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1.
The Summer Surface Energy Balance of the High Antarctic Plateau   总被引：1，自引：0，他引：1  
The summertime surface energy balance (SEB) at Kohnen station, situated on the high Antarctic plateau (75°00′ S, 0°04′ E, 2892m above sea level) is presented for the period of 8 January to 9 February 2002. Shortwave and longwave radiation fluxes were measured directly; the former was corrected for problems associated with the cosine response of the instrument. Sensible and latent heat fluxes were calculated using the bulk method, and eddy-correlation measurements and the modified Bowen ratio method were used to verify these calculated fluxes. The calculated sub-surface heat flux was checked by comparing calculated to measured snow temperatures. Uncertainties in the measurements and energy-balance calculations are discussed. The general meteorological conditions were not extraordinary during the period of the experiment, with a mean 2-m air temperature of −27.5°C, specific humidity of 0.52×10−3kg kg−1 and wind speed of 4.1ms−1. The experiment covered the transition period from Antarctic summer (positive net radiation) to winter (negative net radiation), and as a result the period mean net radiation, sensible heat, latent heat and sub-surface heat fluxes were small with values of −1.1, 0.0, −1.0 and 0.7 Wm−2, respectively. Daily mean net radiation peaked on cloudy days (16 Wm−2) and was negative on clear-sky days (minimum of −19 W m−2). Daily mean sensible heat flux ranged from −8 to +10 Wm−2, latent heat flux from −4 to 0 Wm−2 and sub-surface heat flux from −8 to +7 Wm−2.  相似文献   

2.
The structure of the atmospheric surface layer above the sea is analysed from aircraft turbulence measurements. The data are issued from two experiments performed in 1990 above the Mediterranean sea: Crau and PYREX, and correspond to moderately unstable conditions and to wind velocities ranging from 6 to 20 m/s. Low-altitude straight and level runs were used to compute the variances of the wind components, as well as of the temperature and moisture. Their dependence on the stability index —z/L is analysed. The turbulent fluxes of momentum, sensible heat and latent heat, calculated by the eddy-correlation technique, are used to estimate the neutral bulk coefficients: drag coefficient, Stanton number and Dalton number. The neutral drag coefficient clearly exhibits a dependence on the windspeed, which could be well fitted by the Charnock relation, with a constant of 0.012.  相似文献   

3.
Extremely Weak Mixing in Stable Conditions   总被引：1，自引：2，他引：1  
Transport by extremely weak turbulence occurring on nights with clear skies and weak winds is examined from seven tower levels of eddy-correlation data taken from each of two field programs. The very small flux is systematic, provided that the perturbations are computed from a record-dependent averaging length, which must be as small as 10 s in very stable conditions. With traditional methods for computing the flux, these fluxes were considered too small to estimate, in that the computed values behaved erratically. For extremely weak turbulence, the fluxes decrease systematically with height and often indicate very shallow boundary-layer depths on the order of 10 m. However, in one field program, the turbulence slowly increases with height above the surface flux-based boundary layer apparently due to horizontal advection of stronger turbulence driven by modest surface heterogeneity. For very weak turbulence, the eddy diffusivity for momentum is systematically greater than that for heat in both field programs. The dependence of the turbulence strength and its variability with stability is examined in some detail.  相似文献   

4.
TOGA-COAREIOP海表通量估算   总被引：3，自引：0，他引：3  
姚华栋  李骥  丁一汇 《气象学报》1996,54(6):693-708
使用ＴＯＧＡ－ＣＯＡＲＥＩＯＰ１９９２年１１月５日至１９９３年２月１９日向阳红５号海上船舶（２°Ｓ，１５６°Ｅ）观测资料，通过３种不同的总体方法估算了这个点的海表面通量。首先得出一个中性拖曳系数和风速之间的准线性关系，然后用迭代法处理依赖于稳定度的拖曳系数和输送系数，在此基础上计算了动量、感热和潜热通量。在暖池区与西风相对应的通量远大于东信风相对应的通量，海表通量的量值也依赖于稳定度，特别是在弱风条件下。估算的海表净热量平衡表明从海洋向大气大量的热输送主要是由潜热通量和有效长波辐射产生的。其中潜热通量的数值最大、感热通量数值最小。最后和同期日本Ｒ／ＶＨａｋｕｈｏ考察船（０°，１５６°Ｅ）用涡动相关法得到的直接测量通量比较，作了精确度分析，表明用总体方法估算的通量是可靠的。并用向阳红５号船的资料估算的有效长波辐射和直接测量值作了比较，也和热带西太平洋ＴＯＧＡ调查作了比较分析。  相似文献   

5.
During slightly unstable but still very close to neutral conditions new results from two previous investigations have shown a significant increase of sensible and latent heat fluxes over the sea. The vertical heat transport during these conditions is dominated by detached eddies originating at the top of the boundary layer, bringing relatively cold and dry air to the surface. This effect can be described in numerical models by either enhanced heat transfer coefficients for sensible and latent heat (Stanton and Dalton numbers respectively) or with an additional roughness length, added to the original roughness lengths for heat and humidity. Such new expressions are developed using turbulence measurements from the Baltic Sea valid for wind speeds up to 14 m s−1. The effect of including the increased heat fluxes is investigated using two different numerical models: a regional three-dimensional climate model covering northern Europe, and a process-oriented ocean model for the Baltic Sea. During periods of several days, the latent heat flux can be increased by as much as 100 W m−2. The increase in sensible heat flux is significantly smaller since the process is only of importance in the very near-neutral regime where the sensible heat flux is very small. The long-term average effect over the Baltic Sea is of the order of several W m−2.  相似文献   

6.
Saturation of the Large Aperture Scintillometer   总被引：2，自引：0，他引：2  
The saturation aspects of a large aperture (0.3 m) scintillometer operating over a 10-km path were investigated. Measurements were made over mainly forested, hilly terrain with typical maximum sensible heat fluxes of 300–400 W m −2, and over flat terrain with mainly grass, and typical maximum heat fluxes of 100–150 W m−2. Scintillometer-based fluxes were compared with eddy-correlation observations. Two different schemes for calculating the reduction of scintillation caused by saturation were applied: one based on the work of Hill and Clifford, the other based on Frehlich and Ochs. Without saturation correction, the scintillation fluxes were lower than the eddy-correlation fluxes; the saturation correction according to Frehlich and Ochs increased the scintillometer fluxes to an unrealistic level. Correcting the fluxes after the theory of the Hill and Clifford gave satisfying results  相似文献   

7.
Summary Water vapour flux profiles in the atmospheric boundary layer have been derived from measurements of water vapour density fluctuations by a ground-based Differential Absorption Lidar (DIAL) and of vertical wind fluctuations by a ground-based Doppler lidar. The data were collected during the field experiment LITFASS-2003 in May/June 2003 in the area of Lindenberg, Germany. The eddy-correlation method was applied, and error estimates of ±50 W/m2 for latent heat flux were found. Since the sampling error dominates the overall measurement accuracy, time intervals between 60 and 120 min were required for a reliable flux calculation, depending on wind speed. Rather large errors may occur with low wind speed because the diurnal cycle restricts the useful interval length. In the lower height range, these measurements are compared with DIAL/radar-RASS fluxes. The agreement is good when comparing covariance and error values. The lidar flux profiles are well complemented by tower measurements at 50 and 90 m above ground and by area-averaged near surface fluxes from a network of micrometeorological stations. Water vapour flux profiles in the convective boundary layer exhibit different structures mainly depending on the magnitude of the entrainment flux. In situations with dry air above the boundary layer a positive entrainment flux is observed which can even exceed the surface flux. Flux profiles which linearly increase from the surface to the top of the boundary layer are observed as well as profiles which decrease in the lower part and increase in the upper part of the boundary layer. In situations with humid air above the boundary layer the entrainment flux is about zero in the upper part of the boundary layer and the profiles in most cases show a linear decrease.  相似文献   

8.
Summary The Bowen ratio-energy balance (BREB) and the stability-corrected aerodynamic method were used to estimate turbulent fluxes of sensible and latent heat at an irrigated alfalfa site in a semi-arid valley in northern Utah, U.S.A., during August and September of 1991. Despite inclusion of a generalized stability factor, the aerodynamic method underestimated the daytime (sunrise-sunset) sensible and latent heat fluxes by approximately 30% in comparison with the BREB method. The sum of the aerodynamic estimates of sensible and latent heat seldom balanced the energy avaiable from net radiation and change in storage. Wind speed was low during the experiment (averaging 1.6 m s–1), and so a second analysis was run for data from daytime, non-rainy, turbulent conditions (wind > 1.5 m s–1). This showed that sensible and latent heat were still underestimated by approximately 30% in comparison with the BREB approach. This suggests that underestimation of sensible and latent heat fluxes by the aerodynamic method was not related to the wind speed conditions during the experiment. These results show that the stability-corrected aerodynamic model did not agree with the Bowen ratio method in this experiment. It appears unlikely that the discrepancies resulted from measurement errors. Perhaps the theoretical foundation of the similarity parameters (stability functions) in the aerodynamic model are not sufficiently generalized. The discrepancies found here confirm the necessity of calibration checks on the validity of aerodynamic estimates of the turbulent fluxes.With 7 Figures  相似文献   

9.
We present eddy-correlation measurements of heat and water vapour fluxes made during the Antarctic winter. The surface layer was stably stratified throughout the period of observation and sensible heat fluxes were always directed downwards. However, both upward and downward water vapour fluxes were observed. Their magnitude was generally small and the latent heat flux was not a significant fraction of the surface energy budget. The variation of heat and water vapour fluxes with stability is well described by Monin-Obukhov similarity theory but the scalar roughness lengths for heat and water vapour appear to be much larger than the momentum roughness length. Possible explanations of this effect are discussed.  相似文献   

10.
Aircraft turbulence data from the Autonomous Ocean Sampling Network project were analyzed and compared to the Coupled Ocean–Atmosphere Response Experiment (COARE) bulk parametrization of turbulent fluxes in an ocean area near the coast of California characterized by complex atmospheric flow. Turbulent fluxes measured at about 35 m above the sea surface using the eddy-correlation method were lower than bulk estimates under unstable and stable atmospheric stratification for all but light winds. Neutral turbulent transfer coefficients were used in this comparison because they remove the effects of mean atmospheric conditions and atmospheric stability. Spectral analysis suggested that kilometre-scale longitudinal rolls affect significantly turbulence measurements even near the sea surface, depending on sampling direction. Cross-wind sampling tended to capture all the available turbulent energy. Vertical soundings showed low boundary-layer depths and high flux divergence near the sea surface in the case of sensible heat flux but minimal flux divergence for the momentum flux. Cross-wind sampling and flux divergence were found to explain most of the observed discrepancies between the measured and bulk flux estimates. At low wind speeds the drag coefficient determined with eddy correlation and an inertial dissipation method after corrections were applied still showed high values compared to bulk estimates. This discrepancy correlated with the dominance of sea swell, which was a usually observed condition under low wind speeds. Under stable atmospheric conditions measured sensible heat fluxes, which usually have low values over the ocean, were possibly affected by measurement errors and deviated significantly from bulk estimates.  相似文献   

11.
Using the standard eddy-covariance (EC) method to quantify mass and energy exchange at a single location usually results in an underestimation of vertical eddy fluxes at the surface. In order to better understand the reasons for this underestimation, an experimental set-up is presented that is based on spatial averaging of air temperature data from a network of ground-based sensors over agricultural land. For eight days during the 34-day observational period in May and June 2007, additional contributions to the sensible heat flux of more than 50Wm−2 were measured in the lower surface layer by applying the spatial EC method as opposed to the standard temporal EC method. Smaller but still significant additional sensible heat fluxes were detected for four more days. The additional energy is probably transported in organised convective structures resulting in a mean vertical wind velocity unequal to zero at the tower location. The results show that convective transport contributes significantly to the surface energy budget for measurement heights as low as 2–3 m. Since these structures may be quasi-stationary, they can hardly be captured by a single-location measurement. The spatial EC set-up presented here is capable of quantifying contributions to the sensible heat flux from structures up to the scale of our spatial sensor network, which covered an area 3.5 × 3.5 km. For future experiments aiming at closing the energy balance, the spatial EC method should be employed to measure both the sensible and latent heat fluxes. Experimental determination of the horizontal advection of sensible and latent heat should also be considered, since such transport must occur due to convergence and divergence related to convection.  相似文献   

12.
Sonic anemometers are capable of measuring the wind speed in all three dimensions at high frequencies (10–50 Hz), and are relied upon to estimate eddy-covariance-based fluxes of mass and energy over a wide variety of surfaces and ecosystems. In this study, wind-velocity measurement errors from a three-dimensional sonic anemometer with a non-orthogonal transducer orientation were estimated for over 100 combinations of angle-of-attack and wind direction using a novel technique to measure the true angle-of-attack and wind speed within the turbulent atmospheric surface layer. Corrections to the vertical wind speed varied from −5 to 37% for all angles-of-attack and wind directions examined. When applied to eddy-covariance data from three NOAA flux sites, the wind-velocity corrections increased the magnitude of CO2 fluxes, sensible heat fluxes, and latent heat fluxes by ≈11%, with the actual magnitude of flux corrections dependent upon sonic anemometer, surface type, and scalar. A sonic anemometer that uses vertically aligned transducers to measure the vertical wind speed was also tested at four angles-of-attack, and corrections to the vertical wind speed measured using this anemometer were within ±1% of zero. Sensible heat fluxes over a forest canopy measured using this anemometer were 15% greater than sensible heat fluxes measured using a sonic anemometer with a non-orthogonal transducer orientation. These results indicate that sensors with a non-orthogonal transducer orientation, which includes the majority of the research-grade three-dimensional sonic anemometers currently in use, should be redesigned to minimize sine errors by measuring the vertical wind speed using one pair of vertically aligned transducers.  相似文献   

13.
The impact of the wind forcing temporal resolution in the central Mediterranean Sea is addressed using a numerical ocean circulation model. The model uses interactive surface fluxes based on the ERA-Interim 6-hourly atmospheric reanalyses except for the 10 m wind for which ERA5 hourly reanalyses are used. Additional temporal resolution (2, 3, 6, 12 and 24 h) wind sets are deduced from the ERA5 hourly data. An ensemble of simulations (six members) is then performed where only the temporal resolution of the wind forcing is changed. The impact of the temporal resolution is studied based on this set of simulations. The dependence of the surface wind stress and heat flux on the wind resolution is derived based on an analytical expression where the Weibull distribution is used to characterise the probability density function of the wind speed. Results from the analytical model are found close to those from the numerical model when a linear increase of the exchange coefficients with the wind speed is considered. Power input into the sea and surface heat loss both increase with the increase of the temporal resolution but at lower rates when approaching hourly forcing values. The increase of the latent heat loss at these high resolutions is small (~−0.8 Wm-2) but still important, around 10–20% the Mediterranean basin heat budget (−5 to −7 Wm-2). The increase of the wind forcing temporal resolution decreases the sea surface temperature (SST) and increases the sea surface salinity (SSS) with largest values in the shallow area of the Gulf of Gabès (eastern coast of Tunisia). A decrease of SSS is however noticed in some areas mainly northwest of the Tunisia coast. Hydrographical changes are also found in the Tunisia-Sicily channel. They are characterised by mesoscale structures with no remarkable change of the major water veins.  相似文献   

14.
Summary  The Bowen Ratio-Energy Balance (BREB) and the aerodynamic method were used to estimate turbulent fluxes of sensible and latent heat flux over an irrigated agricultural area (IAA) and over two dry agricultural areas (DAA1 and DAA2). These turbulent fluxes were analysed and particular attention paid to two specific areas. First, a quantitative analysis of sensible and latent heat fluxes obtained by the BREB method was carried out, taking into account different soil type, vegetation and surface conditions. The results showed that in IAA latent heat flux was higher than sensible heat flux, except in summer months, while in DAA1 and DAA2, sensible heat flux was higher except in the months when the vegetation was at the stage of maximum development. Second, sensible and latent heat fluxes estimates from the BREB method were compared with those obtained from the aerodynamic method. In this comparison factors such as soil type, soil vegetation cover, homogeneity or inhomogeneity of terrain and mesoscale effects such as orography and wind patterns were taken into account. The results show that in conditions of light wind, the two methods only concur if the condition of horizontal homogeniety is fulfilled. The influence of inhomogeneity seems to decrease and agreement between methods improves, if the wind is stronger and the effects of meso and synoptic scales are predominant. Received May 18, 1999/Revised March 15, 2000  相似文献   

15.
In weak wind stable conditions, eddy-correlation fluxes calculated using conventional averaging times of 5 min or longer to define the perturbations are severely contaminated by poorly sampled mesoscale motions. A method is developed to identify the averaging time for each individual data record that captures the turbulence while excluding most of the mesoscale motions. The method is based on multiresolution decomposition of the heat flux, and provides an objective procedure for selecting the averaging time for calculating eddy-correlation fluxes. Eddy-correlation data collected in weak turbulence conditions over grass, snow, a pine forest and the ocean are used to demonstrate the approach.When the small-scale turbulence and mesoscale motions are clearly separated by a gap region in the heat flux cospectra, the variable window width reduces the influence of nonstationarity by more effectively filtering out mesoscale motions compared to traditional methods using constant averaging time. For records where turbulence and mesoscale motions overlap in scale, the method is not well posed, although such records occur infrequently for our datasets. These ambiguous cases correspond to significant nonstationarity at scales that overlap with turbulence scales. The improved turbulence fluxes calculated with the proposed method are the appropriate fluxes for evaluating flux-gradient relationships and Monin–Obukov similarity theory for developing improved model parameterizations of turbulence for weakly turbulent flows  相似文献   

16.
Transpiration of a 7 m-high Stika spruce forest was investigated using measurements of net radiation, sensible heat and ground heat fluxes in an energy balance to give latent heat flux, and hence canopy resistance from the Penman-Monteith equation. Sensible heat flux was measured by the eddy-correlation method using a Fluxatron circuit. During six consecutive days of measurement in July/August, canopy resistance typically followed a decreasing trend from high values (≈150 s m−1) at dawn to around 40 s m−1 at midday and then returning steadily to >100 s m−1 at sunset. Transpiration was 3 mm day−1 on average over the period studied and changes in the rate within the day were significantly correlated with changes in net radiation. Comparisons are drawn with published data from other forest sites and the conclusion is reached that it is imprudent to generalise about transpiration rates and canopy resistances of different species at different sites from results gathered at one or two places.  相似文献   

17.
In this study, turbulent heat flux data from two sites within the Baltic Sea are compared with estimates from two models. The main focus is on the latent heat flux. The measuring sites are located on small islands close to the islands of Bornholm and Gotland. Both sites have a wide wind direction sector with undisturbed over-water fetch. Mean parameters and direct fluxes were measured on masts during May to December 1998.The two models used in this study are the regional-scale atmospheric model HIRLAM and the ocean model PROBE-Baltic. It is shown that both models overestimate the sensible and latent heat fluxes. The overestimation can, to a large extent, be explained by errors in the air-water temperature and humidity differences. From comparing observed and modelled data, the estimated 8-month mean errors in temperature and humidity are up to 1 °C and 1 g kg-1, respectively. The mean errors in the sensible and latent heat fluxes for the same period are approximately 15 and 30 W m-2, respectively.Bulk transfer coefficients used for calculating heat and humidity fluxes at the surface were shown to agree rather well with the measurements, at least for the unstable data. For stable stratification, the scatter in data is generally large, and it appears that the bulk formulation chosen overestimates turbulent heat fluxes.  相似文献   

18.
Bulk formulae for wind stress, sensible and latent heat flux are presented that are suitable for strong mesoscale events such as westerly wind bursts that contribute to the El Niño-Southern Oscillation (ENSO). Their exchange coefficients for heat and momentum have a simple polynomial dependence on wind speed and a linear dependence on air–sea temperature difference. The accuracy of these formulae are validated with respect to air–sea fluxes estimated using the standard algorithm adopted by the Tropical Ocean-Global AtmosphereCoupled-Ocean Atmosphere Response Experiment (TOGA COARE). The comparison ismade for observations from 96 Tropical Atmosphere Ocean (TAO) array and National Oceanographic Data Center (NODC) moorings in the equatorial and North Pacific Ocean spanning years 1990–1999. The bulk formulae are shown to have very small median root–mean-square differences with respect to the TOGA COARE estimates: 0.003 N m-2, 1.0 W m-2, and 10.0 W m-2 for the wind stress, sensible heat flux, and latent heat flux, respectively.The variability of air–sea fluxes during the 1997–1998 ENSO is also examined, along with a possible relationship between air–sea fluxes and surface ocean mixed layer depth (MLD). The wind stress and latent heat flux during the 1997 El Niño are found to be greater in the warm pool of the western Pacific than in the central Pacific where the ENSO is most clearly seen. These differences disappear upon the start of La Niña. The MLD in the equatorial Pacific is found to be moderately correlated to air–sea fluxes just before the start of the 1998 La Niña and poorly correlated otherwise.  相似文献   

19.
Transpiration of a 7 m-high Stika spruce forest was investigated using measurments of net radiation, sensible heat and ground heat fluxes in an energy balance to give latent heat flux, and hence canopy resistance from the Penman-Monteith equation. Sensible heat flux was measured by the eddy-correlation method using a Fluxatron circuit.During six consecutive days of measurement in July/August, canopy resistance typically followed a decreasing trend from high values (150 s m-) at dawn to around 40 s m–1 at midday and then returning steadily to > 100 s m–1 at sunset. Transpiration was 3 mm day–1 on average over the period studied and changes in the rate within the day were significantly correlated with changes in net radiation.Comparisons are drawn with published data from other forest sites and the conclusion is reached that it is imprudent to generalise about transpiration rates and canopy resistances of different species at different sites from results gathered at one or two places.  相似文献   

20.
This study documents the variability of surface sensible and latent heat fluxes in five regions of China (Northwest China, the Tibetan Plateau, Northeast China, North China, and Southeast China) using the ERA-40 reanalysis for the years 1960–2000. The surface sensible and latent heat flux variations are remarkably different in Northwest and Southeast China. The seasonal variation of the surface sensible heat fluxes is largest in Northwest China and smallest in Southeast China. In contrast, the seasonal variation in latent heat flux is largest in Southeast China and smallest in Northwest China. The interdecadal variation of surface sensible and surface latent heat fluxes strongly depends on both the region and season. The trends in surface sensible and latent heat fluxes in all four seasons are mainly caused by variations in both the land–air temperature difference and in the specific humidity. There is also a limited contribution of wind speed in some regions, depending on the season.  相似文献   

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