北极低空急流和低层逆温特征观测分析

利用北冰洋冰表面热量平衡计划1997年10月中旬至1998年10月上旬的探空气球探测结果,分析了北极地区近地层逆温和低空急流特征.结果表明,96％的观测时次(11:15和23:15,协调世界时)出现近地层逆温,其中22％的逆温为贴地逆温,70％的逆温厚度在250～850 m之间,冬半年贴地逆温发生频率、逆温层厚度和逆温层内的温度变化都明显要大于夏半年.全年间低空急流出现频率为41％,平均高度为520 m,最大频率出现在150 m附近,70％的急流出现在600m高度以下.急流平均风速为10.6m·s-1,风速在4～13 m·s-1范围内的急流约占总数的75％,东和东南方向为全年急流的主导风向.根据对急流核和地面风速之间转换角分布的分析,惯性震荡可能是北极低空急流的主要成因.     

不同平均时间对LOPEX10资料涡动相关湍流通量计算结果影响的探讨

通过处理涡动相关系统观测的近地层湍流脉动量可以获取地—气间感热和潜热通量,然而选择不同平均时间对通量计算的结果有较大影响。采用黄土高原陆面过程野外观测试验(LOPEX10)期间获得的涡动相关系统观测资料,分析了不同平均时间对湍流通量计算的影响,并采用雷诺平均和分解方法推导了平均时间引起的通量差值的数学表达式(Flux Compensation,FC)。结果表明:(1)FC公式可以说明采用不同平均时间数据之间的关系,也可以直接计算低频涡旋对湍流通量的贡献。FC公式计算的结果与直接计算的不同平均时间通量计算之差的相关系数在0.95以上,并可以确定计算湍流通量的最佳平均时间。(2)通过采用Ogive函数确定了计算LOPEX10期间通量的最佳平均时间长度为30min,印证了利用雷诺平均和分解方法计算湍流通量补偿的准确性。(3)通过进一步的数学变换,证明了平均时间对湍流通量计算的影响直接与湍流低频变化相关,FC公式可以用来确定涡动相关观测数据的最佳平均时间,并且在获得较高时间分辨率的湍流通量数据的同时,补偿因平均时间过短而遗漏的低频信息。     

The Spatial Variability of Energy and Carbon Dioxide Fluxes at the Floor of a Deciduous Forest

Fluxes of carbon dioxide, water and sensible heat were measured using three different eddy covariance systems above the forest floor of a closed deciduous forest (leaf area index approx 6). The primary objective was to examine the representativeness of a single eddy covariance system in estimating soil respiration for time scales ranging from one-half hour to more than one week. Experiments were conducted in which the eddy covariance sensors were in one of three configurations: i) collocated, ii) separated horizontally or iii) separated vertically. A measure of the variation between the three systems (CV',related to the coefficient of variation) for half-hour carbon dioxide fluxes was 0.14 (collocated systems), 0.34 (vertically separated systems at 1, 2 and 4 m above the surface), and 0.57 (systems horizontally separated by 30 m). A similar variation was found for other scalar fluxes (sensible and latent heat). Variability between systems decreased as the number of half-hour sampling periods used to obtain mean fluxes was increased. After forty-eight hours (means from ninety-six half-hour samples), CV' values for carbon dioxide fluxes were 0.07, 0.09 and 0.16 in the collocated, vertically separated and horizontally separated experiments, respectively. The time dependence of variability has implications on the appropriateness of using short-term measurements in modelling validation studies. There are also implications concerning the appropriate number of half-hour samples necessary to obtain reliable causal relationships between flux data and environmental parameters. Based on the longer-term measurements, we also discuss the representativeness of a single eddy covariance system in long-term monitoring of soil respiration and evaporation beneath forest canopies using the eddy covariance method.     

Turbulent variance characteristics of temperature and humidity over a non-uniform land surface for an agricultural ecosystem in China

This paper describes the application of the variance method for flux estimation over a mixed agricultural region in China. Eddy covariance and flux variance measurements were conducted in a near-surface layer over a non-uniform land surface in the central plain of China from 7 June to 20 July 2002. During this period, the mean canopy height was about 0.50 m. The study site consisted of grass (10% of area), beans (15%), corn (15%) and rice (60%). Under unstable conditions, the standard deviations of temperature and water vapor density (normalized by appropriate scaling parameters), observed by a single instrument, followed the Monin-Obukhov similarity theory. The similarity constants for heat (CT ) and water vapor (Cq) were 1.09 and 1.49, respectively. In comparison with direct measurements using eddy covariance techniques, the flux variance method, on average, underestimated sensible heat flux by 21% and latent heat flux by 24%, which may be attributed to the fact that the observed slight deviations (20% or 30% at most) of the similarity “constants” may be within the expected range of variation of a single instrument from the generally-valid relations.     

Comparison of Ozone Fluxes over a Maize Field Measured with Gradient Methods and the Eddy Covariance Technique

Ozone(O_3) fluxes were measured over a maize field using the eddy covariance(EC) technique and gradient methods.The main objective was to evaluate the performance of the gradient methods for measuring the O_3 flux by comparing them with the EC O_3 flux.In this study,turbulent exchange coefficients(K) calculated with three methods were compared.These methods were the aerodynamic gradient(AG) method(in which K is calculated by using wind speed and temperature gradients),the aerodynamic gradient combined with EC(AGEC) method,in which the friction velocity and other variables are based on EC measurements,and the modified Bowen ratio using the EC sensible heat flux and temperature gradient(MBR) method.Meanwhile,the effects of the measurement and calculation methods of the O_3 concentration gradient were analyzed.The results showed that:(1) on average,the transfer coefficient computed by the MBR method was 40% lower,and the coefficient determined with the AG method was 25% higher,than that determined with the AGEC method.(2) The gradient method's O_3 fluxes with the MBR,AGEC,and AG methods were 30.4% lower,11.7% higher,and 45.6% higher than the EC O_3 flux,respectively.(3) The effect of asynchronous O_3 concentration measurements on the O_3 gradient must be eliminated when using one analyzer to cyclically measure two-level O_3 concentrations.The accuracy of gradient methods for O_3 flux is related to the exchange coefficient calculation method,and its precision mainly depends on the quality of the O_3 gradient.     

Testing of the bandpass eddy covariance method for a long-term measurement of water vapour flux over a forest

The bandpass eddy covariance method has been used to measure the turbulent flux of scalar quantities using a slow-responsescalar sensor. The method issimilar in principle to the traditional eddy correlation method but includes the estimation of high-frequency components of the flux on the basis of cospectral similarity in the atmospheric surface layer. In order to investigate the performance of the method, measurements of the water vapour flux over a forest with the bandpass eddy covariance method and the direct eddy correlation method were compared. The flux obtained by the bandpass eddy covariance method agreed with that by the eddy correlation method within ±20% for most cases, in spite of a rather slow sensor-response of the adopted hygrometer. This result supports its relevance to a long-term continuous operation, since a stable, low-maintenance,general-purpose sensor canbe utilized for scalar quantities. Oneweak point of the method isits difficulty in principle to measure the correct flux when the magnitude of the sensible heat flux is very small, because the method uses the sensible heat flux as a standard reference for the prediction of undetectable high-frequency components of the scalar flux. An advanced method is then presented to increase its robustness. In the new method, output signals from a slow-response sensor are corrected using empirical frequency-responsefunctions for the sensor,thereby extending the width of the bandpass frequency region where components of the flux are directly measured (not predicted). The advanced method produced correct fluxes for all cases including the cases of small sensible heat flux. The advanced bandpass eddy covariance method is thus appropriate for along-term measurement of the scalar fluxes.     

Wavelet Analysis of Intermittent Turbulent Transport in the Atmospheric Surface Layer over a Monsoon Trough Region

The structure of the turbulence in the atmospheric surface layer over a monsoon trough region has been studied using structural analysis based on wavelet transform. The observational site is located at the eastern (wet) end of the monsoon trough region, characterized by high moisture in the atmospheric surface layer. On the average relative humidity varied from 70% to 100% during the experiment. The wind and temperature data, collected at Kharagpur (22°25' N, 87°18' E) at six observational hours of a day in June 1990 during the Monsoon Trough Boundary Layer Experiment (MONTBLEX), have been utilized in the study. The wind and instantaneous momentum flux time series were decomposed into 12 scales using the Haar wavelet transform. The eddies exhibited a large temporal variability generating intermittency in the energy and flux distributions. A criterion based on the isotropy has been suggested for separating the large eddies from the small eddies. At the separation scale the isotropy coefficient drops sharply. It is shown that the intermittency in the small eddies resulted from the spatial variation of energy, and deviation of velocity statistics from the Gaussian distribution known as flatness. The deviation from the -5/3 power law has been attributed to the increased mean values of, (i) the coefficient of variation of energy, and (ii) the flatness factor, in the inertial subrange. The decomposition of the instantaneous momentum flux time series reveals that the major contribution to the total flux arises from the large eddies. The quadrant analysis of the momentum flux shows that ejections and sweeps account for a substantial part of the total flux, and quantifies the relative importance of the various spatial scales that contribute to the transport of momentum.     

Effects of the Soil Heat Flux Estimates on Surface Energy BalanceClosure over a Semi-Arid Grassland

Soil heat flux is important for surface energy balance (SEB), and inaccurate estimation of soil heat flux often leads to surface energy imbalance. In this paper, by using observations of surface radiation fluxes and soil temperature gradients at a semi-arid grassland in Xilingguole, Inner Mongolia, China from June to September 2008, the characters of the SEB for the semi-arid grassland were analyzed. Firstly, monthly averaged diurnal variations of SEB components were revealed. A 30-min forward phase displacement of soil heat flux (G) observed by a fluxplate at the depth of 5-cm below the soil surface was conducted and its effect on the SEB was studied. Secondly, the surface soil heat flux (Gs) was computed by using harmonic analysis and the effect of the soil heat storage between the surface and the fluxplate on the SEB was examined. The results show that with the 30-min forward phase displacement of observed G, the slope of the ordinary linear regression (OLR) of turbulent fluxes (H+LE) against available energy (Rn–G) increased from 0.835 to 0.842, i.e., the closure ratio of SEB increased by 0.7%, yet energy imclosure of 15.8% still existed in the SEB. When Gs, instead of G was used in the SEB equation, the slope of corresponding OLR of (H+LE) against (Rn–Gs) reached 0.979, thereby the imclosure ratio of SEB was reduced to only 2.1%.     

Thermodynamics and Microphysical Characteristics of an Extreme Rainfall Event Under the Influence of a Low-level Jet over the South China Coast

In this paper, the data of Automatic Weather Stations (AWSs), ERA5 reanalysis, sounding, wind profile radar, and dual-polarization radar are used to study an extreme rainfall event in the south China Coast on 11 to 12 May 2022 from the aspects of thermodynamics and microphysical characteristics under the influence of low-level jets (LLJs). Results show that: (1) The extreme rainfall event can be divided into two stages: the first stage (S1) from 0000 to 0600 LST on May 12 and the second stage (S2) from 0700 to 1700 LST on the same day. During S1, the rainfall is mainly caused by the upper-level shortwave trough and the boundary layer jet (BLJ), characterized by strong upward motion on the windward side of mountains. In S2, the combined influence of the BLJ and synoptic-system-related low-level jet (SLLJ) increases the vertical wind shear and vertical vorticity, strengthening the rainstorm. In combination with the effect of topography, a warm and humid southwest flow continuously transports water vapor to farther north, resulting in a significant increase in rainfall over the study area (on the terrain’s windward slope). From S1 to S2, the altitude of a divergence center in the upper air decreases obviously. (2) The rainfalls in the two stages are both associated with the mesoscale convergence line (MCL) on the surface, and the wind field from the mesoscale outflow boundary (MOB) in S1 is in the same direction as the environmental winds. Due to a small area of convergence that is left behind the MOB, convection moves eastward quickly and causes a short duration of heavy rainfall. In S2, the convergence along the MOB is enhanced, which strengthens the rainfall and leads to strong outflows, further enhancing the surface convergence near the MOB and forming a positive feedback mechanism. It results in a slow motion of convection and a long duration of heavy rainfall. (3) In terms of microphysics, the center of a strong echo in S1 is higher than in S2. The warm-rain process of the oceanic type characterizes both stages, but the convective intensity in S2 is significantly stronger than that in S1, featuring bigger drop sizes and lower concentrations. It is mainly due to the strengthening of LLJs, which makes small cloud droplets lift to melting levels, enhancing the ice phase process (riming process), producing large amounts of graupel particles and enhancing the melting and collision processes as they fall, resulting in the increase of liquid water content (LWC) and the formation of large raindrops near the surface.     

阿尔卑斯山杉林冠层影响辐射传输的个例分析

利用瑞士Alptal观测站杉树林冠层上方、下方的辐射观测资料,分析了冠层对短波辐射的减弱及对长波辐射的增幅作用及其季节变化。结果表明,对比较密集的常绿针叶林,冠层对入射短波辐射的透过率随着太阳高度的降低而减小,春季以后趋于稳定;冠层对长波辐射的增幅作用随天气状况而变化,这种增幅作用在晴空条件下最显著,可达1.5倍。在冬季,因为太阳辐射较弱,冠层对长波辐射的增幅作用超过对短波辐射的减弱从而增加地面净辐射。在其它季节,太阳辐射比较强,冠层对短波辐射的减弱超过对长波辐射的增幅作用而减少地面净辐射。地面净辐射与冠层上方气温的变化趋势虽然在有些时段一致,但在伴随降雪过程的降温时段,地面净辐射与气温的变化趋势近乎反相,在积雪融化时段,地面净辐射的增加比气温升高更显著,尤其是在白天。     

Consequences of Incomplete Surface Energy Balance Closure for CO2 Fluxes from Open-Path CO2/H2O Infrared Gas Analysers

We present an approach for assessing the impact of systematic biases in measured energy fluxes on CO₂ flux estimates obtained from open-path eddy-covariance systems. In our analysis, we present equations to analyse the propagation of errors through the Webb, Pearman, and Leuning (WPL) algorithm [Quart. J. Roy. Meteorol. Soc. 106, 85–100, 1980] that is widely used to account for density fluctuations on CO₂ flux measurements. Our results suggest that incomplete energy balance closure does not necessarily lead to an underestimation of CO₂ fluxes despite the existence of surface energy imbalance; either an overestimation or underestimation of CO₂ fluxes is possible depending on local atmospheric conditions and measurement errors in the sensible heat, latent heat, and CO₂ fluxes. We use open-path eddy-covariance fluxes measured over a black spruce forest in interior Alaska to explore several energy imbalance scenarios and their consequences for CO₂ fluxes.     

The Influence of Advection on the Short Term CO 2 -Budget in and Above a Forest Canopy

An experimental micrometeorological set-up was established at the CARBOEURO-FLUX site in Tharandt, Germany, to measure all relevant variables for the calculation of the vertical and horizontal advective fluxes of carbon dioxide. The set-up includes two auxiliary towers to measure horizontal and vertical CO₂ and H₂O gradients through the canopy, and to make ultrasonic wind measurements in the trunk space. In combination with the long-term flux tower an approximately even-sided prism with a typical side-length of 50 m was established. It is shown that under stable (nighttime) conditions the mean advective fluxes have magnitudes on the same order as the daily eddy covariance (EC) flux, which implies that they play a significant, but not yet fully understood, role in the carbon budget equation. The two advective fluxes are opposite and seem to cancel each other at night (at least for these measurements). During the day, vertical advection tends to zero, while horizontal advection is still present implying a flow of CO₂ out of the control volume. From our measurements, a mean daily gain of 2.2 gC m^–2 d^–1 for the horizontal advection and a mean daily loss of 2.5 gC m^–2d^–1 for the vertical advection is calculated for a period of 20 days. However the large scatter of the advective fluxes has to be further investigated. It is not clear yet whether the large variability is natural or due to measurement errors and conceptual deficiencies of the experiment. Similar results are found in the few comparable studies.     

近海水汽初值和对流影响一次华南前汛期沿海强降水对流系统发展过程的机理研究

本研究在对华南季风降水试验（SCMREX）观测资料分析的基础上,采用数值模拟试验探讨南海北部区域湿度场初值误差和海上对流对2014年5月8日华南沿海地区的一次强降雨过程的中尺度对流系统（MCS）的发展和移动的影响。加密探空和风廓线观测分析表明在珠江口地区有西南风和偏东风急流形成的辐合区,为对流在该地区增强发展提供了条件。增加和减少近海湿度以及关闭积云和微物理过程潜热释放,所造成的温度场以及风场的变化对广东沿海地区的对流的强度和移动路径都有明显的影响。特别是增加海上关键区的湿度,由于海上对流的发展改变了整个区域的环流,抑制了陆地上对流的发展。关闭海上关键区对流过程潜热的释放,导致低空急流到达更加偏北的位置,对流系统在模拟的后期向东北移动。通过这些试验表明,海上的湿度等要素场和对流活动对沿海地区的降雨预报有着十分重要的影响,需要进一步加强海上观测及其资料同化方法。     

半干旱草原下垫面能量平衡特征及土壤热通量对能量闭合率的影响

土壤热通量在半干旱草原下垫面能量平衡研究中极为重要,土壤热通量估计不够准确是导致地表能量不平衡的一个重要原因。利用2008年6—9月锡林郭勒草原主生长期地表辐射、通量和土壤温度梯度观测资料,研究中纬度半干旱草原下垫面地表能量平衡特征。首先,在分析能量平衡各分量月平均日变化特征的基础上,通过对土壤热流量板观测的5 cm深度土壤热通量(G)的相位前移,研究了土壤热通量相位滞后对地表能量平衡产生的影响;其次,利用谐波分析方法,通过计算地表土壤热通量(Gs),分析了地表到热流量板之间的土壤热量储存对地表能量平衡的影响。结果表明:(1)将土壤热通量相位前移30 min,湍流通量与可利用能量(Rn-G)线性回归的斜率从0.835增加到0.842,地表能量闭合率提高了0.7%,但仍有15.8%的能量不闭合;(2)考虑了地表到热流量板之间的土壤热量储存之后,湍流通量与可利用能量之间的回归斜率达到0.979,能量不闭合程度仅为2.1%。     

夏季风北推和强度对我国北方夏季降水影响的年代际特征

利用近50年NCEP／NCAR的候平均风场和比湿场资料,计算了东亚夏季风北界的位置,发现季风北界存在着明显的年代际变化,季风北界偏北的年份集中于1950年代和1960年代,而偏南的年份则集中于1980年代和1990年代,并且各个季风区的季风北界的年代际变化比较一致。利用这些资料定义了“区域夏季风指数”,更好地反映东亚夏季风的强弱和北方各地区旱涝之间的关系;用合成分析方法比较了夏季风强度与北方各干旱半干旱区域旱涝变化的关系,说明华北地区最为敏感,区域季风指数与7,8月降水量的关系最好。最后用小波分析方法,讨论了这些区域的夏季风强度和夏季降水量在不同时间尺度上的关系,发现区域季风强度指数和各季风区降水量的关系在15年左右的年代际尺度上关系最好,在3～5年的年际尺度上也有比较好的相关。这些结果对于北方地区的气候趋势预测有一定参考价值。