Atmospheric LIDAR Provides Insight into Land Surface-Atmosphere Exchange at AmeriFlux Towers
Abstract
Lower levels of the planetary boundary layer (PBL) tend towards stable conditions at night with turbulent transfer of mass and energy exchange driven solely by mechanically forced turbulence, either from frictional forces near the ground or at the top of the plant canopy or from "top-down" forced intermittent bursts of turbulence generated from wind shear aloft. Nighttime mixing is further complicated by variable topography which creates complex wind flows including strong, along-valley-axis flows (wind direction shifts) and gravity-driven, mountain-valley flows that are particularly strong at night. Our estimates of terrestrial carbon sink or source magnitudes come primarily from flux towers which measure CO2, H2O, and energy exchange between the vegetated surface and the atmosphere with the eddy covariance technique. Flux towers are usually equipped with instrumentation only to the top of the plant canopy or a few meters above, although in reality, nocturnal wind shear can be strong well above the canopy and produce turbulent eddies which intermittently penetrate the canopy. The structures and drivers of these "top-down" forced events are currently being missed at most flux towers due to lack of instrumentation above the canopy. Here, we present over 800 hours of wind flow observations above two tree canopies for understanding nighttime turbulent transfer. Measurements of wind speed, direction and turbulence were taken with a Laser Detection and Ranging (LIDAR) instrument, co-located at the Tonzi AmeriFlux tower, California and the Wind River AmeriFlux tower, Washington. LIDAR provided high-resolution vertical profiles of wind shear and turbulence up to 200 m above the surface. To assess the contribution of turbulent transport to net ecosystem exchange, we assume that the efficiency of turbulence to transport mass and energy can be represented by the magnitude of turbulent kinetic energy (TKE). Although there are limitations to calculating accurate turbulence measurements from LIDAR, as will be discussed, LIDAR does provide observations well above the canopy for identifying drivers of "top-down" forced turbulence. Here, we show evidence of these intermittent turbulent bursts and identify times when they penetrate the plant canopy and influence CO2 exchange. At the Tonzi AmeriFlux tower, we also show evidence of advective flows driven by complex terrain to the east. The contribution of advective terms to CO2 exchange is often ignored in the carbon flux budget although we provide evidence that it should be considered even at sites with gentle local terrain. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2012
- Bibcode:
- 2012AGUFM.A13F0266W
- Keywords:
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- 0315 ATMOSPHERIC COMPOSITION AND STRUCTURE / Biosphere/atmosphere interactions;
- 0426 BIOGEOSCIENCES / Biosphere/atmosphere interactions;
- 0452 BIOGEOSCIENCES / Instruments and techniques