Toshihiko Maitani

On coherent structures in turbulence above and within agricultural plant canopies

Abstract The existence of ramp structures in scalar fields such as air temperature has been reported in laboratory flows over smooth and rough walls, in the atmospheric boundary layer and in flows in and above forests. They have been recognized as the signature of coherent turbulent structures. The aim of this paper is to present some observations and analyses of these features in the agricultural environment. Evidence is given from samples of time traces recorded during experiments conducted in maize crops and orchards. Ramps of air temperature, surface temperature, humidity and CO 2 concentrations are shown to occur under stable, neutral and unstable conditions. Ramp structures are more apparent above short canopies than within them, in contrast to taller tree canopies where ramps are seen most clearly near the canopy top. Under stable conditions, they are sometimes found in association with trapped gravity waves. It is demonstrated that the frequency of occurrence of the coherent structures is related to a wind shear scale characteristic of the canopy flow.

Joint probability analysis of momentum and heat fluxes at a deciduous forest

Turbulent transport processes for momentum and scalar quantities are examined by a joint probability distribution analysis using data observed within and above a deciduous forest. Characteristics of transport processes in the frequency domain were also analyzed using Tukeys procedure. The results confirm that sweep phenomena prevail within and at the top of a tall plant canopy and that downdrafts are more effective for vertical transport of momentum and scalar quantities. On the other hand, updrafts become more efficient for vertical transport in the daytime at levels about twice treetop height. The results show that within the forest, the sweep phenomenon prevails over a wide frequency range, while above the forest, prevalence of the ejection phenomenon is limited to low frequencies. It is again noted that the plant canopy plays an important role in the sweep-ejection cycle as well as in turbulent transport processes.

On the downward transport of turbulent kinetic energy in the surface layer over plant canopies

Downward fluxes of turbulent kinetic energy have been frequently observed in the air layer just above plant canopies. In order to investigate the mechanism for such downward transport, analysis of observational data is attempted. Height-dependency of turbulent kinetic energy flux and turbulence statistics including higher order moments is represented as a function of a non-dimensional height z/H, where z is an observational height and H an average height of plant canopies. Downward fluxes and non-Gaussianity of wind velocity fluctuations are predominant just above plant canopies and decrease with increasing height. The downward flux is closely related to the high intensity of turbulence and the non-Gaussianity of wind velocity fluctuations, especially with a positive skewness in the longitudinal wind and a negative skewness in the vertical wind. The analysis method of conditional sampling and averaging is applied to the present observations. The results show that the predominance of the intermittent inrush phase over the intermittent ejection phase leads to the above-mentioned non-Gaussianity. Finally, a simple explanation is given in order to interpret the turbulent flow structure in the air layer near the plant canopies, which is associated with the downward energy transport process.

Turbulent transport processes of momentum and sensible heat in the surface layer over a paddy field

A sonic anemometer-thermometer was used to measure turbulent fluxes of momentum and sensible heat and related turbulence statistics just above plant canopies in unstable conditions. The stability dependence of transport processes is presented, using observational data in a wide range of instability. The analysis of joint probability distributions of w − u, w − T, w − wu, and w − wT confirmed that just above plant canopies, downdrafts were remarkably efficient for vertical transport of momentum and scalar quantities in near neutral conditions. Furthermore, it was shown that updrafts became more efficient than downdrafts for vertical transport, especially of scalar quantities, in very unstable conditions.

An Observational Study of Wind-Induced Waving of Plants

The motions of individual plants and the turbulence statistics of surface winds measured near the top of a canopy are obtained over a wheat field and a rush field. Two typical cases of motions of individual plants are presented. The displacements of the ear of wheat (the plant height is 1.0 m) showed a natural oscillation in wind speeds of 1.6 m s−1 measured at a height of 30 cm over a wheat canopy, while displacements of the stem of a rush plant were closely related to the fluctuations of surface winds in wind speeds of 1.7 m s−1 measured at the top of the rush plant. The power spectra of displacements of a rush plant seem to support the negative seven-third power hypothesis proposed by Inoue. The frequency responses of displacements of plants to fluctuations of the instantaneous momentum flux are also presented.

Estimates of velocity-pressure and velocity-pressure gradient interactions in the surface layer over plant canopies

The budget equations of turbulent kinetic energy and shear stress contain interaction terms of velocity-pressure and velocity-pressure gradient. These terms were estimated in the surface layer using the air pressure observed at the surface and wind velocity components over plant canopies. The magnitude of the pressure interaction terms was significantly large; it was not negligible compared with the production terms in each budget equation. The present results obtained over a rough surface also confirmed previous results that pressure terms play an important role in the turbulent kinetic energy budgets and the shear stress budget. The height dependency of nondimensional pressure terms versus (z - d)/z0 was not clear.

A comparison of turbulence statistics in the surface layer over plant canopies with those over several other surfaces

Turbulence statistics, including higher order moments, in the surface layer over plant canopies were compared with those observed over several different surfaces, using a nondimensional height (z − d)/z0: The values of (z − d)/z0extend over a very wide range from 10 over plant canopies to 107 over the ocean. Several properties such as intensities of turbulence and skewness factors show a remarkable height-dependency in the air layer below (z − d)/z0 = 102, which is supposed to be much influenced by the underlying surface. In that layer, some peculiar phenomena, such as a downward energy transport and positive flux of shear stress, are frequently observed.

A STUDY OF THE ATMOSPHERIC BOUNDARY LAYER USING RADON AND AIR POLLUTANTS AS TRACERS

Concentrations of radon 222Rn andair pollutants, meteorological parametersnear the surface and vertical profiles of meteorological elements were measured atUchio (Okayama City, Okayama Prefecture, Japan) 12 km north from the coast ofthe Inland Sea of Japan. In the nighttime, the 222Rn concentration increased in the case of weak winds, but did not increase as much in the case of moderate or strong winds, as had been expected. In the daytime, the 222Rn concentrationheld at a slightly higher than average level for the period from sunrise to about 1100 JST. It is considered that this phenomenon is due to a period of morning calm, that is, a transition period from land breeze to sea breeze.NO, which is sensitive to traffic volume,brought information concerning advection.Oxidant concentrations,which reflect the availability of sunlight,acted in the reverse manner to 222Rnconcentrations. Thus, a set of 222Rn and air pollutants could provide useful information regarding the local conditions of the atmospheric boundary layer.

Vertical transport of turbulent kinetic energy in the surface layer over a paddy field

Turbulent kinetic energy and its vertical flux were measured at two heights over a paddy field. The vertical transport of turbulent kinetic energy was always downward right above the paddy field and was frequently downward at higher levels within a few metres above the crop. Contributions to the downward transport arise mainly from the turbulent kinetic energy of horizontal wind velocity components. It is shown from the analysis of probability distributions that appreciable transport takes place intermittently in a few large downward bursts and that these downdrafts are efficient for downward energy transport.In the budget of turbulent kinetic energy, the flux divergence term and the energy dissipation term are the main loss terms under unstable conditions. These terms increase in magnitude with increasing instability. Buoyant production is insufficient to balance these losses. The imbalance term involving the pressure-work term is probably one of the main energy sources in unstable conditions.

Concentrations of 222Rn, Its Short-Lived Daughters And 212Pb And Their Ratios Under Complex Atmospheric Conditions And Topography

Atmospheric activity concentrations of 212Pb and short-lived 222Rndaughters, together with meteorological elements, have been observed continuously atthree sites at Kamisaibara Village in Japan. In addition, atmospheric activity concentrationof 222Rn, equilibrium-equivalent concentration of 222Rn and conditionsof the lower atmosphere were observed for three intensive observation periods at Akawase,one of the three sites in Kamisaibara Village. The equilibrium-equivalent concentration of222Rn is almost the same as the atmospheric activity concentration of short-lived222Rn daughters.The activity concentrations of 212Pb and the short-lived 222Rn daughtersand their ratio were low in the daytime owing to convective mixing, and high at nightowing to the surface-based inversion during periods of no precipitation. Their variationshave several patterns corresponding to the scale of the drainage wind or weak mixing.Mechanical mixing due to strong winds through both day and night during the first andsecond observation periods made the atmospheric activity concentrations of 212Pb and the short-lived 222Rn daughters continuously low. However, their ratios werecontinuously high during the first period yet continuously low during the second period.This difference can be explained by the effect ofextraction of 220Rn and 222Rndue to strong winds and snow cover. There were also cases in which the ratio of theatmospheric activity concentration of 212Pb to that of the short-lived 222Rndaughters at night was equal to or less than the ratio in the daytime. Thisinverse trend, asin the periods of no precipitation mentioned above, is considered to be due to near-neutralconditions on these nights.We find a difference in the ratio of the equilibrium-equivalent concentration of222Rn (the activity concentration of short-lived 222Rn daughters) tothe activity concentration of 222Rn during the first observation period and thatduring the second. The difference can be explained by snow cover on the ground. Wealso find differences among the ratios of the activity concentration of the short-lived222Rn daughters to that of 222Rn during the three observation periods.These differences can be explained by the submergence of paddy fields.