Analysis of the limits of the C-T(2)-profile method for sensible heat fluxmeasurements in unstable conditions

We present a test of the C-T(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 c orrelation measurements performed over a pasture (during the SALSA-Mexico e xperiment) using three identical large aperture scintillometers (LASs) alon g a 330 m propagation path and placed at heights 2.50, 3.45 and 6.45 m is f irst given. Scintillometer derived fluxes using the classical method at one level [Agric. For. Meteorol. 76 (1995) 149] reveal that the three scintill ometers provide consistent measurements but underestimate by 15% the flux o btained with the 3D sonic anemometer. This is attributed to spatial non-hom ogeneities of the experimental site. Considerable scatter (and even the imp ossibility of performing computations) is found when using the C-T(2) -prof ile method which is particularly prone to errors in nearly neutral and high ly unstable conditions. The sensitivity of these errors to the accuracy of scintillometer measurements, the calibration errors and the measurement hei ghts is investigated numerically. Simulations are made assuming a normal di stribution of the relative error for C-N(2) With standard deviations a betw een 2 and 5% and no calibration error in a first step. Only calibration err ors (up to 4% between instruments) are simulated in a second step. They con firm that the profile method degrades very rapidly with the accuracy of C-N (2): for instance the RMS error for H reaches 68 W m(-2) land the cases of impossible computation 28%) for a realistic a = 5% value, with heights 2.50 and 3.45 m. Results appear slightly less sensitive to small calibration er rors. The choice of the measurement heights z(1) and z(2) is also analysed: a ratio z(2)/z(1) similar to 3 or 4 with z(1) > 2m seems the best compromi se to minimise errors in H. Nevertheless the accuracy of the profile method is always much lower than that given by the classical method using measure ments at one level, provided a good estimate of roughness length is availab le. We conclude that the C-T(2)-profile method is not suitable for routine applications. (C) 2000 Elsevier Science B.V. All rights reserved.