A comparison of tropical oceanic heat fluxes determined by airborne eddy correlation and shipboard bulk aerodynamic techniques

Vertical turbulent transports of latent and sensible heat through the tropical marine boundary layer were measured with ship and airborne instruments in the GATE experimental area. The measurements from the two systems are compared for cases of simultaneous sampling in the same locations during undisturbed weather and during the wakes of convective disturbances. The paired average fluxes from the airborne eddy correlation measurements and the shipboard bulk aerodynamic measurements, for undisturbed weather, are related with correlation coefficients in the range of 0.6 to 0.9; the correlations depend primarily on stratifications of the aircraft data according to sampling altitude (15–153 m msl) and flight pattern relative to the mean wind. The agreement between the two types of measurements is best for ship data paired with aircraft samples from the lowest altitudes (15 and 30 m) and from alongwind rather than crosswind flights, as is appropriate since the stationary ships monitor the alongwind characteristics of the turbulence. The use of long (1–3 hr) versus short (10 min) ship samples did not significantly affect the comparability with the aircraft samples during undisturbed periods.The good baseline comparability demonstrated by the undisturbed-weather comparisons was applied to interpret the results from the wakes behind showers. Although these paired measurements were few, not only the sensible heat, but also the latent heat flux was shown to decrease substantially with altitude from 10 m to at least 150 m in the wakes, where the mixing was relatively intense. Variations in the fluxes were much greater at the higher level than nearer the sea surface.     

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.     

A note on bulk aerodynamic coefficients for sensible heat and moisture fluxes

Values are presented for bulk aerodynamic coefficients which were calculated from two rather extensive sets of estimates of sensible heat and moisture fluxes based on profile measurements. These results are compared with a number of other results mainly based on fluxes estimated from eddy-correlation measurements.     

Three-dimensional numerical study of cell broadening during cold-air outbreaks

The boundary-layer development and convection-pattern transition typically occurring in cold-air outbreaks is studied using three-dimensional simulations. The simulations include the secondary-flow transition starting with the relatively small-scale boundary-layer rolls developing during the initial phase and ending with mesoscale cellular convection patterns. The application of a computational grid, whose horizontal mesh size enables the resolution of the small-scale initial patterns and whose domain size is large enough to capture mesoscale convection patterns, overcharges even state-of-the-art supercomputers. In order to bypass the computer storage problem, the horizontal size of the model domain and the horizontal resolution of the computational grid are adjusted to the scale of the dominant convective structures. This enables the simulation of convection cells whose horizontal scales increase up to values exceeding the size of the initial model domain. The model is applied to conditions of a cold-air outbreak observed during the ARKTIS 1991 experiment. The most important characteristics of the observed situation are revealed by the model. Sensitivity studies are performed in order to investigate the relation between cell broadening and various physical processes. The artificial cutoff of liquid-water formation prevents the enlargement of convective scales. Latent heating due to condensation and especially radiative cloud-top cooling are identified as processes leading to cell broadening. We propose a conceptual model that elucidates the mechanism by which cloud-top cooling may generate larger aspect ratios.     

几种计算湍流能量和通量方法的比较

本文利用超声脉动风速仪和白金丝脉动温度仪,分别测量了三维风速u、v、w和温度T的瞬时脉动值.根据这些风、温高频测量值,用几种不同的计算方法计算了湍流能量和动量、热量通量.计算结果表明,由于大气中湍涡尺度很宽,对于不同尺度的湍流量,选用合理的计算方法,能够得到正确的计算结果,并节省计算时间.     

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.     

Inertial-dissipation methods and turbulent fluxes at the air-ocean interface

The use of high frequency atmospheric turbulence properties (inertial subrange spectra, structure function parameters or dissipation rates) to infer surface fluxes of momentum, sensible heat and latent heat is more practical for most ocean going platforms than direct covariance measurement. The relationships required to deduce the fluxes from such data are examined in detail in this paper and several ambiguities and uncertainties are identified. It is noted that, over water, data on water vapor properties (the dimensionless functions for the mean profile, the structure function parameter and the variance transport term) are extremely sparse and the influence of sea spray is largely unknown. Special attention is given to flux estimation on the basis of the structure function formalism. Existing knowledge about the relevant similarity functions is summarized and discussed in light of the ambiguities identified above.     

塔中涡动资料不同坐标旋转方法比较分析

坐标旋转是涡动协方差(EC)通量观测数据后处理当中必须进行的步骤,本文利用塔克拉玛干沙漠腹地塔中西面标校站自然沙丘上3 m高度和塔中本站32 m高度2009年4—10月湍流通量数据,对比分析了不同坐标旋转方法在塔中通量观测数据中的应用。结果表明,塔克拉玛干沙漠腹地塔中湍流各通量在旋转订正前后差别不显著,不订正造成的通量计算误差1%。塔中32 m和塔中西站3 m通量计算误差,与两个站点的采集仪器架设高度差别较大有关。即使对于相对平坦的下垫面,也必须要进行倾斜校正,以修正地势、仪器振动、安装倾斜等产生的虚假信息。本研究对塔中涡动资料进行旋转时采用了二次坐标旋转方法。此结果为塔克拉玛干沙漠腹地EC通量观测的风速仪倾斜校正提供参考。     

Determination of turbulent momentum and heat fluxes by spectral methods

Methods are studied which permit one to evaluate turbulent fluxes from the results of spectral measurements in turbulent laboratory flows and an unstable atmospheric surface layer. The well known dissipation method of flux measurements, which uses spectral data related to the inertial range, is reanalyzed. New theoretical ideas and the latest experimental data are used to specify this method in cases of moderately and very strongly unstable thermal stratifications.Moreover, it is also explained how to estimate momentum and heat fluxes from data in the low frequency parts of the velocity and temperature spectra in the low frequency ranges beyond the lower limit of the inertial range. This permits one to estimate fluxes using rather simple and cheap instruments (e.g., Pilot-tubes and thermocouples in laboratory flows or cup anemometers and crude resistance thermometers in meteorological studies). The equations for flux determination are based in such cases on the recent models by Kader (1987, 1988) and Kader and Yaglom (1990, 1991) of spectral shapes at mesoscale wave numbers; these models agree quite satisfactorily with many (though not all) data of direct spectral measurements. It is shown that estimated momentum and heat fluxes in the laboratory and in an unstably stratified atmospheric surface layer obtained by the method suggested in this paper agree satisfactorily with direct flux measurements.     

Heat fluxes and roll circulations over the western Gulf Stream during an intense cold-air outbreak

Turbulence and heat fluxes in the marine atmospheric boundary layer (MABL) for the roll vortex regime, observed during the Genesis of Atlantic Lows Experiment (GALE) over the western Gulf Stream, have been studied. The spectral analysis suggests that cloud streets (roll vortices) are vertically organized convection in the MABL having the same roll scale for both the cloud layer and subcloud layer, and that the roll spacing is about three times the MABL depth. The roll circulations contribute significantly to the sensible (temperature) and latent heat (moisture) fluxes with importance increasing upward. Near the MABL top, these fluxes are primarily due to roll vortices which transfer both sensible heat and moisture upward in the lower half of the convective MABL. Near the MABL top, the roll circulations transfer sensible heat downward and moisture upward in the clear thermal-street region, but roll vortices influenced by evaporative cooling can transfer sensible heat upward and moisture downward in the cloud-street region. Near the cloud-top, the upward buoyancy flux due to evaporative cooling is highly related to the roll circulations near the inversion.For the lower half of the MABL, the normalized temperature flux decreases upward more rapidly than the humidity flux, which is mainly because potential temperature () increases slightly upward while humidity (q) decreases slightly upward above the unstable surface layer. The gradient production (associated with the gradient) is a source for the temperature flux in the unstable surface layer but changes to a sink in the mixed layer, while the gradient production (associated with the q gradient) acts as a source for the humidity flux in both the unstable surface and mixed layers. The results suggest that the entrainment at the MABL top might affect the budgets of temperature and humidity fluxes in the lower MABL, but not in the unstable surface layer.Caelum Research Corporation, Silver Spring, MD, 20901, U.S.A.     

Derivation of the aerodynamic roughness parameters for a Sahelian savannah site using the eddy correlation technique

Vertical exchange of heat, moisture and momentum above the earth's surface depends strongly on the turbulence generated by surface roughness. This roughness is best specified through the roughness length and the zero plane displacement. The ratio of windspeed to friction velocity was measured at four heights using the eddy correlation technique at a fallow savannah site in the Sahel. The change in this ratio with height was used to derive the zero plane displacement and the roughness length of the surface, together with an estimate of the error in each parameter. These were estimated as 0.93 ± 0.35 m and 0.17 ± 0.01 m, respectively. The method appears to be a more robust alternative to wind profile derivation.     

A parametric model of vertical eddy fluxes in the atmosphere

A scheme for the representation of the vertical eddy fluxes of heat, momentum and water vapour in a forecast model is presented. An important feature of the scheme is the dependence of the diffusion coefficients on the static stability of the atmosphere. Two tests are presented, using the scheme in a one-dimensional model: the simulation of the diurnal cycle, and the transformation of a polar air mass moving over the warm sea.     

A comparison of bowen ratio and eddy correlation sensible and latent heat flux measurements above deciduous forest

Sensible and latent heat flux densities (H and E) were measured above a mature, 18 m deciduous forest during July and August, 1988, using the Bowen ratio-energy balance (BREB) and eddy correlation (EC) methods. EC estimates ofH and E underestimated day-time surface available energy by 11%. EC also partitioned available energy differently than BREB. for/L<0.0, EC favouredH and BREB favoured E. Practical and theoretical limitations of the BREB and EC methods above forests are discussed. The most plausible causes for the failure of EC to close the surface energy balance are a low frequency loss of flux and the failure of a single point measurement to account for the spatial dispersive flux. The most plausible causes of the EC-BREB energy partitioning anomaly are the invalidity of the BREB similarity assumption and the violation of flux-gradient diffusion assumptions in the near-field diffusion region.     

A Comparison Study of the Climatological Air-Sea Heat Fluxes Estimated by Different Computational Schemes of Bulk Formula

In this paper the different results of estimated air-sea heat fluxes by different computational schemes of bulk formula are researched. The varying results may bring another source of the uncertainties of flux climatology.In addition to the classical scheme and the sampling scheme, a revised sampling scheme named semi-sampling scheme is proposed in this paper. In this revised version, the monthly means of and are cal?culated from each simultaneous measurement as that in the sampling method. But the transfer coefficients in this version are defined based on monthly means of and instead of those in each measurement in older to use the files of monthly mean variables and in COADS (Woodruff et al. 1987).The results of a comparison study show that the covariance between and and that between and are less than 2 W / m2 and 5 W / m2 respectively in most areas, which are about one order of magni?tude less than the standard deviation of the fluxes. Therefore the difference between two schemes is not statistically significant on average. However, in the regions off coast of Asian and North American continents, the contribution of covariance between and can be as large as 15 W / m2, which is about one third to a half of the standard deviation. Since there are files of monthly mean and for each year in the COADS, we pro?pose to use these files directly in calculation of climatology. It is not necessary to use every individual report.The flux climatology calculated directly from the long-term mean values of basic meteorological elements shows an significant difference from that calculated from the mean of each year’s flux, particularly for the latent heat flux which is on the order of 10 to 15 W / m2, about 3-4 times of the difference between semi-sampling and classical schemes and about one third of the standard deviation. Therefore it indicates that the time interval of months is per?haps a better one in the calculation of flux climatology. It is not recommended to use the long-term means of the me?teorological variables.     

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.     

Marine cold-air outbreaks in the future: an assessment of IPCC AR4 model results for the Northern Hemisphere

For many locations around the globe some of the most severe weather is associated with outbreaks of cold air over relatively warm oceans, referred to here as marine cold-air outbreaks (MCAOs). Drawing on empirical evidence, an MCAO indicator is defined here as the difference between the skin potential temperature, which over open ocean is the sea surface potential temperature, and the potential temperature at 700 hPa. Rare MCAOs are defined as the 95th percentile of this indicator. Climate model data that have been provided as part of the Intergovernmental Panel on Climate Change (IPCC) Assessment Report Four (AR4) were used to assess the models’ projections for the twenty-first century and their ability to represent the observed climatology of MCAOs. The ensemble average of the models broadly captures the observed spatial distribution of the strength of MCAOs. However, there are some significant differences between the models and observations, which are mainly associated with simulated biases of the underlying sea ice, such as excessive sea-ice extent over the Barents Sea in most of the models. The future changes of the strength of MCAOs vary significantly across the Northern Hemisphere. The largest projected weakening of MCAOs is over the Labrador Sea. Over the Nordic seas the main region of strong MCAOs will move north and weaken slightly as it moves away from the warm tongue of the Gulf Stream in the Norwegian Sea. Over the Sea of Japan there is projected to be only a small weakening of MCAOs. The implications of the results for mesoscale weather systems that are associated with MCAOs, namely polar lows and arctic fronts, are discussed.     

Mean and turbulent structure of a baroclinic marine boundary layer during the 28 January 1986 cold-air outbreak (GALE 86)

Aircraft (NCAR Electra), ship (R/V Cape Hatteras), buoy (NCSU Buoy 2) and satellite (NOAA-7 and 9) measurements have been used to observe the structure of the Marine Boundary Layer (MBL) offshore of Wilmington, North Carolina, during the intense cold-air outbreak of 28 January, 1986, as part of the Genesis of Atlantic Lows Experiment (GALE). Air mass modification processes, driven primarily by the surface turbulent latent and sensible heat fluxes, caused the overlying air mass to warm and moisten as it advected over the warmer waters of the eastern United States continental shelf. Maximum observed total (latent + sensible) heat flux was 1045W/m² (at a height of 49 m) over the core of the Gulf Stream. Heat flux values decreased both east and west of this region, primarily in response to changes in the air-sea temperature difference.MBL height increased steadily in the offshore direction in response to increasing convection. The turbulent structure showed a buoyancy-dominated MBL between 0.1 z/h and 0.8 z/h; whereas shear was important above and below this level, vertical transport of kinetic energy (KE) was dominant as a source term only above 0.8 z/h. The normalized turbulent kinetic energy (TKE) budgets observed at different offshore locations showed general agreement at different flight levels. Thus the findings support the validity of the similarity relations under intense convective conditions.     

Spatial variability of turbulent fluxes and roughness lengths in HAPEX-MOBILHY

Surface-based and aircraft measured fluxes over the heterogeneous surface in HAPEX-MOBILHY are analyzed for the ten flight days when cloud cover above the boundary layer was minimal. The fair-weather climatology of the spatial variation of surface fluxes is estimated to provide an assessment of the generality of previous case studies appearing in the literature. For the 10-day averages, greater heating over the forest generates a forest breeze which leads to rising motion and a modest increase of boundary-layer cloud cover at the forest edge. The exchange coefficients and effective roughness lengths are computed for local averages (15 km scale) and for regional averages (100 km scale) intended to represent a range of grid sizes in numerical models of the atmosphere. The effective roughness length for momentum over the mixed agricultural region for both scales is on the order of 1 m, apparently due to bluff roughness effects associated with scattered trees, edges of small woods and other obstacles. This roughness length value is an order of magnitude larger than values used in numerical models for the same region, which are based on the dominant vegetation type. The spatially varying effective roughness length for heat is computed for use in those models which use surface radiation temperature to estimate surface heat flux. The effective roughness lengths for heat are found to be smaller than those typically used in numerical models of the atmosphere.     

On the bulk parameterization of surface fluxes for various conditions and parameter ranges

A comparison between various parameterizations for the bulk transfercoefficient for heat and momentum is carried out for a wide range ofatmospheric stability and values of the roughness lengths for momentum and heat,z_0m and z_0h respectively.It is confirmed that the parameterization of Launiainen compares wellto a numerical iterative solution for the Obukhov length L as function of the gradients of wind speed and temperatureover a limited range of z_0m/z_0h and stability conditions.For _0m/z_0h > 500, an alternativeinterpolation formulation of Holtslag and Ek,in combination with the formulation of Launiainen, provides a better approximation.     

A comparison of some aerodynamic resistance methods using measurements over cotton and grass from the 1991 California ozone deposition experiment

Measurements of dry, deposition velocities (V _d ) of O₃ (using the eddy correlation technique) made over a cotton field and senescent grass near Fresno California during July and August 1991 were used to test some dry deposition velocity models. Over the cotton field, the observed maximum daytimeV _d was about 0.8 cm s^–1 and the average nighttime value was about 0.2 cm s^–1. Over the grass, daytime values averaged about 0.2 cm s^–1 and nighttime values about 0.05 cm s^–1. Application of a site-specific model known as ADOM (Acid Deposition and Oxidant Model) over the cotton field generally overestimated the observations except for a few hours in the afternoon when the observations were underestimated The overestimation was attributed to inadequacies in the surface resistance formulation and the underestimation to uncertainties in the aerodynamic formulation. Unlike previous studies which focused on the role of surface resistance, we perform additional tests using a large variety of aerodynamic resistance formulae, in addition to those in ADOM, to determine their influence on the modelledV _d of O₃ over, cotton. Over grass, ADOM considerably overestimated the observations but showed improvement when other surface resistance formulations were applied.