The air density correction to eddy flux measurements

Under the usual assumptions for the atmospheric surface layer, we show that air density fluctuations, particularly those due to temperature fluctuations associated with a heat flux, result in a small mean vertical wind velocity. Because of this, there can be a significant correction to eddy flux measurements of passive scalars, for example CO₂, whose average concentration is very large compared to concentration fluctuations associated with the eddy flux.     

A novel ozone sensor for direct eddy flux measurements

A small, lightweight (1.5 kg) and fast-response ozone sensor for direct eddy flux measurements has been built. The basis for detection is the chemiluminescence of an organic dye adsorbed on dry silica gel in the reaction with ozone. The chemiluminescence is monitored with a cheap and small blue-sensitive photomultiplier. At a flow rate of 100 l min^-1 the ozone sensor has a 90% response time of significantly better than 0.1 s with a detection limit lower than 50 ppt at S/N=3. There are no interferences from other atmospheric trace gases like NO_x, H₂O₂ and PAN. Water vapour and SO₂ enhance the chemiluminescence efficiency of the ozone sensor. Since their response times are 22 seconds and 30 minutes, respectively, no correlation between rapid ozone fluctuations and those of these two trace gases is noticed by the ozone sensor when operating at a frequency of 10 Hz.The ozone sensor was tested for several weeks in continuous measurements of ozone fluxes and deposition velocities over different croplands using the eddy correlation technique. Good agreement was found between ozone dry deposition velocities derived from profile measurements and by eddy correlation.     

Experimental study of the atmospheric marine boundary layer from in-situ aircraft measurements (TOSCANE-T Campaign): Variability of boundary conditions and eddy flux parameterization

The need for a thorough knowledge of boundary conditions over the vast oceanic surfaces makes remote sensing appear as the most suitable tool for future development. Remote sensing techniques require calibration data and consequently in-situ experiments to yield those data. Over the sea, aircraft seem to be the most convenient means of conducting validation experiments because they allow exploration of a large range of scales (from local measurements to measurements of pixel scale). This paper reports on some in-situ atmospheric aircraft measurements that were conducted as part of the TOSCANE-T experiment organized by the European Space Agency to calibrate the scatterometer for launch on ERS-1 in the early 1990's for global ocean wind measurement.The data analysed are the thermodynamic and turbulent variables measured by an instrumented aircraft, the Hurel Dubois, flying at a constant level of 50 m over the sea surface. Special attention was drawn to the variability of the boundary conditions within areas of 25 × 30 km. In fact, the two-dimensional fields of fluxes and thermodynamic parameters were inhomogeneous with some rather strong wind distortion.The eddy fluxes were parameterized with the aid of bulk aerodynamic formulations at a basic scale of 30 km samples, which corresponds to the aircraft flux computation legs. The bulk aerodynamic coefficients for momentum and heat were found to remain independent of windspeed (for wind velocities less than 12 m/s). Fluxes and thermodynamic parameters were also investigated at two other scales: the integration scale was either reduced to small areas of which size always remains larger than several characteristic lengths of turbulent transfer, or extended to a large area of 25 × 30km. The results of the bulk aerodynamic relationships appeared to be scale invariant, which would therefore justify the application of average values, within the range of scales under study.     

The effect of water vapour broadening on methane eddy correlation flux measurements

A high resolution tunable diode laser absorption spectrometer (TDLAS) was used to measure the broadening effect of water vapor and other gases (dry air, nitrogen, oxygen, hydrogen and helium) on three methane lines in the v₄ fundamental. The effects on methane eddy correlation flux measurements amount to a few percent for the least broadened line for expected H₂O fluxes, to 10% for the most broadened line for higher H₂O and lower CH₄ fluxes likely to be encountered. The broadening coefficients of methane measured for air, N₂, O₂, and He are in good agreement with recently published values.     

Impact of density fluctuations on flux measurements of trace gases: Implications for the relaxed eddy accumulation technique

A simple and fast approach to determine when density fluctuations are non-negligible in the calculation of the flux of trace gases (F _c ) is proposed. The correction (F _c – F _c (raw)), when expressed as the percentage of the flux, is dependent on the ratio of background concentration of the trace gas over its flux (% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaeikaiabeg% 8aYnaaBaaaleaacaWGJbaabeaakiaab+cacaWGgbWaaSbaaSqaaiaa% dogaaeqaaOGaaeykaaaa!3CBC!\[{\rm{(}}\rho _c {\rm{/}}F_c {\rm{)}}\], on the partitioning of available energy between sensible (F _T ) and latent (F _v ) heat fluxes, and on the flux measuring system. An increase from 100 to 200 W m^-2 in available energy and from 0 to 20% in F _T /(F _T + F _v ) led to a threefold reduction in the required value of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaa0aaaeaacq% aHbpGCdaWgaaWcbaGaam4yaaqabaaaaOGaai4laiaadAeadaWgaaWc% baGaam4yaaqabaaaaa!3B6D!\[\overline {\rho _c } /F_c \] to have a density correction of 10%. A trace gas with a % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaWaaqWaceaaca% WGgbWaaSbaaSqaaiaadogaaeqaaaGccaGLhWUaayjcSdGaai4lamaa% naaabaGaeqyWdi3aaSbaaSqaaiaadogaaeqaaaaaaaa!3E91!\[\left| {F_c } \right|/\overline {\rho _c } \] value above 0.014 m s^-1 has a density correction on flux of less than 10%, for even the worst case scenario. Values of % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaamOramaaBa% aaleaacaWGJbaabeaakiaac+cadaqdaaqaaiabeg8aYnaaBaaaleaa% caWGJbaabeaaaaaaaa!3B6D!\[F_c /\overline {\rho _c } \] for several trace gases computed from typical situations show that the fluxes of N₂O, NO, CO₂, CH₄ and O₃ need to be corrected, while those of pesticides and volatile organic compounds, for example, do not. The corrections required with the newly developed relaxed eddy accumulation technique are discussed and equation development is shown for two sampling systems.Land Resource Research Centre Contribution No 91-61.     

Mean flux estimation and cospectra of airborne carbon dioxide and water vapour eddy flux measurements in the planetary surface layer

CO₂ and H₂O eddy-flux measurements derived from airborne vertical wind and concentration data are examined to determine the accuracy of the eddy-flux estimate in moderately unstable conditions within the surface layer. Integral scale estimation is used to determine the minimum length of a sample required to achieve a given accuracy. Cospectral analysis is used to examine the scales responsible for the transport of water vapour and carbon dioxide in the surface layer. Cospectra are found to broaden as the sampling altitude increases, a well known result. At low altitudes, the cospectrum is found to become negative for large scales. A significant fraction of the flux is carried by this range of scales, which suggests that scales larger than the boundary-layer height may play an important role in the transport at low altitudes.     

Airborne measurements of the vertical flux of ozone in the boundary layer

A fast-response chemiluminescent ozone sensor was mounted in an aircraft instrumented for air motion and temperature measurements. Measurements of the vertical flux of ozone by the eddy correlation technique were obtained after correcting for time delay and pressure sensitivity in the ozone sensor output. The observations were taken over eastern Colorado for two days in April, one a morning and the other an afternoon flight. Since the correlation coefficient of ozone and vertical velocity is small compared to, for example, temperature and vertical velocity in the lower part of the convective boundary layer, an averaging length of the order of 100 km was required to obtain a reasonably accurate estimate of the ozone flux. The measured variance of ozone appeared to be too large, probably mainly due to random noise in the sensor output, although the possibility of the production of ozone fluctuations by chemical reactions cannot be dismissed entirely. Terms in the budget equation for ozone were estimated from the aircraft measurements and the divergence of the ozone flux was found to be large compared to the flux at the surface divided by the boundary-layer height.Part of this work was carried out while a visitor at Risø National Laboratory, Denmark.The National Center for Atmospheric Research is sponsored by the National Science Foundation.     

Turbulent energy flux and eddy diffusivity

The paper deals with the role of the vertical flux of turbulent energy in the atmospheric boundary layer. The influence of the divergence of this flux on the vertical wind profile and on the eddy diffusivity is shown by theoretical considerations.     

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

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

Tests of a robust eddy correlation system for sensible heat flux

Summary Sensible heat flux estimates from a simple, one-propeller eddy correlation system (OPEC) were compared with those from a sonic anemometer eddy correlation system (SEC). In accordance with similarity theory, the performance of the OPEC system improved with increasing height of the sensor above the surface. Flux totals from the two systems at sites with adequate fetch were in excellent agreement after frequency response corrections were applied. The propeller system appears suitable for long periods of unattended measurement. The sensible heat flux measurements can be combined with net radiation and soil heat flux measurements to estimate latent heat as a residual in the surface energy balance.With 6 Figures     

A model for eddy diffusivity in a stable boundary layer

Expressions for the vertical and the lateral diffusivity coefficients were derived from the Local Similarity Theory and the Statistical Diffusion Theory. For such, the spectral density energies for the turbulent velocities were used. The expressions here derived are compared with the diffusivity coefficients for momentum and heat suggested by Sorbjan (from the Minnesota experiments) and Nieuwstadt (from the Cabauw experiments). This comparison allows us to conclude that turbulence is equally efficient in transporting momentum, heat and contaminants in an ideally stable boundary layer.     

Solar ultraviolet flux measurements

Solar ultraviolet fluxes were measured with a ¼> ‐metre Ebert spectrometer flown as part of an Atmospheric Environment Service balloon payload from Fort Churchill on 8 July 1974. The spectral region from 1820 to 3150Å was scanned each 7.5 s with 1.5Å resolution from 1256 to 0025 UT on 10 July. These measurements covered a range of solar zenith angles from 37° (noon) to 69° with the balloon initially at a float altitude of 28.6 km. Fluxes in the window region 1900 to 2200Å were analyzed to determine ozone content above the balloon. Values from 0.069 to 0.11 cm NTP were obtained at heights over a range from 28.6 to 25.3 km.     

Correcting airborne flux measurements for aircraft speed variation

Airplane aerodynamic characteristics correlate aircraft speed with vertical wind velocity, making the time average inappropriate for estimating the ensemble average in airborne eddy-correlation flux computations. The space average, the proper form, is implemented as a time integral by a transformation of variables, which can be interpreted as a ground-speed correction to the time average. The mathematical forms are presented, and the importance of the speed correction is illustrated with airborne data. The computed correction is found to be highly variable, depending on both the turbulent flow encountered and the aircraft used. In general, the speed connection becomes more important as airplane size is reduced. For a small, single-engine Long-EZ airplane, used as an example, the straight time average erred, half the time, by 12%, 10%, 20%, and 15%, respectively, for computed fluxes of momentum, heat, moisture, and CO₂. For a much heavier Twin Otter airplane, also used as an example, the straight time average erred, half the time by only 1%. These errors increased with decreasing altitude for the Long-EZ and with increasing altitude for the Twin Otter.Oak Ridge Associated Universities, assigned to NOAA/ATDD.     

Coupled ocean-atmosphere models with flux correction

A method is proposed for removing the drift of coupled atmosphere-ocean models, which in the past has often hindered the application of coupled models in climate response and sensitivity experiments. The ocean-atmosphere flux fields exhibit inconsistencies when evaluated separately for the individual sub-systems in independent, uncoupled mode equilibrium climate computations. In order to balance these inconsistencies a constant ocean-atmosphere flux correction field is introduced in the boundary conditions coupling the two sub-systems together. The method ensures that the coupled model operates at the reference climate state for which the individual model subsystems were designed without affecting the dynamical response of the coupled system in climate variability experiments. The method is illustrated for a simple two component box model and an ocean general circulation model coupled to a two layer diagnostic atmospheric model.     

Impacts of bias correction of lateral boundary conditions on regional climate projections in West Africa

Biases existing in the lateral boundary conditions (LBCs) influence climate simulations in regional climate models (RCMs). Correcting the biases in global climate model (GCM)-produced LBCs before running RCMs was proposed in previous studies as a possible way to reduce the GCM-related model dependence of future climate projections using RCMs. In this study the ICTP Regional Climate Model Version 4 (RegCM4) is used to investigate the impact of LBC bias correction on projected future changes of regional climate in West Africa. To accomplish this, two types of present versus future simulations are conducted using RegCM4: a control type where both the present and future LBCs are derived directly from the GCM output (as is done in most regional climate downscaling studies); an experiment type where the present-day LBCs are from reanalysis data and future LBCs are derived by combining the reanalysis data and the GCM-projected LBC changes. For each type of simulations, three different sets of LBCs are experimented on: 6-hourly synoptic forcing directly from the reanalysis or GCM, 6-hourly data interpolated from monthly climatology (without diurnal cycle), and 6-hourly data interpolated from the month-specific climatology of diurnal cycles. It is found that the simulations using different LBCs produce similar present-day summer rainfall patterns, but the predicted future changes differ significantly depending on how the LBC bias correction is treated. Specifically, both the bias correction applied at the synoptic scale and the bias correction applied to the monthly interpolated LBCs without diurnal cycle produce a spurious drying signal caused by physical inconsistency in the corrected future LBCs. Interpolated monthly LBCs with diurnal cycle alleviate the problem to a large extent. These results suggest that using bias-corrected LBCs to drive regional climate models may not guarantee reliable future projections although reasonable present climate can be simulated. Physical inconsistencies may be contained in the bias-corrected LBCs, increasing the uncertainties of RCM-produced future projections.     

Airborne measurements of turbulent trace gas fluxes and analysis of eddy structure in the convective boundary layer over complex terrain

Using the new high-frequency measurement equipment of the research aircraft DO 128, which is described in detail, turbulent vertical fluxes of ozone and nitric oxide have been calculated from data sampled during the ESCOMPTE program in the south of France. Based on airborne turbulence measurements, radiosonde data and surface energy balance measurements, the convective boundary layer (CBL) is examined under two different aspects. The analysis covers boundary-layer convection with respect to (i) the control of CBL depth by surface heating and synoptic scale influences, and (ii) the structure of convective plumes and their vertical transport of ozone and nitric oxides. The orographic structure of the terrain causes significant differences between planetary boundary layer (PBL) heights, which are found to exceed those of terrain height variations on average. A comparison of boundary-layer flux profiles as well as mean quantities over flat and complex terrain and also under different pollution situations and weather conditions shows relationships between vertical gradients and corresponding turbulent fluxes. Generally, NO_x transports are directed upward independent of the terrain, since primary emission sources are located near the ground. For ozone, negative fluxes are common in the lower CBL in accordance with the deposition of O₃ at the surface.The detailed structure of thermals, which largely carry out vertical transports in the boundary layer, are examined with a conditional sampling technique. Updrafts mostly contain warm, moist and NO_x loaded air, while the ozone transport by thermals alternates with the background ozone gradient. Evidence for handover processes of trace gases to the free atmosphere can be found in the case of existing gradients across the boundary-layer top. An analysis of the size of eddies suggests the possibility of some influence of the heterogeneous terrain in mountainous area on the length scales of eddies.     

Direct measurements of radiative and turbulent flux convergences in the lowest 1000 m of the convective boundary layer

Results derived from simultaneous measurements of turbulent heat flux and radiation convergence in the daytime convective boundary layer are presented. It is found that the effects of long-wave radiation result in a warming near the surface and cooling at higher levels, in good agreement with infra-red radiative transfer models. Heating rates, roughly 30% of those produced by turbulence, are observed as a result of the absorption of short-wave radiation in the lowest 1000 m of the atmosphere.