We investigated the seasonal patterns of Amazonian forest photosynthetic activity, and the effects\ud thereon of variations in climate and land-use, by integrating data from a network of ground-based eddy\ud flux towers in Brazil established as part of the ‘Large-Scale Biosphere Atmosphere Experiment in Amazonia’\ud project. We found that degree of water limitation, as indicated by the seasonality of the ratio\ud of sensible to latent heat flux (Bowen ratio) predicts seasonal patterns of photosynthesis. In equatorial\ud Amazonian forests (5◦ N–5◦ S), water limitation is absent, and photosynthetic fluxes (or gross ecosystem\ud productivity, GEP) exhibit high or increasing levels of photosynthetic activity as the dry season\ud progresses, likely a consequence of allocation to growth of new leaves. In contrast, forests along the\ud southern flank of the Amazon, pastures converted from forest, and mixed forest-grass savanna, exhibit\ud dry-season declines in GEP, consistent with increasing degrees of water limitation. Although previous\ud work showed tropical ecosystem evapotranspiration (ET) is driven by incoming radiation, GEP observations\ud reported here surprisingly show no or negative relationships with photosynthetically active\ud radiation (PAR). Instead, GEP fluxes largely followed the phenology of canopy photosynthetic capacity (Pc),\ud with only deviations from this primary pattern driven by variations in PAR. Estimates of leaf flush at threeNASA (LBA investigation CD-32 and the LBA-DMIP project, award #NNX09AL52G)National Science Foundation (Amazon-PIRE, NSF award #OISE-0730305)Gordon and Betty Moore Foundation's Andes-Amazon InitiativeFAPESP (02/09289-9)CNPq (Instituto do Milênio-LAB, Ed. Universal-01, Ed. CT-Hidro 03

We investigated the seasonal patterns of water vapor and sensible heat flux along a tropical biome gradient from forest to savanna. We analyzed data from a network of flux towers in Brazil that were operated within the Large‐Scale Biosphere‐Atmosphere Experiment in Amazonia (LBA). These tower sites included tropical humid and semideciduous forest, transitional forest, floodplain (with physiognomies of cerrado), and cerrado sensu stricto. The mean annual sensible heat flux at all sites ranged from 20 to 38 Wm⁻², and was generally reduced in the wet season and increased in the late dry season, coincident with seasonal variations of net radiation and soil moisture. The sites were easily divisible into two functional groups based on the seasonality of evaporation: tropical forest and savanna. At sites with an annual precipitation above 1900 mm and a dry season length less than 4 months (Manaus, Santarem and Rondonia), evaporation rates increased in the dry season, coincident with increased radiation. Evaporation rates were as high as 4.0 mm d⁻¹ in these evergreen or semidecidous forests. In contrast, ecosystems with precipitation less than 1700 mm and a longer dry season (Mato Grosso, Tocantins and São Paulo) showed clear evidence of reduced evaporation in the dry season. Evaporation rates were as low as 2.5 mm d⁻¹ in the transitional forests and 1 mm d⁻¹ in the cerrado. The controls on evapotranspiration seasonality changed along the biome gradient, with evaporative demand (especially net radiation) playing a more important role in the wetter forests, and soil moisture playing a more important role in the drier savannah sites.

We describe the salient features of a field study whose goals are to quantify the vertical distribution of plant-emitted hydrocarbons and their contribution to aerosol and cloud condensation nuclei production above a central Amazonian rain forest. Using observing systems deployed on a 50-m meteorological tower, complemented with tethered balloon deployments, the vertical distribution of hydrocarbons and aerosols was determined under different boundary layer thermodynamic states. The rain forest emits sufficient reactive hydrocarbons, such as isoprene and monoterpenes, to provide precursors of secondary organic aerosols and cloud condensation nuclei. Mesoscale convective systems transport ozone from the middle troposphere, enriching the atmospheric boundary layer as well as the forest canopy and surface layer. Through multiple chemical transformations, the ozone-enriched atmospheric surface layer can oxidize rain forest–emitted hydrocarbons. One conclusion derived from the field studies is that the rain forest produces the necessary chemical species and in sufficient amounts to undergo oxidation and generate aerosols that subsequently activate into cloud condensation nuclei.

A dense Global Navigation Satellite System (GNSS) meteorological network (∼20 stations) in the central Amazon Basin in Brazil is being developed for long‐term studies of deep convection/water vapor interactions and feedback. In this article, the network is described and preliminary results are presented: GNSS‐derived precipitable water vapor is useful for tracking water vapor advection and in identifying convective events and water vapor convergence timescales. Upon network completion (early 2011), 3D water vapor field analyses and participation in the intensive field campaign GPM‐CHUVA will provide unique data sets for initializing, constraining or validating high‐resolution models or refining convective parameterizations. Copyright © 2011 Royal Meteorological Society