M. Irvine

Turbulent Structures in a Pine Forest with a Deep and Sparse Trunk Space: Stand and Edge Regions

Forested landscapes often exhibit large spatial variability in vertical and horizontal foliage distributions. This variability may affect canopy-atmosphere exchanges through its action on the development of turbulent structures. Here we investigate in neutral stratification the turbulent structures encountered in a maritime pine forest characterized by a high, dense foliated layer associated with a deep and sparse trunk space. Both stand and edge regions are considered. In situ measurements and the results of large-eddy simulations are used and analyzed together. In stand conditions, far from the edge, canopy-top structures appear strongly damped by the dense crown layer. Turbulent wind fluctuations within the trunk space, where the momentum flux vanishes, are closely related to these canopy-top structures through pressure diffusion. Consequently, autocorrelation and spectral analyses are not quite appropriate to characterize the vertical scale of coherent structures in this type of canopy, as pressure diffusion enhances the actual scale of structures. At frequencies higher than those associated with canopy-top structures, wind fluctuations related to wake structures developing behind tree stems are observed within the trunk space. They manifest themselves in wind velocity spectra as secondary peaks in the inertial subrange region, confirming the hypothesis of spectral short-cuts in vegetation canopies. In the edge region specific turbulent structures develop just below the crown layer, in addition to canopy-top structures. They are generated by the wind shear induced by the sub-canopy wind jet that forms at the edge. These structures provide a momentum exchange mechanism similar to that observed at the canopy top but in the opposite direction and with a lower magnitude. They may develop as in plane mixing-layer flows, with some perturbations induced by canopy-top structures. Wake structures are also observed within the trunk space in the edge region.

Integrated Sensible Heat Flux Measurements of a Two-Surface Composite Landscape using Scintillometry

The potential of the LAS (large aperture scintillometry) method for measuring sensible heat flux (H) directly integrated over a two-field composite surface is evaluated. We describe a field experiment performed within the Alpilles/ReSeDa project in the south-east of France over a composite surface made up of wheat and bare soil (451 and 216 m long respectively) using two 0.15-m diameter scintillometers mounted at heights of 2.05 and 4.54 m. When compared against reference values obtained by the eddy correlation technique, LAS-measured sensible heat flux reveals a systematic overestimation of about 10%. A simple model describing the integration of the scintillometer signal along the beam for a two-field composite surface is described. A simulation of the experiment confirms that the bias observed isrelated to non-linearities in the integration process in relation with thebell-shape sensitivity curve of the instrument to the structure parameter for the refractive index it measures. The model is used to test the sensitivity of the LAS-derived H values to the composition of the pathlength (ratio of both surfaces) and to the contrast in sensible heat flux and roughness length between the two fields. Sensitivity tests to the aggregation scheme for roughness length (two of them are tested) and to the measurement height are also presented. The composition of the surface in combination with the contrast in sensible heat flux (in direct relation with the contrast in latent heat flux) explains most of the bias, with possible deviations ranging from -50 up to 80 W m-2. A tentative semi-empirical method for correcting the bias is suggested, which only requires a crude estimate of the contrast in component sensible heat fluxes along the pathlength.

Analysis of the limits of the CT2-profile method for sensible heat flux measurements in unstable conditions

We present a test of the CT 2 -profile method described by Hill et al. [J. Atmos. Ocean. Technol. 9 (5) (1992) 526] to estimate the surface sensible heat flux over an homogeneous surface. A comparison with traditional eddy correlation measurements performed over a pasture (during the SALSA-Mexico experiment) using three identical large aperture scintillometers (LASs) along a 330 m propagation path and placed at heights 2.50, 3.45 and 6.45 m is first given. Scintillometer derived fluxes using the classical method at one level [Agric. For. Meteorol. 76 (1995) 149] reveal that the three scintillometers provide consistent measurements but underestimate by 15% the flux obtained with the 3D sonic anemometer. This is attributed to spatial non-homogeneities of the experimental site. Considerable scatter (and even the impossibility of performing computations) is found when using the CT 2 -profile method which is particularly prone to errors in nearly neutral and highly unstable conditions. The sensitivity of these errors to the accuracy of scintillometer measurements, the calibration errors and the measurement heights is investigated numerically. Simulations are made assuming a normal distribution of the relative error for CN 2 with standard deviations between 2 and 5% and no calibration error in a first step. Only calibration errors (up to 4% between instruments) are simulated in a second step. They confirm that the profile method degrades very rapidly with the accuracy of CN 2 : for instance the RMS error for H reaches 68 W m 2 (and the cases of impossible computation 28%) for a realistic D 5% value, with heights 2.50 and 3.45 m. Results appear slightly less sensitive to small calibration errors. The choice of the measurement heights z1 and z2 is also analysed: a ratio z2=z1 3 or 4 with z1 > 2 m seems the best compromise to minimise errors in H. Nevertheless the accuracy of the profile method is always much lower than that given by the classical method using measurements at one level, provided a good estimate of roughness length is available. We conclude that the CT 2 -profile method is not suitable for routine applications.

Impacts of surface bounadry layer turbulence on surface temperature measurements at different spatial resolutions

Surface temperature displays temporal fluctuations driven by turbulence intensity of both surface (SBL) and planetary (PBL) boundary layers. An experiment conceived to evaluate the error induced by these fluctuations on instantaneous satellite measurements at different spatial resolutions is described. It is based on the reconstruction of surface temperature time series from sequences of thermal images acquired using a TIR camera aboard an helicopter in stationary flight over 5 different surface types. It is shown that the spatial averaging performed over the pixel allows one to reduce the contribution of the high frequency SBL turbulence to the error on Ts measurements. This error rapidly decreases with pixel size, and is found to be around ±0.5°C at 50m resolution for the cases we studied. Unfortunately the experimental set up revealed not as well suited to evaluate the contribution of low frequency PBL turbulence to Ts error, and our recommendations are given for future experiments.