1957~2015年我国春季短期温度波动规律

为研究全球变暖背景下我国短期温度波动变化规律,利用全国804个气象站点的逐日气温资料,计算隔日平均温差大于1℃的天数以及隔日温差的变化幅度,分析了1957~2015年我国短期温度波动频率与波动幅度的时空分布与变化规律,探讨了春季短期温度波动与经纬度及海拔之间的关系。结果表明:1957~2015年我国春季短期温度波动大致分为两个阶段:1990年以前我国大部分地区春季短期温度波动频率及幅度没有明显变化趋势,1990年以后我国大部分地区（约70%）春季短期温度波动呈上升趋势。此外,我国春季短期温度波动规律有地域性差异,表现为与经纬度关系密切,波动频率与波动幅度与经纬度等线性相关系数较高。结合波动趋势分布与隔日温度波动的地域分布可知,处于低纬度地区（如广东、广西等地）波动天数和波动幅度均增加,造成人们对于"天气变化无常"的感受强烈。由于生物生长发育等过程与温度密切相关,因此研究结果提示在讨论气候变化对生态系统长期影响时可能还需要关注短期的温度波动情况。     

Paleocommunity Replacements of Benthic Brachiopod in the Middle-Upper Devonian in the Longmenshan Area,Southwestern China:Responses to Sea Level Fluctuations

Twelve paleocommunities dominated by benthic brachiopod are recognized in the Givetian-Frasnian stages of the Devonian in the Longmenshan area, southwestern China, in which two kinds of brachiopod community replacement are classified. One is the abrupt replacement, represented by abrupt alternation between the Leiorhynchus community and Zhonghuacoelia-Striatopugnax community in the Frasnian Tuqiaozi Formation. The other is the gradual one, developed in the Givetian Guanwushan Formation, which had been completed by the shift of the Independatrypa lemma-Uncinulus heterocostellis-Emanuella takwanensis community via the SinospongophyUum irregulare-Pseudomicroplasma fongi community to the Clathrocoilona spissa-Hexagonaria composite reef community. According to analyses of the paleocommunities, either the abrupt or gradual paleocommunity replacement of the Middle-Upper Devonian in the Longmenshan area is suggested as a response to the 5th-order sea level fluctuation due to the replacements of the paleocommunities in a tracts-system of depositional sequence. It is supposed that changes of paleocommunity diversity,one of the results of paleocornmunity replacement, are depended on the range and magnitude of sea level fluctuation, but there is not a linear relationship between them. Furthermore, a suggestion is proposed that the concept of paleocommunity succession seem to be abandoned in the paleocommunity analysis because it almost never be practiced to recover the information of community succession in the geological record at present.     

Groundwater recharge estimation in arid hardrock‐alluvium aquifers using combined water‐table fluctuation and groundwater balance approaches

This paper proposes an approach to estimate groundwater recharge using an optimization‐based water‐table fluctuation method combined with a groundwater balance model in an arid hardrock‐alluvium region, located at the Oman–United Arab Emirates border. We introduce an “effective hardrock thickness” term to identify the percentage of the considered hardrock thickness in which effective groundwater flow takes place. The proposed method is based upon a Thiessen polygon zoning approach. The method includes subpolygons to represent specific geologic units and to enhance the confidence of the estimated groundwater recharge. Two linear and 1 nonlinear submodels were developed to evaluate the model components for the calibration (October 1996 to September 2008) and validation (October 2008 to September 2013) periods. Long‐term annual groundwater recharge from rainfall and return flow over the model domain are estimated as 24.62 and 5.71 Mm³, respectively, while the effective groundwater flow circulation is found to occur in the upper 7% of the known hardrock thickness (42 m), confirming conclusions of previous field studies. Considering a total difference in groundwater levels between eastern and western points of the study area of the order of 220 m and a 12‐year monthly calibration period, a weighted root mean squared error in predicted groundwater elevation of 2.75 m is considered quite reasonable for the study area characterized by remarkable geological and hydrogeological diversity. The proposed approach provides an efficient and robust method to estimate groundwater recharge in regions with a complex geological setting in which interaction between fractured and porous media cannot be easily assessed.     

Detrending methods for fluctuation analysis in hydrology: amendments and comparisons of methodologies

Detrending is a key step in the study of the scaling behaviors using Detrended Fluctuation Analysis (DFA) to explore the long‐range correlation of hydrological series. However, the irregular periodicity and various trends within hydrological series as a result of integrated influences of human activities such as construction of water reservoirs and human withdrawal of freshwater and climate changes such as alterations of precipitation changes in both space and time make difficult the selection of detrending methods. In this study, we attempt to address the detrending problem due to the important theoretical and practical merits of detrending in DFA‐based scaling analysis. In this case, with focus on the irregularity of the periodic trends, a modified DFA, varying parameter DFA (VPDFA), and its combination with adaptive detrending algorithm (ADA) are employed to eliminate the influences of irregular cycles on DFA‐based scaling results. The results indicate that, for streamflow series with no more than 20 cycles, VPDFA is recommended; otherwise, the combined method has to be employed. Comparison study indicates that the scaling behavior of the detrended observed streamflow series by average removed method, when compared to those by DFA, VPDFA, and ADA, is the one of the periodic residues around the averaged annual cycle for the entire series rather than that excluding all annual cycles. However, although the result by VPDFA for short observed streamflow record can well correspond to that for numerically simulated series, the scaling behavior obtained by combined method analyzing long record looks strange and is different from that by numerical analysis. We attribute this difference to the complicated hydrological structure and the possible hydrological alternation due to the increasing integrated impacts of human activities and human activities with the extending record. How to include the most of the important factors into the detrending procedure is still a challenging task for further study in the analysis of the scaling behavior of hydrological processes. Copyright © 2012 John Wiley & Sons, Ltd.     

Water uptake by saltcedar (Tamarix ramosissima) in a desert riparian forest: responses to intra‐annual water table fluctuation

There is considerable interest in naturalizing flow regime on managed rivers to slow the spread of saltcedar (Tamarix ramosissima) invasion in southwestern USA or to preserve riparian forests dominated by saltcedar and other species in northwestern China. However, little is known about the responses of established saltcedar in water sources to frequent intra‐annual fluctuation of water table resulting from this new, more dynamic flow regime. This study investigates how saltcedar at a riparian site in the middle reaches of the Heihe River, northwest China, responds in water sources use to intra‐annual water table fluctuations. Stable oxygen isotope was employed to determine accurate depth at which saltcedar obtains its water supply, and soil moisture monitoring was used to determine sources of plant‐available soil water. We found that the primary zone of water uptake by saltcedar were stable at 25–60 cm depth, but the water sources used by saltcedar switched between groundwater and soil moisture with the water table fluctuations. Saltcedar derived its water from groundwater when water table was at depth less than 60 cm but switched to soil moisture at 25–60 cm depth when water table declined. It is supposed that the well‐developed clay layer at 60–80 cm depth constrained lateral roots of saltcedar to the soil layers above 60 cm, while the fine‐textured soils at this site, which were periodically resaturated by rising groundwater before the stored soil moisture had become depleted, provided an important water reservoir for saltcedar when groundwater dropped below the primary zone of fine roots. The root distribution of saltcedar may also be related to local groundwater history. The quick decline in water table in the early 1980s when the riparian saltcedar had established may strand its roots in the shallow unsaturated zone. We suggested that raising the water table periodically instead of maintaining it invariably above the rooting depth could sustain desired facultative phreatophytes while maximizing water deliveries. Copyright © 2015 John Wiley & Sons, Ltd.     

密集颗粒流中的速度波动及自扩散特征

 密集颗粒流是颗粒介质中非常重要的一种运动状态,其内部单个粒子的速度波动通常是导致系统出现输运、混合、分离等宏观过程的微观基础,因此,如何对其进行定量描述是颗粒介质研究中一个非常关键的问题。基于所建立的环形剪切颗粒流的离散元模型,首先分析了系统达到剪切稳定状态时不同局部区域内微观粒子速度波动的宏观统计特征;通过引入自扩散系数定量研究了剪切颗粒流内粒子扩散运动过程中的局部化和各向异性特征;最后讨论了局部剪切率、初始体积分数以及粒子间的摩擦系数对系统自扩散系数的影响规律。     

一次大暴雨过程中尺度涡旋系统特征分析

利用常规观测资料与加密自动站资料、卫星云图、多普勒雷达与微波辐射计资料以及NCEP/NCAR 1°×1°再分析资料,对2011年7月29—30日发生在天津及华北东部的大暴雨过程进行了分析。结果表明：在有利的环流背景下,迅速发展北上的低空中尺度涡旋是此次大暴雨的直接影响系统。高空槽前正涡度平流加强,造成低层减压、中尺度涡旋发展;涡旋中心向负变压梯度绝对值最大方向移动。分析中尺度涡旋垂直结构表明,开始时涡旋内斜升气流明显,随其发展,低层辐合明显加强、涡旋内转为一致的垂直上升运动,且最大辐合中心与正涡度中心相对应,均位于900 hPa以下。随低层东南气流加强,涡旋右前侧偏东入流显著加强,其不但为涡旋发展提供了有利的动力条件,也为暴雨发生和维持提供了充足的水汽。此次降水的强回波高度较低,钩状回波及中尺度气旋均在低层发展,造成强降水和短时大风,有别于典型钩状回波。水汽密度及液态水含量变化与降水对应非常好,强降水前15～30 min低层大气水汽密度和液态水含量会迅速增大,这可为强降水提前预报预警提供参考依据。     

南京机场降水低能见度雨滴谱特征

对不同种降水对能见度的影响进行研究.选取连续性降水天气和雷暴天气两次过程的雨滴谱资料,计算后发现雷暴降水出现低能见度的雨滴谱较宽,而连续性降水雨滴谱的分布窄.粒子浓度和能见度呈反相关,在粒子浓度降低时能见度值有迅速的好转.在低能见度阶段小粒子的含量明显偏大些,1 mm以下的小粒子浓度对能见度的影响最大,其次是1～2 mm的小粒子,小粒子的高浓度对应着低能见度.出现降水并低能见度的时段时各特征量均为相对高值,能见度好转后突变转好并振荡,呈现相关.计算能见度在高粒子浓度和低能见度时与实测能见度接近.     

一种转阀换向的液压双缸单作用往复泵设计

为解决往复泵恒压力恒流量输出问题,设计了一种液压双缸单作用泵。其液压系统由双油泵供油,采用带单向机构的转阀与普通换向阀相结合的油量分配方式,转阀动作由活塞行程控制,转阀动作精度不受主油泵变量的影响,系统控制可靠。该泵在实际应用中取得了良好的效果。     

Modeling of twin-turbine systems with vertical axis tidal current turbine: Part II—torque fluctuation

We recently showed the advantage of using a numerical system to extract energy from tidal currents by developing a new twin-turbine model (Li and Calisal, 2010a). Encouraged by this result, we decided to use this model to study another important characteristic of the turbine system, torque fluctuation. This effort is summarized in this paper. The torque fluctuation is expected to reduce the fatigue life of tidal current turbines, though potentially it also may deteriorate the power quality of tidal current turbines. In this paper, after reviewing the twin-turbine model, we use it to predict the torque fluctuation of the system with the same configurations as we used to study the power output in Li and Calisal (2010a). Specifically, we investigate the torque fluctuation of twin-turbine systems with various turbine parameters (e.g., relative distance between two turbines and incoming flow angle) and operational condition (e.g., tip speed ratio). The results suggest that the torque of an optimally configured twin-turbine system fluctuates much less than that of the corresponding stand-alone turbine, under the same operating conditions. We then extensively compare the hydrodynamic interaction’s impact on the torque fluctuation and the power output of the system. We conclude that the hydrodynamic interactions pose more constructive impacts on the torque fluctuation than on the power output. The findings indicate that the optimally configured counter-rotating system should be a side-by-side system, and that the optimally configured co-rotating system should have the downstream turbine partially in the wake of the upstream turbine depending on the detailed configuration of the turbines. Furthermore, one must balance the optimal torque fluctuation against the optimal power output.