Intercomparison and assessment of turbulent and physiological exchange parameters of grassland

Abstract

Abstract. Commonly, the micrometeorological parameters that underline the calculations of surface atmosphere exchange fluxes (e.g. friction velocity and sensible heat flux) and parameters used to model exchange fluxes with SVAT-type parameterisations (e.g. latent heat flux and canopy temperature) are measured with a single set of instrumentation and are analysed with a single methodology. This paper evaluates uncertainties in these measurements with a single instrument, by comparing the independent results from nine different institutes during the international GRAMINAE integrated field experiment over agricultural grassland near Braunschweig, Lower Saxony, Germany. The paper discusses uncertainties in measuring friction velocity, sensible and latent heat fluxes, canopy temperature and investigates the energy balance closure at this site. Although individual 15-min flux calculations show a large variability between the instruments, when averaged over the campaign, fluxes agree within 2% for momentum and 11% for sensible heat. However, the spread in estimates of latent heat flux (λE) is larger, with standard deviations of averages of 18%. While the dataset averaged over the different instruments fails to close the energy budget by 30%, if the largest turbulent fluxes are considered, near perfect energy closure can be achieved, suggesting that most techniques underestimate λE in particular. The uncertainty in λE feeds results in an uncertainty in the bulk stomatal resistance, which further adds to the uncertainties in the estimation of the canopy temperature that controls the exchange. The paper demonstrated how a consensus dataset was derived, which is used by the individual investigators to calculate fluxes and drive their models.

An inverse model to estimate ammonia emissions from fields

“…It has been recently possible to extend the EC method to NH 3 with the use of tunable diode lasers (TDL) and quantum cascade lasers (QCL) (Whitehead et al, 2008;Brodeur et al, 2009). The uncertainty of the EC technique itself has been estimated for latent heat as around 18% by Nemitz et al (2009). However, analysis of NH 3 by EC is more difficult because of the 'stickiness' of the gas on the walls of the inlet lines, which causes high-frequency losses.…”

Dynamics of ammonia exchange with cut grassland: synthesis of results and conclusions of the GRAMINAE Integrated Experiment

“…The high relative standard deviation for the sensible heat flux is related to the bi-directional nature of the fluxes as well as the uncertainties occurring under stable atmospheric stratification. The relative standard deviations of average fluxes for the overall experiment were smaller, at 2%, 8%, 18% and 6% for friction velocity, sensible heat, latent heat and net radiation, respectively (Nemitz et al, 2009b). The results imply that for u * and H , the main uncertainty is due to spatial heterogeneity in surface and turbulence, rather than systematic differences between approaches, and hence an ensemble average provides a more robust 15-min value than can be derived with a single setup.…”

“…The measurement foundation of the experiment was a detailed inter-comparison of turbulent exchange estimates (Nemitz et al, 2009b). Instrumentation from nine European institutions was applied to develop "consensus estimates" of each of the turbulent fluxes and components of the energy balance with a 15 min time resolution from 19 May to 15 June 2000.…”

“…The inter-comparison of Nemitz et al (2009b) highlights how uncertainty in individual estimates of sensible and latent heat fluxes would normally propagate to uncertainty in trace gas fluxes in most studies where replicated measurements are not available. For example, the mean relative standard deviation of individual 15 min estimates of the friction velocity (u * ) was 14%, while the values for sensible (H ) and latent (λE) heat flux were 58% (equivalent to 14 W m For R sb , the measured values (points) are compared with a model fit based on total solar radiation and canopy Leaf Area Index (blue line), and with a model fit that also includes the effect of moisture limitation according to vapour pressure deficit (red line).…”

“…Even with such an extensive inter-comparison, the energy balance was only 80% closed. Nemitz et al (2009b) suggest that this may be explained by omission of low turbulence contribution by 15 min averaging and methodological limitations of the eddy covariance method. In addition, uncertainties in net radiation, such as sensor bias or spatial differences between the sampled area and the flux footprint may contribute to differences.…”