Biometeorology, ESPM 129 1Lecture 20 Wind and Turbulence, Part 1, Canopy Air Space: Observations and Principles Instructor: Dennis Baldocchi Professor of Biometeorology Ecosystem Science Division Department of Environmental Science, Policy and Management 345 Hilgard Hall University of California, Berkeley Berkeley, CA 94720 October 17, 2012 A. Processes A. Wind and Turbulence 1. Concepts 2. Conservation equation for wind 3. TKE budget, conceptual B. Variation in Space 1. Mean Wind Profiles within Vegetation 2. Turbulence Statistics in Vegetation L20.1 Introduction I’ll huff and puff and blow your house down Big-Bad Wolf The structure and characteristics of wind and turbulence inside plant canopies is more complex than what is observed in the surface layer above vegetation. A canopy acts as a porous medium. How wind interacts with layers of leaves and stems that are situated above the solid lower boundary, the soil, is much different than how it varies in the surface boundary layer, as has been discussed in the past few lectures. As biometeorologists, we are interested in how wind varies in the vicinity of plants for a number of reasons: 1. Wind affects rates of trace gas and particle exchange through its impact on the thickness of the leaf and soil boundary layers and on the mixing, diffusion and advection of scalars in the canopy air space. 2. It has a mechanical effect on plants by causing them to sway and bend. Honami, the waving of crops, is a wind-generated phenomenon. Stems bend over when aBiometeorology, ESPM 129 2wind gust passes and re-bound in the lee of the gust. The plant’s bend oscillates in accordance to the natural frequency of their stalks. Gusts have streamwise periodicities of 5 to 8 canopy heights. The phase velocity equals 2 U(h). 3. In climates with persistent winds, such forcing will affect the shape of tree crowns. Krummholz trees near the alpine tree line or bent over Cypress trees at Point Reyes are classic examples of wind forcing growth shape. High winds can also cause abrasion, as limbs rub against on another or the movement of sand sandblasts and tears leaves. Under extreme wind loads, catastrophic damage can occur as when tree blow over and crops lodge. Wind affects plant pathology by causing spores to be released and dispersed. 4. Finally, a phenomenon, known as thigmomorphogenesis, has been reported to affect plant growth in the presence or absence of wind. Several types of air flow are associated with biometeorology [Lee, 2000]. These include: 1. wind through homogeneous forest and crop canopies 2. flow into forests and hedges 3. flow exiting forests and hedges 4. flow in isolated clearings 5. flow through forests on complex terrain 6. drainage flow at night on complex terrain and tall vegetation. Physical interactions between plants, soil and wind involve the transfer and conversion of momentum, kinetic energy and work: 1. aerodynamic drag of plant parts extracts momentum from the mean wind flow. 2. plant parts break down large scale eddies into smaller eddies. 3. behind obstructing elements, kinetic energy of the mean flow is converted into turbulent kinetic energy of the wakes. 4. in aeroelastic canopies, mean kinetic energy is used to create waving foliage or wind loads, which can bend or break vegetation In this lecture we will discuss that statistical properties of wind and turbulence in vegetation and how they vary in space. This information is needed to parameterize models and to understand the turbulent transfer and diffusion of material in and outside of canopies. L20.2 Mean Wind Profiles Figure 1 shows a typical profile of wind above and below a crop, soybeans. Several characteristics merit note. Above the canopy one observes the now-familiar logarithmicBiometeorology, ESPM 129 3wind profile. At the canopy-atmosphere interface there is a large shear (du/dz) induced inflexion in the wind profile. This region is followed by zone with an exponential decrease in wind speed. A secondary wind maximum often occurs deep in the canopy in the stem space. Below this level, there is another logarithmic wind profile between the stem space and the ground. Figure 1 wind profile in a soybean canopy. Note the secondary wind maximum. [Baldocchi et al., 1983]. Numerous investigators have attempted to quantify wind profiles within canopies using an exponential relation. The early and classic papers on this topic were conducted in the soybeansAug 4, 1980, 1445 hrwind speed012345height (m)0123Biometeorology, ESPM 129 41960s and early 1970s [Cionco, 1965; Landsberg and Jarvis, 1973; Uchijima and Wright, 1964]. Simple models for wind in canopies follow an exponential decay with depth into the canopy from the top, like light, because cumulative leaf area index exerts drag on the wind and attenuates it: uz uzhh() exp(( ))1 Phenomenological studies have parameterized the wind extinction coefficient as a function of canopy height (h), leaf area, and drag coefficients associated with the canopy and leaf elements haCCdd'2 A survey of values is listed below. Vegetation Reference immature corn 2.8 [Cionco, 1972a] oats 2.8 [Cionco, 1972a] wheat 2.5 [Cionco, 1972a] corn 2.0 [Cionco, 1972a] Sunflower 1.3 [Cionco, 1972a] Larch [Cionco, 1972a] deciduous forest [Baldocchi and Meyers, 1988a] jack pine [Amiro, 1990a] Spruce 2.4 [Amiro, 1990a] Soybean [Baldocchi et al., 1983]Biometeorology, ESPM 129 5Exponential Wind Profileu(z)/u(h)0.0 0.2 0.4 0.6 0.8 1.0z/H0.00.20.40.60.81.0a=3a=2a=1 The secondary wind maximum is a unique aspect of wind flow inside turbulence. It presence had a revolutionary much impact on how we view theoretical transfer of mass and momentum in plant canopies ([Shaw, 1977; N R Wilson and Shaw, 1977]. The earliest citation I have found, so far, on wind profiles in forests and their secondary maxima is attributed to Fons [Fons, 1940] in 1940s! The observation of a secondary wind maxima suggests counter-gradient transfer since momentum transfer is directed downward and is diminishing with depth, despite the local increase in wind velocity. It also led to a major finding that K theory is wrong, when applied in vegetation. Wilson and Shaw [N R Wilson and Shaw, 1977] applied a second order closure model to assess this phenomenon. The budget equation for Reynold's stress (<w'u'>), assuming horizontal homogeneity, steady-state