Abstract. Ejection and sweep eddy motions in the atmospheric surface layer (ASL) are widely accepted as being responsible for much of land surface evaporation, sensible heat flux, and momentum flux; however, less is known about this type of eddy motion within the canopy sublayer (CSL) of forested systems. The present study analyzed the ejectionsweep properties at the canopy-atmosphere interface of a 13 m tall, uniformly aged southern loblolly pine stand and a 33 m tall, unevenly aged hardwood stand using velocity and scalar (temperature, water vapor, and carbon dioxide) fluctuation measurements at the canopy-atmosphere interface. It was found that the measured sweeps and ejections time fractions for scalars and momentum are comparable and are in good agreement with other laboratory and field experiments. This investigation demonstrates that the thirdorder cumulant expansion method (CEM) reproduces the measured relative flux contribution of ejections and sweeps (AS0) and the difference between sweep and ejection time fractions for both momentum and scalars at the canopy-atmosphere interface in contrast to findings from a previous ASL experiment. A linkage between AS 0 and the scalar flux budget is derived and tested via the third-order CEM at the canopy-atmosphere interface for the pine and the hardwood stands. It is shown that AS 0 can be related to the dimensionless scalar flux transport term whose gradient is central to the scalar variance budget. Also, the derived relationship is independent of canopy roughness or scalar sources and sinks. Hence this investigation establishes an analytical linkage between second-order closure models, the ejection-sweep cycle, and third-order CEM at the canopy-atmosphere interface. Dissimilarity between the ejection-sweep cycle for scalar and momentum transport is considered via conditional probability distributions at both forest stands. In contrast to a laboratory heat dispersion experiment, it is shown that while the ejection-sweep cycles for scalar and momentum transport are intimately linked, they are not identical. Therefore the results from momentum ejection-sweeps investigations cannot be extrapolated to scalar transport. Comparisons with other laboratory experiments are also discussed, especially in relation to the scalar ejection and sweep time fractions.

Abstract. Isoprene is usually the dominant natural volatile organic compound emission from forest ecosystems, especially those with a major broadleaf deciduous component. Here we report isoprene emission model performance versus leaf and canopy level isoprene emission measurements made at the Duke University Research Forest near Chapel Hill, North Carolina. Emission factors, light and temperature response, canopy environment models, foliar mass, leaf area, and canopy level isoprene emission were evaluated in the field and compared with model estimates. Model components performed reasonably well and generally yielded estimates within 20% of values measured at the site. However, measured emission factors were much higher in early summer following an unusually dry spring. These decreased later in the summer but remained higher than values currently used in emission models. There was also a pronounced decline in basal emission rates in lower portions of the canopy which could not be entirely explained by decreasing specific leaf weight. Foliar biomass estimates by genera using basal area ratios adjusted for crown form were in excellent agreement with values measured by litterfall. Overall, the stand level isoprene emissions determined by relaxed eddy accumulation techniques agreed reasonably well with those predicted by the model, although there is some evidence for underprediction at ambient temperatures approaching 30øC, and overprediction during October as the canopy foliage senesced. A "Big Leaf" model considers the canopy as a single multispecies layer and expresses isoprene emission as a function of leaf area rather than mass. This simple model performs nearly as well as the other biomass-based models. We speculate that seasonal water balance may impact isoprene emission. Possible improvements to the canopy environment model and other components are discussed.

This paper presents the first micrometeorological-based measurements of methane (CH4) emissions from Asian rice paddies of which we are aware. The research features the tunable diode laser trace gas analyzer system (TGAS) recently developed at the University of Guelph. CH 4 fluxes were measured between March 9 and 24, 1992, from an irrigated rice paddy field at the International Rice Research Institute (IRRI), the Philippines. The daytime CH 4 flux averaged 6.0/xg m -2 s -1 . The CH 4 fluxes displayed a diurnal trend similar to daily soil temperature curves, with peak emissions of about 8/xg m -2 s -1 in the early afternoon. A tenfold increase in CH 4 emissions (to about 70/xg m -2 s -1) during a brief weeding experiment resulted from soil disturbance. Up to 25/xg m -2 s -1 of CH 4 were released during a drying of the field, after which unsuitable soil redox potentials apparently suppressed methanogenesis. The CH 4 flux was also arrested when the field was flooded with oxygen-rich water during a heavy rainstorm. where Fme flux of CH 4 (/xg m -2 s-l); K eddy diffusivity (m2 s -•); ACme change in CH 4 concentration (/xg m-3); Az (m) vertical height difference. TGAS measurements were used to determine a finite CH 4 concentration difference ACme. Three estimates of the eddy diffusivity K were inferred using micrometeorological tech-SIMPSON ET AL.' TUNABLE DIODE LASER MEASUREMENT OF METHANE FLUXES 7285 OF METHANE FLUXES 7289

The underlying mean and variance properties of surface net radiation, soil heat flux, and sensible‐latent heat fluxes are examined over the densely instrumented grassland region encompassing the First ISLSCP Field Experiment (FIFE). Twenty‐two surface flux stations at 20 sites were deployed during the four 1987 intensive field campaigns (IFCs). Flux variability is addressed together with the problem of scaling up to area‐averaged fluxes. Successful parameterization of area‐averaged fluxes in atmospheric models is based on accounting for internal spatial and temporal scales correctly. Mean and variance properties of fluxes are examined in both daily and diurnally averaged frameworks. Results are compared and contrasted for clear and cloudy situations and checked for the influence of surface‐induced biophysical controls (burn and grazing treatments) and topographic controls (slope factors and aspect ratios). Examination of the sensitivity of domain‐averaged fluxes to different averaging procedures demonstrates that this may be an important consideration. The results reveal six key features of the 1987 surface fluxes: (1) cloudiness variability and ample rainfall throughout the growing season led to near‐consistency in flux magnitudes during the first three IFCs; (2) burn treatment, grazing conditions, and topography have clearly delineated influences on the diurnal cycle flux amplitudes but do not alter the evaporative fraction significantly; (3) cloudiness is the major control on flux variability in terms of both mean and variance properties but has little impact on the Bowen ratio or evaporative fraction; (4) spatial weighting of fluxes based on a biophysicaltopographical cross stratification generates a measurable bias with respect to straight arithmetic averaging (up to 20 W m−2 in available heating); (5) structure function analysis demonstrates significant underlying spatial autocorrelation structure in the fluxes, but the observed distance dependence is due to cloudiness controls, not surface controls; (6) Monte Carlo analysis of high resolution vegetation indices obtained from SPOT satellite measurements suggest that the mean domain amplitudes of the diurnal sensible and latent heat flux cycles can be biased up to 30–40 W m −2 by repositioning the 20 site locations within the experimental domain.

During FIFE 1987, surface energy fluxes were measured at 22 flux sites by nine groups of scientists using different measuring systems. A rover Bowen ratio station was taken to 20 of the flux stations to serve as a reference for estimating the instrument‐related differences. The rover system was installed within a few meters from the host instrument of a site. Using linear regression analysis, net radiation, Bowen ratio, and latent heat fluxes were compared between the rover measurements and the host measurements. The average differences in net radiation, Bowen ratio, and latent heat flux from different types of instruments can be up to 10, 30, and 20%, respectively. The Didcot net radiometer gave higher net radiation while the Swissteco type showed lower values, as compared to the corrected radiation energy balance system (REBS) model. The four‐way components method and the Thornthwaite type give similar values to the REBS. The surface energy radiation balance systems type Bowen ratio systems exhibit slightly lower Bowen ratios and thus higher latent heat fluxes, compared to the arid zone evapotranspiration systems. Eddy correlation systems showed slightly lower latent heat flux in comparison to the Bowen ratio systems. It is recommended that users of the flux data take these differences into account.