Estimation of drag coefficient over the western desert sector of the Indian summer monsoon trough

In the estimation of momentum fluxes over land surfaces by the bulk aerodynamic method, no unique value of the drag coefficient (C _D) is found in the literature. The drag coefficient is generally estimated from special observations at different parts of the world. In this study an attempt is made to estimate drag coefficient over the western desert sector of India using data sets of Monsoon Trough Boundary Layer Experiment (MONTBLEX) during the summer monsoon season of 1990. For this purpose, the fast and slow response data sets obtained simultaneously from a 30 m high micro-meteorological tower at Jodhpur are used. All the observations used in this study are confined to a wind speed regime of 2.5–9.0 ms⁻¹. A comparison of momentum fluxes computed by eddy correlation (direct estimation) with profile and bulk aerodynamic (C _D = 3.9 × 10⁻³, Garratt, 1977) methods revealed that though the nature of variation of the fluxes by all these methods is almost similar, both the indirect methods give an under-estimated value of the fluxes. The drag coefficient is estimated as a function of wind speed and surface stability by a multiple regression approach. An average value of the estimated drag coefficient is found to be of the order of 5.43 × 10⁻³. The estimated value ofC _D is validated with a set of independent observations and found to be quite satisfactory. The recomputed momentum fluxes by bulk aerodynamic method using the estimated drag coefficient are in close agreement with the directly estimated fluxes.     

Measured And Simulated Latent And Sensible Heat Fluxes At Two Marine Sites In The Baltic Sea

In this study, turbulent heat flux data from two sites within the Baltic Sea are compared with estimates from two models. The main focus is on the latent heat flux. The measuring sites are located on small islands close to the islands of Bornholm and Gotland. Both sites have a wide wind direction sector with undisturbed over-water fetch. Mean parameters and direct fluxes were measured on masts during May to December 1998.The two models used in this study are the regional-scale atmospheric model HIRLAM and the ocean model PROBE-Baltic. It is shown that both models overestimate the sensible and latent heat fluxes. The overestimation can, to a large extent, be explained by errors in the air-water temperature and humidity differences. From comparing observed and modelled data, the estimated 8-month mean errors in temperature and humidity are up to 1 °C and 1 g kg^-1, respectively. The mean errors in the sensible and latent heat fluxes for the same period are approximately 15 and 30 W m^-2, respectively.Bulk transfer coefficients used for calculating heat and humidity fluxes at the surface were shown to agree rather well with the measurements, at least for the unstable data. For stable stratification, the scatter in data is generally large, and it appears that the bulk formulation chosen overestimates turbulent heat fluxes.     

Linking moisture and near-surface wind with winter temperature to reveal the Holocene climate evolution in arid Xinjiang region of China

An increasing number of palaeo-climatic records have been reported to identify the Holocene climate history in the arid Xinjiang region of northwest China. However, few studies have fully considered the internal linkages within the regional climate system, which may limit our understanding of the forcing mechanisms of Holocene climate change in this region. Here, we systematically consider three major issues of the moisture/precipitation, temperature and near-surface wind relevant to the Holocene climate history of Xinjiang. First, despite there still has debated for the Holocene moisture evolution in this region, more climatic reconstructions from lake sediments, loess, sand-dunes and peats support a long-term regional wetting trend. Second, temperature records from ice cores, peats and stalagmites demonstrate a long-term winter warming trend during the Holocene in middle- to high-latitudes of Asia. Third, recent studies of aeolian sedimentary sequences reveal that the near-surface winds in winter gradually weakened during the Holocene, whereas the winter mid-latitude Westerlies strengthened in the Tienshan Mountains. Based on this evidence, in the arid Xinjiang region we propose an early to middle Holocene relatively cold and dry interval, with strong near-surface winds; and a warmer, wetter interval with weaker near-surface winds in the middle to late Holocene during winter. Additionally, we develop a conceptual model to explain the pattern of Holocene climate changes in this region. From the early to the late Holocene, the increasing atmospheric CO₂ content and winter insolation, and the shrinking of high-latitude continental ice-sheets, resulted in increasing winter temperatures in middle to high latitudes in the Northern Hemisphere. Subsequently, the increased winter temperature strengthened the winter mid-latitude Westerlies and weakened the Siberian high-pressure system, which caused an increase in winter precipitation and a decrease in near-surface wind strength. This scenario is strongly supported by evidence from geological records, climate simulation results, and modern reanalysis data. Our hypothesis highlights the important contribution of winter temperature in driving the Holocene climatic evolution of the arid Xinjiang region, and it implies that the socio-economic development and water resources security of this region will face serious challenges presented by the increasing winter temperature in the future.     

鄱阳湖湿地典型植被群落地下水—土壤—植被—大气系统界面水分通量及水源组成

地下水-土壤-植被-大气系统(GSPAC)界面水分传输是湿地生态水文过程研究的关键.本文选取鄱阳湖湿地高位滩地的2种典型植被群落:茵陈蒿(Artemisia capillaris)和芦苇(Phragmites australis)群落为研究对象,运用HYDRUS-1D垂向一维数值模拟,量化了湿地GSPAC系统界面水分通量,阐明了典型丰水年(2012年)和枯水年(2013年)鄱阳湖湿地植被群落的蒸腾用水规律和水源组成.结果表明:(1)茵陈蒿和芦苇群落土壤-大气界面的年降水入渗量为1570~1600 mm,主要集中在雨季4-6月,占年总量的60%;植物-大气界面的年蒸腾总量分别为346~470 mm和926~1057 mm,其中7-8月植被生长旺季最大,占年总量的40%~46%;地下水-根区土壤界面的向上补给水量受不同水文年水位变化的影响显著,地下水年补给量分别为15~513 mm和277~616 mm,主要发生在蒸散发作用强烈和地下水埋深较浅的时段.(2)植被蒸腾用水分为生长初期(4-6月)和生长旺季(7-10月)2个阶段,丰水年植被的整个生长期蒸腾用水充足,枯水年植被生长旺季的蒸腾用水受到严重水分胁迫,实际蒸腾量仅为潜在蒸腾量的一半左右.(3)不同水文年湿地植被生长旺季的水源贡献不同:丰水年茵陈蒿群落以地下水补给为主,芦苇群落以湖水和地下水补给为主;枯水年茵陈蒿群落以降水和前期土壤水储量为主,芦苇群落以地下水补给为主.本研究结果有助于揭示湿地植被的水分利用策略,为阐明湖泊水情变化与植被演替的作用机理提供参考依据.     

Understanding of the Common Land Model performance for water and energy fluxes in a farmland during the growing season in Korea

The Common Land Model (CLM) is one of the most widely used land surface models (LSMs) due to the practicality of its simple parameterization scheme and its versatility in embracing a variety of field datasets. The improved assessment of land surface water and energy fluxes using CLM can be an alternative approach for understanding the complex land–atmosphere interactions in data‐limited regions. The understanding of water and energy cycles in a farmland is crucial because it is a dominant land feature in Korea and Asia. However, the applications of CLM to farmland in Korea are in paucity. The simulations of water and energy fluxes by CLM were conducted against those from the tower‐based measurements during the growing season of 2006 at the Haenam site (a farmland site) in Korea without optimization. According to the International Geosphere–Biosphere Programme (IGBP) land cover classification, a homogeneous cropland was selected initially for this study. Although the simulated soil moisture had a similar pattern to that of the observed, the former was relatively drier (at 0·1 m³ m^?3) than the latter. The simulated net radiation showed good agreement with the observed, with a root mean squared error (RMSE) of 41 W m^?2, whereas relatively large discrepancies between the simulation and observation were found in sensible (RMSE of 66 W m^?2) and latent (RMSE of 60 W m^?2) heat fluxes. On the basis of the sensitivity analysis, soil moisture was more receptive to land cover and soil texture parameterizations when compared to soil temperature and turbulent fluxes. Despite the uncertainty in the predictive capability of CLM employed without optimization, the initial performance of CLM suggests usefulness in a data‐limited heterogeneous farmland in Korea. Further studies are required to identify the controls on water and energy fluxes with an improved parameterization. Copyright © 2010 John Wiley & Sons, Ltd.     

Fluid fluxes during low-temperature alteration: implications of multi-style alteration assemblages from the Welsh Borderland,UK

Localized (domainal) low-temperature alteration may indicate variations in the chemistry of alteration fluids and/or discrete fluid flow paths during metamorphism. Occurrences of epidote- and pumpellyite-dominated domains are often used as evidence for large fluxes of Ca-rich fluids. However, comparative studies of two domainal alteration styles from basaltic to andesitic lavas and volcaniclastic rocks at Builth Wells, Wales, UK suggest that such interpretations cannot be applied universally. Here, only one set of domains can be attributed to large fluxes of Ca-rich fluids. In contrast, the second set of domains formed where the host rock supplied the necessary Ca, and fluid/rock ratios were relatively low. These domains are hosted by rocks which show a Caledonian regional metamorphic imprint, characterized by the alteration assemblage albite+chlorite+titanite±prehnite±pumpellyite±calcite±muscovite±quartz (considered to have formed at c. 200°C and ≤2.5 kbar). Type 1 domains lie along fluid channelways, such as veins. Pumpellyite is the dominant secondary mineral, but the alteration assemblage is variable with pumpellyite±prehnite±calcite±titanite±chlorite (±quartz). The domains formed at temperatures of 130–230°C during Ordovician hydrothermal activity, before the peak of regional metamorphism. In contrast, Type 2 domains are not associated with obvious fluid channelways. A central prehnite-dominated zone typically has a rim with the assemblage pumpellyite+calcite±prehnite±chlorite±titanite±K-rich phyllosilicate. These domains probably formed from pelitic xenoliths which contained a volcaniclastic component at temperatures of 130–230°C and at relatively low fluid/rock ratios. However, the timing of domain formation is uncertain. Pyrobitumen, in veins and disseminated throughout the rock, may have influenced the formation of both types of domain by stabilizing pumpellyite in preference to epidote and causing metamorphic calcite formation through the release of CO₂ to the alteration fluids. © 1996 John Wiley & Sons, Ltd.     

南大洋普里兹湾基于234Th/238U不平衡法的POC输出通量研究

中国第22次南极科学考察(2005年11月至2006年3月)期间,测定了南极普里兹湾海域5个站位的从表层至150 m水深的不同层位水样中溶解态和颗粒态234Th,238U的放射性比活度以及颗粒有机碳.利用234Th/238U在上层水体中的不平衡,计算了南极普里兹湾上层水体中234Th的平均停留时间和输出通量.结果显示,随着纬度的增加,上层水体中颗粒态和溶解态234Th的平均停留时间总体趋向减小,并在中纬度站位出现了最低值,分别为1~8和29~48 d,而颗粒态和溶解态234Th的输出通量则在中纬度站位出现了最大值,分别为21~38和26~39 dpm/(m3·d).运用箱型清除模式,利用两种不同的方法估算了各水柱中从真光层底部输出的POC通量,平均值分别达到104.7 mmol/(m2·d)(E法)和120.6 mmol/(m2·d)(B法),表明南极普里兹湾夏季存在很高的新生产力,它将会对该海域碳的生物泵过程产生重要作用.     

Simultaneous water and vapour transfer in an unsaturated mudstone in badlands terrain,southwestern Taiwan

The simultaneous transfer of pore fluid and vapour was studied in the unsaturated shallow subsurface of a Plio-Pleistocene marine mudstone badland slope in southwestern Taiwan during the dry season using field monitoring data and numerical simulations. Data from field monitoring show mass-basis water contents of ~0.05 to ~0.10 that decrease towards the unsaturated ground surface and were invariant during the middle part of the dry season, except for daily fluctuations. In addition, the observed daily fluctuations in water content correlate with fluctuations in bedrock temperature, especially at depths of 2.5–5.0 cm. Periodic increases in water content occurred most notably during the day, when the bedrock temperature showed the greatest increase. Water contents then decreased to the previous state as bedrock temperature decreased during the night. Calculated vapour fluxes within the mudstone during the day increased up to 6 × 10⁻⁶–1 × 10⁻⁵ kg m⁻² s⁻¹, deriving a 0.01–0.02 increase in mass-basis water content at 2.5 cm depth for a 12-h period. This agrees with field monitoring data, suggesting that increases in water content occurred due to vapour intrusions into the bedrock. Pore water electrical conductivity (EC) showed periodic variations due to vapour intrusion, and gradually increased between the ground surface and depths of 2.5–5.0 cm. In contrast, pore water EC gradually decreased between 15 and 40 cm depth. Calculated water fluxes at depths of 2.5–40.0 cm varied from −4 × 10⁻⁶ to −2 × 10⁻⁹ kg m⁻² s⁻¹. These fluxes generated an increase in solute concentrations at the ground surface, with negative values of water flux indicating an upwards movement of water towards the surface. We show that the increase in solute content due to solute transfer from depth is highly dependent on variations in water flux with depth. © 2020 John Wiley & Sons, Ltd.     

前汛期北江洪水过程水汽汇与河水流量的关系

本文分析了１９８０－１９８４年４－６月和１９９４年６月洪水过程北江流域的大气水汽汇特征及其与河水流量的关系。结果表明：前汛期洪水过程的前期大乞一般连续存在超过一定值的较强水汽汇，时间为１周左右，洪水的发生是强水汽汇连续产生的径流的积累所致；北江流域洪水期水汽汇的极大峰一般比石角站的洪峰流量超前２天左右，本文结果为洪水的预报提供一条新思路。     

Modelling of the vertical fluxes of nitric acid,ammonia, and ammonium nitrate

Results from numerical investigations regarding the exchange of HNO₃, NH₃, and NH₄NO₃ between the atmosphere and the biosphere are presented. The investigations were performed with a modified inferential method which is based on the generally accepted micrometeorological ideas of the transfer of momentum, sensible heat and matter near the Earth's surface and the chemical reactions among these nitrogen compounds. This modified inferential method calculates the micrometeorological quantities (such as the friction velocity and the fluxes of sensible and latent heat), the height-invariant fluxes of the composed chemically conservative trace species with group concentrationsc ₁=[HNO₃]+[NH₄NO₃] (total nitrate),c ₂=[NH₃]+[NH₄NO₃] (total ammonia), andc ₃=[HNO₃]-[NH₃] as well as the fluxes of the individual nitrogen compounds. The parameterization of the fluxes is based on the flux-gradient relationships in the turbulent region of the atmospheric surface layer. The modified inferential method requires only the data of wind velocity, temperature, humidity and concentrations (HNO₃, NH₃, and NH₄NO₃) measured at a reference height by stations of a monitoring network.