Turbulent Flux Measurements Research Articles

Influence of Thermodynamic State Changes on Surface Cloud Radiative Forcing in the Arctic: A Comparison of Two Approaches Using Data From AFLUX and SHEBA

AbstractThe cloud radiative forcing (CRF) quantifies the warming or cooling effects of clouds. To derive the CRF, reference values of net (downward minus upward) irradiances in cloud‐free conditions are required. There are two groups of techniques to estimate these reference values; one is based on radiative transfer modeling, and a second group uses measurements in cloud‐free situations. To compare both approaches, we first look at a case study from the airborne measurements of radiative and turbulent FLUXes of energy and momentum in the Arctic boundary layer (AFLUX) campaign, where a moving cloud field with a sharp edge separating a cloudy boundary layer from an adjacent evolving cloud‐free area was probed. These data enabled the quantification of the impact of changing atmospheric and surface properties relevant for the reference net irradiances in cloud‐free conditions. The systematically higher surface albedo below clouds compared to cloud‐free conditions, results in a 11 W·m−2 smaller shortwave cooling effect by clouds estimated from the radiative transfer approach compared to the measurement‐based one. Due to the transition of thermodynamic parameters between the cloudy and cloud‐free atmospheric states, a 20 W·m−2 stronger warming effect is estimated by the radiative transfer approach. In a second step, radiative transfer simulations based on radiosoundings from the Surface Heat Budget of the Arctic Ocean campaign are used to quantify the impact of the vertical profiles of thermodynamic properties on the CRF. The largest difference between the longwave CRF estimated by the two methods is found in autumn with up to 25 W·m−2.

Examining relationships between entrainment-driven scalar dissimilarity and surface energy balance underclosure in a semiarid valley

A surface energy budget underclosure, equivalent to an undersampling of sensible and latent heat fluxes at the surface, is commonly observed in the daytime convective boundary layer over land. The two mechanisms currently held responsible for such underclosure are, on one hand, entrainment-induced reduction of heat fluxes due to low-frequency circulations spanning the depth of the convective boundary layer, and on the other hand, horizontal advection effects induced by surface horizontal heterogeneity. Given the involvement of both heat fluxes in the budget equation, it is expected that the turbulent perturbations of variables involved in these fluxes (potential temperature θ′ and specific humidity q′) contain the signature of both mechanisms. Here, we assess to what extent non-local entrainment effects contribute to the budget underclosure in complex terrain using a quadrant analysis and multiresolution flux decomposition approach.We use data collected in the semi-arid Owens Valley, CA, USA, during the Terrain-Induced Rotor Experiment (T-REX) campaign. The budget components are sampled near the surface of the valley floor and the adjacent slope, with turbulence flux measurements, pyranometers, pyrgeometers, as well as ground heat flux measurements. We found that the budget underclosure was within the range of underclosures reported in past studies, with a systematically poorer closure over the valley slope. Although quadrant analysis revealed a dominance of scalar-similar events, associated with convective boundary layer overturning, a high degree of disagreement was found between the scalar-dissimilar quadrants which represent the counter-gradient motions. Specifically, those turbulent motions associated with entrainment of free tropospheric air to the surface, were found to be of minor importance in degrading scalar similarity. Instead, we postulate that local horizontal advection and hectometre-scale land-use heterogeneity in the vicinity of both the valley floor and slope sites, are the major factors in controlling the relationship between the budget underclosure and scalar similarity.

Observation of wave-driven air–water turbulent momentum exchange in a large but fetch-limited shallow lake

Abstract. Wind-induced waves play a key role in air–sea momentum and heat exchange. Fetch-limited shallow lakes differ significantly from open ocean circumstances since the wave field is characterized by young and growing waves that (i) are steeper and can collapse by white-capping at lower wind speeds, and (ii) travel with lower phase velocity. Consequently, momentum (and heat) flux estimation methods arising from oceanographic observations cannot be directly applied; however, few attempts have been made to describe air–water turbulent exchange in case of large, but still fetch-limited shallow lakes. Within a Croatian-Hungarian measurement campaign, turbulent flux measurements were performed in Lake Balaton. Momentum and heat fluxes were measured with eddy-covariance technique at an offshore station, while waves were simultaneously recorded with underwater acoustic surface tracking. Momentum fluxes were also recorded at two further stations closer to the shore. In this study, we analyze the measured wind stress and surface waves to reveal surface drag in case of highly fetch-limited conditions. We compare our results with relevant model formulations that attempt to estimate momentum flux using different wave state parameterizations (i.e. wave age and wave slope modified Charnock formulations) and show that derived drag and roughness length parameterizations differ significantly from oceanographic formulas.

Evapotranspiration over Land from a Boundary-Layer Meteorology Perspective

The precise determination of evapotranspiration rate is challenging because it is a quantity that is difficult to measure and to parametrize. Direct estimates include the determination of the change of mass of a volume of soil and vegetation that evapotranspirates using lysimeters, or direct measurements of turbulent water vapour fluxes by eddy-covariance systems. Parametrized estimates that make use of the Monin–Obukhov similarity theory use vertical gradient measurements of temperature and moisture at one point, and line or area averages by means of scintillometers operating at high frequency. A relation for the evapotranspiration from well-watered surfaces was initially developed by Penman and later expanded for vegetated surfaces and for heterogeneous croplands. A popular simplified expression was obtained by Priestley and Taylor. The current challenge is to find expressions for the evapotranspiration in non-saturated conditions, which are common in arid and semi-arid climates, and for heterogeneous terrain. In numerical models, the estimated actual evapotranspiration over land is obtained as the result of the explicit representation of the different involved sub-processes taking place in the soil and the canopy, using so-called land-surface models. Usually these mechanisms are described in a simplified manner and rely on a number of adjustable parameters. The improvement of such descriptions relies in the availability of experimental measurements to make the physical models more complete and robust.

Effects of Gentle Topography on Forest‐Atmosphere Gas Exchanges and Implications for Eddy‐Covariance Measurements

AbstractThe interpretation of tower‐based eddy‐covariance (EC) turbulent flux measurements above forests hinges on three key assumptions: (1) steadiness in the flow statistics, (2) planar homogeneity of scalar sources or sinks, and (3) planar homogeneity in the flow statistics. Large eddy simulations (LESs) were used to control the first two so as to explore the break‐down of the third for idealized and real gentle topography such as those encountered in Amazonia. The LES runs were conducted using uniformly distributed sources inside homogeneous forests covering complex terrain to link the spatial patterns of scalar turbulent fluxes to topographic features. Results showed strong modulation of the fluxes by flow features induced by topography, including large area with negative fluxes compensating “chimney” regions with fluxes almost an order of magnitude larger than the landscape flux. Significant spatial heterogeneity persisted up to at least two canopy heights, where most eddy‐covariance measurements are performed above tall forests. A heterogeneity index was introduced to characterize and contrast different scenarios, and a topography categorization was shown to have predictive capabilities in identifying regions of negative and enhanced fluxes.

Surface Turbulent Fluxes during Persistent Cold-Air Pool Events in the Salt Lake Valley, Utah. Part II: Simulations

AbstractRealistically representing the land–atmosphere interactions during persistent cold-air pools (PCAPs) is critical in simulating the strength of PCAPs, where uncertainties in simulating the PCAP strength will impact the ability to model the poor air quality. To quantify the model performance for land–atmosphere exchange, measurements of surface turbulent and radiative energy fluxes during two PCAPs, one weak and one strong, in Utah were compared with simulations from the Weather Research and Forecasting (WRF) Model. The results show that the WRF Model simulated the surface energy fluxes well in the weak PCAP case and that the performance degraded in the strong PCAP case. The significantly overestimated surface sensible heat flux H and latent heat flux (LE) in the strong PCAP were related, in part, to the overestimated net radiation and soil moisture and unsuitable turbulence parameterizations. The simulation using the Mellor–Yamada–Nakanishi–Niino planetary boundary layer scheme produced the least bias in both net radiation and surface turbulent fluxes for the strong PCAP case, which is expected because of the local higher-order (2.5) turbulence closure scheme. The surface exchange coefficient (CH), a crucial variable used to calculate H, was overall overestimated by the WRF Model. The underestimation of the nondimensional vertical temperature gradient in the Monin–Obukhov stability function was responsible for the overestimated CH, where the stability functions deviate significantly from expected values from observations for the stable atmospheric boundary layer. Our study highlights the need to improve the flux–profile parameterizations under stable conditions over complex terrain by including impacts due to mountainous terrain, such as surface radiative flux divergence and the diurnal mountain wind system.

Comparison of sensible heat fluxes by large aperture scintillometry and eddy covariance over two contrasting−climate vineyards

Measurements of turbulent energy fluxes between land surface and the atmosphere are commonly carried out applying the eddy covariance (EC) method. Nevertheless, in some cases (e.g. surface heterogeneity, remote sensing or model validation), the use of a Large Aperture Scintillometer (LAS) is more appropriate. We compared LAS and EC measurements of sensible heat flux (H) over two contrasting vineyards of Southern (MIN site) and Northern (LIS site) Italy, mainly differentiating for climate (i.e. dry and humid), water management (i.e. irrigated and rainfed), and LAS setup. The relationship between H by LAS (HLAS) and by EC (HEC) was good at both sites, but with slightly higher HLAS. At MIN, where multiple EC stations were deployed, the best agreement was found with the station in the center of LAS path. Surface heterogeneity due to irrigation had different impact on HLAS depending on location of the source area. The canopy growth at LIS during the study period caused the approaching of LAS beam to the roughness sublayer, resulting in a reduction of HLAS compared to HEC. The closure of energy balance was higher (79%) at MIN and lower (64%) at LIS, but the agreement between HLAS and HEC was similar at both sites, suggesting that particular attention should be given to latent heat flux measurements by EC in order to better understand the energy balance closure issue.

Eddy covariance measurements of turbulent fluxes in the surf zone

Turbulent eddies play a critical role in oceanic flows. Direct measurements of turbulent eddy fluxes beneath the sea surface were taken to study the direction of flux-carrying eddies as a means of supplementing our understanding of vertical fluxes exchange processes and their relationship to tides. The observations were made at 32 Hz at a water depth of ~1.5 m near the coast of Sanya, China, using an eddy covariance system, which mainly consists of an acoustic doppler velocimeter (ADV) and a fast temperature sensor. The cospectra-fit method—an established semi-empirical model of boundary layer turbulence to the measured turbulent cospectra at frequencies below those of surface gravity waves—was used in the presence of surface gravity waves to quantify the turbulent eddy fluxes (including turbulent heat flux and Reynolds stress). As much as 87% of the total turbulent stress and 88% of the total turbulent heat flux were determined as being at band frequencies below those of surface gravity waves. Both the turbulent heat flux and Reynolds stress showed a daily successive variation; the former peaked during the low tide period and the later peaked during the ebb tide period. Estimation of roll-off wavenumbers, k0, and roll-off wavelengths, λ0 (where λ0=2π/k0), which were estimated as the horizontal length scales of the dominant flux-carrying turbulent eddies, indicated that the λ0 of the turbulent heat flux was approximately double that of the Reynolds stress. Wavelet analysis showed that both the turbulent heat flux and the Reynolds stress have a close relationship to the semi-diurnal and diurnal tides, and therefore indicate the energy that is transported from tides to turulence.

Air—Surface Turbulent Heat Exchange in the Antarctic Coastal Zone Derived from Instrumental Observations

The results of direct measurements of turbulent fluxes at Bellingshausen station (the Antarctic) in the summer of 2002, 2003, 2007, and 2009 are considered. The features of energy exchange are studied for these seasons. The interannual and intraseasonal variability of energy exchange characteristics is investigated. It is found that heat fluxes depend on the surface state and on the wind direction. Also, as a rule, they have a clear diurnal course with negative or near-zero values at night and with positive values in the daytime, when intensive convective fluxes are observed over open ground. The momentum flux is defined by the wind direction and wind speed and by stratification conditions. The main factors affecting energy exchange between the atmosphere and the surface in the Antarctic coastal zone are synoptic conditions and the surface state (snowless season duration and moss cover presence).

Glacial Aerodynamic Roughness Estimates: Uncertainty, Sensitivity, and Precision in Field Measurements

AbstractCalculation of the sensible and latent heat (turbulent) fluxes is required in order to close the surface energy budget of glaciers and model glacial melt. The aerodynamic roughness length, z0, is a key parameter in the bulk approach to calculating sensible heat flux; yet, z0 is commonly considered simply as a tuning parameter or generalized between surfaces and over time. Spatially and temporally distributed observations of z0 over ice are rare. Both direct (from wind towers and sonic anemometers) and indirect (from microtopographic surveys) measurements of z0 are subject to sensitivities and uncertainties that are often unstated or overlooked. In this study, we present a quantitative evaluation of aerodynamic profile‐based and microtopographic methods and their effect on z0 using data collected from Storglaciären and Sydöstra Kaskasatjäkkaglaciären, Tarfala Valley, Arctic Sweden. Aggressive data filters discard most of the wind tower data but still produce realistic z0 values of 1.9 mm and 2 mm. Despite uncertainty introduced by scale and resolution dependence, microtopographic methods produced estimates of z0 comparable to wind tower values and those found on similar surfaces. We conclude that (1) in the absence of direct turbulent flux measurements from sonic anemometers, the profile and microtopographic methods provide realistic z0 values, (2) both 2D and 3D microtopographic methods are dependent on scale, resolution, and the chosen detrending method, and (3) careful calibration of these parameters could enable glacier‐wide investigations of z0 from remotely sensed data, including those increasingly available from satellite platforms.

Applicability of a closed-path quantum cascade laser spectrometer for eddy covariance (EC) flux measurements of nitric oxide (NO) over a cropland during a low emission period

Croplands are important sources of atmospheric nitric oxide (NO). However, high-frequency measurements of NO fluxes over croplands using the eddy covariance (EC) technique are still scarce, mainly due to instrumental limitation. In this study, a closed-path NO analyzer based on a quantum cascade laser (QCL) absorption spectrometer was employed for EC flux measurements over a subtropical vegetable field during a two-month summer period with the lowest NO emission intensity of the year. The purpose was to investigate the detection limit of the EC system based on this NO analyzer and evaluate its applicability for measuring the turbulent fluxes of NO under field conditions. The performance of the analyzer was stable, showing an average precision (0.1 s) of 0.338 nmol mol−1 and a corresponding flux detection limit of 5.6 μg N m−2h−1 at the 95% confidence interval. The measured turbulent NO fluxes ranged from −7.1 to 61.4 μg N m−2h−1 (median: 3.5 μg N m−2h−1), with a relative random error of 386% before field ploughing and 76% thereafter. The systematic errors due to the high-frequency loss and the use of lag times of carbon dioxide for NO flux calculation were estimated at 12% and 3%, respectively. During the measurement period, 37% of the observed half-hourly fluxes were larger than the detection limit; the magnitude of these fluxes is comparable with that measured by the static chambers. Nevertheless, this EC system could be still qualified for measuring turbulent NO fluxes over common croplands if the flux averaged at daily or longer timescales are of interest, because either flux detection limit or random error would decrease by an order of n, wherein n is the number of half-hourly fluxes being taken for averages. This study shows that the closed-path dual-QCL analyzer could be an effective option for EC measurements of turbulent NO fluxes with the advantages of (i) stable performance, (ii) high precision and fast response, and (iii) feasible instrumental maintenance for long-term field measurements. However, the observed turbulent fluxes still underestimated the soil NO emissions likely due to chemical reaction loss of NO below the sensor height. Further studies are necessary to address this systematic error.

Estimation of sensible heat flux at a tropical location: a performance evaluation of half-order time derivative method

Measurement of surface energy balance (SEB) components such as sensible heat flux is a very fundamental input in weather forecast, hydrological, and air quality models among many other applications. The instrumentation for measuring SEB components is however resource-intensive and highly susceptible to damage under field-experiment conditions. A simple, less equipment-intensive, cost-effective, and relatively accurate method is thus required to overcome these setbacks. The present study, therefore evaluates the performance of a rather simple half-order time derivative (HTD) method in estimating sensible heat flux at a tropical location in West Africa (Ile-Ife, Nigeria 7°33′N, 4°33′E). HTD estimates of sensible heat flux were made from near surface atmospheric parameters measured during Phase I of the Nigerian Micrometeorological Experiment. At the same site, direct turbulence flux measurements from an eddy covariance system comprising a 3-D ultrasonic anemometer (USA-1) and a krypton hygrometer (KH20) were used as standard benchmark values for the HTD performance evaluation. Within a diurnal air temperature range of 18–34 °C, estimated daytime sensible heat flux reached a peak of 318.7 W m−2 and a lowest nighttime value of − 15.8 W m−2, indicating surface cooling. Statistical tests performed; mean bias error (MBE <2 W m−2), root mean square error (RMSE <7.3 W m−2), mean absolute error (MAE< 2 W m−2), and percent mean relative error (PMRE <1%) indicated a good accuracy of the HTD method with an overall significant correlation (R2 = 0.89) with benchmark values. The HTD method is found most suitable for daytime convective conditions than stable nighttime periods at the tropical location.

Recovering Evapotranspiration Trends from Biased CMIP5 Simulations and Sensitivity to Changing Climate over North America

Abstract Future projections of evapotranspiration (ET) are of critical importance for agricultural and freshwater management and for predicting land–atmosphere feedbacks on the climate system. However, ET from phase 5 of the Coupled Model Intercomparison Project (CMIP5) simulations exhibits substantial biases, bolstering little confidence in future ET projections. Despite poor predictive skill and large bias of ET from the global climate models, the information content necessary to calculate ET offline is available in the models’ archived outputs: temperature T, water vapor pressure e, atmospheric pressure P, and surface net radiation R. A relatively simple three-source energy balance model [Penman–Monteith (PM)], along with the mean annual cycle of remotely sensed vegetation properties, can then be used to reconstruct ET with a substantially reduced bias relative to in situ turbulent heat flux measurements. This methodology is used here to reconstruct ET projections from 2006 through 2100 over North America using output from selected CMIP5 models and to attribute projected ET trends to specific atmospheric controls. CMIP5 ET exhibits substantial bias in annual ET relative to in situ flux measurements across North America (38%–73%; 2006–15), but ET reconstructed from the CMIP5 meteorology with the PM method greatly reduces this bias (−8% to +14%). Present-day North American ET is more sensitive to changes in atmospheric demand for ET (temperature and water vapor pressure) than energy limitation (net radiation), and to a lesser extent vegetation properties (leaf area index). Accordingly, ET is projected to increase 0.26–0.87 mm yr−1 yr−1 over North America through 2100 driven primarily by trends in temperature.

An Assessment of Coordinate Rotation Methods in Sonic Anemometer Measurements of Turbulent Fluxes over Complex Mountainous Terrain

The measurement of turbulent fluxes in the atmospheric boundary layer is usually performed using fast anemometers and the Eddy Covariance technique. This method has been applied here and investigated in a complex mountainous terrain. A field campaign has recently been conducted at Alpe Veglia (the Central-Western Italian Alps, 1746 m a.s.l.) where both standard and micrometeorological data were collected. The measured values obtained from an ultrasonic anemometer were analysed using a filtering procedure and three different coordinate rotation procedures: Double (DR), Triple Rotation (TR) and Planar Fit (PF) on moving temporal windows of 30 and 60 min. A quality assessment was performed on the sensible heat and momentum fluxes and the results show that the measured turbulent fluxes at Alpe Veglia were of a medium-high quality level and rarely passed the stationary flow test. A comparison of the three coordinate procedures, using quality assessment and sensible heat flux standard deviations, revealed that DR and TR were comparable, with significant differences, mainly under low-wind conditions. The PF method failed to satisfy the physical requirement for the multiple planarity of the flow, due to the complexity of the mountainous terrain.

First measurements of the ocean-atmosphere CO2 fluxes at the Cabo Frio upwelling system region, Southwestern Atlantic Ocean

This study presents novel results about ocean-atmosphere CO2 exchanges based upon novel direct in situ measurements of turbulent fluxes using the eddy covariance (EC) methodology. This study was carried out on the Southeast coast of Brazil and the adjacent open ocean during two days of the austral winter of 2015, on board the RV Vital de Oliveira. This is the first time that the EC technique was used in this region for CO2 flux studies. The results presented illustrate contrasting magnitudes of CO2 fluxes measured on the open sea region (1.22 ± 2.08 μmol m−2 s−1), the Brazil Current (−2.92 ± 1.25 μmol m−2 s−1) and the coastal region (−3.11 ± 1.67 μmol m−2 s−1). The main cause of negative CO2 fluxes, indicating a mean CO2 sink behavior of this region, is due to the occurrence of the coastal upwelling phenomenon and the atmospheric conditions. We found that lower atmosphere local modulation due to sea surface temperature (SST) variations along the study region combined with the large-scale atmospheric synoptic conditions contributed to the flux behavior.

Differences in Eddy‐Correlation and Energy‐Balance Surface Turbulent Heat Flux Measurements and Their Impacts on the Large‐Scale Forcing Fields at the ARM SGP Site

AbstractDifferences in the surface turbulent heat fluxes measured by the eddy correlation flux measurement system (ECOR) and the energy balance Bowen ratio system (EBBR) are examined using 12 years of continuous measurements collected at multiple stations in the Atmospheric Radiation Measurement (ARM) program Southern Great Plains (SGP) surface observational network. The flux measurements are found strongly impacted by their upwind surface types, which vary with wind direction. For collocated ECOR and EBBR at the central facility, the fluxes measured by ECOR and EBBR agree much better when their upwind fetches are over the same surface type. Among all stations at SGP, ECOR measures more over winter wheat fields while EBBR measures mostly over grassland. The different seasonality of growth cycles between winter wheat and grass causes systematic differences in measured fluxes between ECOR and EBBR. These differences impact the derived large‐scale forcing as illustrated in a constrained variational analysis, in which the state variables have to be adjusted according to different fluxes to keep the column‐integrated energy and moisture budgets in balance. This impact prevails in summertime on nonprecipitating days. A single‐column model test shows that model‐simulated boundary layer development is impacted by using the large‐scale forcing data of different surface turbulent fluxes. It is recommended to include both ECOR and EBBR measurements to better represent the domain‐mean turbulent fluxes and atmospheric budgets of energy and water vapor at the SGP.

Inverse distributed modelling of streamflow and turbulent fluxes: A sensitivity and uncertainty analysis coupled with automatic optimization

The interactions of hydrological variables in the terrestrial hydrological cycle are complex. To better predict the variables, distributed and physically based models are used as they account for the complexity of interactions. In this study, we addressed the joint simulation of water- and energy fluxes and the potential benefit of flux measurements in the parameter estimation process. For this purpose, we applied the hydrological model GEOtop to a prealpine catchment in southern Germany (River Rott, 55 km2) over two recent summer episodes, as a test case. Due to its complexity, the model is computationally demanding and only a limited number of forward runs can be afforded in inverse modelling and parameter estimation. We applied the gradient-based nonlinear Gauss-Marquardt-Levenberg (GML) parameter estimation method and linked the GEOtop model to the Parameter ESTimation tool (PEST). Using this developed GEOtop-PEST interface, we particularly investigated the value added by including turbulent flux data in the parameter estimation process, and analyse the impact of the additional flux data on the uncertainty bounds of the parameters. To better understand the interplay of the model parameters and to identify the dominating parameters in the calibration process, we also conducted a Principal Component Analysis (PCA). We were able to identify a set of model parameters that reproduced both observed streamflow and turbulent heat fluxes reasonably well. The majority of the estimated parameters were highly sensitive to the considered variables. We showed that the confidence bounds of estimated parameters are narrowed significantly when considering not only streamflow observations but also turbulent flux measurements in the calibration process. In this manner, correlations between estimated parameters could also be reduced.

Improved Spatiotemporal Representativeness and Bias Reduction of Satellite‐Based Evapotranspiration Retrievals via Use of In Situ Meteorology and Constrained Canopy Surface Resistance

AbstractEvapotranspiration (ET) is a key component of the atmospheric and terrestrial water and energy budgets. Satellite‐based vegetation index approaches have used remotely sensed vegetation and reanalysis meteorological properties with surface energy balance models to estimate global ET (MOD16 ET). We reconstructed satellite retrievals using in situ meteorology (Argonne‐ET) and evaluated them using a dense network of surface turbulent heat flux measurements. Argonne‐ET resolves spatial heterogeneity of ET across the U.S. Southern Great Plains that is not well characterized by MOD16; MOD16 ET exhibits spatial autocorrelation (Moran's I significantly &gt;0 in May–Aug.), whereas in situ and Argonne‐ET exhibit characteristics of a random spatial process (Moran's I not significantly different from 0). The skill in resolving ET temporal variability is not significantly different between MOD16 and Argonne‐ET (correlation coefficient = 0.75 and 0.72, respectively). However, the root‐mean‐square errors were significantly lower for Argonne‐ET (36 W/m2) than MOD16 (43 W/m2), and MOD16 exhibits substantial bias in annual ET relative to in situ measurements (−38%). This is attributed to overestimation in the dry canopy surface resistance (rs) parameterization. Using rs constrained to the range of typical measured values, Argonne‐ET substantially reduces the bias in the annual ET (+1%). The improved ET estimates are critical for regional water budget analyses. The methodology presented herein also demonstrates the ability to retrieve high temporally resolved (30 min; cf. 8‐day MOD16) ET that can be used for development of processed‐based diagnostics of model biases and to elucidate avenues to improve ET model parameterizations.

Estimating the storage term in eddy covariance measurements: the ICOS methodology

Abstract In eddy covariance measurements, the storage flux represents the variation in time of the dry molar fraction of a given gas in the control volume representative of turbulent flux. Depending on the time scale considered, and on the height above ground of the measurements, it can either be a major component of the overall net ecosystem exchange or nearly negligible. Instrumental configuration and computational procedures must be optimized to measure this change at the time step used for the turbulent flux measurement. Three different configurations are suitable within the Integrated Carbon Observation System infrastructure for the storage flux determination: separate sampling, subsequent sampling and mixed sampling. These configurations have their own advantages and disadvantages, and must be carefully selected based on the specific features of the considered station. In this paper, guidelines about number and distribution of vertical and horizontal sampling points are given. Details about suitable instruments, sampling devices, and computational procedures for the quantification of the storage flux of different GHG gases are also provided.

Sensitivity of surface drag predictions using Monin–Obukhov similarity theory in non-ideal flow conditions

Turbulent flux measurements in the atmospheric surface layer (ASL), acquired from a vertical tower of sonic anemometers, are used to investigate the transient behavior of the drag coefficient over a relatively flat, homogeneous, desert playa surface. The goal of the study is to quantify the extent to which Monin–Obukhov similarity theory (MOST) successfully predicts the empirical drag coefficient when the data are not necessarily restricted to idealized conditions. Here, “idealized” refers to the conditions under which MOST is assumed to be valid, namely that the ASL exhibits statistically stationary behavior and stresses are constant with height. In order to quantify the agreement between the empirical and theoretical drag coefficients, uncertainty bands as a function of time of day were calculated for each. These uncertainty bands account for random errors and nonstationarity effects in the momentum flux, horizontal wind velocity, and stability parameter, as well as uncertainties associated with the model parameters appearing in MOST, i.e., the von Karman constant and aerodynamic roughness length. Results show that the average uncertainties in the empirical and theoretical drag coefficients are about $$\pm \,30\%$$ and $$\pm \,20\%$$ , respectively. In the case of the former, the major contribution lies from uncertainty in the momentum flux measurements; while, the latter is dominated by uncertainty in the von Karman constant. Joint probability density functions reveal that large differences between the empirical and theoretical drag coefficient have a high probability of occurrence when the ASL is nonstationary and/or exhibits a non-constant stress layer.