Comparison of three models to estimate evapotranspiration for a temperate mixed forest

Forest evapotranspiration is one of the main components in the regional water budget. A comparison between measured and estimated eddy covariance (EC) data, considering the Katerji–Perrier (KP), Todorovic (TD) and Priestley–Taylor (PT) actual evapotranspiration methods, was carried out. These models, relying on more easily obtainable data, are valuable when long‐term direct measurements are not available. The objective of this paper is to compare the effectivity of these three models. In this paper, experimental data were obtained within the temperate mixed forest of broad‐leaved and coniferous trees of the Changbai Mountains in northeastern China during the growing seasons of 2003 to 2005. The KP method gave the most effective values for half‐hourly and daily evapotranspiration computed by summing up half‐hourly estimates, and the TD method overestimated evapotranspiration by about 30%. The diurnal courses of estimated and measured evapotranspiration showed bell curves, similar to that of net radiation, except for a slight increase at about 14:30 solar time due to a peak value of vapour pressure deficit (VPD). For the case of daily evapotranspiration using daily mean micrometeorological variables, the PT method presented the closest values to the measurements. Accuracy of estimation related to VPD negatively (especially for VPD > 1·5 kPa). The KP parameters, considered to be vegetation dependent, were a = 0·545 and b = 1·31 at the experimental site. A constant PT parameter (α = 1·18) was applied to estimated evapotranspiration. Daily values of α responded to VPD (negatively) more strongly than to soil moisture (positively) in this forest. The experiment showed the inherent limits and advantages of the three methods. The KP method, a semi‐empirical approach, was preferred to estimate half‐hourly evapotranspiration. The TD method was a mechanistic approach to estimate reference evapotranspiration and always overestimated actual evapotranspiration. The PT method, being site dependent and the simplest approach, was effective enough to estimate large time‐scale (at least daily) evapotranspiration. Copyright © 2008 John Wiley & Sons, Ltd.     

Characteristics and Drivers of Methane Fluxes from a Rice Paddy Based on the Flux Measurement

Rice paddies are an important anthropogenic source of methane (CH₄) to the atmosphere, which aggravate the global warming greatly. CH₄ fluxes from a rice paddy in Central China were continuously measured with the eddy covariance method in 2018. The characteristics, dynamics and drivers of the observed CH₄ fluxes from this paddy field were subsequently analyzed. The results indicated that \mathrm{①} a distinct seasonal variation of daily CH₄ fluxes was found over the whole observed period. Daily CH₄ fluxes were the highest in the vegetative period, then decreased gradually, and became the lowest in the fallow period; \mathrm{②} observed CH₄ fluxes had a clear single-peak diurnal pattern during the vegetative and reproductive periods, and reached daily peaks at about 14:00-16:00. However, no obvious diurnal variation in CH₄ fluxes was observed during the fallow period; \mathrm{③} air temperature was the most important drivers that controlled the seasonal variation of CH₄ fluxes from this paddy field, and Vapor Pressure Deficit (VPD) was also found related to the CH₄ emissions; \mathrm{④} the largest daily CH₄ flux was 0.69 μmol/(m²·s), occurred in the late of vegetative period, and the total amount of CH₄ emissions over the whole observed period was about 28 g C/m².     

An estimation of the height system bias parameter <Emphasis Type="Italic">N</Emphasis><Subscript>0</Subscript> using least squares collocation from observed gravity and GPS-levelling data

This paper deals with the analysis of gravity anomaly and precise levelling in conjunction with GPS-Levelling data for the computation of a gravimetric geoid and an estimate of the height system bias parameter N_o for the vertical datum in Pakistan by means of least squares collocation technique. The long term objective is to obtain a regional geoid (or quasi-geoid) modeling using a combination of local data with a high degree and order Earth gravity model (EGM) and to determine a bias (if there is one) with respect to a global mean sea surface. An application of collocation with the optimal covariance parameters has facilitated to achieve gravimetric height anomalies in a global geocentric datum. Residual terrain modeling (RTM) technique has been used in combination with the EGM96 for the reduction and smoothing of the gravity data. A value for the bias parameter N_o has been estimated with reference to the local GPS-Levelling datum that appears to be 0.705 m with 0.07 m mean square error. The gravimetric height anomalies were compared with height anomalies obtained from GPS-Levelling stations using least square collocation with and without bias adjustment. The bias adjustment minimizes the difference between the gravimetric height anomalies with respect to residual GPS-Levelling data and the standard deviation of the differences drops from 35 cm to 2.6 cm. The results of this study suggest that N_o adjustment may be a good alternative for the fitting of the final gravimetric geoid as is generally done when using FFT methods.     

Interannual variability of Gulf Stream warm-core rings in response to the North Atlantic Oscillation

Analysis of a quality-controlled database of Gulf Stream warm-core rings (WCRs) between 75° and 50°W during 1978–1999 demonstrates a significant correlation between WCR occurrences and variations in large-scale atmospheric forcing related to the state of the North Atlantic Oscillation (NAO). The mechanisms for linking the NAO with the rate of WCR occurrences are two-fold: (1) the influence of the NAO on Gulf Stream (GS) position, which could affect the interaction of the Gulf Stream with the New England Seamounts chain and thus allow for a higher/lower number of WCR occurrences; (2) the NAO-induced eddy kinetic energy (EKE) variability in the Gulf Stream region (GSR), which is indicative of the baroclinic instability processes necessary for WCR formation. Variability in GS movement is studied by analyzing annual mean positions of the Gulf Stream North Wall obtained from satellite-derived sea surface temperature (SST) frontal charts. Response of GSR EKE to fluctuations in the state of the NAO is examined with a numerical simulation of the North Atlantic basin from 1980–1999. The North Atlantic basin is simulated using a 1/6°-resolution eddy-resolving Regional Ocean Modeling System (ROMS) model that spins up with Southampton Oceanography Center (SOC) ocean-atmosphere atlas-derived atmospheric forcing fields. Model-derived EKE estimates are observed to be in good agreement with TOPEX/Poseidon altimeter-based EKE estimates as well as with results from other modeling studies for the North Atlantic basin. We suggest that lateral movement of the GS may not be the primary mechanism causing variation in the rate of WCR occurrences, because GS position is observed to respond at a lag of one year, whereas annual rates of WCR occurrences respond at 0-year lag to the NAO. Based on results from numerical simulations of the North Atlantic basin, adjustment to NAO-induced wind forcing is seen to impact the GSR EKE intensity and possibly the related baroclinic instability structure of the GS at 0-year lag. These results suggest that NAO-induced interannual variability in GSR EKE is the most likely mechanism affecting WCR occurrences. Numerical simulations show that high (low) phases in the state of the NAO exhibit higher (lower) EKE in the GSR, providing a greater (lesser) source of baroclinic instability to the GS front, possibly resulting in higher (lower) occurrences of WCRs.     

2013年吕宋暖涡的生消过程及特征研究

基于 HYCOM（HYbrid Coordinate Ocean Model）海洋模式,采用全球到西北太平洋,西北太平洋到南海两层嵌套的数值模拟方法,以 2013 年为例,获得了高精度 （1/25° × 1/25° × cosθ） 的南海海洋动力数据资料。详细刻画了吕宋暖涡的演变过程及三维结构,并对吕宋暖涡演化过程中伴随的能量变化进行了分析。研究发现,2013 年 6 月中旬吕宋暖涡生成于吕宋岛西北侧,该涡先向西移动,8 月底滞留约一个月,随后沿 200 m 等深线向西南移动,最终于 12 月中旬消亡。以 2013 年8 月 4 日为例,吕宋暖涡的影响深度可从海表面到达海底,涡的形状类似于圆柱状,涡中心存在倾斜,从海表面至海底涡的中心可倾斜约 110 km;吕宋暖涡最大流速可达 0.5 m/s。吕宋暖涡所在区域的温度高于周围区域 2~6 ℃,盐度低于周围区域约 0.1~0.3 ‰,最大温度异常值为 6.5℃,盐度异常值可达-1 ‰,都出现在深度 150 m 左右的海域。吕宋暖涡的总能量可达左右,涡势能约是涡动能的 6 倍;正压能量转化与斜压能量转化之和占涡总能量变化的 76%,对涡能量变化起决定性作用。     

流动沙丘涡度相关通量的坐标旋转订正与对比研究

复杂下垫面导致近地面大气运动具有特殊的动力学和热力学特征,进而影响物质和能量的交换过程。以科尔沁流动沙丘7月份涡度数据为例,分析由于地形引起的风速、旋转角度β的变化,采用二次旋转法（DR）和平面拟合法（PF）对水热通量、数据质量对比分析,结果表明：（1）地形平缓区（180°~330°）垂直风速>0,β在1.677 3°左右;地形陡峭区（330°~180°）垂直风速<0,β在-1.648 6°左右。（2）DR法和PF法对潜热通量影响很大,分别增加17%和减小15%,DR法订正前后水热通量相关性均高于PF法。（3）DR法和PF法订正后的数据分别有89.6%和89.3%通过湍流发展的充分性检验;DR法订正前后摩擦风速相关性高于PF法;能量闭合率分别提高了3.47%和2.78%。（4）整个生长季中,6、8月订正后涡度数据质量有较好的提高。因此,DR法更适用于起伏较大的流动沙丘。研究结果充分考虑地形对涡度数据影响,为科尔沁沙地及具有相似地形的地区涡度数据的合理使用提供参考。     

吉尔吉斯柯孜尔河上游河谷草场能量闭合分析

通过分析了2014 年8 月13~11 月12 号吉尔吉斯斯坦克孜尔苏山地生态观测站的湍流通量和有效能量的日、月变化特征,并且通过对能量平衡残差（D）以及能量平衡比率（EBR）两种主要方法,分析了该研究区域的能量闭合情况和月变化特征。结果表明：有效能量和湍流通量呈单峰模式的日变化特征,并且在观测期间随着月份的变化,有效能量和湍流通量的月平均值在显著减小;通过对研究区域的能量平衡分析,发现其能量不闭合的情况,并且有研究表明,土壤热通量计算不准确是主要的因素之一。通过对能量平衡残差（D）分析,得出这个观测期间基本上在8~16 点之间能量平衡残差大于零,大概在8 点和16 点的时候能量达到了平衡;用能量平衡比率（EBR）研究能量闭合情况得出,能量平衡比率8月份的0.75,9月份最小为0.7左右,10月份的为0.85。     

双目视觉三维点云重建的误差度量模型

从图像中快速提取并构建三维模型是GIS空间数据获取的重要途径之一,作为智慧城市建设的基础数据,如何度量和控制图像三维模型质量成为阻碍智慧城市发展的首要问题。本文以立体像对为数据源,在研究三维点云重建模型的基础上,结合矩阵微分理论和协方差传播律构建了双目视觉三维点云误差度量模型及量化表达方法,为图像三维建模质量控制和精度评估提供理论依据。     

Microseismic Event Detection Kalman Filter: Derivation of the Noise Covariance Matrix and Automated First Break Determination for Accurate Source Location Estimation

Since 1972, Weir-Jones Engineering Consultants (WJEC) has been involved in the development and installation of microseismic monitoring systems for the mining, heavy construction and oil/gas industries. To be of practical value in an industrial environment, microseismic monitoring systems must produce information which is both reliable and timely. The most critical parameters obtained from a microseismic monitoring system are the real-time location and magnitude of the seismic events. Location and magnitude are derived using source location algorithms that typically utilize forward modeling and iterative optimal estimation techniques to determine the location of the global minimum of a predefined cost function in a three-dimensional solution space. Generally, this cost function is defined as the RMS difference between measured seismic time series information and synthetic measurements generated by assuming a velocity structure for the area under investigation (forward modeling). The seismic data typically used in the source location algorithm includes P- and S-wave arrival times, and raypath angles of incidence obtained from P-wave hodogram analysis and P-wave first break identification. In order to obtain accurate and timely source location estimates it is of paramount importance that the extraction of accurate P-wave and S-wave information from the recorded time series be automated—in this way consistent data can be made available with minimal delay. WJEC has invested considerable resources in the development of real-time digital filters to optimize extraction, and this paper outlines some of the enhancements made to existing Kalman Filter designs to facilitate the automation of P-wave first break identification.