Biosphere-atmosphere Exchange: Insights from Measurements and Models
Edward G. Patton
National Center for Atmospheric Research
Forests cover a significant fraction of Earth’s land surface and play a critical role in Earth’s climate through their influence on energy, water, and carbon cycles, as well as through exchanges of reactive species that place stringent controls on the atmosphere’s oxidative capacity [or cleansing ability]. For these reasons, understanding the processes controlling turbulent exchange of energy, momentum, and scalars between the vegetation and the atmosphere has never been more important. Vegetation canopies produce turbulence that is qualitatively different to that over a rough surface, which ultimately results from canopies absorbing momentum over a distributed height range rather than at the ground surface. Within the canopy airspace, the distribution of the mean velocity and the turbulence is controlled by the interplay of downward turbulent transport of momentum and canopy drag modulated by diabatic influences. The aerodynamic drag of the canopy varies spatially based upon the distribution of the canopy elements, their efficiency at extracting momentum, and the velocity field itself. Similarly, the balance between turbulent transfer and the distribution of scalar sources and sinks determines within-canopy distributions of scalars like heat, water vapor, and carbon dioxide. These, in turn, respond to solar radiation as it attenuates through the foliage, the biological state of the plants (e.g., their access to soil water), the ambient concentration of the particular scalar in the canopy airspace, and, in the case of reactive scalars, their reaction rates. Current theory describing canopy exchange largely hinges on the hydrodynamic instability associated with an inflection point in the vertical profile of the horizontal wind at canopy top (sometimes called an inflection-point instability) induced through the canopy’s distributed momentum absorption. Parameterizations built upon this theory are showing great promise in predicting flux–gradient relationships in a canopy’s vicinity. However because the theory relies on the presence of wind speed shear at canopy top, its applicability across the broad stability variation that occurs outdoors remains uncertain. Utilizing a combination of measurements and models, this talk will: 1) present our current understanding of biosphere-atmosphere exchange, 2) highlight some new insights into atmospheric stability’s role in determining the spatial structure and distribution of motions controlling turbulent transfer at the canopy- atmosphere interface, and 3) discuss implications for parameterization of biosphere-atmosphere exchange in weather and climate models.
Wednesday, 5 October 2016, 2:00 PM
Refreshments 1:45 PM
3450 Mitchell Lane
Bldg 2 Large Auditorium (Rm1022)