Hydrometeorological processes in semi-arid western India: insights from long term isotope record of daily precipitation

Long term (2005–2016) daily precipitation isotope data (δ18O, δD and d-excess) from Ahmedabad in semi-arid Western India are examined in light of various meteorological parameters and air parcel trajectories to identify prominent patterns in the isotopic character and discern the underlying hydrometeorological processes. One of the most prominent and systematic annual patterns is the isotopic depletion (average δ18O: − 2.5‰ in Jun–Jul; − 5.2‰ in Aug–Sept) in the second half of the Indian Summer Monsoon (ISM), which is observed in the 11 out of the 12 years of this study. Four geographically feasible causal factors have been examined if they contribute to observed late monsoon isotopic depletion. These factors are: (1) increased contribution of terrestrially recycled vapor; (2) intra-seasonal change in sea-surface, surface-air and cloud base temperatures; (3) increased rain-out fraction from marine vapor parcel; and (4) increase in relative proportion of convective rain. It is inferred from the present study that isotopic depletion in the second half of ISM is associated with: (1) increased contribution (45% from 36%) of terrestrially recycled moisture; (2) 1.9° C lower cloud base temperature; (3) increased rainout fraction due to decreased wind velocity (6.9 m/s from 8.8 m/s); and (4) an increase of 22.3% in the proportion of convective rain. Daily rain events with atypical isotopic composition (20‰ < d-excess < 0‰) are ascribed mainly to local weather perturbations causing sudden updraft of moist air facilitating terrestrial recycling of water vapor.     

Synoptic and mesoscale controls on the isotopic composition of precipitation in the western United States

We present a new event-scale catalog of stable isotopic measurements from 5?years of storm events at 4 sites in southern California, which is used to understand the storm to storm controls on the isotopic composition of precipitation and validate the event-scale performance of an isotope-enabled GCM simulation (IsoGSM) (Yoshimura et?al. 2008). These analyses are motivated to improve the interpretation of proxy records from this region and provide guidance in testing the skill of GCMs in reproducing the hydrological variability in the western US. We find that approximately 40% of event-scale isotopic variability arises from the percentage of precipitation that is convective and the near surface relative humidity in the days prior to the storms landfall. The additional isotopic variability arises from the fact that storms arriving from different source regions advect moisture of distinct isotopic compositions. We show using both field correlation and Lagrangian trajectory analysis that the advection of subtropical and tropical moisture is important in producing the most isotopically enriched precipitation. The isotopic catalog is then used along with satellite-derived δD retrievals of atmospheric moisture to benchmark the performance of the IsoGSM model for the western US. The model is able to successfully replicate the observed isotopic variability suggesting that it is closely reproducing the moisture transport and storm track dynamics that drive the large storm-to-storm isotopic range. Notably, we find that an increase in moisture flux from the central tropical Pacific leads to a convergence of isotopically enriched water vapor in the subtropics and consequently an increase in δ¹⁸O of precipitation at sites along the entire west coast. Changes in poleward moisture flux from the central Tropical Pacific have important implications for both the global hydrological cycle and regional precipitation amounts and we suggest such changes can be captured through instrumental and proxy-reconstruction of the spatiotemporal isotopic patterns in the precipitation along the west coast of the US.     

Snow precipitation at four ice core sites in East Antarctica: provenance, seasonality and blocking factors

Snow precipitation is the primary mass input to the Antarctic ice sheet and is one of the most direct climatic indicators, with important implications for paleoclimatic reconstruction from ice cores. Provenance of precipitation and the dynamic conditions that force these precipitation events at four deep ice core sites (Dome C, Law Dome, Talos Dome, and Taylor Dome) in East Antarctica were analysed with air mass back trajectories calculated using the Lagrangian model and the mean composite data for precipitation, geopotential height and wind speed field data from the European Centre for Medium Range Weather Forecast from 1980 to 2001. On an annual basis, back trajectories showed that the Atlantic-Indian and Ross-Pacific Oceans were the main provenances of precipitation in Wilkes Land (80%) and Victoria Land (40%), respectively, whereas the greatest influence of the ice sheet was on the interior near the Vostok site (80%) and in the Southwest Ross Sea (50%), an effect that decreased towards the coast and along the Antarctic slope. Victoria Land received snowfall atypically with respect to other Antarctica areas in terms of pathway (eastern instead of western), seasonality (summer instead of winter) and velocity (old air age). Geopotential height patterns at 500 hPa at low (>10 days) and high (2–6 days) frequencies during snowfall cycles at two core sites showed large positive anomalies at low frequencies developing in the Tasman Sea-Eastern Indian Ocean at higher latitudes (60–70°S) than normal. This could be considered part of an atmospheric blocking event, with transient eddies acting to decelerate westerlies in a split region area and accelerate the flow on the flanks of the low-frequency positive anomalies.     

Comparison of trends and frequencies of drought in central North China and sub-Saharan Africa from 1901 to 2010

This study compares the trends and frequencies of drought between central North China(CNC) and(SSA) for the periods 1901–2010 and 1951–2010. The Standardized Precipitation Evapotranspiration Index(SPEI) and Self-Calibrating Palmer Drought Severity Index(sc-PDSI) are used to assess the drought trends and frequencies. In general, the results exhibit downward trends of drought index values and upward trends of drought frequencies over CNC and SSA. A high rate of the trends' slopes for the drought index and a low rate of the frequencies' slopes is found over CNC with respect to SPEI and sc-PDSI. Furthermore, some abrupt changes are revealed after applying the sequential Mann–Kendall test to detect change points. These findings offer insight into the trends and frequencies of drought over the regions studied. Further analysis needs to be undertaken to understand the mechanisms underlying the occurrence of drought in these areas.     

Assessment of future olive crop yield by a comparative evaluation of drought indices: a case study in western Turkey

A comparative performance analysis was studied on well-known drought indices (Standardized Precipitation Index, Palmer Drought Severity Index (PDSI) and its moisture anomaly index (Orig-Z), and self-calibrated Palmer Drought Severity Index (SC-PDSI) and its moisture anomaly index (SC-Z)) to determine the most appropriate index for assessing olive (O. europaea L.) yield for oil in seven crop regions (Mu?la, Ayd?n, ?zmir, Manisa, Bal?kesir, ?anakkale, and Bursa) in western Turkey and to evaluate the vulnerability of olive yield for oil to climate change with future projections provided by the Hadley Centre for Climate Prediction and Research ENSEMBLES project (HadCM3Q0). A series of curvilinear regression-based crop yield models were developed for each of the olive-growing regions based on the drought indices. The crop yield model that performed the best was the SC-PDSI model in Mu?la, Ayd?n, ?zmir, and Manisa regions and the PDSI model in ?anakkale, Bal?kesir, and Bursa regions. The SC-PDSI index-based model described 65%, 62%, 61%, and 62% of the measured variability of olive yield in Mu?la, Ayd?n, ?zmir, and Manisa regions, respectively. The PDSI index-based model explained 59%, 58%, and 64% of the measured variability of olive yield in Bal?kesir, ?anakkale, and Bursa regions, respectively. The vulnerability of the olive yield for oil to HadCM3Q0 future climate projections was evaluated for Ayd?n and ?anakkale regions due to the resolution of the regional climate model. In terms of the future scenarios, the expected decrease in olive yield residuals was 2.5?ton (10³ trees)^?1 and 1.78?ton (10³ trees)^?1 in Ayd?n and ?anakkale regions, respectively.     

Changes in precipitation extremes over arid to semiarid and subhumid Punjab,Pakistan

Asymmetrical monsoons during the recent past have resulted into spatially variable and devastating floods in South Asia. Analysis of historic precipitation extremes record may help in formulating mitigation strategies at local level. Eleven indices of precipitation extremes were evaluated using RClimDex and daily time series data for analysis period of 1981–2010 from five representative cities across Punjab province of Pakistan. The indices include consecutive dry days, consecutive wet days, number of days above daily average precipitation, number of days with precipitation ≥10 mm, number of days with precipitation ≥20 mm, very wet days, extremely wet days, simple daily intensity index, maximum 1-day precipitation quantity, maximum 5 consecutive day precipitation quantity, and annual total wet-day precipitation. Mann-Kendall test and Sen’s slope extremes were used to detect trends in indices. Droughts and excessive precipitation were dictated by elevation from mean sea level with prolonged dry spells in southern Punjab and vice versa confirming spatial trends for precipitation extremes. However, no temporal trend was observed for any of the indices. Summer in the region is the wettest season depicting contribution of monsoons during June through August toward devastating floods in the region.     

台风“巴威”外围致山东半岛西部强降水过程的中尺度特征及环境条件

利用自动气象站观测资料、青岛雷达产品以及“天衍”雷达拼图产品和ERA5再分析资料,对台风“巴威”外围致山东半岛西部强降水过程的中尺度特征及环境条件进行分析。结果表明：1）“巴威”在黄海北上期间,其外围暖湿气流与冷空气在山东半岛西部到鲁东南交汇,对流层中低层形成东北—西南向深厚的切变线,高层处于高空急流入口区右侧,低层辐合、高层辐散有利于产生强降水,强降水位于850 hPa切变线及其右侧偏东风一侧。2）前期降水回波先后表现为两条有组织的线形回波带,其形成、发展和移动与850 hPa切变线密切相关;后期切变线右侧偏东风气流中γ中尺度辐合不断触发单体新生,青岛即墨境内组合反射率因子CR、差分反射率因子ZDR、差分相移率KDP均显著增大,导致即墨南泉连续两个小时雨强大于100 mm?h-1。3）切变线附近垂直上升运动深厚,850 hPa以下水汽通量辐合较强,为中尺度系统提供了触发条件和水汽条件;850 hPa,θse暖舌位置与切变线一致,暖舌中心达352 K,为中尺度系统发生、发展提供了能量条件;对流层中高层弱冷空气对触发强对流天气起到一定作用。4）850 hPa以下水汽通量辐合量值≤-8×10-7 g?cm-2?hPa-1?s-1的区域与暴雨区基本吻合,水汽通量辐合中心及垂直上升运动中心越低越有利于出现强降水。     

Summer maximum temperature in northern France over the past century: instrumental data versus multiple proxies (tree-ring isotopes, grape harvest dates and forest fires)

Changes in maximum spring and summer temperature are expected to have impacts on plant phenology and the occurrence of forest fires. Homogenised instrumental records of maximum spring and summer temperature are available in northern France for the past century, as well as documentary records of grape harvest dates and forest fire frequencies. Here we provide a new proxy of seasonal climate obtained by the analysis of latewood tree ring cellulose isotopic composition (δ¹⁸O, δ¹³C and δD), from 15 living oak trees (Quercus petraea) sampled in the Fontainebleau forest, near Paris. For the past 30 years, we have conducted a study on the inter-tree (for oxygen isotopes) and inter-station (for oxygen and hydrogen) isotopic variability. Multiple linear regression statistical analyses are used to assess the response function of documentary and tree-ring isotopic records to a variety of climatic and hydrological parameters. This calibration study highlights the correlation between latewood tree-ring δ¹⁸O and δ¹³C, grape harvest dates and numbers of forest fire starts with maximum growing season (April to September) temperature, showing the potential of multiple proxy reconstructions to assess the past fluctuations of this parameter prior to the instrumental period.     

Variations in vapor pressure and standardized precipitation evapotranspiration index since AD 1861 over the western Himalaya in India: inference from tree ring-width records

A regional tree ring-width index chronology prepared from various tree core samples of the western Himalaya has been analyzed in relation to climate fluctuations. The correlation analysis of tree ring chronology shows significant positive correlations with regional rainfall and standardized precipitation evapotranspiration index (SPEI) and negative correlations with temperature and vapor pressure (VP) during the spring season. The correlation coefficients (CCs) of tree ring-width index chronology with rainfall, temperature, SPEI, and VP during 1901–1990 are 0.50, −0.49, 0.65, and −0.51, respectively. All CCs are significant at 0.1% level. The highly significant CCs between tree ring-width index chronology and SPEI indicate that tree growth over the western Himalaya is more sensitive to soil moisture availability than rainfall, whereas the rising VP is found to have a significant moisture stress condition to tree growth by accelerating the evapotranspiration, which is not conducive for the development of tree growth in the region. So, based on the strong association between tree ring-width index chronology and SPEI; the reconstructions of SPEI and VP are developed back to AD 1861, that show the long period of dryness during 1936–1963.     

Changes in winter cyclone frequencies and strengths simulated in enhanced greenhouse warming experiments: results from the models participating in the IPCC diagnostic exercise

The effect of enhanced greenhouse warming on the behaviour of mid-latitude cyclones is examined for changes in the total number of cyclone events and for changes in the number of intense events using the daily averaged mean sea level pressure simulated by coupled climate models participating in the IPCC AR4 (Fourth Assessment Report) diagnostic exercise. Results are presented for a set of scenarios which were produced using a wide range of increasing levels of greenhouse gases. For the enhanced greenhouse warming experiments, the models simulated a reduction in the total number of events and an increase in the number of intense events. This is a robust result, which essentially all the models exhibit. Comparison of the results for each of the scenarios shows that the magnitude of the changes in the number of simulated events increases with increasing levels greenhouse gas forcing used in the scenarios. Even though the numbers of events change, there is no apparent change seen in the geographical distribution of the events, i.e. there is no obvious change in the positions of the storm tracks seen on hemispheric charts. This was also evident in the results for the filtered variance of the meridional wind which was used as a proxy for cyclone activity. In spite of this, it is possible that small shifts in the storm tracks, which are difficult to resolve with the relatively coarse grid used for analysis, could occur.     

Seasonal relationship between temperature,precipitation and snow cover in a mountainous region

Summary An analysis of correlation coefficients for climatological data covering the period 1901–1994 or 1931–1994 for six locations in Switzerland has been made in order to highlight the relationships between temperature, precipitation (rain and snow) and snow in summer and in winter. The results show that colder summers tend to be associated with more precipitation, mainly in terms of the frequency of occurrence of precipitation, but also in terms of its abundancy. In winter, sites located at lower altitudes behave differently from those at higher elevations. At lower altitudes, warmer winters tend to be rainier and to have less snow (only a small part of winter precipitation falls in the form of snow). Above 1000–1500 m, correlations between temperature on the one hand, and precipitation or snow on the other, tend to be weaker than at lower elevations; warmer winters are associated with less snow but also with less precipitation in general, while the relationship between precipitation and snow is stronger.These results confirm that during cold periods of the past, such as Löbben Phase (1400 BC — 1230 BC) cold summers were probably linked to frequent and abundant precipitation. These conditions led to increased mortality as well as to population migrations. In terms of potential future global warming, if the current temperature/precipitation relationships remain unchanged, then warmer summers will likely be linked to a decrease in precipitation. Higher winter temperatures can be expected to lead to a general decrease of snow and to a decrease in precipitation, but only at higher elevations; warmer winters would conversely be associated with an increase in precipitation at lower altitudes.With 4 Figures     

2017年苏南一次特大暴雨高分辨率模拟及特征分析

利用高分辨率(水平3 km)数值模式,结合FNL分析及预报场资料、地面加密自动站观测资料和多普勒雷达资料,对2017年6月9-10日苏南一次因江淮气旋沿切变线东移造成的特大暴雨过程进行研究.首先对模式模拟结果做了降水、风场等多方面的验证,随后利用模拟结果和观测资料对该特大暴雨过程的对流系统结构及其发展维持机制进行分析....     

1953-2010年浙西南山区降水与旱涝变化特征

根据1953-2010年浙西南山区逐月降水资料,在对不同时间尺度降水进行气候分析的基础上,利用降水距平百分率研究年、季和月尺度上旱涝分布特征,并通过Morlet连续小波变化探讨旱涝演变趋势。结果表明：近58 a来,浙西南山区降水量和雨日均呈下降趋势,且雨日减少幅度较明显,约每10 a减少3.39 d。其中降水量以8月增多最显著,5月下降最明显;雨日则以5月和12月的减少最为突出。用Morlet小波分析得出旱涝存在不同尺度的周期分量,并且各周期强度有所不同,其中以4-8 a周期和16-20 a周期最为强烈,2010年后浙西南山区旱涝将向偏涝方向发展。年际尺度上,浙西南山区旱涝主要表现为轻旱和轻涝;季和月尺度上,以8-12月旱涝发生最为频繁,其中干旱概率多于洪涝,但洪涝强度大于干旱。