A. D. Culf, J. A. Elbers, A. C. Araujo, A. D. Nobre and P. Stefani

1. ufba Energy and carbon fluxes at the Manaus K34 site Instrument performance and error analysis A. D. Culf, J. A. Elbers, A. C. Araujo, A. D. Nobre and P. Stefani Introduction The new forest tower at the site which has become known as Manaus K34 (see poster by Araujo et al. for further details) was erected in May 1999 and flux instrumentation installed in July. In October 1999 there were several tree falls in the vicinity of the tower but, luckily, there was no long term damage to the tower structure or instrumentation. The flux measurement system in use is based on a Solent 3 dimensional sonic anemometer and LiCor 6262 infra-red gas analyser. The signals are logged on an HP palmtop computer running software developed at Alterra (Wageningen, Netherlands). This system is similar to others used previously in Brazil, the Euroflux program and in numerous other campaigns. All raw (10Hz) data are recorded by the system and fluxes are calculated offline, allowing for complete reprocessing of the data if improved flux calculation methods are devised or if standardisation across LBA sites is required for future scaling-up exercises or inter-comparisons. Energy Closure Energy closure, as measured by the so called “recovery ratio”, (H+LE)/Rn calculated from daily totals, is generally poor (a mean value of ~80% for December 1999 for example). These low values are similar to values obtained in many other studies which have used similar instrumentation but are a cause of concern and require further investigation. There appears to be a relationship between the recovery ratio and rainfall, with the lowest recovery ratios (excluding days with rainfall) occurring on days immediately following rainfall. This relationship is mirrored in the ratio LE/Rn, but not in H/Rn, suggesting that it is an artifact of the measurement system rather than some real effect of enhanced energy storage in the forest following rain for example. Frequency Response corrections Frequency response corrections have been applied to the data in the normal way following the method first proposed by Moore (1986). These corrections are large, especially in the daytime. For December 1999 for example, 40% of the half hourly values of sensible heat flux included a correction greater than 30% of the measurement. Similar corrections apply to carbon dioxide (as shown in the pie chart) and water vapour. The reason for the large size of these corrections (usually quoted as being less than 10%) is a combination of high measurement height, low windspeeds and relatively short time constant (200s) in the running mean used to calculate the background trend in the flux calculations. At low windspeed and high measurement height the correction process assumes a large proportion of the flux occurs at extremely low frequencies which, if they exist at all, are eliminated from the measurements by the running mean applied to the data. Approximations to the flux at these low frequencies are calculated and included in the final flux values. The frequency response corrections are almost exclusively at the low frequency end of the spectrum where measurements exist and could be used. An additional worry with large low frequency response corrections is that the co-spectra on which they are based have considerable uncertainty at the low frequency end of the spectrum. For the stability range -2<z/L < 0 Kaimal et al. (1972) present an area rather than a single line on a plot of co-spectral energy versus frequency. The standard corrections are based on an average curve through this area. The uncertainty in the low frequency corrections can be gauged by using the upper and lower edges of the shaded area instead of the average curve. For sensible heat flux we find that the average flux for December 1999 ranges from 94% to 112% of the value calculated using the standard, average curve. Similar results are expected for water vapour and carbon dioxide fluxes. Conclusions As currently calculated, the fluxes of sensible heat, water vapour and carbon dioxide contain considerable contributions from low frequency flux loss corrections. These corrections are likely to be inappropriate at the lowest frequencies and further work is required to determine more appropriate cospectra to be used for frequency loss corrections in tropical forest conditions. The low frequency corrections contain significant uncertainties even if their current form is accepted as being applicable. The corrections add uncertainty to the daytime flux measurements of the order of +/- 10%. If we want to include low frequency fluctuations in our flux estimates, we should use the measurements themselves, rather than rejecting slow fluctuations by using relatively rapidly adjusting running means and then adding low frequency corrections back in during the analysis stage. For more information please contact: Alistair Culf CEH-Wallingford Wallingford Oxon Tel +44 (0)1491 692456 UK email adc@ceh.ac.uk