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1.
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  相似文献   

2.
The objective of this study is to produce two-dimensional maps of the sensible and the latent heat fluxes from airborne measurements, based on the analysis of a flight pattern, called grid flights. A footprint model with along-wind and cross-wind components was used to project the measured fluxes onto the surface map. The method was applied to measurements over Arctic tundra during the Mackenzie Area GEWEX (Global Energy and Water Cycle Experiment) Study (MAGS) 1999. The resulting flux estimates were computed by integration of a wavelet transform, and corrected for long wavelength losses using information from 100 km long regional runs that were conducted close to the grid flights. The random flux error was estimated based on the flight length that is represented in each map element, and a map resolution of 3 × 3 km was chosen in order to keep the average random error of the latent heat flux below 25%. The random error of the sensible heat flux was smaller by a factor of 1.4 on average. An analysis of airborne flux measurements at different altitudes showed no significant increase of flux estimates for measurement heights below 90 m. Thus, the fluxes measured at heights between 48 and 64 m were not corrected for vertical flux divergence. The resulting flux maps provide quantitative two-dimensional estimates of the energy exchange between the surface and the atmosphere during the snow melt period in an Arctic environment, which are well-suited for calibration and validation of numerical models.  相似文献   

3.
In June 1991 the EFEDA-field experiment (ECHIVAL Field Experiment in a Desertification-Threatened Area) was carried out in the Spanish province Castilla-La Mancha, to improve the understanding of the interactions between the soil, the vegetation and the atmosphere.Here results of energy balance studies at the Barrax site are given, one of the three intensively studied experimental sites within Castilla-La Mancha. This area is characterized by a large fraction of irrigated fields (40%) while the remaining 60% was fallow land at the end of June 1991. The energy balances over these two characteristic land-use classes totally differ. While for the irrigated fields the evapotranspiration is dominant, for the non-irrigated fields the sensible and the soil heat fluxes dominate and the latent heat flux is nearly negligible.In order to achieve areally averaged turbulent fluxes, surface, SODAR and aircraft data have been used. Comparing the surface fluxes from all three facilities, it can be found that:The sensible heat flux estimation from SODAR (w-method) gives reliable results when a calibration of w is done with another independent system (e.g. aircraft).Aircraft measurements in conjunction with energy budget methods yield surface fluxes of sensible heat, which are about 20% lower than the areally averaged values calculated by the surface measurements. The areally averaged latent heat fluxes from aircraft and surface measurements agree better than the sensible heat fluxes.  相似文献   

4.
Summary An aircraft-based experimental investigation of the atmospheric boundary layer (ABL) structure and of the energy exchange processes over heterogeneous land surfaces is presented. The measurements are used for the validation of the mesoscale atmospheric model “Lokal-Modell” (LM) of the German Weather Service with 2.8 km resolution. In addition, high-resolution simulations using the non-hydrostatic model FOOT3DK with 250 m resolution are performed in order to resolve detailed surface heterogeneities. Two special observation periods in May 1999 show comparable convective boundary layer (CBL) conditions. For one case study vertical profiles and area averages of meteorological quantities and energy fluxes are investigated in detail. The measured net radiation is highly dependent on surface albedo, and the latent heat flux exhibits a strong temporal variability in the investigation area. A reduction of this variability is possible by aggregation of multiple flight patterns. To calculate surface fluxes from aircraft measurements at low altitude, turbulent energy fluxes were extrapolated to the ground by the budget method, which turned out to be well applicable for the sensible heat flux, but not for the latent flux. The development of the ABL is well captured by the LM simulation. The comparison of spatiotemporal averages shows an underestimation of the observed net radiation, which is mainly caused by thin low-level clouds in the LM compared to observed scattered CBL clouds. The sensible heat flux is reproduced very well, while the latent flux is highly overestimated especially above forests. The realistic representation of surface heterogeneities in the investigation area in the FOOT3DK simulations leads to improvements for the energy fluxes, but an overestimation of the latent heat flux still persists. A study of upscaling effects yields more structures than the LM fields when averaged to the same scale, which are partly caused by the non-linear effects of parameter aggregation on the LM scale.  相似文献   

5.
The performance of a combined large aperture scintillometer (LAS) and a millimetre wave scintillometer (MWS) for estimating surface fluxes of sensible and latent heat over natural landscape is investigated, using data gathered during LITFASS-2003. For this purpose the LAS–MWS system was installed in a moderate heterogeneous landscape over a path length of 4.7 km with an effective beam height of 43 m. The derived surface fluxes have been compared with aggregated eddy-covariance (EC) measurements. The fluxes of sensible and latent heat from the LAS–MWS combination, as well as sensible heat fluxes of the single LAS, agreed fairly well with the EC-based fluxes, considering the uncertainties of the similarity stability functions and observed energy imbalance.  相似文献   

6.
利用苏州地区2011年12月20日—2012年8月13日的湍流观测资料对不同季节、高温、台风强天气过程下的湍流特征进行分析。结果表明:城市地区不同季节动量通量、感热通量、潜热通量日变化明显,各通量的夏季平均值、最大值均高于冬春季,夏季感热通量日最大值为160.2 W·m-2,感热在城市地表能量平衡中的作用大于潜热,各季节潜热通量平均值仅为感热通量的40%~45%。降水量和植被覆盖度影响地表能量平衡,尤其影响地表热量在感热和潜热之间的分配。在高温天气过程中,感热通量增加明显,其峰值约是夏季平均的1.93倍。由于水汽较少,潜热通量明显减少,约为夏季日平均值的60%。速度三分量谱中u谱与w谱在低频区存在两个峰值,说明在城市复杂下垫面里,湍流激发机制中存在低频过程的影响。在台风天气过程中,动量通量大且变化快,感热输送弱,潜热输送波动大。速度谱w基本不符合"-5/3"次律,惯性子区最小且向高频移动,这和台风内部的复杂上升下沉气流有关。  相似文献   

7.
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   

8.
The SEMAPHORE (Structure des Echanges Mer-Atmosphère, Propriétés Océaniques/ Recherche Expérimentale) experiment, which took place between 04 Oct. and 17 Nov. 1993, was conducted over the oceanic Azores current located in the Azores basin. The SST (Sea Surface Temperature) field was characterized in the SEMAPHORE area (31°–38°N; 21°–28°W) by a large meander with a SST gradient of about 1°C per 100 km. In order to study the evolution of the MABL (Marine Atmospheric Boundary Layer) over the ocean, the mean and the turbulent data were evaluated by the measurement with two aircraft and a ship in different meteorological conditions. Three cases of low pressure and three cases of high pressure are mainly presented here. For the six cases, the satellite images (NOAA) did not show any relation between the SST field and the cloud cover. At each flight level, the decrease of the SST with the altitude due to the divergence of the infrared radiation flux from the ocean is 0.25°C per 100 m. For the comparison between the two aircraft, the mean thermodynamic and dynamic parameters show a good agreement except for the temperature. The dispersion of the sensible heat flux is larger than that of the latent heat flux due to the weak sensible heat flux over the ocean both in the intercomparison between two aircraft and in the comparison between the aircraft and the ship.  相似文献   

9.
A variational technique (VT) is applied to estimate surface sensible and latent heat fluxes based on observations of air temperature, wind speed, and humidity, respectively, at three heights (1 m, 4 m, and 10 m), and the surface energy and radiation budgets by the surface energy and radiation system (SERBS). The method fully uses all information provided by the measurements of air temperature, wind, and humidity profiles, the surface energy budget, and the similarity profile formulae as well. Data collected at Feixi experiment station installed by the China Heavy Rain Experiment and Study (HeRES) Program are used to test the method. Results show that the proposed technique can overcome the well-known unstablility problem that occurs when the Bowen method becomes singular; in comparison with the profile method, it reduces both the sensitivities of latent heat fluxes to observational errors in humidity and those of sensible heat fluxes to observational errors in temperature, while the estimated heat fluxes approximately satisfy the surface energy budget. Therefore, the variational technique is more reliable and stable than the two conventional methods in estimating surface sensible and latent heat fluxes.  相似文献   

10.
With the marine ship observation data set obtained by three cruise-phases of Chinese Xi-angyanghong 5 from November 5,1992 to February 19,1993 in the TOGA-COARE IOP at 2°S,156°E.the sea surface fluxes at this point are estimated by three different bulk schemes.Firstly.aquasi-linear relation is found between the neutral drag coefficients and wind speed.Then,the sta-bility-dependent drag and heat transfer coefficients are solved in the iterative method.Based onthus-derived transfer coefficients,the momentum,sensible and latent heat fluxes are calculated.In the warm pool region,the fluxes corresponding to the westeily winds are much greater thanthose of the easterly trade winds.The magnitude of sea surface fluxes depends upon the atmo-spheric stability as well,in particular in the case of weak wind condition.The estimated sea sur-face net heat budget shows that considerable amount of heat transport from ocean to atmosphere ismainly produced by the effective longwave radiation,latent and sensible heat fluxes.Among themthe value of latent heat flux is the largest and the sensible heat flux is the smallest.Finally,an ac-curacy analysis is made by direct measurements with the eddy-correlation method on the JapaneseR/V Hakuho board at the same time.It is shown that the bulk-derived fluxes are acceptable withmuch confidence.The estimated effective longwave radiation is used to compare with those by di-rect observations on Xiangyanghong 5.This research is compared with TOGA investigation in thetropical western Pacific.  相似文献   

11.
High-resolution simulations with a mesoscale model are performed to estimate heat and moisture budgets of a well-mixed boundary layer. The model budgets are validated against energy budgets obtained from airborne measurements over heterogeneous terrain in Western Germany. Time rate of change, vertical divergence, and horizontal advection for an atmospheric column of air are estimated. Results show that the time trend of specific humidity exhibits some deficiencies, while the potential temperature trend is matched accurately. Furthermore, the simulated turbulent surface fluxes of sensible and latent heat are comparable to the measured fluxes, leading to similar values of the vertical divergence. The analysis of different horizontal model resolutions exhibits improved surface fluxes with increased resolution, a fact attributed to a reduced aggregation effect. Scale-interaction effects could be identified: while time trends and advection are strongly influenced by mesoscale forcing, the turbulent surface fluxes are mainly controlled by microscale processes.  相似文献   

12.
We compute the interannual fluctuations of the surface heat budget of the North Atlantic using the trimmed monthly summaries of the Comprehensive Ocean-Atmosphere DataSet (COADS) for the period 1950–1979. The presence of long-period trends in the heat budget imply large variations of the northward cross-equatorial heat transport over the years. To assess the reliability of these variations, we compare the COADS climate signal to that derived from the ocean weather stations (OWSs) of the North Atlantic. The sea surface temperature, air temperature and sea level pressure show good correlation between the anomaly time series derived from the merchant ship monthly summaries of COADS, and those derived from OWS monthly summaries, except for northernmost locations during winter. In contrast, the sensible and latent heat parameters, which require simultaneous measurements of various variables, have merchant ships and ocean weather stations anomaly time series that are poorly correlated. Only in heavily travelled latitudes and during winter, when the air-sea heat exchange anomalies are large, are the merchant ship measurements able to reproduce the interannual fluctuations of the heat fluxes. The long-period trends in the surface heat budget of North Atlantic equatorward of 40° N implied by COADS thus appear unrepresentative of true climate trends. The COADS trends result from a gradual increase in the magnitude of the reported winds over the years due probably to variations in the ratio of measured to estimated winds, as well as from long period fluctuations in the near surface vertical temperature and humidity gradients. Offprint requests to: R Michaud  相似文献   

13.
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   

14.
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.  相似文献   

15.
Boundary-layer heat and moisture budgets from fife   总被引：1，自引：0，他引：1  
Aircraft stacks were flown upwind and downwind of the First ISLSCP Field Experiment (FIFE) site in Kansas to measure the heat and moisture budgets of the boundary layer under fairly clear skies for four daytime periods. In this paper, we evaluate the terms in the conservation equation. The vertical flux divergence and advection do not account for the difference between surface and low-level aircraft flux estimates. Budget estimates of the surface fluxes using the aircraft data agree well with surface flux measurements, but extrapolation of the aircraft fluxes gives surface fluxes that are too low. With the 5 km cutoff filter used, the aircraft underestimate for sensible heat flux is about 40%, and for the latent heat flux about 30%. Part of the underestimation is attributable to long-wavelength contributions (longer than the 5 km filter), but more investigation is needed.  相似文献   

16.
曾剑  张强  王春玲 《气象学报》2016,74(6):876-888
东亚夏季风边缘摆动区既是气候敏感区,也是生态脆弱区和农牧交错带,其特殊陆面能量空间分布格局和演变特征对理解该区域天气和气候变化有重要意义。然而受限于陆面观测资料缺乏,对这部分陆面特征的认识仍非常有限。通过对34 a陆面模拟集成产品的分析,发现夏季风边缘摆动区内潜热和感热通量在空间上表现出明显的过渡特征,由摆动区外的相对均衡状态进入到摆动区内的“突变转换”;陆面能量平衡具有明显的区域特征,能量平衡各分量在纬向和经向都表现出了“阶梯型”的变化。就演变而言,区域平均感热和潜热没有表现出规律性的递减或递增趋势,波动幅度在±20%以内,但在20世纪末存在一个较为明显的摆动相位转换:1997年之前夏季风边缘摆动区夏季风相对活跃,潜热通量总体高于其气候值而感热通量则低于其气候值,之后出现了相反的现象。此外,区内感热和潜热通量对气候环境干湿性质非常敏感,两者存在明显的线性关系。   相似文献   

17.
沙漠地区沙尘天气近地层湍流输送特征分析   总被引：12，自引：3，他引：12  
利用1996－04－20－50－10在内蒙古自治区吉兰泰气象站以及1997－04－05－27在内蒙古自治区阿拉善右旗气象站观测到的风，温，湿廓线资料，应用空气动力学方法，计算了每天各观测时次的流动量通量，湍流感热通量和湍流潜热通量，并且比较分析了荒漠下垫面不同沙尘天气状况下的湍流动量通量，感热通量和潜热通量的日变化。结果表明，在沙漠地区春季无降水天气状况下，晴空条件下，白天热力湍流基于动力湍流，在近地层最重要的物质交换是湍流感热交换，浮尘天气条件下，近地层湍流输送较晴空天气条件下弱；扬沙天气条件下，近地层湍流感热交换和湍流动力量流感热通量的峰值大于交换，并且湍流动量通量的峰值较晴空峰值大将近一个量级，湍流感热通量的峰值大于浮尘天气峰值，但小于晴空条件下峰值，沙尘暴天气条件下，湍流动量通量和感热通量一样都是最重要的湍流交换，湍流交换强于扬沙天气，强的感热对沙漠地局地性沙尘暴的产生和加强起着重要作用。  相似文献   

18.
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   

19.
The estimation of spatial patterns in surface fluxes from aircraft observations poses several challenges in the presence of heterogeneous land cover. In particular, the effects of turbulence on scalar transport and the different behaviour of passive (e.g. water vapour) versus active (e.g. temperature) scalars may lead to large uncertainties in the source area/flux-footprint estimation for sensible (H) and latent (LE) heat-flux fields. This study uses large-eddy simulation (LES) of the land–atmosphere interactions to investigate the atmospheric boundary-layer (ABL) processes that are likely to create differences in airborne-estimated H and LE footprints. We focus on 32~m altitude aircraft flux observations collected over a study site in central Oklahoma during the Southern Great Plains experiment in 1997 (SGP97). Comparison between the aircraft data and traditional model estimates provide evidence of a difference in source area for turbulent sensible and latent heat fluxes. The LES produces reasonable representations of the observed fluxes, and hence provides credible evidence and explanation of the observed differences in the H and LE footprints. Those differences can be quantified by analyzing the change in the sign of the spatial correlation of the H and LE fields provided by the LES model as a function of height. Dry patterns in relatively moist surroundings are able to generate strong, but localized, sensible heating. However, whereas H at the aircraft altitude is still in phase with the surface, LE presents a more complicated connection to the surface as the dry updrafts force a convergence of the surrounding moist air. Both the observational and LES model evidence support the concept that under strongly advective conditions, H and LE measured at the top of the surface layer (≈50 m) can be associated with very different upwind source areas, effectively contradicting surface-layer self-similarity theory for scalars. The results indicate that, under certain environmental conditions, footprint models will need to predict differing source area/footprint contributions between active (H) and passive (LE) scalar fluxes by considering land-surface heterogeneity and ABL dynamics.  相似文献   

20.
Intercomparison of three urban climate models   总被引：1，自引：0，他引：1  
An intercomparison of the surface energy budgets from three urban climate models was made to assess the comparability of results, and to evaluate the surface energy fluxes from each model. The three models selected spanned the continuum of approaches currently employed in the treatment of the effects of urban geometry. The first model was an urban canopy-layer model which explicitly examined urban canyon geometry. The second model treated the city as a warm, rough, moist plate but included greatly simplified parameterizations of urban geometry. Neither model included a dynamic link to the urban boundary-layer. The third model was a one-dimensional urban boundary-layer model which utilized a simple warm, rough, moist plate approach but included a dynamic coupling of the urban surface layer to the urban boundary-layer.Results showed considerable disagreement between the three models in regards to the individual energy fluxes. Average rankings of the energy fluxes in terms of comparability from high-to-low similarity were: (1) solar radiation, (2) sensible heat flux, (3) conduction, (4) latent heat flux, (5) longwave re-radiation, and (6) longwave radiation input. In general, the urban canopy-layer model provided more realistic results, although each model demonstrated strong and weak points. Results indicate that current urban boundary-layer models may produce surface energy budgets with lower sensible heat fluxes and substantially higher latent heat fluxes than is supported by field evidence from the literature.  相似文献   

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