（Cloud Water Path,CWP）、水云云滴有效粒子半径（Cloud Effective Radius Water,CERW）、冰云云滴有效粒子半径（Cloud Effective Radius Ice,CERI）和降水强度变化范围分别为0.21—1.05、15.01—24.02、151.98—219.20 g·m^-2、12.93—15.37 μm、28.85—39.14 μm和0.44—8.54 mm·d^-1;黄土高原和四川盆地AOD有显著降低趋势,年倾向分别为-2.30%和-3.20%,长江三角洲COD年增幅为29.11%,华北平原、长江三角洲和珠江三角洲CERI及塔克拉玛干沙漠CERW变化趋势分别为-21.60%、-15.77%、-18.94%和-10.31%;AOD与COD和CWP呈正相关,与云滴有效粒子半径（Cloud Effective Radius,CER）关系较为复杂,受水汽影响较大,在云层含水量较低的情况下,CERI（CERW）与AOD呈负（正）相关,而在云层含水量较高的情况下,二者呈正（负）相关;气溶胶和降水关系复杂,整体来看,气溶胶促进了中国地区的夏季降水。     

Underestimation of the Warming Trend over the Tibetan Plateau during 1998–2013 by Global Land Data Assimilation Systems and Atmospheric Reanalyses

Accurate surface air temperature (T2m) data are key to investigating eco-hydrological responses to global warming. Because of sparse in-situ observations, T2m datasets from atmospheric reanalysis or multi-source observation-based land data assimilation system (LDAS) are widely used in research over alpine regions such as the Tibetan Plateau (TP). It has been found that the warming rate of T2m over the TP accelerates during the global warming slowdown period of 1998–2013, which raises the question of whether the reanalysis or LDAS datasets can capture the warming feature. By evaluating two global LDASs, five global atmospheric reanalysis datasets, and a high-resolution dynamical downscaling simulation driven by one of the global reanalysis, we demonstrate that the LDASs and reanalysis datasets underestimate the warming trend over the TP by 27%–86% during 1998–2013. This is mainly caused by the underestimations of the increasing trends of surface downward radiation and nighttime total cloud amount over the southern and northern TP, respectively. Although GLDAS2.0, ERA5, and MERRA2 reduce biases of T2m simulation from their previous versions by 12%-94%, they do not show significant improvements in capturing the warming trend. The WRF dynamical downscaling dataset driven by ERA-Interim shows a great improvement, as it corrects the cooling trend in ERA-Interim to an observation-like warming trend over the southern TP. Our results indicate that more efforts are needed to reasonably simulate the warming features over the TP during the global warming slowdown period, and the WRF dynamical downscaling dataset provides more accurate T2m estimations than its driven global reanalysis dataset ERA-Interim for producing LDAS products over the TP.     

Trends in monthly precipitation over the northwest of Iran (NWI)

Increasing global temperatures during the last century have had their own effects on other climatic conditions, particularly on precipitation characteristics. This study was meant to investigate the spatial and temporal monthly trends of precipitation using the least square error (LSE) approach for the northwest of Iran (NWI). To this end, a database was obtained from 250 measuring stations uniformly scattered all over NWI from 1961 to 2010. The spatial average of annual precipitation in NWI during the period of study was approximately 220.9–726.7 mm. The annual precipitation decreased from southwest to northeast, while the large amount of precipitation was concentrated in the south-west and in the mountainous areas. All over NWI, the maximum and minimum precipitation records occurred from March to May and July to September, respectively. The coefficient of variation (CV) is greater than 44 % in all of NWI and may reach over 76 % in many places. The greatest range of CV, for instance, occurred during July. The spatial variability of precipitation was consistent with a tempo-spatial pattern of precipitation trends. There was a considerable difference between the amounts of change during the months, and the negative trends were mainly attributed to areas concentrated in eastern and southern parts of NWI far from the western mountain ranges. Moreover, limited areas with positive precipitation trends can be found in very small and isolated regions. This is observable particularly in the eastern half of NWI, which is mostly located far from Westerlies. On the other hand, seasonal precipitation trends indicated a slight decrease during winter and spring and a slight increase during summer and autumn. Consequently, there were major changes in average precipitation that occurred negatively in the area under study during the observation period. This finding is in agreement with those findings by recent studies which revealed a decreasing trend of around 2 mm/year over NWI during 1966–2005.

     

Variability of the Antarctic Ozone Anomaly in 2011–2018

The variability of parameters of the Antarctic ozone anomaly is studied using data of the TOMS/OMI satellite monitoring of the ozone layer, MERRA-2 reanalysis, and balloon sounding of the vertical distribution of ozone and temperature at the South Pole. The dynamic processes in the Antarctic stratosphere which define conditions for the significant ozone layer destruction are analyzed. Despite the decrease in the concentration of ozone-depleting substances, the significant ozone loss in the recent 8 years was observed in the Antarctic in 2011 and 2015.     

2015—2019年甘肃平凉地区夏季短时强降水时空分布及天气形势特征

选取了甘肃平凉地区2015-2019年6-8月发生的27次短时强降水事件。利用自动站逐小时降水资料和高空探测资料,按照短时强降水阈值分类统计法和常规天气分析方法对甘肃平凉地区夏季短时强降水时空分布、影响天气系统及大气环境背景进行了统计分析。结果表明：6月短时强降水频次少,强降水高发区为六盘山山区;7月频次明显增多,活跃地区为静宁、崇信、泾川一带;8月频次及强度达到峰值,密集区为平凉北部的崆峒山区、崇信南部河谷地带。6月短时强降水日变化呈现单峰型特征,以午后居多,1 h降水量在20-30 mm段发生频次最高,占6月频次的80%以上;7月和8月日变化呈多峰型结构,夜间短时强降水频次增多,1 h降水量大于30 mm的频次显著增加,约占7月和8月总频次的40%。分析总结了平凉地区短时强降水天气类型,即高空低槽类、副热带高压类、西北气流类。另外,对表征动力、水汽、不稳定条件的环境参数统计分析,结果显示K指数、CAPE、T₈₅₀-T₅₀₀、Q₈₅₀、θ_se-850等物理量平均特征值对平凉地区强降水预报有较好的指示意义。     

Shift changes and monotonic trends in autocorrelated temperature series over Iran

Temperature data from 29 synoptic stations in Iran for a period of 40?years (1966–2005) were analyzed to test for the existence of monotonic trends and shift changes in the annual, seasonal, and monthly mean air temperature series using the Mann–Kendall and Mann–Whitney tests. The influences of significant lag-1 serial correlation were eliminated from data by the trend-free pre-whitening method prior to the trend analysis. The magnitude of the temperature trends was derived from the Theil–Sen’s slope estimator. It was found that annual mean air temperature increased at 25 out of the 29 stations, of which 17 stations showed significant monotonic trends. The magnitude of the annual mean air temperature trends averagely was (+)0.224°C per decade. Most of the stations with the significant positive monotonic trends had a significant upward shift change. The analysis indicated that the change point year of the significant upward shift changes was 1972 for the whole stations except the coastal ones. Moreover, the strongest monotonic increasing trends and upward shift changes were observed in summer especially in August and September. The spatial analysis of the mean air temperature trends revealed the highest numbers of significant monotonic trends in the big cities of Iran. These findings provide more insights for better understanding of regional temperature behavior in the study area.     

Low precipitation over Greece during 1989–1990

Summary The unusual low precipitation observed during 1989–1990 in Greece is the subject of the present study. Statistical comparisons of this drought period with other dry periods in the historical records, possible statistical cycles, (periodicities), or general trends, together with the behaviour of the long time series are investigated. Power spectra of the annual rainfall series are also plotted considering the 95 years of data for the most reliable station in Greece. Furthermore, the drought persistence between successive months is also analysed using standard methods. The analysis identified periodicities of 2.1 and 3.6 years while no significant trends were observed in almost all cases. Finally the persistency of successive dry days was found to be insignificant after the fourth day.With 5 Figures     

Temperature and Precipitation Changes in China During the Holocene

We review here proxy records of temperature and precipitation in China during the Holocene, especially the last two millennia. The quality of proxy data, methodology of reconstruction, and uncertainties in reconstruction were emphasized in comparing different temperature and precipitation reconstruction and clarifying temporal and spatial patterns of temperature and precipitation during the Holocene. The Holocene climate was generally warm and wet. The warmest period occurred in 9.6-6.2 cal ka BP, whereas a period of maximum monsoon precipitation started at about 11.0 cal ka BP and lasted until about 8.0-5.0 cal ka BP. There were a series of millennial-scale cold or dry events superimposed on the general trend of climate changes. During past two millennia, a warming trend in the 20th century was clearly detected, but the warming magnitude was smaller than the maximum level of the Medieval Warm Period and the Middle Holocene. Cold conditions occurred over the whole of China during the Little Ice Age （AD 1400-AD 1900）, but the warming of the Medieval Warm Period （AD 900-AD 1300） was not distinct in China, especially west China. The spatial pattern of precipitation showed significant regional differences in China, especially east China. The modern warm period has lasted 20 years from 1987 to 2006. Bi-decadal oscillation in precipitation variability was apparent over China during the 20th century. Solar activity and volcanic eruptions both were major forcings governing the climate variability during the last millennium.     

Spatiotemporal changes in the size and shape of heat waves over North America

Heat waves are occurring more frequently across the globe and are likely to increase in intensity and duration under climate change. Much work has already been completed on attributing causes of observed heat waves and on modeling their future occurrence, but such efforts are often lacking in exploration of spatial relationships. Based on principles of landscape ecology, we utilized fragmentation metrics to examine the spatiotemporal changes in heat wave shape and occurrence across North America. This methodological approach enables us to examine area, shape, perimeter, and other key metrics. The application of these shape metrics to high-resolution historical (1950–2013) climate data reveals that the total number and spatial extent of heat waves are increasing over the continent, but at an individual heat wave patch level, they are becoming significantly smaller in extent and more complex in shape, indicating that heat waves have become a more widespread and fragmented phenomena.     

Homogeneity analysis of precipitation series in Iran

Assessment of the reliability and quality of historical precipitation data is required in the modeling of hydrology and water resource processes and for climate change studies. The homogeneity of the annual and monthly precipitation data sets throughout Iran was tested using the Bayesian, Cumulative Deviations, and von Neumann tests at a significance level of 0.05. The precipitation records from 41 meteorological stations covering the years between 1966 and 2005 were considered. The annual series of Iranian precipitation were found to be homogeneous by applying the Bayesian and Cumulative Deviations tests, while the von Neumann test detected inhomogeneities at seven stations. Almost all the monthly precipitation data sets are homogeneous and considered as “useful.” The outputs of the statistical tests for the homogeneity analysis of the precipitation time series had discrepancies in some cases which are related to different sensitivities of the tests to break in the time series. It was found that the von Neumann test is more sensitive than the Bayesian and Cumulative Deviations tests in the determination of inhomogeneity in the precipitation series.