TOGA-COARE IOP期间的海气通量观测结果

在TOGA－COAREIOP期间用涡度相关法对海气热通量进行了船载直接观测．对垂直风速、温度和湿度湍流脉动观测数据的谱分析显示它们在高频区基本满足"－2／3次方律"．对船体简谐震荡影响的讨论从理论上证明该影响在热通量计算中可被忽略．根据以此方法得到的通量求出了中性层结条件下感热和潜热的整体输送系数分别为2．25×10^－3和1．26×10^－3．对海气边界层特性的分析表明该海域的近海层主要呈中性或弱不稳定层结．海气通量的变化与背景环流形势密切相关,潜热通量主要受海面风场强度的影响,而感热通量变化除了风场的影响外,层结变化也是一个重要因素．用整体输送法计算TOGA－COAREIOP期间以及TOGA期间8个航次的通量结果而得到的Bowen比约为0．1,显示潜热通量是暖池大气的主要热源．     

Effects of atmospheric stability conditions on heat fluxes from small water surfaces in (semi-)arid regions

Atmospheric stability conditions over the water surface can affect the evaporative and convective heat fluxes from the water surface. Atmospheric instability occurred 72.5% of the time and resulted in 44.7 and 89.2% increases in the average and maximum estimated evaporation, respectively, when compared to the neutral condition for a small shallow lake (Binaba) in Ghana. The proposed approach is based on the bulk-aerodynamic transfer method and the Monin-Obukhov similarity theory (MOST) using standard meteorological parameters measured over the surrounding land. For water surface temperature, a crucial parameter in heat flux estimation from water surfaces, an applicable method is proposed. This method was used to compute heat fluxes and compare them with observed heat fluxes. The heat flux model was validated using sensible heat fluxes measured with a 3-D sonic anemometer. The results show that an unstable atmospheric condition has a significant effect in enhancing evaporation alongside the sensible heat flux from water surfaces.     

戈壁地区夏季碳和水热循环及其湍流特征

本文利用2006年夏季大气边界层观测资料,采用涡动相关法分析了我国西北地区戈壁下垫面碳收支及水热循环的规律和特征,并分析了大气湍流特征.结果表明:夏季白天的CO₂湍流通量呈逆输送特征,即CO₂白天向下输送,夜间向上输送,平均数值为-0.199 mg·m^-2·s^-1,整体上表现为碳汇;戈壁地区湿度小,其数值受水平来流的影响较大,日变化特征不明显;温度的归一化标准差与稳定度参数的关系满足Monin-Obukhov Similarity (MOS)理论;温度和CO₂的能谱相似;互谱uc与uθ,wc与wθ相似;水汽和CO₂的输送主要受水平方向湍流的影响.     

绿洲和戈壁近地面层热量和物质输送特征对比

利用甘肃金塔绿洲和戈壁10 m涡旋相关法的观测资料对比研究温度、湿度和二氧化碳浓度(CO₂)的平流和湍流通量特征.结果表明,在典型绿洲-戈壁复杂下垫面上,热量和物质的平流通量比湍流通量的值大1到2个量级;在绿洲和戈壁,受平流输送影响和下垫面强迫,不同性质标量湍流的水平和垂直通量分布具有不同的特征.     

Estimation of spatially distributed surface energy fluxes using remotely‐sensed data for agricultural fields

Land surface energy fluxes are required in many environmental studies, including hydrology, agronomy and meteorology. Surface energy balance models simulate microscale energy exchange processes between the ground surface and the atmospheric layer near ground level. Spatial variability of energy fluxes limits point measurements to be used for larger areas. Remote sensing provides the basis for spatial mapping of energy fluxes. Remote‐sensing‐based surface energy flux‐mapping was conducted using seven Landsat images from 1997 to 2002 at four contiguous crop fields located in Polk County, northwestern Minnesota. Spatially distributed surface energy fluxes were estimated and mapped at 30 m pixel level from Landsat Thematic Mapper and Enhanced Thematic Mapper images and weather information. Net radiation was determined using the surface energy balance algorithm for land (SEBAL) procedure. Applying the two‐source energy balance (TSEB) model, the surface temperature and the latent and sensible heat fluxes were partitioned into vegetation and soil components and estimated at the pixel level. Yield data for wheat and soybean from 1997 to 2002 were mapped and compared with latent heat (evapotranspiration) for four of the fields at pixel level. The spatial distribution and the relation of latent heat flux and Bowen ratio (ratio of sensible heat to latent heat) to crop yield were studied. The root‐mean‐square error and the mean absolute percentage of error between the observed and predicted energy fluxes were between 7 and 22 W m⁻² and 12 and 24% respectively. Results show that latent heat flux and Bowen ratio were correlated (positive and negative) to the yield data. Wheat and soybean yields were predicted using latent heat flux with mean R² = 0·67 and 0·70 respectively, average residual means of −4·2 bushels/acre and 0·11 bushels/acre respectively, and average residual standard deviations of 16·2 bushels/acre and 16·6 bushels/acre respectively (1 bushel/acre ≈ 0·087 m³ ha⁻¹). The flux estimation procedure from the SEBAL‐TSEB model was useful and applicable to agricultural fields. Copyright © 2005 John Wiley & Sons, Ltd.     

近地层能量闭合度对陆面过程模式影响

大量近地层观测试验表明,利用涡动相关法观测的湍流通量小于近地层可利用能量,即近地层能量是不闭合的,这种不闭合度一般为20%甚至更高.而陆面过程模式是基于地气间能量平衡建立,并且模式中的湍流边界层参数化方案通常根据实际观测的湍流通量来确定,因此能量不闭合必将对陆面过程模式造成一定的影响.本文利用2007年春季SACOL站的近地层观测资料,依据能量守恒将能量不闭合中的残余能量通过波文比分配到观测的湍流通量中,即修正涡动相关法观测的湍流通量使得近地层能量达到平衡;之后分别利用观测和修正的湍流通量,建立了能量不闭合和闭合情形下的湍流参数化方案,借助陆面过程模式SHAW,通过数值模拟和对比分析方法考察近地层能量闭合度对陆面过程模式的影响.研究结果表明近地层能量闭合对陆面过程模式有显著的影响:在陆面过程数值模拟中,当应用近地层能量不闭合的湍流通量形成的湍流参数化方案时,陆面过程模式会明显高估地表长波辐射及土壤温度;但当应用修正湍流通量使得近地层能量达到闭合形成的湍流参数化方案后,在不改变任何地表土壤物理生化属性的情况下,陆面过程模式能较好地模拟地表长波辐射和土壤温度.     

Evaluation of energy balance closure adjustment methods by independent evapotranspiration estimates from lysimeters and hydrological simulations

Non‐closure of the surface energy balance is a frequently observed phenomenon of hydrometeorological field measurements, when using the eddy‐covariance method, which can be ascribed to an underestimation of the turbulent fluxes. Several approaches have been proposed in order to adjust the measured fluxes for this apparent systematic error. However, there are uncertainties about partitioning of the energy balance residual between the sensible and latent heat flux and whether such a correction should be applied on 30‐min data or longer time scales. The data for this study originate from two grassland sites in southern Germany, where measurements from weighable lysimeters are available as reference. The adjusted evapotranspiration rates are also compared with joint energy and water balance simulations using a physically based distributed hydrological model. We evaluate two adjustment methods: the first one preserves the Bowen ratio and the correction factor is determined on a daily basis. The second one attributes a smaller portion of the residual energy to the latent heat flux than to the sensible heat flux for closing the energy balance for every 30‐min flux integration interval. Both methods lead to an improved agreement of the eddy‐covariance based fluxes with the independent lysimeter estimates and the physically based model simulations. The first method results in a better comparability of evapotranspiration rates, and the second method leads to a smaller overall bias. These results are similar between both sites despite considerable differences in terrain complexity and grassland management. Moreover, we found that a daily adjustment factor leads to less scatter than a complete partitioning of the residual for every half‐hour time interval. The vertical temperature gradient in the surface layer and friction velocity were identified as important predictors for a potential future parameterisation of the energy balance residual.     

Discussion and reassessment of the method used for accepting or rejecting data observed by a Bowen ratio system

The Bowen ratio energy balance method often produces extremely inaccurate magnitudes of the flux due to resolution limits of the instruments. We analysed the criteria used for rejecting inaccurate data observed using a Bowen ratio system and the resolution limit of the sensors to analytically determine the reliable values of the Bowen ratio (β) and the latent and sensible heat fluxes. The formula used to calculate the error limit of the Bowen ratio (β) was corrected based on the theory of error analysis. An example was proposed for the common case with 0.2 °C resolution limit of temperature measurement and 0.08 kP_a resolution limit of water vapour pressure measurement, to show the steps of accepting or rejecting data observed by a Bowen ratio system. The acceptance or rejection of data observed by a Bowen ratio system is a dynamic process, which should be performed based on the excluded interval of the Bowen ratio and the qualitative relationships among the data observed by a Bowen ratio system. The excluded interval of the Bowen ratio can be structured based on the accuracy of the sensors used. Data are excluded first if they do not satisfy the qualitative relationships between the vapour pressure difference, the temperature difference, Bowen ratio, and the available energy, whereas the data in the rejection region range of the Bowen ratio are excluded second. It is necessary to improve the accuracy of the temperature and humidity probes to improve the acceptance rate for data collected using the Bowen ratio system, apart from improving the observed precision of available energy. Copyright © 2013 John Wiley & Sons, Ltd.     

Land Surface Layer Characteristics During Monsoon at a Tropical Station

—The atmospheric surface layer over land may behave differently in the tropics, particu larly during the monsoon. A preliminary attempt is made to observe the behavior of surface layer characteristics such as fluxes of momentum, sensible heat and latent heat, friction velocity, friction temperature, M-O length scale, Richardson number and Bowen’s ratio over Kharagpur (22°20′N, 87°18′E), a typical moist tropical station. The diurnal and day-to-day variations have been studied. It is observed that during the active phase of the monsoon the sensible heat flux and Bowen’s ratio are low. The diurnal variation is apparent for most parameters. Mostly near neutral conditions are observed.     

Energy balance closure at ChinaFLUX sites

Network of eddy covariance observation is measuring long-term carbon and water fluxes in contrasting ecosystems and climates. As one important reference of independently evaluating scalar flux estimates from eddy covariance, energy balance closure is used widely in study of carbon and water fluxes. Energy balance closure in ChinaFLUX was evaluated by statistical regression of turbulent energy fluxes (sensible and latent heat) against available energy (net radiation, soil heat flux, canopy heat storage) and the energy balance ratio (EBR) and the frequency distribution of relative errors of energy balance (δ). The trends of diurnal and seasonal variation of energy balance in ChinaFLUX were analyzed. The results indicated that the imbalance was prevalent in all observation sites, but there were little differences among sites because of the properties variation of sites. The imbalance was greater during nocturnal periods than daytime and closure was improved with friction velocity intensifying. Generally the results suggested that estimates of the scalar turbulent fluxes of sensible and latent heat were underestimated and/or that available energy was overestimated. Finally, we discussed certain factors that are contributed to the imbalance of energy, such as systematic errors associated with the sampling mismatch, systematic instrument bias, neglected energy sinks, low and high frequency loss of turbulent fluxes and advection of heat and water vapor.     

Combining the dissipation method and surface renewal analysis to estimate scalar fluxes from the time traces over rangeland grass near Ione (California)

The dissipation method, the method preferred for estimating scalar surface fluxes over open water has not traditionally been used by agronomists, whereas the surface renewal (SR) theory in conjunction with the analysis of the scalar time trace offers tremendous advantages for estimating fluxes over agronomic crops. For a steady and horizontally homogeneous flow, it is shown that the dissipation method and SR analysis are closely related. As a consequence, a new dissipation–SR analysis expression for estimating scalar surface fluxes was derived. The new equation requires no calibration, and the scalar time trace measured at a frequency capable of identifying canopy‐scale coherent structures (typically 4–10 Hz in agriculture) is the only input required. Sensible and latent heat flux estimates obtained from 10 Hz air temperature and water vapour concentration measurements in the inertial sub‐layer (2 m height) over short, homogeneous rangeland grass at a site where similarity does not hold gave similar results to those measured with the eddy covariance (EC) method. For unstable cases, the new equation provided a root mean square error of 57 W m^?2 for the surface energy‐balance closure. For stable cases, the performance was difficult to evaluate because the EC fluxes were similar in magnitude to the sensor error. It is concluded that the proposed method can contribute to a better understanding of hydrological processes and water requirements by providing an accurate, less costly, alternative method to indirectly estimate evapotranspiration as the residual of the energy balance equation. Copyright © 2009 John Wiley & Sons, Ltd.     

一种基于地表能量平衡的遥感干旱监测新方法及其在甘肃河东地区干旱监测中的应用初探

目前遥感干旱监测方法的精度普遍不高,探求新的遥感干旱监测方法有助于干旱监测预警技术的提升与发展.波文比是感热通量与潜热通量之比,能综合反映地表水热特征,可尝试将其引入到遥感干旱监测领域加以利用.应用甘肃河东地区的EOS-MODIS卫星资料和同步地面气象资料,基于地表能量平衡原理构建了波文比干旱监测模型,对比分析了波文比(β)指数、温度植被指数(TVX)与土壤水分的相关性,并以典型晴空影像(2014年10月5日)为例初步建立了β的干旱分级标准,对研究区进行了旱情评估.结果表明:β与土壤相对湿度呈现出高度负相关,相比于当下广泛应用的TVX,β与0~20cm平均土壤相对湿度具有更好的相关性,监测精度得到了显著提高.用β干旱分级标准评估的研究区干湿状况与前期降水空间分布吻合得相当好,评估表明2014年10月5日研究区基本为适宜(无旱),与2014年9月的降水距平百分率特征一致.基于地表能量平衡的波文比(β)指数在干旱监测中效果突出,具有很好的应用前景.     

海面动量、感热和水汽输送的动力学分析

基于大气边界层动量、感热和水汽通量的基本方程,定量地计算了波动海面的动量、感热和水汽通量。首先,应用Prandtl的混合长概念,推导出贴海面大气层中风速、位温和比湿的涡动交换率及其贴海面层厚度,并且证明了波面上位温或比湿贴海面层厚度与速度贴海面层厚度的比值,和平面上的相应比值完全相等。随后,利用空气动力学理论讨论了贴海面动量、感热和水汽输送的参数化问题。最后,对现有五种理论模式进行了比较说明。     

It goes both ways: measurements of simultaneous evapotranspiration and fog droplet deposition at a montane cloud forest

Fluxes of latent heat, sensible heat, and water vapor, including turbulent deposition of fog droplets, were measured for two months in autumn 2005 within a subtropical montane cypress forest in Taiwan. The goal of the study was to determine whether significant evapotranspiration can occur during foggy conditions. Water vapor fluxes, Q_W, as determined with the Bowen Ratio method, were compared to those simultaneously measured with the eddy covariance method. The median Bowen Ratio was 1.06, and the median Q_W flux was 5 · 2 × 10^?5 kg m^?2 s^?1. The vertical gradients of temperature and specific humidity over the forest, ΔT and Δq, peaked around noon during days without fog, and were reduced during foggy conditions. For 66% of the data points, ΔT and Δq were negative, corresponding to positive (upward) fluxes of sensible heat Q_H and latent heat Q_E. A Monte Carlo simulation proved that statistically significant evapotranspiration rates, i.e., upward water vapor fluxes, occurred during fog. At the same time, deposition fluxes of fog droplets occurred. Our results show that even during fog events, significant evapotranspiration may occur. Copyright © 2008 John Wiley & Sons, Ltd.     

Turbulent heat fluxes by profile and inertial dissipation methods: analysis of the atmospheric surface layer from shipboard measurements during the SOFIA/ASTEX and SEMAPHORE experiments

Heat flux estimates obtained using the inertial dissipation method, and the profile method applied to radiosonde soundings, are assessed with emphasis on the parameterization of the roughness lengths for temperature and specific humidity. Results from the inertial dissipation method show a decrease of the temperature and humidity roughness lengths for increasing neutral wind speed, in agreement with previous studies. The sensible heat flux estimates were obtained using the temperature estimated from the speed of sound determined by a sonic anemometer. This method seems very attractive for estimating heat fluxes over the ocean. However allowance must be made in the inertial dissipation method for non-neutral stratification. The SOFIA/ASTEX and SEMAPHORE results show that, in unstable stratification, a term due to the transport terms in the turbulent kinetic energy budget, has to be included in order to determine the friction velocity with better accuracy. Using the profile method with radiosonde data, the roughness length values showed large scatter. A reliable estimate of the temperature roughness length could not be obtained. The humidity roughness length values were compatible with those found using the inertial dissipation method.     

黄土高原复杂地形上高质量湍流通量数据获取方法

利用兰州大学半干旱气候与环境观测站(Semi-Arid Climate and Environment Observatory of Lanzhou University,SACOL)湍流观测资料,分析了二次坐标旋转(double rotation,DR)、平面拟合(planar fit,PF)和分风区平面拟合(fetch planar fit,FPF)在复杂地形上的适用性,总结出一套适用于SACOL的总体湍流特征参数化方案.经过超声虚温订正、坐标旋转、空气密度脉动订正以及平稳性检验、总体湍流特征检验、总体质量分级处理,摩擦速度(u*)、感热通量、潜热通量、CO_2通量高质量数据所占比例分别为45%~62%、66%~68%、62%~65%、52%~54%.采用DR得到的高质量数据比例与采用PF相比,u*提高了17%,后三种通量略降低2%~3%.PF和FPF两种结果的差别主要体现在u*上,只考虑主导风向数据DR得到的u*质量仍最好.综合兼顾数据质量和计算工作量,在复杂地形上处理湍流观测资料的最优坐标旋转方法是DR.     

Estimation of seasonal crop water consumption in a vineyard using Bowen ratio‐energy balance method

Information about seasonal crop water consumption is useful to develop the appropriate irrigation scheme. Measurements of energy balance components using the Bowen ratio method were made for a complete growing season at a vineyard in the arid region of northwest China. Vine in the experiment was furrow‐irrigated using a trellis system. The measured evapotranspiration was compared with estimates using the soil water balance method. It is shown that the Bowen ratio method provided accurate estimates of evapotranspiration from the vineyard and this requires that the Bowen ratio system is appropriately installed. The energy balance components showed typical diurnal pattern with peaks that occurred around the midday, except for the ground heat flux which delayed its peak by 2–3 h. The sensible heat flux was greater than the latent heat flux and followed the net radiation closely. The ratio of the latent heat flux to net radiation was low in the early growing season and increased over time. Under the limited irrigation experienced in the vineyard, the latent heat flux was controlled by available soil moisture and the total evapotranspiration in the growing season was 253 mm. The seasonal progression of the crop coefficient is similar to that reported in the literature, with the maximum occurring during the month of September. The crop coefficient can be estimated as a non‐linear function of day of year (DOY) and used to estimate evapotranspiration from vineyards in the region. Copyright © 2007 John Wiley & Sons, Ltd.     

Evaporation and energy balance of a sub‐Arctic hillslope in northern Finland

This paper presents measurements of the energy balance (radiation, sensible heat flux, evaporation) from a sub‐arctic hillside in northern Finland for a summer season. Comparisons are also made with a nearby wetland site. The hillslope measurements show an equal partition of the radiant energy into sensible and latent heat flux. The evaporative ratio of just over one half was remarkably constant throughout the season, despite very large day‐to‐day and diurnal variations of temperature, humidity deficit and radiation input. This conservative behaviour of the evaporation was caused by a strong rise in effective surface resistance to evaporation with increasing vapour pressure deficit. This suggests a strong physiological control on the evaporation, with stomata closing at times of high evaporative demand. There was no obvious impact of soil‐water stress on the evaporation. However, a comparison with the evaporation measured at a nearby mire site in 1997 suggests that the mire has a significantly lower surface resistance, even when the impact of a significantly lower humidity deficit in the earlier year is taken into account. The measurements are used to test, off‐line, the performance of MOSES (Meteorological Office Surface Exchange Scheme), a simple, but comprehensive, land surface model. The sensitivity of the energy exchanges to the thermal properties of the top soil layer (a surrogate for the upper soil/vegetation layer) is investigated with the use of the model. It is found that the evaporation is insensitive to these properties; they do, however, influence the partition of energy between the sensible heat flux and the ground heat flux (and hence the soil temperatures). It is suggested that the model needs to represent the thermal properties of the canopy more realistically. Copyright © 2002 John Wiley & Sons, Ltd.     

Natural integration of scalar fluxes from complex terrain

Large eddy simulations of turbulent flow and transport in the atmospheric boundary layer were conducted over heterogeneous sources of heat and water vapor to identify the blending properties of the turbulent mixing in an unstably stratified boundary layer. The numerical simulations show that the concept of blending in the ABL is in fact a useful one, even under convective conditions, for a range of surface conditions. Since the transport eddies that are responsible for the blending have sizes that are constrained by the boundary layer depth, and since the vertical motion is so important under the unstable density stratification studied here, we see that when the length scales of the source variability on the land surface become significantly greater than the ABL depth the blending is lost. In this case the source fields remain relatively uncoupled by the important eddy motion. However, for smaller surface length scales, the dynamic eddy motion couples the surface patches. Hence, there is good reason that the land surface exchange phenomenon would not be scale invariant over the entire range of scales. Because of the active role of temperature the effects of inhomogeneous surface sources of sensible heat persist higher into the ABL than do the effects of surface sources from more passive scalars, such as water vapor. Moreover, the mean fields of potential temperature and specific humidity blend at much lower heights than do the vertical turbulent flux fields of these two scalars. We propose a useful measure of blending efficiency for simulation studies and show how this bridges from the dynamics responsible for the blending to the horizontal homogeneity of scalar flux fields at measurement heights in the ABL.