Variabilidade em escala da turbulência atmosférica acima de floresta na Amazônia Ocidental

Abstract

Daily and seasonal variability of the three components of turbulent wind speed (u, v, w), potential virtual temperature (Өv), specific humidity (q) and CO2 (c) concentration fields have been studied. For this, fast response data measured in a 65 high tower, erected above primary forest, obtained during the wet (1999) and dry to wet transition (2002) season intensive campaigns of the LBA Experiment, in Jarú Biological Reserve in Rondônia, have been used. The wavelet transform (with the Morlet function) has been used to project the available time series on 32 scales in order to investigate the scale variability of statistical turbulent variables. Thus, some characteristics of diurnal and nocturnal turbulent kinetic energy, as well as the variances and covariances, correlation coefficients, asymmetry and flattening parameters have been investigated. The results have shown that in both, wet and dry season, horizontal wind velocity variance contributions to generate turbulent kinetic energy (TKE) are greater during the morning comparatively to the afternoon period. However, when night and early morning periods have been compared, the results show a significant TKE increase during the wet season early morning. During the wet season, important low-frequency contributions for the covariances have been detected, probably due to occurrence of mesoscale atmospheric circulations, which are frequent during this period. Daytime correlation coefficients obtained during the wet season have shown greater variability on time scales lesser than 15 s, suggesting that in this period, the local eddies are crucial in generating turbulent mixing between forest and just above atmospheric surface layer. At night, a similar kind of variability pattern has been observed during the wet season, but at scales greater than 17.27 min, where the low frequency energy contributions are more important. The occurrence of such phenomena could explain the observed asymmetries in the wind and scalars field, as well as the increase in the intermittency of small scale oscillations, which are reflected in the flattening of the scalar coefficients, particularly during the mornings of the wet season, when TKE values have considerably increased. As a consequence of the existence of a spectral gap, night-time relative minimum values for the u, v, Өv, and q, variances, have been detect near 8.7 min at night, during both seasons, while in early morning, minimum values have been detected near the scale of 10.7 min during the dry season and 23.4 min during the wet season. In general, the correlation coefficients did not reach ±1 values. However, the correlation coefficients between w and Өv, q reached values greater than 0.5 during the day. At night, except for rcq, all calculated correlation coefficients presented values lesser than ±0.5. Such results indicate Monin-Obukhov Similarity failure for Rebio-Jarú surface boundary layer, probably due to the existence of a roughness sublayer above the forest (particularly during the night-time) and should be taken into account in simulations of atmospheric flow in the region.