This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that:(i) The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy.(ii) In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top ofthe canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss.(iii) In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller.(iv) Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence.(v) Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.

Models of mass and energy exchanges between the biosphere and the atmosphere generally contain a nonlinear dependence between fluxes and model parameters, and thus estimation of these parameters from measurements in a heterogeneous landscape depends on the scale of the observations. The scale‐dependence of a typical surface‐exchange model (the CSIRO Biospheric Model, CBM) is examined using the diurnal variation of hourly fluxes of CO2, latent heat, sensible heat and soil heat. The fluxes were measured using micrometeorological techniques over six sites in a grazing/pasture system in SE Australia during a period of three weeks in 1995. Nonlinear parameter inversion was used to determine model parameters. Analysis of the covariance of the estimates of the parameters and the unexplained residuals of the model showed that a maximum of three or four parameters could be determined independently from the observations for all six sites. Estimates of a key model parameter, jmax, the mean of maximum potential electron transport rate of all leaves within the canopy, was best determined by the measurements of net CO2 flux at all sites examined. Measurements of ground heat flux provide little information about any of the model parameters in CBM. Because of nonlinearities in the surface exchange model, calculated fluxes will be in error if parameters for the component vegetation types are simply averaged in proportion to their areal fraction. The magnitude of these errors was examined for CBM using a hypothetical land surface consisting of two surface types, each with different parameter values. Predictions of net CO2, latent heat and ground heat fluxes using a linear combination of model parameters for the two surface types were quite similar with those found using optimal estimates of the parameters for the landscape, but were significantly poorer for sensible heat fluxes.

Micrometeorological measurements, including direct eddy‐correlation measurements of heat and moisture fluxes, have been made from shipboard under light‐wind conditions in the western equatorial Pacific warm pool. Air‐sea temperature differences were typically 1.5°–2°C, that is, 1°–1.5°C larger than long‐term averages from merchant ship data. A sea surface “cool skin” of about 0.3°C was observed. Bulk transfer coefficients for both fluxes agree well with the predictions of Liu et al. (1979) in the convective wind speed regime below 4 m s−1. Between 4 and 6 m s−1 values of the neutral exchange coefficients were CEN ‐ 0.89 × 10−3; CHN = 1.03 × 10−3; CDN = 1.16 × 10−3. At zero mean wind speed, latent heat flux is maintained at about 25 W m−2 by convective exchange. Inertial dissipation estimates of the latent heat flux are about 20% lower than the directly measured eddy‐correlation values below 4 m s−1.

This paper describes a wind-tunnel experiment on the dispersion of trace heat from an effectively planar source within a model plant canopy, the source height being h, = 0.80 h,, where h, is the canopy height. A sensor assembly consisting of three coplanar hot wires and one cold wire was used to make simultaneous measurements ofthe temperature and the streamwise and vertical velocity components. It was found that:(i) The thermal layer consisted of two parts with different length scales, an inner sublayer (scaling with h, and h,) which quickly reached streamwise equilibrium downstream of the leading edge of the source, and an outer sublayer which was self-preserving with a length scale proportional to the depth of the thermal layer.(ii) Below 2h,, the vertical eddy diffusivity for heat from the plane source (KHP) was substantially less than the far-field limit of the corresponding diffusivity for heat from a lateral line source at the same height as the plane source. This shows that dispersion from plane or other distributed sources in canopies is influenced, near the canopy, by turbulence 'memory' and must be considered as a superposition of both near-field and far-field processes. Hence, one-dimensional models for scalar transport from distributed sources in canopies are wrong in principle, irrespective of the order of closure. (iii) In the budgets for temperature variance, and for the vertical and streamwise components of the turbulent heat flux, turbulent transport was a major loss between h, and h, and a principal gain mechanism below h,, as also observed in the budgets for turbulent energy and shear stress. (iv) Quadrant analysis of the vertical heat flux showed that sweeps and ejections contributed about equal amounts to the heat flux between h, and h,, though among the more intense events, sweeps were dominant. Below h,, almost all the heat was transported by sweeps.

Accurate estimates of long-term linear trends of wind speed provide a useful indicator for circulation changes in the atmosphere and are invaluable for the planning and financing of sectors such as wind energy.Here a large number of wind observations over Australia and reanalysis products are analyzed to compute such trends. After a thorough quality control of the observations, it is found that the wind speed trends for 1975-2006 and 1989-2006 over Australia are sensitive to the height of the station: they are largely negative for the 2-m data but are predominantly positive for the 10-m data. The mean relative trend at 2 m is 20.10 6 0.03% yr 21 (20.36 6 0.04% yr 21 ) for the 1975-2006 (1989-2006) period, whereas at 10 m it is 0.90 6 0.03% yr 21 (0.69 6 0.04% yr 21 ) for the 1975-2006 (1989-2006) period. Also, at 10 m light winds tend to increase more rapidly than the mean winds, whereas strong winds increase less rapidly than the mean winds; at 2 m the trends in both light and strong winds vary in line with the mean winds. It was found that a qualitative link could be established between the observed features in the linear trends and some atmospheric circulation indicators (mean sea level pressure, wind speed at 850 hPa, and geopotential at 850 hPa), particularly for the 10-m observations. Further, the magnitude of the trend is also sensitive to the period selected, being closer to zero when a very long period, 1948-2006, is considered. As a consequence, changes in the atmospheric circulation on climatic time scales appear unlikely.