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THE CANOPY HORIZONTAL ARRAY TURBULENCE STUDY B Y Ed W a R D G

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THE CANOPY HORIZONTAL ARRAY TURBULENCE STUDY B Y Ed W a R D G THE CANOPY HORIZONTAL ARRAY TURBULENCE STUDY BY EDWARD G. PATTON , THOMAS W. HORST , PE T E R P. SULLIVAN , DONALD H. LE NSCHOW , ST E V E N P. ONCL E Y , WILLIAM O. J. BROWN , SE AN P. BURNS , AL E X B. GU E NTH E R , ANDR E AS HE LD , THOMAS KARL , SHAN E D. MAYOR , LUCIANA V. RIZZO , SCOTT M. SP UL E R , JI E LUN SUN , ANDR E W A. TURNI P S ee D , EU ge N E J. ALLWIN E , ST E V E N L. EDBUR G , BRIAN K. LAMB , RONI AVISSAR , RONALD J. CALHOUN , JAN KL E ISSL , WILLIAM J. MASSMAN , KYAW THA PAW U, AND JE FFR E Y C. WE IL Observations from a California walnut orchard—before and after leaves emerged— should help advance understanding, simulation capabilities, and modeling of coupled vegetation–atmosphere–land surface interactions. OTIVATION. Vegetation covers nearly 30% absorbed throughout the canopy depth as drag on the of Earth’s land surface and influences climate plant elements rather than as friction on the ground. M through the exchanges of energy, water, carbon Over the years, it has become clear that the bulk of the dioxide, and other chemical species with the atmo- exchange between the canopy layers and aloft occurs sphere (Bonan 2008). The Earth’s vegetation plays a through turbulent eddies that are of similar scale to critical role in the hydrological, carbon, and nitrogen the canopy itself rather than to the scale of the indi- cycles and also provides habitat and shelter for biota vidual canopy elements (e.g., Gao et al. 1989). Recent that deliver essential ecosystem services, such as polli- studies postulate that these eddies are produced by nation. In addition, living foliage produces a variety of larger PBL-scale turbulent motions triggering an in- chemical compounds that can significantly influence stability associated with the inflection in the velocity the oxidation capacity (cleansing ability) of the atmo- profile induced by plant canopy drag (e.g., Raupach sphere (e.g., Fuentes et al. 2000; Guenther et al. 2006) et al. 1996; Finnigan et al. 2009). and global aerosol distributions (e.g., Hallquist et al. The ground and vegetation can act as both scalar 2009). Plant–atmosphere interactions can also have sources and sinks. Within the soil, heterotrophic and negative effects with billions of dollars each year lost autotrophic respiration is a source of CO2 throughout from wind damage to forests and crops. Therefore, the year that varies seasonally with soil tempera- understanding the processes controlling vegetation– ture and moisture (e.g., Ryan and Law 2005), while atmosphere exchange at the most fundamental level decomposing litter and soil microbes can produce is of critical importance for weather, climate, and volatile organic compounds (VOCs) (e.g., Leff and environmental forecasting as well as for agricultural Fierer 2008). The distribution of canopy sources/ and natural resource management. sinks depends on the amount and state of the canopy foliage, which varies throughout the seasonal cycle Turbulent exchange within and above vegetation. Tur- for deciduous trees: from bare limbs in winter (no bulence in plant canopies is unique from turbulence photosynthesis and an open canopy) to rapid growth elsewhere in the planetary boundary layer (PBL), in in spring (increasing photosynthesis and canopy that the majority of the atmosphere’s momentum is density) to maturity in summer (more constant AMERICAN METEOROLOGICAL SOCIETY MAY 2011 | 593 photosynthesis and canopy density) to senescence In canopy-resolving LES,1 the pressure and viscous and leafdrop in fall (decreasing photosynthesis and drag associated with the vegetation elements serve canopy density). Thus, a broad spectrum of different as an important momentum sink, which induces conditions occurs through the year, and dynamical an inflection in the velocity profile and introduces and scalar fluxes exhibit height dependence and sea- turbulent length scales such as the canopy depth or sonal variability. State-of-the-art analytic or closure the eddy penetration depth (e.g., Finnigan et al. 2009). models describing these processes are only begin- Intriguingly, increased grid resolution improves the ning to address these complexities (e.g., Harman and accuracy of the simulation by better resolving the Finnigan 2008). velocity gradient at canopy top; contrast this with LES over unresolved roughness where increased Linking models and measurements. Large-eddy simu- resolution continually reduces the scale of the peak lation (LES) is one of the best available numerical in the vertical velocity energy spectrum, which tools capable of linking turbulent motions over scales forces continued reliance on SFS parameterizations ranging from the microscale to the mesoscale (e.g., (Sullivan et al. 2003). Wyngaard 1984). Using a three-dimensional grid, LES solves the time-dependent filtered Navier–Stokes Previous HATS campaigns. Despite its myriad contri- equations to simulate all resolved scales of motion as butions to understanding turbulent flows, LES does they evolve in space and time while using a subfilter- have shortcomings and needs to be validated and scale (SFS) model to parameterize only the smallest improved to deal with complex flows, especially for scales (Pope 2000). Another important component surface layers where dependence on the SFS model of LES is that active, passive, and reactive scalars increases. To address this issue, the National Center can be incorporated. As numerical techniques and for Atmospheric Research (NCAR) in collaboration computational capabilities have improved over the with several university groups recently carried out last 30 years, LES results have become a key comple- two pioneering observational studies to improve ment to measurements. subfilter-scale parameterizations over flat terrain with sparse low-lying vegetation [Horizontal Array Turbulence Study (HATS)] and over the ocean [Ocean HATS (OHATS)] (e.g., Sullivan et al. 2003; Horst AFFILIATIONS: PATTON , HORST , SULLIVAN , LE NSCHOW , ONCL E Y , et al. 2004). These studies applied a technique first BROWN , BURNS , GU E NTH E R , HE LD ,* KARL , MAYOR ,* RIZZO ,* SP UL E R , SUN , AND TURNI P S ee D —National Center for Atmospheric proposed by Tong et al. (1998) that uses horizontal Research,+ Boulder, Colorado; ALLWIN E , EDBUR G ,* AND LAMB — arrays of 14–18 sonic anemometers/thermometers Washington State University, Pullman, Washington; AVISSAR *— deployed at two levels to measure spatially filtered Duke University, Durham, North Carolina; CALHOUN —Arizona variables and their gradients. These datasets provide State University, Tempe, Arizona; KL E ISSL —University of an observational basis for validating and developing California, San Diego, La Jolla, California; MASSMAN —USDA Forest SFS closure approximations. Service, Fort Collins, Colorado; PAW U—University of California, Davis, Davis, California; WE IL —University of Colorado, Boulder, Canopy influence on SFS motions. Colorado Vegetation adds *CURREnt affiliations: HE LD —University of Bayreuth, complexity to SFS motions. Canopy elements are Bayreuth, Germany; MAYOR —California State University, Chico, nonuniformly distributed spatially (often occur- California; RIZZO —University of Sao Paulo, Sao Paulo, Brazil; ring in clumps) and thereby spread sources/sinks EDBUR G —University of Idaho, Moscow, Idaho; AVISSAR —University of momentum and scalars throughout the canopy of Miami, Miami, Florida layer. For example, large-scale turbulence interacting + The National Center for Atmospheric Research is sponsored by with clumped elements can rapidly break down into the National Science Foundation. smaller wake-scale motions, thereby short-circuiting CORRESPONDING AUTHOR: Edward Patton, National Center for Atmospheric Research, P.O. Box 3000, Boulder, CO, the inertial energy cascade (Finnigan 2000). In 80307–3000 addition, depending on the density of the canopy E-mail: [email protected] elements, the spatially distributed plant structures The abstract for this article can be found in this issue, following the intercept (and are heated by) solar radiation during table of contents. DOI:10.1175/2010BAMS2614.1 1 Canopy-resolving LES means that the equations of motion In final form 15 October 2010 are spatially filtered in the presence of solid canopy elements, ©2011 American Meteorological Society which generates terms in the equations accounting for the canopy-induced pressure and viscous drag on the fluid. 594 | MAY 2011 daytime and radiatively cool faster than a bare surface on SFS motions but lack the ability to assess the impact at night. As a result, in daytime (nighttime) the leaves of vegetation on scalar transport and to characterize can create stable (unstable) conditions within the the impact of the larger scales of motion and thermal canopy when the overlying atmosphere is unstably stratification in the planetary boundary layer. (stably) stratified. Given the fundamentally different mechanisms of momentum and heat transport to and Hence, CHATS. In practice, most canopy measure- from the foliage (where momentum transport occurs ments are typically limited to time averages from ver- largely through pressure drag and heat transport tical arrays of sensors at a single location. Exceptions occurs through the much slower process of molecular to this have become more common in recent years, diffusion), within-canopy turbulence can also com- with explicit spatial averaging in wind tunnel model pletely collapse and decouple from the above-canopy canopies (e.g., Böhm et al. 2000) and multiple tower turbulence (Belcher et al.
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