Acoustic sounding of the atmospheric boundary layer at Halley, Antarctica

The records obtained from a monostatic acoustic sounder run at Halley, Antarctica, have been analysed with the use of data from instruments on a 32 m mast and from radiosonde ascents. Echoes representing ground-based layers, waves, and shallow gravity currents are discussed. The spiky ground-based echo is related to a westerly surface wind, whilst a layered wavy flow is related to surface easterlies. Such relationships are consistent with the sloped inversion wind regime at Halley

Carbon dioxide transfer over a Central Amazonian rain forest

Tropical rain forests are among the most important and least monitored of terrestrial ecosystems. In recent years, their influence on atmospheric concentrations of carbon dioxide and water vapor has become the subject of much speculation. Here we present results from a yearlong study of CO2 fluxes at a tropical forest in central Amazonia, using the micrometeorological technique of eddy covariance. The diurnal cycle of CO2 flux was consistent with previous short-term studies in tropical rain forests, implying that the Amazonian rain forest shows a fair degree of spatial uniformity in bulk ecophysiological characteristics. Typical peak daytime photosynthesis rates were 24-28 μmol CO2 m-2 s-1, and respiration rates were 6-8 μmol CO2 m-2 s-1. There was significant seasonality in peak photosynthesis over the year, which appeared strongly correlated with soil moisture content. On the other hand, there was no evidence of strong seasonality in soil respiration. Central Amazonia has only a short, 3-month dry season, not atypical of tropical rain forest, and it is therefore likely that large areas of Amazonia exhibit significant seasonality in photosynthetic capacity. The gross primary production was calculated to be 30 t C ha-1 yr-1. An analysis of data quality is included in the appendix. Copyright 1998 by the American Geophysical Union

Carbon dioxide measurements in the noctural boundary layer over Amazonian forest

Measurements of carbon dioxide concentration, temperature and windspeed were made in the nocturnal boundary layer over a tropical forest near Manaus, Brazil using a tethered balloon system. The measurements were made up to a maximum height of 300 m on ten consecutive nights in November 1995. Simultaneous surface flux and in-canopy concentration measurements were made at the surface close to the site. The observation period included several different types of conditions. Generally strong windshear and relatively weak temperature gradients prevented the formation of a strong capping inversion to the nocturnal boundary layer. On some nights, however, the inversion was sufficiently strong that the CO2 concentration at 100 m above the surface exceeded 400 ppm. The concentration within the canopy was largely controlled by the presence of an inversion very close to the canopy surface. The temperature and wind profiles are contrasted with conditions in Rondonia, Brazil, where the windshear was found to be weaker and higher carbon dioxide concentrations were observed in the early morning. The difference in carbon dioxide concentrations in the nocturnal boundary layer between dusk and dawn is used to estimate the regional nighttime flux of carbon dioxide. The value obtained generally exceeds the measured surface flux and sometimes exceeds the sum of the surface flux and the in-canopy storage made at the tower site. The reasons for the discrepency are not clear; either one of the methods is in error or the regional carbon dioxide budget differs significantly from the local budget measured at the tower site

Biogeochemical cycling of carbon, water, energy, trace gases, and aerosols in Amazonia: The LBA-EUSTACH experiments

The biogeochemical cycling of carbon, water, energy, aerosols, and trace gases in the Amazon Basin was investigated in the project European Studies on Trace Gases and Atmospheric Chemistry as a Contribution to the Large-Scale Biosphere-Atmosphere Experiment in Amazonia (LBA-EUSTACH). We present an overview of the design of the project, the measurement sites and methods, and the meteorological conditions during the experiment. The main results from LBA-EUSTACH are: Eddy correlation studies in three regions of the Amazon Basin consistently show a large net carbon sink in the undisturbed rain forest. Nitrogen emitted by forest soils is subject to chemical cycling within the canopy space, which results in re-uptake of a large fraction of soil-derived NOx by the vegetation. The forest vegetation is both a sink and a source of volatile organic compounds, with net deposition being particularly important for partially oxidized organics. Concentrations of aerosol and cloud condensation nuclei (CCN) are highly seasonal, with a pronounced maximum in the dry (burning) season. High CCN concentrations from biomass burning have a pronounced impact on cloud microphysics, rainfall production mechanisms, and probably on large-scale climate dynamics. Copyright 2002 by the American Geophysical Union