Eddy Correlation Flux Measurements Research Articles

Meteorological and geological controls on dust devil activity: Initial results from a field study at Smith Creek Valley, Nevada, USA

We present initial results of an investigation into meteorological and geological controls on the formation of dust devils (i.e., dust-filled vortices formed in the daytime dry convective boundary layer). During a 2-week field campaign in June 2019 at Smith Creek Valley (SCV), Nevada, USA, we conducted automated time-lapse stereo imaging of dust devils (DDs), while monitoring local meteorological conditions with a broad suite of instruments. Counts of imaged dust devils from two near-cloudless days were compared with a standard suite of atmospheric measurements from a weather tower, eddy correlation flux measurements, and ceilometer backscatter returns. DDs forming in moderate winds (5–8.5 m/s) were more likely to be relatively wide and disorganized, with qualitatively low dust opacity, whereas those forming in weaker winds were more likely to be coherent, dusty, well-formed conical or cylindrical structures. The daily maximum DD counts at SCV occurred shortly after their onset in late morning (11:00–12:00 local time), coinciding with a surge in CBL growth that was likely delayed by the thermal properties of the playa. This late morning peak contrasts with previous studies conducted elsewhere that typically observed peak DD counts in the afternoon. As observed in previous field studies, DDs formed in highly convective conditions, when the heat flux (H) and friction velocity (u∗) were elevated and the convective ratio w∗/u∗ exceeded ∼ 4 (i.e., -h/L exceeded ∼ 25). However, values of>4w∗/u∗ also occurred in mid-morning, prior to DD formation and CBL growth, suggesting that this metric is not the sole condition required for DD generation. Aside from the late morning maxima, DD counts fluctuated considerably throughout the afternoon at timescales of 0.5–2 h–correlating poorly with fluctuations in H and u∗, and not at all with either w∗/u∗ or the Monin-Obukhov length (L). Several factors, such as local variations in surface thermal properties and meteorology, may be responsible for these short-term fluctuations.

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.

Mixing efficiency in the thermocline of lakes observed from eddy correlation flux measurements

AbstractVertical mixing in the thermocline of lakes is poorly understood and most of the current knowledge is based on ex situ methods like laboratory measurements and simulations. Here we used the eddy correlation technique (EC) to directly measure oxygen and buoyancy fluxes in the thermocline of two lakes (Lake Scharmützelsee and Lake Arendsee in 2012 and 2013, respectively). Additionally, sets of temperature microstructure profiles (SCAMP) were measured during the EC deployments. We used these data to quantify the mixing efficiency as well as the turbulent diffusivity. The derived turbulent diffusivities from EC for the Prandtl number of DO were one order of magnitude higher than predicted by commonly applied parameterization, while the diffusivities for the Prandtl number of heat confirmed the parameterization. The results from EC and SCAMP showed strong differences which we attribute to the fact that SCAMP measurements reflect snapshots of the instantaneous turbulence field while EC provides a temporal average of the prevailing turbulence. Finally, we discuss problems of the EC and the inertial dissipation method in a strongly stratified environment and propose how they could be improved to resolve the full temporal variability of mixing in thermoclines.

An Assessment of the Precision and Confidence of Aquatic Eddy Correlation Measurements

AbstractThe quantification of benthic fluxes with the aquatic eddy correlation (EC) technique is based on simultaneous measurement of the current velocity and a targeted bottom water parameter (e.g., O2, temperature). High-frequency measurements (64 Hz) are performed at a single point above the seafloor using an acoustic Doppler velocimeter (ADV) and a fast-responding sensor. The advantages of aquatic EC technique are that 1) it is noninvasive, 2) it integrates fluxes over a large area, and 3) it accounts for in situ hydrodynamics. The aquatic EC has gained acceptance as a powerful technique; however, an accurate assessment of the errors introduced by the spatial alignment of velocity and water constituent measurements and by their different response times is still needed.Here, this paper discusses uncertainties and biases in the data treatment based on oxygen EC flux measurements in a large-scale flume facility with well-constrained hydrodynamics. These observations are used to review data processing procedures and to recommend improved deployment methods, thus improving the precision, reliability, and confidence of EC measurements. Specifically, this study demonstrates that 1) the alignment of the time series based on maximum cross correlation improved the precision of EC flux estimations; 2) an oxygen sensor with a response time of <0.4 s facilitates accurate EC fluxes estimates in turbulence regimes corresponding to horizontal velocities < 11 cm s−1; and 3) the smallest possible distance (<1 cm) between the oxygen sensor and the ADV’s sampling volume is important for accurate EC flux estimates, especially when the flow direction is perpendicular to the sensor’s orientation.

A sea spray aerosol flux parameterization encapsulating wave state

Abstract. A new sea spray source function (SSSF), termed Oceanflux Sea Spray Aerosol or OSSA, was derived based on in-situ sea spray aerosol measurements along with meteorological/physical parameters. Submicron sea spray aerosol fluxes derived from particle number concentration measurements at the Mace Head coastal station, on the west coast of Ireland, were used together with open-ocean eddy correlation flux measurements from the Eastern Atlantic Sea Spray, Gas Flux, and Whitecap (SEASAW) project cruise. In the overlapping size range, the data for Mace Head and SEASAW were found to be in a good agreement, which allowed deriving the new SSSF from the combined dataset spanning the dry diameter range from 15 nm to 6 μm. The OSSA source function has been parameterized in terms of five lognormal modes and the Reynolds number instead of the more commonly used wind speed, thereby encapsulating important influences of wave height, wind history, friction velocity, and viscosity. This formulation accounts for the different flux relationships associated with rising and waning wind speeds since these are included in the Reynolds number. Furthermore, the Reynolds number incorporates the kinematic viscosity of water, thus the SSSF inherently includes dependences on sea surface temperature and salinity. The temperature dependence of the resulting SSSF is similar to that of other in-situ derived source functions and results in lower production fluxes for cold waters and enhanced fluxes from warm waters as compared with SSSF formulations that do not include temperature effects.

The effects of spatial and temporal variability at the sediment surface on aquatic eddy correlation flux measurements

The eddy correlation technique has become a widely used approach to measure ecosystem scale fluxes in aquatic benthic environments; however, there are few theoretical studies designed to guide deployment planning, the characterization of flux variability, or how to correctly interpret measured fluxes. Here, a three‐dimensional numerical model for the turbulent transport and mixing in the near‐bottom water was used to examine how well variations in vertical flux due to heterogeneities in the benthic community are integrated in eddy correlation measurements. The results showed that an appropriate choice of measuring height above the sediment surface is crucial to obtain an accurate average of the vertical flux from heterogeneous benthic communities. Through regression analysis of modeling results, a set of simple analytical expressions were derived for this threshold measuring height as a function of two parameters: heterogeneity patch size and sediment surface roughness. The same model was also used to examine the time lag between when a change in vertical flux occurs at the sediment surface and when it reaches the measuring point. The results showed that this response time varies by several orders of magnitude depending on flow velocity, sediment surface roughness, and measuring height. Simple analytical expressions were also derived for the response time. Overall, the findings indicate that both the spatial variability in benthic communities and the response to temporal changes are important factors that should be considered in planning eddy correlation measurements and interpreting eddy correlation fluxes.

FLOHOF 2007: an overview of the mesoscale meteorological field campaign at Hofsjökull, Central Iceland

The FLOHOF field campaign took place in the period July 21 to August 24, 2007 on and in the surroundings of Hofsjokull glacier in Central Iceland. During the campaign, 18 automatic weather stations (AWS) recording temperature, humidity, wind speed, wind direction, pressure, and precipitation were deployed on and around the glacier. In addition, atmospheric soundings were performed N and S of Hofsjokull by a tethered balloon, pilot balloons, and two unmanned aerial systems (UAS). An energy balance station, consisting of a net radiometer and an eddy correlation flux measurement station, has also been installed. This paper describes the experimental setup of the campaign and presents first results of the data analysis with respect to transience of mountain-induced gravity waves, the extension of katabatic winds into the surrounding of the glacier, the occurrence of katabatic microfronts, and report on novel approaches to probe the vertical structure of the atmospheric boundary layer by UAS. The observed pressure perturbations related to transient gravity wave activity due to changing inflow conditions were between −2 and 2 hPa in general, with positive values upstream and negative values downstream. Differential heating of the glacier and its surrounding is triggering daytime katabatic flow from the glacier into its surrounding. During the campaign, those katabatic winds typically reached out 4–7 km from the edge of the glacier. During late night in clear sky conditions, frontal-like microstructures have been observed frequently with typical repetition times in the order of 30–60 min indicating the interaction of large-scale synoptic and nighttime katabatic density flows close to the ground. The first research application of the newly developed small unmanned meteorological observer proved the applicability of the system for atmospheric boundary layer research by successfully profiling the atmosphere up to 3.5 km above ground.

Simulation of Evapotranspiration and Its Response to Plant Water and CO2 Transfer Dynamics

Abstract Evapotranspiration (ET) is controlled by atmospheric demand, plant and soil hydraulic constraints, and the plant physiological activities that determine canopy resistance. This paper introduces a new ET scheme developed for the Ecological Assimilation of Land and Climate Observations (EALCO) model that integrates these controls into one dynamic system. This scheme is based on solving the governing equation system that represents the coupled canopy energy–water–CO2 transfer dynamics, where the canopy temperature Tc, plant water potential ψc, and leaf intercellular CO2 concentrations Ci are simultaneously obtained and used in ET calculations. Modeled ET was compared with eddy correlation flux measurement at a boreal aspen forest. Results showed that the correlation coefficient (R) between modeled and measured daily ET was greater than 0.96. The average absolute error was approximately 0.3 mm day−1. Modeled ET was generally higher than measured ET by 10%. This is consistent with the energy balance closure analyses from observations that showed that turbulent energy flux was frequently less than 90% of the total available energy. The effects of the plant CO2 and water transfer dynamics on ET simulations were investigated by running the model in two additional settings. These were 1) static Ci—where the ratio of Ci to atmospheric CO2 concentration was set to a constant value, and 2) static ψc—where the ψc was linearly related to soil water potential. The dynamic CO2 transfer scheme and the static Ci scheme produced relatively small differences in ET that mainly occurred at a subdaily time scale. Differences in ET produced using the dynamic water transfer scheme and the static ψc scheme depended on ecosystem water conditions and were more significant when the plant was under water stress. Ignoring the dynamic water transfer process in the model decreased the correlation coefficient between modeled and measured ET more significantly in drier years. This implies that the dynamic water transfer scheme is of more importance for ET estimates in arid or semiarid ecosystems.

Eddy correlation flux measurements: The sediment surface area that contributes to the flux

We investigated the size and shape of the area on the sediment surface, the so‐called footprint, that contributes to the flux in subaqueous eddy correlation measurements. Tracer tracking simulations were performed for a dissolved conservative tracer released from the sediment surface into a current‐driven flow not affected by density stratifications and surface waves. Simulations revealed that the footprint length (l) can be calculated as l = −2.783 − 158.7h + 159.2h2 − 120.8h log(z0) (all units in m) for eddy correlation measurements heights (h) between 0.05 and 0.3 m above the sediment surface and for sediment surface roughness parameter (z0) values between 7.04 × 10−6 and 0.01 m. The upstream distance (xmax) to the location that contributes the strongest flux signal can likewise be estimated as xmax = −0.09888 − 11.53h + 10.25h2 ‐ 6.650h log (z0). Because vertical turbulent mixing scales with mean current velocity, l and xmax are independent of current velocity. The footprint width (w) can be calculated as w = 6.531h. These expressions were developed for water depths (H) of H > 27h. In the depth interval 6.7h > H > 27h, l can be calculated by multiplying the length, as given above, by the factor 1 + 8.347exp(−0.2453 H/h), whereas xmax is independent of H. For H > 6.7h, the tracer transfer rate over the air‐water interface controls the size and shape of the footprint. All expressions are valid for isotropic turbulence, but as a first‐order estimate, the expressions for l and xmax also hold for anisotropic conditions. In contrast, w scales with √(Ey/Ez), where Ey and Ez are the transverse and the vertical eddy diffusivity, respectively. Finally, we describe how site‐specific values of z0 and levels of anisotropy in a turbulent near‐bottom flow can be extracted directly from eddy correlation measurements.

Airborne aerosol flux measurements with eddy correlation above the ocean in a coastal environment

This work is a first pilot study of particle eddy correlation flux measurements from an aircraft using a condensation particle counter. The study reports aerosol flux measurements from an airplane conducted at approximately 33 m altitude above the ocean in the coastal region of Monterey Bay. The systematic and random uncertainties of such measurements are evaluated. Covariance was calculated for 200 s time series sampled at 10 Hz at 50 m s - 1 airspeed. Systematic and random uncertainties of flux measurements errors are originated from turbulence measurements errors, counter slow response time and particle counting statistics. The flux error was found to be on order of 0.1– 0.3 × 10 6 particles m - 2 s - 1 , for ambient aerosol concentration in the range of 1000–1500 particles cm - 3 . The direct effect of oceanic emissions (sea-salt or organic aerosol) on measured local aerosol fluxes was minimal in the studied coastal region due to ship emitted abundant anthropogenic particles.

High precision measurements of atmospheric nitrous oxide and methane using thermoelectrically cooled mid-infrared quantum cascade lasers and detectors

A compact, fast response, mid-infrared absorption spectrometer using thermoelectrically (TE) cooled pulsed quantum cascade (QC) lasers and TE detectors has been developed to demonstrate the applicability of QC lasers for high precision measurements of nitrous oxide and methane in the earth’s atmosphere. Reduced pressure extractive sampling with a 56 m path length, 0.5 l volume, multiple pass absorption cell allows a time response of <0.1 s which is suitable for eddy correlation flux measurements for these gases. Precision of 0.3 ppb (rms, 1 s averaging time) or 0.1% of the ambient concentration for N 2O (4 ppb or 0.2% of ambient for CH 4), has been demonstrated using QC lasers at 4.5 μm (7.9 μm for CH 4), corresponding to an absorbance precision of 4×10 −5 Hz −1/2 (8×10 −5 Hz −1/2 for CH 4). Stabilization of the temperature of the optical bench and the pulse electronics results in a minimum Allan variance corresponding to 0.06 ppb for N 2O with an averaging time of 100 s (0.7 ppb with an averaging time of 200 s for CH 4). The instrument is capable of long-term, unattended, continuous operation without cryogenic cooling of either laser or detector.

Modelling carbon dynamics of boreal forest ecosystems using the Canadian land surface scheme

The ecosystem carbon (C) and nitrogen (N) simulations recently implemented in the Canadian Land Surface Scheme (CLASS) are presented. The main calculations include plant photosynthesis, autotrophic respiration, root N uptake, litterfall, plant growth, and soil heterotrophic respiration. Model experiments are made on two boreal forest ecosystems, deciduous (aspen) and conifers (black spruce). Simulated plant, soil, and ecosystem CO2 exchanges are analysed on half-hourly, daily, and annual time scales and compared with tower eddy correlation flux measurements and estimates from various authors. Modeled daily ecosystem CO2 exchange explained86% and 54%, respectively, of the observed variance of eddy correlationflux at the aspen site and at the black spruce site. Annual results show that the aspen ecosystem was simulated as a C sink in both 1994 (+164 g Cm−2) and 1996 (+142 g C m−2), and the black spruce ecosystem wassimulated as a C sink in 1994 (+39 g C m−2) and 1995 (+25 g Cm−2), but as a C source in 1996 (–27 g C m−2).

Sampling error in eddy correlation flux measurements

Sampling errors in eddy correlation flux measurements arise from the small number of large eddies that dominate the flux during typical sampling periods. Several methods to estimate sampling, or random error in flux measurements, have been published. These methods are compared to a more statistically rigorous method which calculates the variance of a covariance when the two variables in the covariance are auto‐ and cross‐correlated. Comparisons are offered between the various methods. Compared to previously published methods, error estimates from this technique were 20 to 25% higher because of the incorporation of additional terms in the estimate of the variance. This new approach is then applied to define the random error component of representative eddy correlation flux measurements of momentum, sensible and latent heat, carbon dioxide, and ozone from five field studies, three over agricultural crops (corn, soybean, and pasture), and two from towers over forests (deciduous and mixed). The mean normalized error for each type of flux measurement over the five studies ranged from 12% for sensible heat flux to 31% for ozone flux. There were not large or significant differences between random errors for fluxes measured over crops versus those measured over forests. The effects of stability, flux magnitude, and wind speed on measurement error are discussed.

10.2172/1020274

The eddy correlation (ECOR) flux measurement system provides in situ, half-hour measurements of the surface turbulent fluxes of momentum, sensible heat, latent heat, and carbon dioxide (CO2) (and methane at one Southern Great Plains extended facility (SGP EF) and the North Slope of Alaska Central Facility (NSA CF). The fluxes are obtained with the eddy covariance technique, which involves correlation of the vertical wind component with the horizontal wind component, the air temperature, the water vapor density, and the CO2 concentration.

10.2172/1245980

A Surface Energy Balance System (SEBS) has been installed collocated with each deployed Eddy Correlation Flux Measurement System (ECOR) at the Atmospheric Radiation Measurement (ARM) Climate Research Facility’s Southern Great Plains (SGP) site, North Slope of Alaska (NSA) site, first ARM Mobile Facility (AMF1), second ARM Mobile Facility (AMF2), and third ARM Mobile Facility (AMF3) at Oliktok Point (OLI). A SEBS was also deployed with the Tropical Western Pacific (TWP) site, before it was decommissioned. Data from these sites, including the retired TWP, are available in the ARM Data Archive. The SEBS consists of upwelling and downwelling solar and infrared radiometers within one net radiometer, a wetness sensor, and soil measurements. The SEBS measurements allow the comparison of ECOR sensible and latent heat fluxes with the energy balance determined from the SEBS and provide information on wetting of the sensors for data quality purposes.

Test of a modified Shuttleworth–Wallace estimate of boreal forest evaporation

A modified version of the two-layer Shuttleworth–Wallace approach was used to simulate evaporation from a mixed 50–100 years old boreal forest stand in central Sweden. It was tested against eddy correlation flux measurements from the Northern hemisphere climate Processes land-surface Experiment (NOPEX). The soil regulation on canopy resistance was taken into account when estimating transpiration. The generality of the model performance was attained by using model parameters that represented typical boreal forest stand and not the specific stands at the site. The analysis was done for two growing seasons, 1994 and 1995 having quite different weather conditions. Despite the rough parameterisation, the model performance was good. Problems regarding representativity of the measured data used for the model validation are discussed. It was concluded that the model could accurately simulate evaporation components on a seasonal time scale and is suitable to be incorporated as a submodel in larger forest ecosystem models.

Comparison of the spatial and temporal distribution of fluxes of sensible heat, latent heat and CO2 from grid flights in BOREAS 1994 and 1996

Analysis of airborne eddy correlation flux measurements of heat (H), moisture (LE) and CO2 (C) over two 16 km × 16 km heterogeneous grid sites in BOREAS 1994 (IFC‐2) and 1996 are compared in order to examine persistence and variability in the distributions of surface characteristics and fluxes between the two years. The data used were obtained in grid patterns flown at 30 m above ground level, under generally clear sky and thermally unstable conditions. Maps of fluxes and surface characteristics were constructed by block averaging over 2 km windows along the flight lines, analyzed for similarities, and used to quantify spatial variability of the fluxes. Sensitivity analysis suggested minor effects of boundary layer variability and window size on the main features of the source/sink distributions. Incident radiation was more highly correlated with grid‐averaged values of C than with H and LE. The dominant role of surface inhomogeneity, as opposed to local variations in solar energy input, on spatial variation of flux distributions was confirmed, and mesoscale motion was found negligible, probably because of the small sizes of homogeneous subareas with sufficient surface contrast to induce thermally generated motion. CO2 flux and greenness index were highly correlated, but correlation was site‐ and time‐specific. The previously observed low correlation between sensible heat flux and surface minus air temperature difference (Ts‐Ta), primarily over old black spruce, was confirmed. The high Bowen ratio over the forest contributed to the growth and development of the observed deep boundary layers over the sites, but no clear correlation emerged between boundary layer depth and observed near‐surface fluxes.

Eddy correlation fluxes of trace gases using a tandem mass spectrometer

This paper describes a field evaluation of a tandem mass spectrometer (TAGA) for use in measuring turbulence fluxes of trace gases. Measurements were made over a two-day period in a fallow farm field west of Columbus, OH. The results show that for weakly adsorbing species such as acetone, the effective time constant for the TAGA is <0.1 s, and the device can be used for eddy correlation flux measurements with little need for corrections. The effective time constant is longer for adsorbing species, such as ammonia, but spectral corrections can permit flux estimates for these species as well. This paper presents the first reported measurements of fluxes of acetone, formic acid, and ammonia using the eddy correlation technique.

The Lagrangian Stochastic Model for fetch and latent heat flux estimation above uniform and nonuniform terrain

A Lagrangian stochastic model was used to estimate the fetch and latent heat flux above a nonuniform grass‐covered forest clearing site at the Duke Forest, in Durham, North Carolina, and an irrigated bare soil patch at the University of California in Davis. The latent heat flux predictions by the Lagrangian model compared well with eddy correlation flux measurements. In order to apply the Lagrangian model to a nonuniform grass‐covered forest clearing, the surface was treated as an imaginary “equivalent” uniform terrain subjected to surface roughness and turbulence statistics (i.e., mean, variance, and covariance) of velocities and scalars identical to those measured above the nonuniform terrain. At the irrigated bare soil site the equilibrium distance of the air from dry to wet was well defined, and its influence on the water vapor flux internal boundary layer was considered. In the Lagrangian model, five different schemes to account for inhomogeneous turbulent flows were compared in terms of estimating scalar fluxes. Our comparisons demonstrate that the five different schemes produce similar scalar fluxes despite the fact that some of them do not satisfy the well‐mixed criterion. Also, the analytical solution to the advection‐diffusion equation was used to predict the fetch and latent heat flux under neutral conditions and compared to the Lagrangian model.

Latent and sensible heat flux predictions from a uniform pine forest using surface renewal and flux variance methods

A surface renewal model that links organized eddy motion to the latent and sensible heat fluxes is tested with eddy correlation measurements carried out in a 13m tall uniform Loblolly pine plantation in Duke Forest, Durham, North Carolina. The surface renewal model is based on the occurance of ramp-like patterns in the scalar concentration measurements. To extract such ramp-like patterns from Eulerian scalar concentration measurements, a newly proposed time-frequency filtering scheme is developed and tested. The time-domain filtering is carried out using compactly-supported orthonormal wavelets in conjunction with the “Universal Wavelet Thresholding” approach of Donoho and Johnstone, while the frequency filtering is carried out by a band-pass sine filter centered around the ramp-occurrence frequency as proposed by other studies. The method was separately tested for heat and water vapour with good agreement between eddy correlation flux measurements and model predictions. The usefulness of the flux-variance method to predict sensible and latent heat fluxes is also considered. Our measurements suggest that the simple flux-variance method reproduces the measured heat and momentum fluxes despite the fact that the variances were measured within the roughness sublayer and not in the surface layer. Central to the predictions of water vapour fluxes using the flux-variance approach is the similarity between heat and water vapour transport by the turbulent air flow. This assumption is also investigated for this uniform forest terrain.