Aircraft turbulence measurements to estimate surface heat fluxes from the mixed layer variance methods over semi-arid grassland
Ayumi Kotani (1), Michiaki Sugita (1)
(1) Graduate School of Life and Environmental Sciences, University of Tsukuba
Turbulence data obtained by aircraft observations in convective boundary layer (CBL) were analyzed to estimate the regional surface heat fluxes through the variance methods. The observation was carried out from June to October of 2003 as part of RAISE (Rangelands Atmosphere-Hydrosphere-Biosphere Interaction Study Experiment) project. The flight paths covered the experimental area of Kherlen river basin in Mongolia, and several heights within and above CBL (the height of CBL is estimated to be 700 - 1200m for each flight) were flown repeatedly above the ground based observation sites in relatively homogeneous semi-arid grassland. Within the area, two flux stations and four automatic weather stations (AWS) were in operation during the flights to produce reference values.
The vertical profiles of the second order moments, i.e., the variance, of temperature followed in general the functional curves proposed by previous studies based on CBL observations or numerical modeling, although, similar to those studies, at higher level of CBL (higher than 80% from the surface), the scatter of data became larger. These variance statistics were applied to the variance formulations to estimate surface heat fluxes. The equations have some experimentally determined constants and these constants were calibrated with the current data set. The resulting rms error of the estimated sensible heat flux was improved from about 80 Wm-2 (before calibration) to 40 Wm-2 (after calibration). Finally, these equations with locally calibrated coefficients were applied to the data over the other areas with the AWS, where surface fluxes have been obtained by the bulk similarity method. Although the rms error increased, it was probably because of larger uncertainty of the ground based fluxes derived by the bulk method.
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Ayumi Kotani
28-Jul-04-16:16:47
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Graduate School of Life and Environmental Sciences, University of Tsukuba