Comparative study of MLT mean winds using MF radars located at 16.8°N and 8.7°N

The main aim of the paper is to describe mesosphere and lower thermosphere (MLT) wind field observed between June 2000 and May 2002 by medium frequency (MF) radars at two locations near the equatorial region and at tropical latitude. We have presented and compared observations of mean horizontal winds in the mesosphere and lower thermosphere (MLT) region over Kolhapur (16.8°N, 74.2°E) and Tirunelveli (8.7°N, 77.8°E). Our analysis reveals annual oscillation in the zonal wind and semi-annual oscillation in the meridional wind over Kolhapur. On the other hand, results over Tirunelveli reveal semi-annual oscillation (SAO) in the zonal wind and annual oscillation in the meridional wind. Also we have observed enhanced magnitude of wind speed in spring equinox period of 2002 exhibiting the signatures of the quasi-biennial oscillation (QBO) over Tirunelveli.     

Upper and lower tropospheric energetics of standing and transient eddies in wave number domain during summer monsoon of 1991

Kinetic energy exchange equations (Saltzman 1957) in wave number domain are partitioned into standing, transient and standing-transient components following Murakami (1978, 1981). These components are computed for the 1991 summer monsoon using dailyu andv grid point data at 2.5° latitude-longitude interval between the equator and 40°N at 200 hPa and 850 hPa levels for the period June through August. The data are obtained from NCMRWF, New Delhi. The study shows that at 200 hPa wave number 1 over Region 3 (30°N to 40°N), wave number 2 over Region 2 (15°N to 30°N) and wave number 3 over Region 1 (equator to 15°N) dominate the spectrum of transport of momentum and wave to zonal mean flow interaction. Wave number 1 over Region 1 and Region 3 and wave number 2 over Region 2 are the major sources of kinetic energy to other waves via wave-to-wave interaction. At 850 hPa wave number 1 over Region 3 has maximum contribution in the spectrum of transport of momentum and kinetic energy and more than 90% of its contribution is from the standing component. This indicates that standing wave number 1 over Region 3 plays a very important role in the dynamics of monsoon circulation of the lower troposphere. The study further shows that although the circulation patterns at 200 hPa and 850 hPa levels are opposite in character, a number of energy processes exhibit a similar character at these levels. For example, (i) transport of momentum by most of the waves is northward, (ii) small scale eddies intensify northward, (iii) eddies are sources of kinetic energy to zonal mean flow over Region 1 and (iv) standing eddies are sources of kinetic energy to transient eddies. Besides the above similarities some contrasting energy processes are also observed. Over Region 2 and Region 3 standing and transient eddies are sources of kinetic energy to zonal mean flow at 200 hPa, while at 850 hPa the direction of exchange of kinetic energy is opposite i.e. zonal mean flow is a source of kinetic energy to standing as well as transient eddies. L(n) interaction indicates that at 200 hPa waves over R2 maintain waves over R1, while at 850 hPa waves over R1 maintain waves over R2. It has been found that the north-south gradient of zonal mean of zonal wind is the deciding factor of wave to zonal mean flow interaction.     

Last Glacial Maximum to Early Holocene Wind Strength in the Mid-latitudes of the Southern Hemisphere from Aeolian Dust in the Tasman Sea

Dust transported by Southern Hemisphere mid-latitude westerly winds from Australia and deposited in the Tasman Sea shows no evidence for stronger winds during the last glacial maximum (LGM), compared to the Holocene. Features of the particle-size distributions of the dust do, however, indicate enhanced dry deposition of dust in the LGM changing to rainfall scavenging during deglaciation and the early Holocene as climate ameliorated. From these results it appears that activation of desert dunefields over 40% of Australia during the LGM was the result of a reduction in stabilizing vegetation and more frequent episodes of sand movement rather than of increased wind strength. The LGM climate of inland Australia must have been considerably more stressful for plants as a result of lower precipitation and/or carbon dioxide stress to achieve the implied levels of surface destabilization. Enhanced atmospheric dust loads in the Southern Hemisphere and deposition over Antarctica were most likely the result of greatly expanded source areas in the mid-latitude southern continents and a weaker hydrological cycle rather than greater entrainment or more efficient transport by stronger winds. During the LGM wind strength appears to have varied regionally, and predominantly in high latitudes, rather than uniformly for all zonal winds.     

Equatorial modes observed in atmospheric variables

Wavenumber-frequency spectral analysis of different atmospheric variables has been carried out using 25 years of data. The area considered is the tropical belt 25°S–25°N. A combined FFT-wavelet analysis method has been used for this purpose. Variables considered are outgoing long-wave radiation (OLR), 850 hPa divergence, zonal and meridional winds at 850, 500 and 200 hPa levels, sea level pressure and 850 hPa geopotential height. It is shown that the spectra of different variables have some common properties, but each variable also has few features different from the rest. While Kelvin mode is prominent in OLR and zonal winds, it is not clearly observed in pressure and geopotential height fields; the latter two have a dominant wavenumber zero mode not seen in other variables except in meridional wind at 200 hPa and 850 hPa divergences. Different dominant modes in the tropics show significant variations on sub-seasonal time scales.     

黑河上游降水同位素特征及其水汽来源分析

为了加深对黑河上游水循环过程的理解, 以研究区5个站点2015年8月至2016年8月的降水同位素实测数据和气象数据为基础, 除对降水同位素特征进行分析外, 主要利用TrajStat软件中的后向轨迹计算模块与潜在源贡献因子分析（PSCF）方法, 对研究区降水的水汽来源进行了分析, 并结合水汽通量等方法进行了补充分析。结果表明： 降水同位素呈夏高冬低趋势, 大气水线斜率（8.02）和截距（11.02）均高于全球大气水线的斜率（8.00）和截距（10.00）, 存在温度效应（δ¹⁸O=0.43x-10.82, r=0.54, P<0.0001）, 不存在降水量效应（δ¹⁸O=-0.05x-7.81, r=0.03, P<0.0001）; 研究区降水受多种水汽影响, 西风水汽影响最大。夏季除受西风水汽影响外, 还受东南季风水汽影响显著且水汽来源复杂; 研究区夏季的潜在蒸发源地集中在一些相对湿度和蒸发量较大的地区, 如祁连山区、 河西地区、 柴达木盆地北部、 青藏高原东南部及酒泉地区西南部等; 当降水量小、 温度高时, 持续性降水的大气水线方程的斜率和截距较小, 暴雨稳定同位素值较贫化, 受东南季风水汽影响最大, 其次是北方和西风水汽, 多种水汽辐合是暴雨事件发生的必要条件。     

Simulation of marine boundary layer characteristics using a 1-D PBL model over the Bay of Bengal during BOBMEX-99

The characteristic features of the marine boundary layer (MBL) over the Bay of Bengal during the southwest monsoon and the factors influencing it are investigated. The Bay of Bengal and Monsoon Experiment (BOBMEX) carried out during July–August 1999 is the first observational experiment under the Indian Climate Research Programme (ICRP). A very high-resolution data in the vertical was obtained during this experiment, which was used to study the MBL characteristics off the east coast of India in the north and south Bay of Bengal. Spells of active and suppressed convection over the Bay were observed, of which, three representative convective episodes were considered for the study. For this purpose a one-dimensional multi-level PBL model with a TKE-ε closure scheme was used. The soundings, viz., the vertical profiles of temperature, humidity, zonal and meridional component of wind, obtained onboard ORV Sagar Kanya and from coastal stations along the east coast are used for the study. The temporal evolution of turbulent kinetic energy, marine boundary layer height (MBLH), sensible and latent heat fluxes and drag coefficient of momentum are simulated for different epochs of monsoon and monsoon depressions during BOBMEX-99.The model also generates the vertical profiles of potential temperature, specific humidity, zonal and meridional wind. These simulated values compared reasonably well with the observations available from BOBMEX.     

Harmonic analysis of summer mean wind at 200 mbar level during contrasting monsoon years over India

Summer (June–August) mean zonal and meridional wind components at 200 mbar level are subjected to harmonic analysis for the years 1970, 1971, 1972 and 1979. It is found that the small scale disturbances are intense during normal monsoon years. The westerlies in the belt 10°S to 30°S are stronger during drought years. During normal monsoon years (1970, 1971) the northward transport of westerly momentum by wave number 1 at 19.6°N is large as compared to drought years (1972, 1979). The transport of westerly momentum by standing eddies is northward for all the years between 5°S and 28.7°N but large during the normal monsoon years.     

Role of barotropic,baroclinic and combined barotropic-baroclinic instability for the growth of monsoon depressions and mid-tropospheric cyclones

A detailed barotropic, baroclinic and combined barotropic-baroclinic stability analysis has been carried out with mean monsoon zonal currents over western India, eastern India and S.E. Asia. The lower and middle tropospheric zonal wind profiles over western India are barotropically unstable. The structure and growth rate of these modes agree well with the observed features of the midtropospheric cyclones. Similar profiles over eastern India and S.E. Asia, however, are barotropically stable. This is attributed to weak horizontal shear, inherent to these profiles. The upper tropospheric profiles, on the other hand, are barotropically unstable throughout the whole region. The features of these unstable modes agree with those of observed easterly waves. The baroclinic and combined barotropic-baroclinic stability analyses show that the baroclinic effects are not important in tropics. Though the barotropic instability of the mean zonal current seems to be res ponsible for the initial growth of the mid-tropospheric cyclones, neither barotropic nor baroclinic instability of the mean zonal current seem to explain the observed features of the monsoon depressions.     

末次盛冰期太平洋赤道辐合带对暖池外热带海表温度变化的敏感性

利用美国NCAR CAM3大气环流模式,分析了末次盛冰期（LGM）两个不同的热带海表温度重建方案中,北半球冬季热带中、西太平洋对流活动及大气环流对暖池外（赤道东太平洋和热带大西洋）热带SST异常的敏感性。结果表明：  1）SST异常首先引起大气环流的改变。  赤道东太平洋对流层下沉增强,而作为经向补偿,副热带东太平洋上升运动增强,其中南半球尤为明显,同时南半球热带中、西太平洋上升运动增强,加剧了该区纬向逆时针环流,说明冰期热带海气耦合过程受气候背景场（如SST）影响很大;   2）大气环流格局改变引起热带中西太平洋的大气加热、对流活动、表层风场及降雨的巨大变化。  140°E以西的婆罗洲和菲律宾区域,总的大气加热减少是由于对流与辐射加热减少所致,对应于该区风场辐散和降雨减少;   而140°E以东的南半球热带中、西太平洋,大气吸收热量增加,对流与辐射加热均增强,总降雨量也随之增加,反映该区赤道辐合带南移并增强。该项研究为探索热带太平洋在冰期/间冰期旋回中的古海洋学变化提供了新的数据支撑。此外,不同重建SST对赤道辐合带的影响比较大,因此利用重建SST进行数值模拟或者利用耦合模式研究LGM热带海气相互作用时,应该十分重视全球热带SST分布特征。     

Surface wind speed probability distribution in the Southeast Pacific of Marine Stratus and Stratocumulus regions

Probability distributions of surface wind speeds (SWS) near coastal regions are needed for applications such as estimating offshore wind power and ocean surface fluxes and for offshore wind risk assessments. Ocean surface wind speed probability distribution (PDF) is characterized using three-year QuikSCAT and AIRS satellite observations in the southeast Pacific of marine stratus and stratocumulus (MSC) regions. Seasonal variation is removed from wind statistics. It was found that the observed SWS standard deviation has a linear positive relationship with its mean SWS; while the SWS skewness decreases with mean SWS in regimes of strong winds and increases with mean SWS in regimes of weak winds. A simple 1D conceptual model is developed near the Peruvian region, which successfully reproduces the observed relationship between higher moments of SWS and its mean value. The model based physical picture among ocean surface winds, SST, and marine boundary clouds are supported by three-year QuikSCAT surface wind observations and fifteen-year ERA40 re-analysis data. Model sensitive tests suggest that large-scale divergence, and strengths of momentum and cloud fluctuations have significant effects on the ocean SWS-PDF in marine stratus and stratocumulus regions.     

Wind gradients at meteor heights over mid-latitude stations

The gradients of amplitude and phase of diurnal and semidiurnal components of zonal and meridional winds in the height intervals of 80–90 km and 90–100 km are studied using the meteor wind radar data collected at Atlanta (34°N, 84°W) during the period August 1974–March 1978. The results are compared with those at Adelaide (35°S, 139°E). It is found that the gradients vary in an opposite manner between the two height intervals.     

陆面过程模式对不同土壤物理性质的敏感性研究

使用NCAR陆面过程模式（LSM）和荷兰Cabauw1987的大气观测资料，考察了在11种不同的土壤条件下陆面过程模式所模拟的地表能量及水分循环的差异。结果表明，粘土含量较高的土壤具有较好的持水能力，蒸发量和径流量都比较大，而在含砂量高的土壤中，水分大量的下渗，蒸发量和径流量小，从而到地表热通量的分配，在不考虑大气反馈的情况下，各种土壤造成的差异主要出现在春季的4、5月份。     

青藏高原雨季降水的水汽条件研究

利用1961—2017年青藏高原109站降水量资料、NCEP全球逐月再分析资料,讨论了雨季期间高原的水汽输送特征.结果表明:高原雨季降水呈显著的年际变化特征,高原雨季降水主模态为南北反向型和全区一致型.气候态高原雨季的水汽输送路径为来自阿拉伯海的偏西风水汽输送,在孟加拉湾附近分为三支水汽输送气流:一支向北输送,自高原南...     

南水北调对北方干旱化趋势可能影响的初步分析

使用地表土壤水量平衡模式和大气能量平衡模式，对南水北调中线工程对华北地区的可能局地气候效应进行了虚拟模拟试验，得出了在不同调水方案和气候情景假定下华北地区可能出现的局地土壤水分、蒸发、温度和降水的变化。结果表明，不同调水量在不同时间、不同气候条件下的气候效应是有差别的，并与降水的季节变化有关。总体而言，在给定气候背景下调入水可以改变土壤水含量和径流，并使局地夏半年温度降低、冬半年温度有所升高；夏半年局地降水量和蒸发量有所增加，而冬半年变化不大。因此，在目前情况下南水北调中线工程的实施不仅可以减缓华北地区水资源紧缺的矛盾，而且有可能在一定程度上改善当地的局地干旱气候环境，进而有利于生态环境的良性循环和有序人类活动的可持续发展。     

Progress in the Research on the Impact of Wind Farms on Climate and Environment

Wind power has become one of the fastest growing renewable energy. With the large-scale deployment of wind farms in the world, people have started to pay attention to the impact of wind farms on the ecological environment and climate. This paper summarized the impact of wind farms on climate and ecological environment by investigating relevant literature: In the areas of wind farms, on the one hand, the set-up of wind turbines changes original aerodynamic roughness height and strengthens the dragging of the land surface against turbulence, directly affecting the turbulent motion of the boundary layer, resulting in the changes of intensity and pattern of material energy and water vapor exchange between land surface and near-surface atmosphere, further affecting the atmospheric circulation and climate. On the other hand, wind turbines convert the majority of the wind kinetic energy into electric energy, which produces the wake effect of the wind turbine. The budget patterns and spatial and temporal distribution of large-scale kinetic energy in the boundary layer are changed correspondingly, generating changes in various fluxes (heat flux, water vapor flux, etc.) in the atmosphere, which affect temperature, precipitation, and wind speed. Generally, the warming or cooling effect of wind farms on the near-surface is related to the stability of atmosphere. However, simulations in the global climate model showed that the average impact of wind farms on global climate is small, much smaller than the expected changes in greenhouse gas emissions and the interannual changes in natural climate.Wind power emits almost no carbon dioxide and pollutants. Compared with other traditional energy sources, it reduces water consumption but may generate some negative ecological impacts such as animal habitats, bird collisions, and noise, vision impact. However, some measures can be taken to mitigate these adverse effects.     

Earth rotation,ocean circulation and paleoclimate

During the 20 Ka glaciation maximum Earth's rate of rotation was significantly faster. The subsequent glacial eustatic rise in sea level meant an increase of the equatorial radius and hence led to a general deceleration in the Earth's rate of rotation. At about 6000 BP the glacial eustatic rise in sea level finished and a new situation began which was characterized by feedback interchanges of angular momentum between the solid Earth and the hydrosphere. There is a strong linkage between Earth's rate of rotation — total as well as differential — and the changes in ocean surface circulation. The ocean circulation changes are, in their turn, strongly linked to the paleoclimatic evolution of the boardering land masses. This is due to the high heat-storing capacity of the oceans, the ocean/ atmosphere heat flux, and the ocean/land interaction via heat transport by the winds. Consequently, we see a causal connection between Earth's rotation, oceanic circulation, ocean/atmosphere heating, atmospheric (wind) heat transport and continental paleoclimatic changes. We propose that the paleoclimatic changes on the decadal to millennial time scale are primarily driven by this mechanism. Observational data of changes in ocean water masses and paleoclimate are presented for the 20 Ka situation, for the high-amplitude changes 13-10 Ka ago, for the decadal-to-century changes during the Holocene, for the last centuries' instrumental data and for the ENSO-events. This implies that the oceanic system (the ocean surface circulation system) has a much more important role than previously appreciated.     

The Maximum Entropy Production Approach for Estimating Evapotranspiration: Principle and Applications

This review introduces a novel method for modeling evapotranspiration and surface heat fluxes built on the theory of Maximum Entropy Production (MEP) as an application of the maximum entropy principle to non-equilibrium thermodynamic systems. The formulation of the MEP model uses the Bayesian probability theory, information theory through the concept of information entropy, and the similarity theory of the atmospheric boundary-layer turbulence. The MPE model provides simultaneous solution of latent, sensible and surface medium heat fluxes using only three input variables: net radiation, surface temperature and specific humidity. A unique feature of the MEP model is that the surface energy balance is closed at a range of space and time scales. The model does not require data of temperature and water vapor gradient, wind speed and surface roughness. It does not include empirical tunable parameters such as atmospheric and stomatal conductance. The MEP model is a promising new approach for the study of water and energy cycles of the Earth system across space-time scales.     

最大熵增地表蒸散模型:原理及应用综述

精确估算地表蒸散一直是地球系统科学中的难点问题。经典的蒸散模型大多建立在水汽输送及能量平衡约束等基础上,相关的基础理论研究进展缓慢。最大熵增地表蒸散(E-MEP)模型是在综合借鉴贝叶斯概率论、信息熵概念、非平衡态热力学理论和大气边界层湍流相似性理论的基础上,建立的全新地表蒸散理论框架,克服了经典模型的主要缺陷,包括:离散梯度模型不满足能量守恒条件,Penman模型针对饱和土壤,Penman-Monteith模型需要率定经验参数等。E-MEP模型具有3个显著特点:①同时给出地表(包括水面、雪面和冰面)蒸散量、感热通量和介质表面热通量,且在所有时间空间尺度上满足能量平衡方程;②模型公式中没有可调经验参数,不依赖于温度梯度和水汽梯度变量,不需要输入风速和表面粗糙度;③适用于任何土壤含水量和植被覆盖条件。由于E-MEP模型建立在坚实的数学物理基础上,并具有解析表达式,简单易用,其输入变量和模型参数少于传统蒸散模型使用。地表辐射、表面温度、表面比湿等模型输入变量易于实地观测获取,且可通过遥感反演获得。检验分析表明,E-MEP模型优于Penman和Penman-Monteith等传统蒸散模型。这一全新的地表蒸散模型已被用于大尺度地表水热的遥感反演和过程监测,并用于改进气候模式的参数化方案。     

Climate variability during the Last Glacial Maximum in eastern Australia: evidence of two stadials?

A high‐resolution, multiproxy record encompassing the last glacial–interglacial transition is presented for Native Companion Lagoon, a coastal site in subtropical eastern Australia. Rates of aeolian sedimentation in the lake were established by trace element analyses of lacustrine sediments and used as a proxy for aridity. In conjunction with sediment moisture content, charcoal and pollen these provide a multi‐decadal record of palaeoenvironmental variability for the period 33–18 k cal. yr BP. Results indicate that the Last Glacial Maximum in eastern Australia spanned almost 10 k cal. yr, and was characterised by two distinct cold dry events at approximately 30.8 k cal. yr BP and 21.7 k cal. yr BP. Provenance of selected sediment samples by trace element geochemical fingerprinting shows that continental sourced aeolian sediments originated primarily from South Australia during these cold events and from sites in central Australia during the intervening time. Used in combination with a pollen record, the provenance of long‐travelled dust to mainland sites shows that the two cold events were characterised by frequent meridional dry southwesterly winds rather than zonal westerly airflow as previously believed. The intervening period was cool and humid, which we infer as being associated with more frequent southeasterly winds of maritime origin. These results lend support to previous research that indicates the Southern Hemisphere experienced a period of widespread climatic amelioration at the height of the last glacial known as the Antarctic Isotopic Maximum. Copyright © 2008 John Wiley & Sons, Ltd.     

乌鲁木齐河山区流域径流形成的实验研究

乌鲁木齐河山区流域较之山前平原区有着充沛的降水，是径流形成的基础。降水量随海拔增加而增加，高山带降水以固态为主，使冰川成为山区流域重要的水储存形式。降水观测误差对流域降水量和蒸发量的估算有较大影响。冰雪消融可以依据常规气象要素的参数化热通量表达式来计算。在高山草甸蒸发量的观测、计算中，蒸渗器称重法与波恩比率法较之通量－梯度法有较高的观测精度。山区流域径流系数随流域平均海拔增加而增加，高山冰川作用区在径流形成和调节中起着重要作用。