Atmospheric turbulent fluxes over snow

On March 26, 1971, eddy fluxes of momentum, sensible heat and water vapour were measured over Lake Mendota, Wisconsin, U.S.A., which was covered by an extensive snowfall. An evaporation rate of about 0.7mm day^–1 (2.2 mW cm^–2) was detected. Wind speeds were light and the atmosphere near the surface was highly stable. In these conditions, the average sensible heat transfer and Reynolds stress were -0.9 mW cm^–2 and 0.10 dyn cm^–2, respectively. Comparison with measured gradients of wind speed, temperature and humidity yield a drag coefficient of about 0.54 × 10^–3, and bulk transfer coefficients for sensible and latent heat of 0.41 × 10^–3 and 0.78 × 10^–3, respectively, applied to 10-m data. When corrected for the effect of atmospheric stability, these three coefficients become (in the same order) 1.2 × 10^–3, 0.9 × 10^–3 and 2.5 × 10^–3. The errors in these estimates are such that the drag coefficient is not significantly different from that corresponding to an aerodynamically smooth surface, while the heat coefficients are similar to those normally applied over liquid water surfaces.     

Evaluation of turbulent fluxes over Maitri,Antarctica

Turbulent fluxes have been evaluated for clear sunny days over the Indian Antarctic station, Maitri, using the basic meteorological data recorded at four levels of a 28 m tower. The data are supplemented with radiation data. The surface layer over Maitri remains thermally stratified during the hours of minimum solar insolation, the so-called nighttime period. The surface winds during this period are generally very strong resulting in high momentum fluxes. In particular, for high winds (>12 m s^–1), the temperature gradient is found to be less positive than for moderate winds (4 to 7 m s^–1). Solar insolation provided the daytime heating necessary for the diurnal variation of atmospheric stability, and hence, for the turbulent fluxes. Thus, on clear days daytime conditions are marked by upward transport of heat with reduced momentum flux, while stable nighttime conditions are marked by a downward heat flux with increased momentum fluxes.     

湍流通量计算方法和误差的比较研究

近地层的风、温、湿梯度以及辐射和土壤热通量资料在过去的几十年间被广泛地用于计算陆-气感热、潜热通量.然而,不同的计算方法之间存在一些差异,随意的选用可能会造成计算结果存在较大的误差.文中利用绿洲系统能量与水分循环过程观测试验的2005年6月甘肃金塔绿洲中部观测资料,在分析了观测期间近地层微气象特征的基础上,对鲍恩比法、空气动力学法(包含两种不同计算方案)和变分法计算的绿洲农田下垫面的湍流通量进行了比较,并通过敏感性实验和理论分析探讨了不同方法间计算结果的差异和误差的来源.结果表明:不同方法计算的湍流通量特征存在明显差异.鲍恩比法从理论上满足能量平衡关系,但在昼夜交替和夜晚时出现了计算不稳定现象,因此在这些时段不适用.变分法避免了鲍恩比法计算夜间湍流通量的不稳定性,而且二者计算的白天湍流通量有较好的相关性;同时变分法由于加入了辐射和土壤热通量信息,明显改善了空气动力学法白天的能量平衡状况;变分法计算得到的潜热通量对辐射和微气象条件有较为合理的响应;对数据误差的敏感性试验表明,鲍恩比法和空气动力学法应用于绿洲下垫面上对观测误差较为敏感,而变分法相对比较稳定.进一步的分析表明,鲍恩比法在-2<β<0的范围内由于方法本身局限而适用性较差;而空气动力学方法计算结果的差异和误差来源于所应用的相似性函数.因而与鲍恩比法和空气动力学法计算的湍流通量值相比较,变分法的计算结果更加合理和稳定,在绿洲下垫面有较好的适用性.该文研究方法和结果可为这些方法的使用提供参考.     

Airborne measurements of turbulent trace gas fluxes and analysis of eddy structure in the convective boundary layer over complex terrain

Using the new high-frequency measurement equipment of the research aircraft DO 128, which is described in detail, turbulent vertical fluxes of ozone and nitric oxide have been calculated from data sampled during the ESCOMPTE program in the south of France. Based on airborne turbulence measurements, radiosonde data and surface energy balance measurements, the convective boundary layer (CBL) is examined under two different aspects. The analysis covers boundary-layer convection with respect to (i) the control of CBL depth by surface heating and synoptic scale influences, and (ii) the structure of convective plumes and their vertical transport of ozone and nitric oxides. The orographic structure of the terrain causes significant differences between planetary boundary layer (PBL) heights, which are found to exceed those of terrain height variations on average. A comparison of boundary-layer flux profiles as well as mean quantities over flat and complex terrain and also under different pollution situations and weather conditions shows relationships between vertical gradients and corresponding turbulent fluxes. Generally, NO_x transports are directed upward independent of the terrain, since primary emission sources are located near the ground. For ozone, negative fluxes are common in the lower CBL in accordance with the deposition of O₃ at the surface.The detailed structure of thermals, which largely carry out vertical transports in the boundary layer, are examined with a conditional sampling technique. Updrafts mostly contain warm, moist and NO_x loaded air, while the ozone transport by thermals alternates with the background ozone gradient. Evidence for handover processes of trace gases to the free atmosphere can be found in the case of existing gradients across the boundary-layer top. An analysis of the size of eddies suggests the possibility of some influence of the heterogeneous terrain in mountainous area on the length scales of eddies.     

青藏高原大气水汽探测误差及其成因

青藏高原大气水汽分布对区域天气气候有很大影响,其探测资料的可靠性备受关注。以地基全球定位系统(GPS)遥感的大气可降水量为对比参照,分析了1999—2008年拉萨和2003年那曲探空观测大气可降水量的误差及其原因。结果表明,近10年拉萨站探空观测的可降水量比GPS遥感的可降水量明显偏小,偏小程度随使用不同的探空仪而异。GZZ-2型机械探空仪和GTS-1型电子探空仪多年平均的大气可降水量相对偏差分别为8.8%和4.4%,随机误差分别为19.8%和13.3%。近10年大气可降水量探测偏差具有减少的趋势,从12.7%减少至2.4%,主要是由探空仪性能改进所致。分析发现青藏高原大气可降水量探测偏差具有明显的日变化,12时(世界时)比00时大。拉萨站GZZ-2型和GTS-1型探空仪在12时多年平均的大气可降水量探测偏差分别为15.8%和8.3%,00时分别为1.6%和0.5%。那曲站GZZ-2型探空仪在12和00时的大气可降水量探测偏差分别为12.4%和0.3%。大气可降水量探测偏差还具有季节变化,夏季大,冬季小。对大气可降水量探测偏差日变化和季节变化的成因分析表明,12时气温比00时气温高以及夏季比冬季气温高是造成大气可降水量探测偏差日变化和季节变化的重要原因。     

The influence of water vapor fluctuations on turbulent fluxes

The influence of water vapor fluctuations on vertical turbulent fluxes is examined. It is shown that effects on density and, consequently, buoyancy are insignificant. However, because specific heat of air is a function of specific humidity, these fluctuations are found to influence sensible heat flux significantly. The critical parameter is the Bowen ratio, and a formulation relating the heat flux assuming dry air to the true value is given. The implications of this analysis to flux-gradient relationships in the surface layer are commented on.     

Impact of coherent eddies on airborne measurements of vertical turbulent fluxes

During the Hydrological-Atmospheric Pilot Experiment (HAPEX)-Sahel, which took place in Niger in the transitional period between the wet and dry seasons, two French aircraft probed the Sahelian boundary layer to measure sensible and latent heat fluxes. The measurements over the Niamey area often revealed organised structures of a few km scale that were associated with both thermals and dry intrusions. We study the impact of these coherent structures using a single day’s aircraft-measured fluxes and a numerical simulation of that day with a mesoscale model. The numerical simulation at high horizontal resolution (250 m) contains structures that evolve from streaks in the early morning to cells by noon. This simulation shows distribution, variance and skewness similar to the observations. In particular, the numerical simulation shows dry intrusions that can penetrate deeply into the atmospheric boundary layer (ABL), and even reach the surface in some cases, which is in accordance with the observed highly negatively skewed water vapour fluctuations. Dry intrusions and thermals organised at a few km scale give skewed flux statistics and can introduce large errors in measured fluxes. We use the numerical simulation to: (i) evaluate the contribution of the organised structures to the total flux, and (ii) estimate the impact of the organised structures on the systematic and random errors resulting from the 1D sampling of the aircraft as opposed to the 2D numerical simulation estimate. We find a significant contribution by the organised structures to the total resolved fluxes. When rolls occur, and for a leg length of about 30 times the ABL depth, the 1D sampled flux is shown to be sometimes 20% lower than the corresponding 2D flux when the 1D sampling direction is the same as the main axis of the rolls, whereas the systematic error is much lower when the direction of the leg is transverse to the rolls. In the case of cells, an underestimate of around 10% can still be observed with the 1D approach independent of direction, due to poor sampling of the energy-containing scales.     

The respective effects of water vapor and temperature on the turbulent fluxes of sensible and latent heat

Various aspects of the Brook correction for the effects of moisture fluctuations or gradients on atmospheric specific heat and, consequently, on the vertical flux of sensible heat are discussed, and additional forms of the complete and approximate equations are derived. Corollary expressions for the influence of temperature fluctuations or gradients on vertical latent heat flux are presented. Errors due to neglecting these temperature and moisture effects on the respective fluxes are compared in terms of the Bowen ratio. Either of these normally neglected effects can change the direction (sign) and very substantially affect the magnitude of the corresponding flux. The effects sometimes compensate in the total, sensible plus latent, heat flux. Calculations include practical examples from the very different climates of the tropical Atlantic Ocean and the Great Plains of Nebraska.     

Direct measurements of nitrogen oxides and ozone fluxes over grassland

Using the eddy correlation method, fluxes of nitric oxide, nitrogen dioxide, ozone, water, and sensible heat were measured at a site 20 km north of Denver, Colorado over mature crested wheat grass, 0.75 m high in late June and early July. During this period the weather was fair with no synoptic disturbances. In the early morning a well-mixed diluted urban pollution plume traversed the site, by late morning aged pollution had mixed downward into the local boundary layer, and by afternoon the air came from a relatively unpolluted area of the high plains. The mean trace gas concentrations reflect this repeated pattern of local air flow. The fluxes of the trace gases were influenced both by the variation of the means and by other factors including temperature and biological activity. Ozone fluxes were found to be always negative and proportional to the mean, with an average deposition velocity for this case of about 0.006 m s^-1. For the oxides of nitrogen this simple treatment was not appropriate. Both deposition and emission were observed, generally deposition predominated in the morning and emission in the afternoon with observed variations in the fluxes of NO_x=NO+NO₂ from –0.3 to +0.2 ppbv m s^-1.The National Center for Atmospheric Research is sponsored by the National Science Foundation     

Derivation of water vapor fluxes from Lidar measurements

Two techniques are described by which the flux of water vapor can be derived from concentration measurements made by a Raman-Lidar. Monin-Obukhov similarity theory and dissipation techniques are used as the basis for these methods. The resulting fluxes are compared to fluxes from standard point instruments. The techniques described are appropriate for measuring the flux of any scalar quantity using Lidar measurements in the inner region of the boundary layer.     

Eddy-correlation measurements of sensible heat fluxes over a grass surface

Direct measurements of sensible heat fluxes were conducted over a grass surface at Ladner, British Columbia, using yaw-sphere-thermometer eddy-correlation systems. The results show that for half-hour averaging periods, there is no phase-lag between sensible heat and net radiation flux densities. Field comparison of two yaw-sphere-thermometer systems gave good and consistent agreement. At a height of 2 m above ground and a horizontal crosswind separation of 1.5 m, less than 5% variability was noted in the measured heat fluxes. For a 19-m horizontal separation, the variability was less than 20%. The aridity index (α) advanced by Priestley and Taylor (1972), is shown to be a potentially useful climatic indicator.     

Comparative measurements of the energy fluxes over a pine forest

During the summer of 1974, simultaneous heat flux measurements were made over a pine forest in three ways:

1. Using the Bowen ratio - Energy Budget technique, with the Bowen ratio estimated from temperature and humidity profiles measured with two pairs of wet-and dry-bulb thermometers at six levels.
2. Using the Bowen ratio - Energy Budget technique, with the Bowen ratio estimated from temperature and humidity measurements using pairs of wet-and dry-bulb thermometers mechanically interchanged every ten minutes.
3. Using the Eddy Correlation - Energy Budget technique, with a direct measurement of the sensible heat flux made by a simple eddy correlation apparatus.

Methods (b) and (c) are shown to give similar results apart from a systematic difference of about 25% in the measured sensible heat flux. This corresponds to an underestimate of the sensible heat flux by the eddy correlation apparatus. Although sometimes consistent with the other two methods, on occasions method (a) gave results which were significantly different from both (b) and (c). When differences occurred, they tended to be systematic and persistent over individual days; but they could change magnitude and sign if the particular sensors used at each level in the profile were rearranged. The experimental program used to collect these and previous data involved the rearrangement of sensors on a regular (two day) time scale. It is shown that, when averaged over several such rearrangements, method (a) produces median values of surface resistance which are more in keeping with those produced by the other methods. This is taken to imply that data previously gathered in this way can be used to produce physically reasonable results providing they are averaged over several days. On the basis of the results presented, recommendations are made on future experimental work in forest micrometeorology.     

Airborne measurements of turbulent fluxes during LITFASS-98: Comparison with ground measurements and remote sensing in a case study

Summary ?Simultaneous flight measurements with the research aircraft Do 128 and the helicopter-borne turbulence probe Helipod were performed on 18 June 1998 during the LITFASS-98 field experiment. The area-averaged turbulent vertical fluxes of momentum, sensible, and latent heat were determined on a 15 km × 15 km and a 10 km × 10 km flight pattern, respectively. The flights were carried out over heterogeneous terrain at different altitudes within a moderately convective boundary layer with Cumulus clouds. Co-spectra-analysis demonstrated that the small scale turbulent transport was completely sampled, while the comparatively small flight patterns were possibly of critical size regarding the large-scale turbulence. The phygoide of the airplane was identified as a significant peak in some co-spectra. The turbulent fluxes of momentum and sensible heat at 80 m above the ground showed systematic dependence on the location of the flight legs above the heterogeneous terrain. This was not observed for the latent heat flux, probably due to the vertical distribution of humidity in the boundary layer. Statistical error analysis of the fluxes F showed that the systematic statistical error ΔF was one order of magnitude smaller than the standard deviation σ _F . The difference between area-averaged fluxes derived from simultaneous Helipod and Do 128 measurements was much smaller than σ _F , indicating that the systematic statistical error was possibly over-estimated by the usual method. In the upper half of the boundary layer the airborne-measured sensible heat flux agreed well with windprofiler/RASS data. A linear fit was the best approximation for the height dependence of all three fluxes. The linear extrapolations of the latent and sensible heat fluxes to the ground were in good agreement with tower, scintillometer, and averaged ground-station measurements on various surface types. Systematic discrepancies between airborne and ground-based measurements were not found. Received June 18, 2001; revised December 21, 2001; accepted June 3, 2002     

Comparison of NCEP turbulent heat fluxes with in situ observations over the south-eastern Arabian Sea

Turbulent surface heat fluxes (latent and sensible heat) are the two most important parameters through which air–sea interaction takes place at the ocean–atmosphere interface. These fluxes over the global ocean are required to drive ocean models and to validate coupled ocean–atmosphere global models. But because of inadequate in situ observations these are the least understood parameters over the tropical Indian Ocean. Surface heat fluxes also contribute to the oceanic heat budget and control the sea surface temperature in conjunction with upper ocean stratification and ocean currents. The most widely used flux products in diagnostic studies and forcing of ocean general circulation models are the ones provided by the National Centres for Environment Prediction (NCEP) reanalysis. In this study we have compared NCEP reanalysed marine meteorological parameters, which are used for turbulent heat fluxes, with the moored buoy observation in the south-eastern Arabian Sea. The NCEP latent heat flux (LHF) and sensible heat flux (SHF) derived from bulk aerodynamic formula are also compared with that of ship and buoy derived LHF and SHF. The analysis is being carried out during the pre-monsoon and monsoon season of 2005. The analysis shows that NCEP latent as well as sensible heat fluxes are largely underestimated during the monsoon season, however, it is reasonably comparable during the pre-monsoon period. This is largely due to the underestimation of NCEP reanalysis air temperature (AT), wind speed (WS) and relative humidity (RH) compared to buoy observations. The mean differences between buoy and NCEP parameters during the monsoon (pre-monsoon) period are ~21% (~14%) for WS, ~6% (~3%) for RH, and ~0.75% (0.9%) for AT, respectively. The sudden drop in AT during rain events could not be captured by the NCEP data and, hence, large underestimations in SHF. During the pre-monsoon period, major contribution to LHF variations comes from WS, however, both surface winds and relative humidity controls the LHF variations during the monsoon. LHF is mainly determined by WS and RH during the monsoon and, WS is the main contributor during the pre-monsoon.     

Simultaneous measurements of turbulence over land and water

Turbulence and mean flow variables under unstable conditions are examined with special emphasis on the consequences of roughness and surface elevation change. An interpolation formula for _w ², between neutral and free convection, is shown to bring order to the data. The spectral distribution of vertical wind variance is found to be in good agreement with results over horizontal homogeneous terrain, both with respect to form and position. In particular, the length scale _m corresponding to the maximum of nS _w(n) is unchanged. Another turbulent length scale, (k/)z, is found to be substantially reduced in the disturbed zone of the internal boundary layer. To a first approximation, the flow-acceleration effect on the non-dimensional wind shear can be separated from the diabatic effect.     

On the influence of sensor inertia on the estimation of turbulent fluxes from aircraft measurements

If no correction is performed, the inertia of the sensors for temperature and humidity in research aircraft leads to an error in the calculation of turbulent fluxes. Simple considerations give an estimation of the order of magnitude of these errors. A simple method for the in-situ estimation of temperature sensor inertia is given.