Monique Y. Leclerc

Redefinition and global estimation of basal ecosystem respiration rate

Basal ecosystem respiration rate (BR), the ecosystem respiration rate at a given temperature, is a common and important parameter in empirical models for quantifying ecosystem respiration (ER) globally. Numerous studies have indicated that BR varies in space. However, many empirical ER models still use a global constant BR largely due to the lack of a functional description for BR. In this study, we redefined BR to be ecosystem respiration rate at the mean annual temperature. To test the validity of this concept, we conducted a synthesis analysis using 276 site-years of eddy covariance data, from 79 research sites located at latitudes ranging from similar to 3 degrees S to similar to 70 degrees N. Results showed that mean annual ER rate closely matches ER rate at mean annual temperature. Incorporation of site-specific BR into global ER model substantially improved simulated ER compared to an invariant BR at all sites. These results confirm that ER at the mean annual temperature can be considered as BR in empirical models. A strong correlation was found between the mean annual ER and mean annual gross primary production (GPP). Consequently, GPP, which is typically more accurately modeled, can be used to estimate BR. A light use efficiency GPP model (i.e., EC-LUE) was applied to estimate global GPP, BR and ER with input data from MERRA (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate resolution Imaging Spectroradiometer). The global ER was 103 Pg C yr (-1), with the highest respiration rate over tropical forests and the lowest value in dry and high-latitude areas.

EXPERIMENTAL EVALUATION OF ANALYTICAL AND LAGRANGIAN SURFACE-LAYER FLUX FOOTPRINT MODELS

Three surface-layer flux footprint models have been evaluated with the results of an SF6 tracer release experiment specifically designed to test such models. They are a Lagrangian stochastic model, an analytical model, and a simplified derivative of the analytical model. Vertical SF6 fluxes were measured by eddy correlation at four distances downwind of a near-surface crosswind line source in an area of homogeneous sagebrush. The mean fluxes were calculated for 136 half-hour test periods and compared to the fluxes predicted by the footprint models. All three models gave similar predictions and good characterizations of the footprint over the stability range -0.01 < z0/L < 0.005. The predictions of the three models were within the limits of the uncertainty of the experimental measurements in all but a few cases within this stability range. All three models are unconditionally recommended for determining the area defined by the footprint over short vegetative canopies in this range. They are also generally appropriate for estimating flux magnitudes within the limits of experimental uncertainties. Most of the mean differences observed between the measured and predicted fluxes at each of the four towers reflect a tendency for the measured fluxes to be greater than those predicted by the three models. Rigorous verification of the models in strongly stable conditions was complicated by the need to obtain very accurate measurements of small fluxes in only marginally stationary conditions. Verification in strongly unstable conditions was hampered by the limited number of appropriate data.

Observations and large‐eddy simulation modeling of footprints in the lower convective boundary layer

Scalar source footprint predictions inside and above the unstable surface layer are performed using a large-eddy simulation (LES) model. LES footprint results were evaluated against a tracer field experiment at one level and against other models beyond the 10 m measurement level in the surface layer. This paper further reports on an extension of the range over which current footprint models can be used by presenting LES calculations of flux mapping to the downwind region from the source location beyond the surface layer. Our LES flux mapping results show a gradual decoupling with increasing distance from the surface. Our results also indicate that the footprints, cumulative fluxes, and concentration distributions calculated using LES in unstable conditions are in general agreement with field observations and with those of other models. The present work shows promise for the use of LES as a tool to model more complex footprint cases such as those over heterogeneous surfaces.

Assessment of the soil CO2 gradient method for soil CO2 efflux measurements: comparison of six models in the calculation of the relative gas diffusion coefficient

This paper uses a refined soil gradient method to estimate soil CO2 efflux. Six different models are used to determine the relative gas diffusion coefficient (ξ ). A weighted harmonic averaging is used to estimate the soil CO2 diffusion coefficient, yielding a better estimate of soil CO2 efflux. The resulting soil CO2 efflux results are then compared to the soil CO2 efflux measured with a soil chamber. Depending on the choice of ξ model used, the estimated soil CO2 efflux using the gradient method reasonably approximates the efflux obtained using the soil chamber method. In addition, the estimated soil CO2 efflux obtained by this improved method is well described by an exponential function of soil temperature at a depth of 0.05 m with the temperature sensitivity (Q10) of 1.81 and a linear function of soil moisture at a depth of 0.12 m, in general agreement with previous findings. These results suggest that the gradient method is a practical cost-effective means to measure soil CO2 emissions. Results from the present study suggest that the gradient method can be used successfully to measure soil CO2 efflux provided that proper attention is paid to the judicious use of the proper diffusion coefficient.

Modelling the turbulence structure in the canopy layer

Abstract The large-eddy simulation (LES) is used to investigate the canopy flow structure and the transfer of TKE, momentum and heat within and above a tall canopy in full leaf. This paper reports on a comparison of three simulations performed, one in neutral conditions and two with different thermal loads on the canopy layer. The instantaneous flow structures observed in the upper crown region, the horizontally averaged statistics of the flow, the second-order equations of the Reynolds stress, TKE and heat fluxes, and the quadrant analyses of the Reynolds stress and heat flux are examined for three atmospheric conditions. The LES results of vertical profiles, temperature, momentum and heat fluxes, velocity variances and skewnesses generally agree well with observations. The effects of buoyancy forces on the velocity variances and skewnesses are discussed. LES results of instantaneous turbulent flow fields reveal basic characteristics of turbulence structures particularly near the top of the canopy layer. The LES simulation results show that near the treetop, shear production constitutes the main source of TKE. The pressure transport, together with the canopy drag are important energy source and sink respectively throughout most of the canopy. In the bottom two-third of the canopy layer, the pressure transport term exceeds other production terms. The turbulent transport term shows an export of TKE from the upper canopy to the region above the canopy and to the deeper regions of the forest. Thermal effects inside the canopy layer are generally small in that budget in unstable conditions, except for the buoyancy production term which peaks at about two-thirds of the layer. This is in contrast with the corresponding term in the Reynolds stress budget where buoyancy production is of almost the same magnitude as the turbulent transport term in unstable conditions, corresponding to about 15% of the shear production term near the canopy top. In the Reynolds stress budget of the upper canopy region, pressure destruction is balanced by shear production. Turbulent transport and subgrid-scale effects are largest near the canopy top. Thermal effects impact mostly on buoyancy production and on the turbulent transport term. These effects are most pronounced near the top of the canopy layer. In the vertical heat flux budget, gradient production and pressure covariance are the main production and destruction of heat flux terms respectively and the gradient production is the largest source term producing an upward heat flux in the crown layer and a downward heat flux in the lower canopy. Unlike its corresponding terms in the TKE and Reynolds stress budgets, this term becomes large and negative in the lower canopy. The importance of the turbulent transport term increases substantially with increasing thermal instability. The magnitude of the pressure covariance, gradient production, turbulent transport and buoyancy production increases with increased thermal load on the canopy layer. Quadrant analysis obtained from our LES results both for momentum and heat fluxes show that above and below the treetop, the contribution to momentum and to a lesser extent to heat transfer, is chiefly controlled by the sweep/ejection mechanisms in the cases studied. In these regions, the ejections contribution increases with unstable conditions while the sweeps contribution to the Reynolds stress decreases. Results are similar although more subdued in the case of the heat flux. For both momentum and heat flux, in the lower canopy, thermal stability significantly alters the structure of the flow mostly by shifting the predominance of the inward interaction quadrant in neutral conditions to the outward interaction quadrant in unstable conditions.

Impact of non-local advection on flux footprints over a tall forest canopy: a tracer flux experiment

Since the early 1990s, in the planning and execution of their flux measurement campaigns, micrometeorologists have used the footprint concept to determine site quality with respect to both fetch and surface homogeneity. While the usefulness of these models has been demonstrated time and time again, there are cases which clearly call for caution. For instance, over tall forest canopies, footprint models have been applied without prior experimental validation, thus causing uncertainties in the interpretation of flux data particularly close to sources and sinks. Another example is that, despite recent advances aimed at identifying both the spatial extent of upwind sources and their individual weight to a point flux measurement, little attention has been given to the possible contribution of sources well outside the footprint region to flux measurements. This paper evaluates footprint models applied to fluxes above tall forest canopies with a tracer experiment. This work further identifies the contribution of sources outside the footprint region to an in situ flux measurement. A field campaign was performed over an 1 l-year old slash pine canopy and a passive tracer (SF6) was released from a line source deployed near the treetop. Vertical SF6 fluxes were measured using the eddy covariance technique at two positions downwind from the source. Measured fluxes during near-neutral, unstable and very unstable conditions were compared against a Lagrangian simulation and an analytical solution to the diffusion equation, two methods commonly used to predict the footprints. Both the formulations compare favorably with SF6 flux measurements except in cases characterized by wind direction-specific advection. Results suggest that, in addition to local source inhomogeneities within the footprint, in some conditions, non-local, larger scale forcings originating hundreds of meters outside the footprint envelope can contribute significantly to flux measurements. This finding is supported by sodar measurements of the three-dimensional velocity field. This finding further suggests caution to experimentalists involved with flux measurements and illustrates the fact that flux measurements must be made with an awareness of landscape-wide surface properties.

Comparison between measured tracer fluxes and footprint model predictions over a homogeneous canopy of intermediate roughness

Abstract Fast response tracer flux measurements are compared against flux footprint predictions from both a Lagrangian stochastic simulation and an analytical solution to the equation of diffusion over a canopy of intermediate roughness. A turbulent tracer flux experiment was conducted over a peach orchard for a range of mildly unstable to very unstable conditions. For this purpose, a line source was used to release sulfur hexafluoride at treetop and fast response tracer flux instrumentation placed in the roughness sub-layer was mounted on four towers perpendicular to the line source to measure the vertical tracer flux. There was excellent agreement between Lagrangian simulated fluxes and their experimental counterparts. The analytical solution to the advection–diffusion equation used also shows a good agreement with the tracer fluxes particularly far from the tracer source. These results suggest that for canopies of intermediate roughness, the flux footprint predictions from both models presented work very well, despite their simplifying assumptions.

Footprints in Micrometeorology and Ecology

History and definition.- Surface layer properties and parameterizations.- Classification of footprint models.- Footprint studies.- Model validation.- Land surface - coupled footprints.- Application of footprint models to different measurement techniques.- Practical applications of footprint techniques.- Looking forward to the next generation of footprint models.

Consistent Two-Equation Closure Modelling for Atmospheric Research: Buoyancy and Vegetation Implementations

A self-consistent two-equation closure treating buoyancy and plant drag effects has been developed, through consideration of the behaviour of the supplementary equation for the length-scale-determining variable in homogeneous turbulent flow. Being consistent with the canonical flow regimes of grid turbulence and wall-bounded flow, the closure is also valid for homogeneous shear flows commonly observed inside tall vegetative canopies and in non-neutral atmospheric conditions. Here we examine the most often used two-equation models, namely