Gabriel Pita

Redefinition and global estimation of basal ecosystem respiration rate

Basal ecosystem respiration rate (BR), the ecosystem respiration rate at a given temperature, is a common and important parameter in empirical models for quantifying ecosystem respiration (ER) globally. Numerous studies have indicated that BR varies in space. However, many empirical ER models still use a global constant BR largely due to the lack of a functional description for BR. In this study, we redefined BR to be ecosystem respiration rate at the mean annual temperature. To test the validity of this concept, we conducted a synthesis analysis using 276 site-years of eddy covariance data, from 79 research sites located at latitudes ranging from similar to 3 degrees S to similar to 70 degrees N. Results showed that mean annual ER rate closely matches ER rate at mean annual temperature. Incorporation of site-specific BR into global ER model substantially improved simulated ER compared to an invariant BR at all sites. These results confirm that ER at the mean annual temperature can be considered as BR in empirical models. A strong correlation was found between the mean annual ER and mean annual gross primary production (GPP). Consequently, GPP, which is typically more accurately modeled, can be used to estimate BR. A light use efficiency GPP model (i.e., EC-LUE) was applied to estimate global GPP, BR and ER with input data from MERRA (Modern Era Retrospective-Analysis for Research and Applications) and MODIS (Moderate resolution Imaging Spectroradiometer). The global ER was 103 Pg C yr (-1), with the highest respiration rate over tropical forests and the lowest value in dry and high-latitude areas.

Coherent structures in the near field of round jets

The present paper reports measurements of axial and radial velocities obtained with laser-Doppler anemometry in the transitional region of a round jet. The predominant frequencies of the coherent structures detected in the non-artificially excited jet, correspond to Strouhal numbers, around 0.33 and 0.55 and an interpretation of these observations is made. The use of external excitation can modify some characteristics of the jets disturbing the balance between the different flow structures, namely those corresponding to the two values of St. The jet diameter was 30.0 mm and the velocities ranged from near zero up to 15 m/s, corresponding to Reynolds number of 2.87 × 104.To obtain the velocity probability distributions, power spectral distributions and auto-correlations, together with mean and rms velocities a LDA with double Bragg cell system was used. The Doppler signal was analysed with a counter interfaced to an Apple II microcomputer and a Racal magnetic tape.

Winter respiratory C losses provide explanatory power for net ecosystem productivity

M. Haeni, R. Zweifel, W. Eugster, A. Gessler, S. Zielis, C. Bernhofer, A. 7 Carrara, T. Grünwald, K. Havránková, B. Heinesch, M. Herbst, A. Ibrom, A. 8 Knohl, F. Lagergren, B.E. Law, M. Marek, G. Matteucci, JH. McCaughey, S. 9 Minerbi, L. Montagnani, E. Moors, J. Olejnik, M. Pavelka, K. Pilegaard, G. 10 Pita, A. Rodrigues, M. J. Sanz Sánchez, M.-J. Schelhaas, M. Urbaniak, R. 11 Valentini, A. Varlagin, T. Vesala, C. Vincke, J. Wu, and N. Buchmann 12

Errors induced by catalytic effects in premixed flame temperature measurements

The evaluation of instantaneous temperature in a premixed flame using fine-wire Pt/Pt-13%Rh thermocouples was found to be subject to significant errors due to catalytic effects. An experimental study was undertaken to assess the influence of local fuel/air ratio, thermocouple wire diameter, and gas velocity on the thermocouple reading errors induced by the catalytic surface reactions. Measurements made with both coated and uncoated thermocouples showed that the catalytic effect imposes severe limitations on accuracy of mean and fluctuating gas temperature in the radical-rich flame zone.<<ETX>>