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Progress in understanding land-surface-atmosphere coupling from LBA research

LBA research has deepened our understanding of the role of soil water storage, clouds and aerosols in land-atmosphere coupling. We show how the reformulation of cloud forcing in terms of an effective cloud albedo per unit area of surface gives a useful measure of the role of clouds in the surface energy budget over the Amazon. We show that the diurnal temperature range has a quasi-linear relation to the daily mean longwave cooling; and to effective cloud albedo because of the tight coupling between the near-surface climate, the boundary layer and the cloud field. The coupling of surface and atmospheric processes is critical to the seasonal cycle: deep forest rooting systems make water available throughout the year, whereas in the dry season the shortwave cloud forcing is reduced by regional scale subsidence, so that more light is available for photosynthesis. At sites with an annual precipitation above 1900 mm and a dry season length less than 4 months, evaporation rates increased in the dry season, coincident with increased radiation. In contrast, ecosystems with precipitation less than 1700 mm and a longer dry season showed clear evidence of reduced evaporation in the dry season coming from water stress. In all these sites, the seasonal variation of the effective cloud albedo is a major factor in determining the surface available energy. Dry season fires add substantial aerosol to the atmosphere. Aerosol scattering and absorption both reduce the total downward surface radiative flux, but increase the diffuse/direct flux ratio, which increases photosynthetic efficiency. Convective plumes produced by fires enhance the vertical transport of aerosols over the Amazon, and effectively inject smoke aerosol and gases directly into the middle troposphere with substantial impacts on mid-tropospheric dispersion. In the rainy season in Rondônia, convection in low-level westerly flows with low aerosol content resembles oceanic convection with precipitation from warm rain processes and little electrification. But in the same region in the dry season, widespread fires produce a high aerosol loading with high numbers of cloud condensation nuclei from biomass burning; and convection is then dominated by ice-phase processes giving deep clouds with frequent lightning and convective tops in the lower stratosphere. Recent studies based on measurements of CCN and cloud droplet distributions have successfully modeled this wide range of convective regimes, and shown the fundamental link between cloud droplet spectra, convective structure and precipitation. The regional scale circulation responds to precipitation and aerosol forcing, as well as the memory provided by soil moisture. The field observations from LBA have been essential to identifying the interactions of critical processes, and for the development and evaluation of our models of the coupled system.

Plain English Discussion

This is a review of what we learnt in the past ten years about the links between the Amazon forest and the atmosphere during the LBA experiment (LBA stands for Large-scale Biosphere-Atmosphere). Many things are involved:

  • heavy seasonal rainfall which fills deep reserves of water in the soil, so that the forest can survive through the dry season
  • diffuse light (scattered by clouds and aerosols) which can penetrate deep into forest canopies and increase photosynthesis and the uptake of CO2
  • reflection of sunlight by clouds
  • dry season fires, when the forests burn and put lots of smoke and aerosols into the atmosphere, which produce many smaller droplets in the clouds.

The accuracy of our daily weather forecasts for S. America, and our climate models predictions for this century, depend on how well we can model all these complex processes.

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Full Citation

Betts, A. K. and M. A. F. Silva Dias (2010), Progress in understanding land-surface-atmosphere coupling over the Amazon: a review. J. Adv. Model. Earth Syst. 2(6), 20 pp., doi:10.3894/JAMES.2010.2.6. [Published Online 30 Jun. '10] http://james.agu.org/index.php/JAMES/article/view/v2n6/JAMES.2010.2.6