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The Amazonian boundary layer and mesoscale circulations

The interactions between the Amazonian boundary layer, the surface, atmospheric convection, aerosols, and larger-scale circulations are complex. The field experiments in Amazonia have provided rich insights into the daytime and nighttime boundary layer in different regions and seasons over both forest and pasture and into the coupling between the surface fluxes, the boundary layer, precipitation, and cloud radiative forcing. We discuss the typical diurnal cycle of Amazonian convection, the self-organization into mesoscale systems in different synoptic regimes, and the role of forest and river breeze circulations. We review the coupling between aerosols, smoke, and convection in the dry season; ozone transports by deep convection; and microphysical and electrical impacts on convection.

Understanding the complex interaction of clouds, rain and the biosphere in the Amazon was the reason for the Large-Scale Atmosphere-Biosphere Experiment in Amazonia (LBA) [Silva Dias et al., 2002a]. The focus of this chapter is the Amazonian boundary layer (BL) and its coupling with the surface, atmospheric convection and larger-scale circulations. An early BL study using radiosoundings and tethered balloons was made during the GTE-ABLE-2B (Global Tropospheric Experiment Amazon Boundary Layer Expedition) [Martin et al., 1988] at a tropical forest site near Manaus; followed by a longer study at contrasting forest and pasture sites during the Rondônia Boundary Layer Experiment (RBLE) [Nobre et al., 1996]. During the LBA experiment, two main field campaigns took place to characterize the structure and evolution of the BL [Fisch et al., 2004]. The first was during the wet season in early 1999: the Wet-Season Atmospheric Mesoscale Campaign (WETAMC), and the coincident Tropical Rainfall Measuring Mission (TRMM). The second field program was during the dry-to-wet transition season, September to November, 2002; and it also had two components, the study of Radiation, Cloud, and Climate Interactions (RACCI-LBA), and the study of the interaction of SMOke aerosols, Clouds, rainfall and Climate (LBA-SMOCC).

Plain English Discussion

This paper summarizes what we have learnt from an international experiment that has been studying how the forest, convective clouds, rain and smoke from fires are linked together over the Amazon forest. We review the climate over the Amazon and the difference between the wet and dry seasons. The forest has deep roots that can draw up soil water in the dry season. We show how evaporation from the forest is linked to the surface relative humidity and to cloud base and cloud cover. We discuss the role of thunderstorms and the transport of smoke, aerosols and ozone over the Amazon.

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Full Citation

Betts, A.K., G. Fisch, C. von Randow, M.A.F. Silva Dias, J.C.P. Cohen, R. da Silva and D.R. Fitzjarrald (2009), The Amazonian boundary layer and mesoscale circulations. P163-181 in “Amazonia and Global Change”, Michael Keller, Mercedes Bustamante, John Gash, and Pedro Silva Dias, Editors, Geophys. Monogr. Ser., 186, 576 pp., hardbound, 2009, ISBN 978-0-87590-476-4.