IV.7 Biosphere–Atmosphere Interactions in Landscapes
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IV.7 Biosphere–Atmosphere Interactions in Landscapes F. I. Woodward OUTLINE 1. Feedbacks of landscapes on the atmosphere: Theory 2. Observing feedbacks—comparing different sites 3. Time series of feedbacks 4. The nocturnal boundary layer 5. Conclusion 6. Overview This chapter investigates some of the ways in which a vegetated landscape can influence its own climate within the planetary boundary layer. During the daytime, impacts may be exerted up to 1–2 km above the surface and are caused by changes in energy exchange, predominantly evapotranspiration, such as that resulting from deforestation or leafing out in deciduous forests. At night, it has been shown that desert vegetation can significantly warm the air above, probably by the emission of greenhouseactive hydrocarbons. A major problem exists in identifying cause and effect in the recorded changes of the planetary boundary layer, as there appears to be no certainty that the landscape is causing all of the described effects, especially where climate is subjected to chaotic dynamics of varying origins. This feature is also true for climatic change studies at the global scale, where it is difficult to identify with certainty the causes of current climatic changes. GLOSSARY evapotranspiration. The evaporation of water vapor from surfaces plus the evaporation of water through the plant and leaf stomata by transpiration inversion layer. The cap of the planetary boundary layer, where there is little or no vertical mixing and where the temperature may increase or remain constant latent energy exchange. The exchange of energy by the evaporation of water planetary boundary layer (PBL). The lowest part of the atmosphere where the surface influences wind movements, humidity, and temperature over time periods of about 1 hour and up to 1 to 2 km above the surface sensible heat exchange. The exchange of energy as heat At the scale of a few meters, it is quite possible to experience feedbacks between the biosphere and the atmosphere. Under bright summer sunshine the air moving past your legs will be warmer if it has moved across dark dry sand than if it has passed through vegetation , primarily because of differences in radiation absorption and evapotranspiration. At the global scale, modifications of carbon dioxide exchange by the terrestrial and marine biospheres can change the global climate. Landscape-scale interactions are somewhere between these two extremes of scale, in the range of 10 to about 20,000 km2 . 1. FEEDBACKS OF LANDSCAPES ON THE ATMOSPHERE: THEORY The relevant part of the atmosphere that interacts with the landscape scale defined above is the lower part of the troposphere, the planetary boundary layer (PBL). This typically extends up to 2 to 3 km above the surface during the day. Convective exchange by buoyant thermals and mechanical turbulence by wind transport lead to energy and mass being exchanged between the surface of the biosphere and the PBL. At this time the surface is warmer than the air above. The top of the PBL is defined by sudden changes in temperature and decreases in specific humidity between it and the free atmosphere above. This inversion layer acts as a cap to the PBL and may incorporate clouds. The PBL grows in depth during the day, driven by increases in solar radiation , which warm the surface and lower layers of the PBL, increasing turbulence and entraining air from the free atmosphere above. At around sunset, thermals cease to form as solar radiation reaches zero, and the PBL collapses to form the nocturnal boundary layer (NBL). At this time, the surface generally becomes colder than the air above, and a shallow inversion layer tens to hundreds of meters in depth forms above the landscape. Plants in the landscape have the potential to influence the PBL both day and night, with active water and energy exchange during the day exerting influences on PBL development. Topography can also exert a marked and obvious effect, and valley bottoms may often be shrouded in cloud at sunrise. This results from the drainage of cold and dense air into the valley bottom ; this layer underlies warmer air above, and water condensation occurs where the warmer air is cooled to dew point temperature. The PBL, therefore, is the part of the atmosphere that is influenced in some way by the underlying landscape . In the typically heterogeneous landscapes on Earth, lateral wind movements combine with the convective upward and downward exchanges in the PBL (figure 1). The PBL consists of several layers that...