Comparative measurements of carbon dioxide fluxes from two nearby towers in a central Amazonian rainforest: the Manaus LBA site

Forests around Manaus have staged the oldest and the longest forest-atmosphere CO2 exchange studies made anywhere in the Amazon. Since July 1999 the exchange of CO2, water, and energy, as well as weather variables, have been measured almost continuously over two forests, 11 km apart, in the Cuieiras reserve near Manaus, Brazil. This paper presents the sites and climatology of the region based upon the new data sets. The landscape consists of plateaus dissected by often waterlogged valleys, and the two sites differ in terms of the relative areas of those two landscape components represented in the tower footprints. The radiation and wind climate was similar to both towers. Generally, both the long-wave and short-wave radiation input was less in the wet than in the dry season. The energy balance closure was imperfect (on average 80%) in both towers, with little variation in energy partitioning between the wet and dry seasons; likely a result of anomalously high rainfall in the 1999 dry season. Fluxes of CO2 also showed little seasonal variation except for a slightly shorter daytime uptake duration and somewhat lower respiratory fluxes in the dry season. The net effect is one of lower daily net ecosystem exchange (NEE) in the dry season. The tower, which has less waterlogged valley areas in its footprint, measured a higher overall CO2 uptake rate. We found that on first sight, NEE is underestimated during calm nights, as was observed in many other tower sites before. However, a closer inspection of the diurnal variation of CO2 storage fluxes and NEE suggests that at least part of the nighttime deficits is recovered from either lateral influx of CO2 from valleys or outgassing of soil storage. Therefore there is a high uncertainty in the magnitude of nocturnal NEE, and consequently preliminary estimates of annual carbon uptake reflecting this range from 1 to 8 T ha-1 y-1, with an even higher upper range for the less waterlogged area. The high uptake rates are clearly unsustainable and call for further investigations into the integral carbon balance of Amazon landscapes

Estimativa do índice de área Foliar (IAF) e biomassa em pastagem no estado de Rondônia, Brasil

Medidas mensais da altura da pastagem, biomassa total, variações de biomassa viva e morta, a área específica foliar (SLA) e o Índice de Área de Folha (IAF) de fevereiro de 1999 a janeiro de 2005 na Fazenda Nossa Senhora (FNS) e em Rolim de Moura (RDM) entre Fevereiro a Março de 1999, Rondônia, Brasil. A pastagem predominante é Urochloa brizantha (Hochst. ex A. Rich) R. D. Webster (99% na FNS e 76% em RDM), com pequenas manchas de Urochloa humidicula (Rendle). A altura média anual da grama foi de ~0,16 m. Com o pastejo, o mínimo mensal foi de 0,09 m (estação seca) e máximo de 0,3 m sem pastejo (estação úmida). O IAF, biomassa total, material morto, vivo e SLA tiveram valores médios de 2,5 m2 m-2 , 2202 kg ha-1, 2916 kg ha-1 e 19 m2 kg-1 respectivamente. A média mensal da biomassa foi 4224 kg ha-1 em 2002 e 6667 kg ha-1 em 2003. Grande variação sazonal do material vivo e morto, sendo mais alto o vivo durante a estação úmida (3229 contra 2529 kg ha-1), sendo o morto maior durante a seca (2542 contra 1894 kg ha-1). O nível de água no solo variou de -3,1 a -6,5 m durante as estações. Em médias anuais os IAF foram de 1,4 em 2000 a 2,8 em 2003 e o SLA entre 16,3 m2 kg-1 em 1999 e 20,4 m2 kg-1 em 2001. As observações do Albedo variaram de 0,18 para 0,16 em relação aos altos valores de IAF

Observations of boundary layer development during the HAPEX-Sahel intensive observation period

The radiosonde measurement programme of the boundary layer during the Hydrologic Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) is described. Typical examples of boundary layer evolution at two sites for wet and dry periods are shown. The Central Site was consistently dryer and warmer than the Southern Site and also reached higher boundary layer depths in the dry period in October. A brief analysis of surface flux variability is used to determine the errors associated with using a single average value for boundary layer input. Budget calculations using a mixed layer model show that advection plays an important role in the early morning budgets of heat and moisture. In the dry period subsidence also contributed to boundary layer heating

Variability in boundary layer structure during HAPEX-Sahel wet-dry season transition

The variability of the Sahelian boundary layer has been studied with streamline analyses, rainfall measurements, and upper air soundings during its transition from wet to dry season. The 1992 rainy season ended prematurely because of the early arrival of westerly troughs over West Africa. The change in the circulation is related to global-scale atmospheric circulations as successive westerly troughs over this region can be traced back upstream on a planetary scale. Once the upper level easterlies changed to westerlies, the large-scale circulation brought the surface northeasterly flow southward, which led to the retreat of the Southwest Monsoon in Niger. The boundary layer responded quickly to this transition of synoptic events from wet to dry seasons. During the wet period, the boundary layer was relatively cool and moist because evapotranspiration dominated, keeping the surface cool and preventing significant direct sensible heating of the boundary layer. During the transition period, less extensive showery weather allowed the boundary layer to have more time to recover from rainfall episodes, leading to a warming and drying trend. During the dry period, soil moisture contents dropped rapidly. With more sensible heat flux made available for boundary layer heating and energetics, the boundary layer reached its maximum temperatures and minimum moisture contents during the Hydrological Atmospheric Pilot Experiment in the Sahel (HAPEX-Sahel) intensive observational period. Budget calculations indicate that the horizontal advection and vertical flux divergence terms were most important during the wet period, whereas during the dry period, the subsidence and vertical flux divergence terms were most important. From wet to dry seasons, the vertical wind shear of the zonal wind was reduced from 23 m s−1 to 16 m s−1, consistent with vertical wind shear differences between wet and dry years as reported in the literature. Similarities and differences with the First ISLSCP Field Experiment (FIFE-89) boundary layer are also examined. It is hypothesized that the retreat of the southwesterly monsoon could be upheld by a sustained secondary circulation if the wet season rainfall pattern imprints an organized south to north soil moisture gradient maintaining a concurrent reverse gradient in surface sensible heating. The boundary layer circulation that would be established in response to the heating gradient would reinforce surface southwesterlies, as well as reinforcing mid-level easterlies of which the African Easterly Jet is a part, and thus help uphold the intrusion of westerlies and the monsoon retreat. Such a mechanism, whose effectiveness would be a function of how distinct the south-north soil moisture gradient develops from the wet season precipitation pattern, could help explain the large interannual variability of rainfall over the Sahel

A comparison of surface fluxes at the HAPEX-Sahel fallow bush sites

The variability between surface flux measurements at the fallow sites of the three HAPEX-Sahel supersites is examined over periods of three or four consecutive days. A roving eddy correlation instrument provided a common base for comparison at each supersite. The inhomogeneity of the surface and the instrumental layout did not provide the conditions to allow the separation of the effects of instrument error from those due to the spatial variability of vegetation cover and soil moisture. Surface fluxes of sensible and latent heat and energy balance terms were intercompared at each supersite over summation timescales of 1 hour and 3 days. It is shown that, generally, HAPEX-Sahel hourly sensible heat flux and latent heat values have confidence limits of 15% and 20% respectively. The three-day period energy balance shows the combined sensible and latent heat fluxes to have a confidence limit of 3%. It is concluded that, due to the averaging effect of longer time periods and larger flux footprints on spatial inhomogeneity, confidence in the surface flux measurements increases with longer summation periods and with neutral atmospheric surface layers which characterise the rainy period of the Intensive Observation Period

A comparison of surface fluxes at the HAPEX-Sahel fallow bush sites

The variability between surface flux measurements at the fallow sites of the three HAPEX-Sahel supersites is examined over periods of three or four consecutive days. A roving eddy correlation instrument provided a common base for comparison at each supersite. The inhomogeneity of the surface and the instrumental layout did not provide the conditions to allow the separation of the effects of instrument error from those due to the spatial variability of vegetation cover and soil moisture. Surface fluxes of sensible and latent heat and energy balance terms were intercompared at each supersite over summation timescales of 1 hour and 3 days. It is shown that, generally, HAPEX-Sahel hourly sensible heat flux and latent heat values have confidence limits of 15% and 20% respectively. The three-day period energy balance shows the combined sensible and latent heat fluxes to have a confidence limit of 3%. It is concluded that, due to the averaging effect of longer time periods and larger flux footprints on spatial inhomogeneity, confidence in the surface flux measurements increases with longer summation periods and with neutral atmospheric surface layers which characterise the rainy period of the Intensive Observation Period