鄱阳湖夏季水热通量特征及环境要素影响分析

气候变化加速了全球水文循环过程,然而,气候变化如何影响水体蒸发及其水热通量交换仍然不清楚.基于涡度相关系统观测鄱阳湖水体水热通量过程,在小时和日尺度分析了水热通量的变化规律及其主要影响因子.研究表明,潜热通量日变化波动剧烈,大部分为正值,变化范围在-50~580 W/m2之间.而感热通量数值较小,变化范围在-50~50 W/m2之间.8月份潜热通量和感热通量均呈波动下降趋势,均值分别为167.4和15.9 W/m2.8月份日平均潜热通量和感热通量之和大于净辐射,这是由于这一时段储存在水体中的热量释放并补充潜热通量和感热通量.小时尺度上潜热通量日变化在相位上与净辐射无显著相关性,而与风速显著相关.在日尺度变化趋势上,8月份日平均潜热通量仍主要受到风速和水温的影响,感热通量则主要受到风速和饱和水汽压差的影响.     

洱海湖气界面水汽和二氧化碳通量交换特征

基于2012年涡动相关法取得的洱海湖气之间湍流通量资料,计算了湖面反照率、空气动力学粗糙度和整体输送系数等湖气交换过程的基本物理参数;分析高原湖泊表面动量通量、感热通量、潜热通量和二氧化碳通量的变化特征及其主要的控制因子;采用神经网络法对缺失蒸发量数据进行填补,估算了洱海湖面全年蒸发量。2012年全年蒸发量为（1165±15）mm,大于年实际降水量（2012年的年降水量为818mm）。洱海局地环流在全年范围内较显著;全年主导风向为东南（谷风／湖风）和西北风（山风／陆风）。高原湖泊感热通量通常只有每平方米几十瓦,通常午后感热通量为负值;即湖面向大气输送热量。夏季湖泊大气界面感热通量最大值出现在清晨,与湖气温差的出现时间一致;在白天湖面的有效能量主要分配为潜热通量;湖气温差和水汽压差分别是感热通量和潜热通量日变化的主要控制因子。湖气界面二氧化碳通量除夏季存在弱的吸收外,其余季节（冬季）表现为弱的排放。湖面反照率的季节变化规律与太阳高度角的季节变动有关,同时湖面反照率与水的浑浊度等有关。与实际观测得到的湖面反照率相比,CLM4湖泊模式在冬季低估（夏季高估）了湖面反照率。     

CLM4-LISSS模型对太湖多时间尺度水热通量模拟性能的评估

湖泊模型为数值天气预报模型提供热量通量、水汽通量和动量通量等下边界条件,但是不同时间尺度上湖泊水热通量变化的控制因子不同,因此有必要对湖泊模型进行多时间尺度上的离线评估.本文利用2012-2016年太湖中尺度通量网避风港站的气象资料和辐射数据驱动CLM4-LISSS模型(Community Land Model version 4-Lake,Ice,Snow and Sediment Simulator),并与涡度相关观测(Eddy Covariance,EC)结果进行对比,以年平均潜热通量模拟结果最佳为目标调整了模式中的消光系数、粗糙度长度方案,研究了该模型从半小时到年尺度上对湖表温度和水热通量的模拟性能.结果表明:模型对湖表温度的模拟在各时间尺度上均比较理想,但是模拟的日较差较小;从半小时到年尺度上潜热通量的变化趋势都能被很好地模拟出来,但在季节尺度上,潜热通量的模拟出现了秋冬季偏高、春夏季偏低的情况,季节变化模拟不准确.湖表温度和潜热通量模拟偏差的原因可能是消光系数的参数化方案.相比之下,感热通量尽管年际变化趋势的模拟值与观测值一致,但是从半小时到年尺度均被高估.特别地,冷锋过境期间,模型能较好地模拟出潜热通量和感热通量的变化趋势,但对于高风速条件下的感热通量模拟效果不佳.本文的研究结果能为湖泊模式的应用与发展提供有用信息.     

非均匀灌溉棉田能量平衡特征研究

运用国际能量平衡实验(EBEX-2000)的湍流、净辐射和土壤观测资料,运用涡动相关法分析了非均匀灌溉引起的热内边界层发展条件下近地层感热、潜热通量特征,并对有无灌溉两种条件下的能量闭合度进行了对比分析.在计算感热、潜热通量过程中,分别将Schotanus订正和Webb订正纳入了考虑范围,研究了两种订正方法对计算湍流热通量的影响.研究结果发现,由于非均匀灌溉生成的热内边界层使得近地层感热通量受到抑制,潜热通量出现波动,该现象在8.7 m比2.7 m 更为显著.非均匀灌溉导致的热内边界层的存在使得近地层能量闭合度偏低,能量平衡比率约为0.65;而没有热内边界层存在时,近地层能量平衡比率约为0.70.本实验中,Schotanus订正使得感热通量显著减小,其订正量日平均值约为-8 W/m²,占净辐射的近4%;Webb订正量日平均值约为2 W/m²,对能量平衡的影响较小.     

夏季中国西部地区辐射特征的观测分析和能量收支的模式计算

在2000年6-7月新疆阜康地区野外观测资料和美国宇航局Goddard地球观测系统资料同化系统计算相应时间能量收支基础上分析了夏季我国西部干旱地区辐射特征和能量收支.新疆阜康地区观测的夏季中午平均吸收辐射为664 W/m2左右,反照率为17%,夜间长波净辐射值为60W/m2左右.模式计算的相应地区的辐射特征与观测结果相当接近.模式计算结果表明,夏季新疆、甘肃西部、内蒙一带为感热通量高值区,其最大值在华北、东北地区,其值可达300W/m2.广大西北地区潜热通量都很低,其高值区在华东、华南、西藏东部和四川一带,最大值可达250-300W/m2.     

太湖小时尺度水面蒸发特征及3种模型模拟效果对比

小时尺度水面蒸发可影响水面大气边界层热力和动力结构,分析湖泊小时尺度水面蒸发主要影响因素,选取准确模拟其特征的蒸发模型,将有助于改善流域天气预报和空气质量预报.基于太湖避风港站2012—2013年通量、辐射和气象观测数据,分析太湖小时尺度水面蒸发主要影响因子和3个模型(传统质量传输模型、Granger and Hedstrom经验模型、DYRESM模型)的模拟效果.结果表明:影响太湖小时尺度水面蒸发的主要因子为水气界面水汽压差和风速的乘积,而非净辐射.传统质量传输模型、Granger and Hedstrom经验模型、DYRESM模型模拟值与全年实测值的一致性系数分别为0.92、0.87和0.89,均方根误差分别为28.35、41.58和38.26 W/m~2.传统质量传输模型对太湖小时尺度水面蒸发的日变化和季节动态模拟效果最佳,其夜间模拟相对误差小于3%,除秋季外,其他季节的模拟绝对误差均小于4 W/m~2.Granger and Hedstrom经验模型系统性地高估太湖潜热通量,在大气较为稳定的午后(高估22~32 W/m~2)和冬季(高估72%)高估最为明显,模拟效果最差.DYRESM模型也系统地高估太湖潜热通量,模拟效果居中.考虑水汽交换系数随风速的变化特征将有助于改善传统质量传输模型和DYRESM模型对太湖小时尺度水面蒸发的模拟精度.     

藏南羊卓雍错湖面大气湍流特征观测分析

湍流运动是大气边界层的本质特征,是地表与大气之间能量和物质交换的主要方式.本文利用2016和2017年4-10月藏南羊卓雍错湖泊涡动观测资料,分析了湖面大气湍流方差和湍流特征量的统计和变化特征.结果表明：（1）不稳定层结下,三维风速分量和超声虚温、水汽密度、CO₂密度的无量纲标准差随稳定度变化符合Monin-Obukhov相似理论的"1/3"或"-1/3"次幂律,垂直风速的拟合效果最好;稳定层结下,除CO₂密度无量纲标准差与稳定度无明显关系外,其他量基本上满足相似性规律;中性条件下,以上物理量的无量纲标准差分别趋近常数：3.57、3.93、0.77、20.91、6.35和11.96.（2）水平方向平均湍流强度（0.60和0.58）大于垂直方向（0.13）,三维方向湍流强度与平均风速的变化呈显著负相关,相关系数分别为-0.39、-0.42和-0.34.（3）湖面湍流动能随风速呈线性增长,增长率达0.45 m/s;近中性层结时湍流动能最大,层结越稳定或不稳定湍流动能均减小.（4）湖泊下午到傍晚动量输送较强,13：00-22：30时间段平均动量通量达0.091 kg/（m·s²）;热量输送以潜热为主,潜热通量日平均值（77.3 W/m²）是感热通量（14.6 W/m²）的5.3倍,感热和潜热通量日变化峰值分别出现在5：30（22.4 W/m²）和16：00（106.6 W/m²）.     

夏季中国西部地区辐射特片的观测分析和能量收支的模式计算

在2000年6-7月新疆阜康地区野外观测资料和美国宇航局Goddard地球观测系统资料同化系统计算相应时间能量收支基础上分析了夏季我国西部干旱地区辐射特征和能量收支。新疆阜康地区观测的夏季中午平均吸收辐射为664W/m^2左右，反照率为17%，夜间长波净辐射值为60W/m^2左右。模式计算的相应地区的辐射特征与观测结果相当接近。模式计算结果表明，夏季新疆、甘肃西部、内蒙一带为感热通量高值区，其最大值在华北、东北地区，其值可达300W/m^2。广大西北地区潜热通量都很低，其高值区在华东、华南、西藏东部和四川一带，最大值可达250-300W/m^2。     

夏季亚洲大地形双加热及近对流层顶位涡强迫的激发Ⅱ:伊朗高原-青藏高原感热加热

基于诊断和数值模拟,研究了夏季青藏高原和伊朗高原热力强迫的相互作用,其对亚洲副热带季风区水汽通量辐合的贡献及对欧亚大陆上空高对流层顶和平流层低层冷中心形成的影响.结果表明两大高原感热加热存在相互影响和反馈,伊朗高原感热加热减少青藏高原的表面加热,而青藏高原的感热加热则增加伊朗高原的表面加热;形成了观测到的伊朗高原感热加热-青藏高原感热加热和凝结潜热释放-大气垂直环流之间的准平衡耦合系统(TIPS),影响大气环流.青藏高原上的感热-潜热相互反馈在这个TIPS耦合系统中起主要作用.两大高原感热加热对其他地区的影响有相互加强也有相互抵消;青藏高原感热加热引发的对亚洲副热带季风区水汽通量辐合贡献率为伊朗高原感热影响的2倍以上;伊朗高原和青藏高原的感热加热共同作用对亚洲副热带季风区的水汽辐合作出最主要的贡献.TIPS的加热作用使对流层温度升高,并抬升了其上空的对流层顶,造成了那里平流层下层温度偏低;与欧亚大陆大尺度热力强迫共同作用,形成了对流层上层的暖性但在平流层下层为冷性的强大的反气旋环流南亚高压,从而影响区域和全球的天气气候.     

黄土高原冬季湍流相干结构在地表能量平衡中的作用

地表能量不平衡问题是近40年一直困扰微气象学界的重要难题.本文利用黄土高原定西干旱气象与生态环境试验站的相关数据,通过将湍流分解为相干结构部分和非相干结构部分,在涡动相关法中引入相干结构(Coherent structure,简称为CS)的贡献,探讨了CS在地表能量平衡中的作用.研究发现：(1)CS出现频次高,间歇性显著,在30 min平均时间中出现的数目为38个,出现时间与未出现时间分别占40%和60%左右;(2)CS对通量影响显著,引入CS贡献后,白天垂直动量通量、感热、潜热和湍流通量(感热与潜热之和)等4个通量计算的准确性均获得显著提高,分别提高达26%(0.01 m²·s^-2)、24%(25.33 W·m^-2)、31%(6.23 W·m^-2)和30%(31.66 W·m^-2).而不考虑CS的传统方法对这4个通量低估达19%、22%、21%和23%;(3)CS对4个通量的贡献分别为36%、41%、33%和50%;(4)传统方法得到的地表能量平衡闭合率EBR只有0.76、能...     

大型浅水湖泊与大气之间的动量和水热交换系数——以太湖为例

湖泊水面与大气之间垂直方向的动量、水汽和热量通量与风速、湿度和温度梯度之间存在比例关系,因此在湖泊水-气相互作用研究中,这比例系数(交换系数)是关键因子.在以往的研究中,交换系数通常直接采用水面梯度观测法或海洋大气近地层的参数化方案进行计算.本文采用涡度相关系统和小气候系统仪器在太湖平台上直接观测的通量和气象要素,对上述交换系数(最小均方差原则)进行优化,结果为:动量交换系数C_D10N=1.52×10^-3、水汽交换系数C_E10N=0.82×10^-3、热量交换系数CH10N=1.02×10^-3,与其他内陆湖泊涡度相关观测数据的推导结果一致.本文的研究结果表明:与海洋参数化方案相比,在相同的风速条件下,湖面的空气动力学粗糙度比海洋高,这可能是由于受到水深的影响;如果采用海洋参数化方案,会导致湖泊年蒸发量的估算值偏大40%.太湖的动量、水汽和热量交换系数可以视为常数,可以不考虑稳定度和风速的影响.这是因为本文中83%的数据为近中性条件.敏感性分析表明:如果考虑稳定度的影响,LE模拟值的平均误差降低了0.5 W/m²,H的平均误差降低了0.4 W/m²,u*的计算值没有变化;如果考虑风速的影响,u*模拟值的平均误差降低了0.004 m/s,LE的平均误差升高了1.3 W/m²,H的模拟结果几乎不受影响.这一结果能为湖气相互作用研究提供参考.     

Seasonal variations between sampling and classical mean turbulent heat flux estimates in the eastern North Atlantic

The two commonly used statistical measures of the air-sea heat flux, the sampling and classical means, have been compared using hourly reports over a 7-year-period from a weather ship stationed in the NE Atlantic. The sampling mean is the average over all flux estimates in a given period, where individual flux estimates are determined from ship reports of meteorological variables using the well-known bulk formulae. The classical mean is the flux derived by substituting period-averaged values for each of the meteorological variables into the bulk formula (where the averaging period employed is the same as that over which the fluxes are to be determined). Monthly sampling and classical means are calculated for the latent and sensible heat fluxes. The monthly classical mean latent heat flux is found to overestimate the sampling mean by an amount which increases from 1–2 W m⁻² in summer to 7 W m⁻² in winter, on average, over the 7-year-period. In a given winter month, the excess may be as great as 15 W m⁻², which represents about 10% of the latent heat flux. For the sensible heat flux, any seasonal variation between the two means is of the order of 1 W m⁻² and is not significant compared to the interannual variation. The discrepancy between the two means for the latent heat flux is shown to arise primarily from a negative correlation between the wind speed and sea-air humidity difference, the effects of which are implicitly included in the sampling method but not in the classical. The influence of the dominant weather conditions on the sign and magnitude of this correlation are explored, and the large negative values that it takes in winter are found to depend on the typical track of the mid-latitude depressions with respect to the position sampled. In conclusion, it is suggested that sampling means should be employed where possible in future climatological studies.     

Advective contributions to the heat balance of the german bight (LV Elbe 1) and the central north sea (OWS Famita)

Summary The mean annual cycle of the net energy flux through the sea surface and of the heat storage are investigated in detail using observations of the Light Vessel LV Elbe 1 for the period 1962-1986 in the German Bight and at Ocean Weather Ship OWS Famita for the period 1965-1978 in the central North Sea. The investigation confirms the general geographical picture of the heat budget of the German Bight that shows a net loss to the atmosphere by a long-term mean of -15 W m^-2. The radiative surface input of 113 W m^-2 is balanced by -62 W m^-2 net back radiation, -51 W m^-2 of latent heat flux and -15 W m^-2 of sensible heat flux. The heat advection calculated as the residual of the heat storage rate and surface energy balance is 16 W m^-2. The mean annual cycles of heat storage and surface energy balance are nearly equal, and the temperature variations are mainly driven by local heat input. The small differences build up the annual advection cycle. Warm water advection occurs from October to April and cold water advection in summer from May to September. The seasonal advection variability is extreme in winter and summer, and the ranges slow down in spring and autumn, when the sign of the heat balance changes. The OWS Famita is situated also in an area of net energy loss to the atmosphere, showing a long-term annual mean loss of -16 W m^-2. The surface radiation input of 105 W m^-2 is mainly balanced by outgoing long wave back radiation of -60 W m^-2 and a latent heat flux of -49 W m^-2. A minor contribution to the heat balance is the sensible heat flux of -12 W m^-2. Warm water advection occurs in winter and spring. Variability is greater than at LV Elbe 1. Calculated monthly fluxes show the dominance of the energy gain of incoming solar radiation. Net long-wave radiation is nearly constant with time. The sensible heat flux serves as a heat source only at LV Elbe 1 from May to June. The latent heat flux is a loss term all the year. The heat storage cycle is nearly equal to the surface energy balance at LV Elbe 1 ; the differences are more irregular at OWS Famita. The temperature variations are mainly driven by local heat input. The simplified one-dimensional balance holds generally for the heating period in both regions, although for some months the magnitude of the advection is up to a third of the net surface fluxes or the storage rate. At LV Elbe 1 from April to December, the heat budget is dominated by local dynamics. At OWS Famita the advective contribution is less than 30% of net surface heat input from May to October. The dominance of solar radiation in determining the surface heat fluxes, and the annual cycles of the storage rate in phase justify the use of one-dimensional models as a first approximation of the temperature dynamics in these regions. Comparisons of simulations of the temperature cycle at both sites with observations give sufficient precision during most parts of the seasonal cycle. Suitable data sets to drive and validate these models are now available and different models should be tested.

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Advektive beitr?ge zur w?rmebilanz der deutschen bucht (feuerschiff elbe 1) und zur w?rmebilanz der zentralen nordsee (Wetterschiff Famita)

Zusammenfassung Untersucht wurde der mittlere Jahresgang vom W?rmeeintrag durch die Meeresoberfl?che und vom W?rmeinhalt der Wassers?ule. Dazu wurden Messungen aus der Deutschen Bucht vom Feuerschiff Elbe 1 für die Jahre 1962-1986 und Messungen in der zentralen Nordsee vom Wetterschiff Famita für die Jahre 1965-1978 verwendet. Die Untersuchung best?tigt das generelle Bild einer W?rmeabgabe an die Atmosph?re von -15 W m^-2 im langj?hrigen Mittel für die Deutsche Bucht. Die kurzwellige Einstrahlung von 113 W^-2 wird durch -62 W m^-2 langwellige Ausstrahlung, -51 W m^-2 latenten W?rmefluβ und -15 W m^-2 sensiblen W?rmefluβ nahezu balanciert. Die berechnete W?rmeadvektion als Residuum aus W?rmeinhalt und Nettow?rmefluβ an der Meeresoberfl?che betr?gt 16 W m^-2 Der Jahresgang des W?rmeinhaltes und der Jahresgang des Nettow?rmeflusses an der Oberfl?che sind fast gleich, so daβ der Temperaturjahresgang haupts?chlich durch den lokalen W?rmeeintrag gesteuert wird. Kleine Abweichungen hiervon bestimmen den Jahresgang der W?rmeadvektion. Warmwasseradvektion tritt von Oktober bis April auf. Kaltwasseradvektion liegt im Sommer von Mai bis September vor. Die Variabilit?t der W?rmeadvektion ist im Winter und Sommer am gr?βten, w?hrend die Variabilit?t im Frühjahr und Herbst geringer ist, wenn sich das Vorzeichen der W?rmebilanz ?ndert. Das Wetterschiff Famita befindet sich ebenfalls in einer Region, in der W?rme an die Atmosph?re abgegeben wird. Die W?rmeabgabe betr?gt -16 W m^-2 im langzeitlichen Mittel. Die kurzwellige Einstrahlung von 105 W m^-2 wird haupts?chlich durch -60 W m^-2 langwellige Ausstrahlung, -49 W m^-2 latenten W?rmefluβ und -12 W m^-2 sensiblen W?rmefluβ balanciert. Warmwasseradvektion tritt im Winter und Frühjahr auf. Die Variabilit?t der W?rmeadvektion ist gr?βer als bei Feuerschiff Elbe 1. Die berechneten monatlichen Energieflüsse zeigen, daβ die solare Einstrahlung den Jahresgang der W?rmebilanz dominiert. Die effektive Ausstrahlung ist nahezu konstant. Die sensible W?rme wirkt nur bei Feuerschiff Elbe 1 von Mai bis Juni als W?rmequelle. Der latente W?rmefluβ ist w?hrend des gesamten Jahres negativ. Für Feuerschiff Elbe 1 ist der W?rmeinhalt der Wassers?ule mit dem Energieeintrag an der Oberfl?che in Phase, w?hrend bei Wetterschiff Famita Differenzen auftreten. Die Temperaturvariationen sind haupts?chlich durch den lokalen W?rmeeintrag bestimmt. Diese vereinfachten Verh?ltnisse gelten für beide Regionen, obwohl für einige Monate die W?rmeadvektion bis zu einem Drittel des Nettow?rmeflusses an der Oberfl?che betragen kann. Bei Feuerschiff Elbe 1 wird die W?rmebilanz von April bis Dezember durch die lokale Dynamik bestimmt. Bei Wetterschiff Famita ist die W?rmeadvektion von Mai bis Oktober kleiner als 30% vom Oberfl?cheneintrag. Die Dominanz der solaren Einstrahlung für die W?rmebilanz an der Oberfl?che und der phasengleiche Jahresgang des W?rmeinhaltes rechtfertigen es, eindimensionale Wassers?ulenmodelle für die Region zu verwenden, um die Dynamik der Temperatur zu berechnen. So zeigt der Vergleich von simulierten und gemessenen Temperaturjahresg?ngen an beiden Positionen eine ausreichende Genauigkeit über weite Teile des Jahres. Damit stehen neben der gezeigten W?rmebilanzabsch?tzung zwei Datens?tze zur Verfügung, um Modelle zu betreiben, zu validieren und verschiedenartige Modelle zu vergleichen.

     

三峡水库香溪河库湾水-气界面热交换过程及水体稳定性

水库或湖泊的热分层结构是其动力与环境过程的重要研究方面,虽然很多学者针对水体分层结构和演变机理开展了大量研究,但水体通过水-气界面与大气进行热交换的过程,各气象因子的贡献机理等研究成果还很缺乏。本文基于三峡水库香溪河库湾2019年3月-2020年2月期间的水温、水位及气象等监测数据,针对水-气界面热交换过程如何影响水温垂向结构及表层水体湍流混合作用开展研究。结果表明,(1)香溪河水体年内呈高温期分层、低温期混合的基本特征,高温期混合层深度小于8 m,低温期混合层深度超过30 m。(2)太阳短波辐射是香溪河水体的主要热源,潜热通量和长波辐射是香溪河水体的主要冷源,感热通量贡献极小。(3)香溪河平均风速较弱,约为1.6 m/s,主要通过增强潜热和感热通量的方式影响水体垂向稳定性结构特征,其机械扰动作用较弱。(4)表层水体湍能通量在高温期较低(10^-7m³/s³量级),此时水体处于分层状态,风应力大概率主导表层水体湍流发育;低温期表层水体湍能通量较高(10^-6 m³/s^3<...     

Thermische Energieflüsse an der Wasseroberfläche: Beispiel Sempachersee

Data collected every 20 minutes for 18 months by a meteorological buoy moored on Lake Sempach in Switzerland (maximum depth 86 m, surface area 14.1 km²) are used to calculate different processes contributing to the net heat flux between water and atmosphere. The processes considered are shortwave and longwave radiation, evaporation/condensation and sensible heat transfer. The temporal resolution of the measurements allows the evaluation of the processes occurring on three different time scales: diurnal variations, weather events of a few days and yearly cycles. The heat content of the lake is calculated from quasi-continuous measurements of water temperature at different depths. The yearly amplitude of the heat content is 1100·10⁶ J/m². Short-time variations of the heat flux determined from water temperature analysis agree well with the flux variations modeled using meteorological data. However, the latter generally underestimates the measured net heat flux in the long term. Wind measurements, together with the net heat flux, are used to calculate the Flux Richardson Number and the Monin-Obukov Length. Examples are given to show the predominant influence of the wind on the stratification of the upper water column and thus on the surface water temperature.      

Verification of land surface evapotranspiration estimation from remote sensing spatial contextual information

Estimation of evapotranspiration (ET) is of great significance in modeling the water and energy interactions between land and atmosphere. Negative correlation of surface temperature (T_s) versus vegetation index (VI) from remote sensing data provides diagnosis on the spatial pattern of surface soil moisture and ET. This study further examined the applicability of T_s–VI triangle method with a newly developed edges determination technique in estimating regional evaporative fraction (EF) and ET at MODIS pixel scale through comparison with large aperture scintillometer (LAS) and high‐level eddy covariance measurements collected at Changwu agro‐ecological experiment station from late June to late October, 2009. An algorithm with merely land and atmosphere products from MODIS onboard Terra satellite was used to estimate the surface net radiation (R_n) and soil heat flux. In most cases, the estimated instantaneous R_n was in good agreement with surface measurement with slight overestimation by 12 W/m². Validation results from LAS measurement showed that the root mean square error is 0.097 for instantaneous EF, 48 W/m² for instantaneous sensible heat flux, and 30 W/m² for daily latent heat flux. This paper successfully presents a miniature of the overall capability of T_s–VI triangle in estimating regional EF and ET from limited number of data. For a thorough interpretation, further comprehensive investigation needs to be done with more integration of remote sensing data and in‐situ surface measurements. Copyright © 2011 John Wiley & Sons, Ltd.     

Treatment of anchor pixels in the METRIC model for improved estimation of sensible and latent heat fluxes

Abstract

Reliable estimation of sensible heat flux (H) is important in energy balance models for quantifying evapotranspiration (ET). This study was conducted to evaluate the value of adding the Priestley-Taylor (PT) equation to the METRIC (Mapping Evapotranspiration at high Resolution with Internalized Calibration) model. METRIC was used to estimate energy fluxes for 10 Landsat images from the 2005, 2006 and 2007 crop growing seasons in south-central Nebraska, USA, where each image owing to recent rainfall exhibited high residual moisture content even at the hot pixel. The METRIC model performed satisfactorily for net radiation (R_n ) and soil heat flux (G) estimation with a root mean square error (RMSE) of 52 and 24 W m^-2, respectively. A RMSE of 122 W m^-2 for H indicated the limitation of the METRIC model in estimating H for high residual moisture content of the hot pixel (Alfalfa reference ET fraction, ET _r F > 0.15). The modified METRIC model (wet METRIC or wMETRIC) incorporating the PT equation was applied to calculate H at the anchor pixels (hot and cold) for high residual moisture content of the hot pixel. The α coefficient of the PT equation was locally calibrated using hourly meteorological data from an automatic weather station and R_n and G data from a Bowen ratio flux tower. The mean α coefficient value was 1.14. The wMETRIC model reduced the RMSE of H from 122 to 64 W m^-2 and that of latent heat flux, LE, from 163 to 106 W m^-2. The RMSE of daily ET decreased from 1.7 to 1.1 mm d^-1 with wMETRIC. The results indicate that treatment of anchor pixels for high residual moisture content with the PT approach gives improved estimation of H, LE and daily ET. It is recommended that the wMETRIC model be used for estimating ET if the hot pixel has high residual moisture (i.e. reference ET fraction > 0.15).

Citation Singh, R. K. & Irmak, A. (2011) Treatment of anchor pixels in the METRIC model for improved estimation of sensible and latent heat fluxes. Hydrol. Sci. J. 56(5), 895–906.     

A detailed energy budget analysis of river supercooling and the importance of accurately quantifying net radiation to predict ice formation

River supercooling and ice formation is a regular occurrence throughout the winter in northern countries. The resulting frazil ice production can obstruct the flow through intakes along the river, causing major problems for hydropower and water treatment facilities, among others. Therefore, river ice modellers attempt to calculate the river energy budget and predict when supercooling will occur in order to anticipate and mitigate the effects of potential intake blockages. Despite this, very few energy budget studies have taken place during freeze-up, and none have specifically analysed individual supercooling events. To improve our understanding of the freeze-up energy budget detailed measurements of air temperature, relative humidity, barometric pressure, wind speed and direction, short- and longwave radiation, and water temperature were made on the Dauphin River in Manitoba. During the river freeze-up period of late October to early November 2019, a total of six supercooling events were recorded. Analysis of the energy budget throughout the supercooling period revealed that the most significant heat source was net shortwave radiation, reaching up to 298 W/m², while the most significant heat loss was net longwave radiation, accounting for losses of up to 135 W/m². Longwave radiation was also the most significant heat flux overall during the individual supercooling events, accounting for up to 84% of the total heat flux irrespective of flux direction, highlighting the importance of properly quantifying this flux during energy budget calculations. Five different sensible (Q_h) and latent (Q_e) heat flux calculations were also compared, using the bulk aerodynamic method as the baseline. It was found that the Priestley and Taylor method most-closely matched the bulk aerodynamic method on a daily timescale with an average offset of 8.5 W/m² for Q_h and 10.1 W/m² for Q_e, while a Dalton-type equation provided by Webb and Zhang was the most similar on a sub-daily timescale with average offsets of 20.0 and 14.7 W/m² for Q_h and Q_e, respectively.