Marcelo Chamecki

A study of spectra, structure and correlation functions and their implications for the stationarity of surface-layer turbulence

The related concepts of stationarity and the existence and values of integral timescales are central to the ability of analyzing micrometeorological data within theframework of Monin–Obukhov similarity theory. Not only does the theory strongly hinge on the stationarity assumption, the estimation of turbulence moments and their accuracies are dependent on the values of the correspondent integral time scales. In spite of the general importance of these concepts, there are relatively few studiesconcerned with them. Moreover, although each turbulence variable has its ownintegral scale, this fact is often overlooked when numerical values are estimated.In this work we study three daytime events of surface inversion formation, that is,events where a nonstationary period is clearly present. Our analysis reveals alow-frequency component in the temperature data that is not totally removed bya simple (but often used in turbulence data analysis) first-order recursive filter.This component has to be filtered out in the frequency domain, after which we areable to recover similarity between temperature and humidity statistical descriptors(in this case, the structure function). After applying a simple criterion to estimatenumerical values of the integral time scales, we are able to assess the relationshipsbetween the existence of integral scales and the stationarity of the correspondingprocess. Finally, we find out that in the case of second-order moments the Sarmanovtheorem does not always apply. The implications for accuracy estimates of thesemoments are then briefly discussed.

Pollen clumping and wind dispersal in an invasive angiosperm

Pollen dispersal is a fundamental aspect of plant reproductive biology that maintains connectivity between spatially separated populations. Pollen clumping, a characteristic feature of insect-pollinated plants, is generally assumed to be a detriment to wind pollination because clumps disperse shorter distances than do solitary pollen grains. Yet pollen clumps have been observed in dispersion studies of some widely distributed wind-pollinated species. We used Ambrosia artemisiifolia (common ragweed; Asteraceae), a successful invasive angiosperm, to investigate the effect of clumping on wind dispersal of pollen under natural conditions in a large field. Results of simultaneous measurements of clump size both in pollen shedding from male flowers and airborne pollen being dispersed in the atmosphere are combined with a transport model to show that rather than being detrimental, clumps may actually be advantageous for wind pollination. Initial clumps can pollinate the parent population, while smaller clumps that arise from breakup of larger clumps can cross-pollinate distant populations.

Estimating the Random Error in Eddy-Covariance Based Fluxes and Other Turbulence Statistics: The Filtering Method

A spatially local decomposition of turbulent fluxes based on properties of spatial filters is used to develop a new method of estimating random error in turbulent moments of any order. The proposed error estimation method does not require an estimate of the integral time scale, which can be highly sensitive to the method used to calculate it. The error estimation method is validated using synthetic flux data with a known ensemble mean and intercompared with existing methods using data from the Advection Horizontal Array Turbulence Study (AHATS). Typical errors for a 27.3-min block of data collected at a height of 8 m are found to be approximately 10% for the heat flux and 7–15% for variances. The error in the momentum flux increases rapidly with increasing atmospheric instability, reaching values of 40% or greater for unstable conditions. A new method based on filtering is also proposed to estimate integral time scales of turbulent quantities.

The influence of local meteorological conditions on the circadian rhythm of corn (Zea mays L.) pollen emission

Field experiments were performed to study the diurnal cycle of corn pollen emission and its relation to local meteorological conditions, including temperature, relative humidity, solar radiation, mean wind speed, and turbulence quantities. Pollen concentrations were measured from canopy height to twice this level using four Rotorod samplers located on a pole in the middle of the corn field. The measured pollen concentration at canopy height was used as a surrogate for the pollen source strength while the concentrations above canopy represented the pollen transported upwards from canopy height. At twice the canopy height, the pollen concentration decreased considerably to about 30% of the canopy height values. During the mornings, pollen was emitted in large quantities while during the afternoons, airborne pollen concentrations decreased and no significant atmospheric pollen was measured from about 2 h prior to sunset until sunrise the next morning. The actual time that airborne pollen was first recorded differed from day-to-day and depended on the time required for the anthers to dry and open, as well as there being sufficiently strong winds to entrain the pollen away from the plants. On four consecutive mornings diurnal atmospheric pollen concentration distributions were bi-modal in time. The first pollen concentration peak happened shortly before the direct irradiance peak on the anthers suggesting that direct solar irradiation might be important for drying the anthers. The subsequent dip in pollen concentration seemed to be linked to a lull in mean and turbulent wind conditions. Analysis of the vertical velocity fluctuations, sw, showed that the fraction of pollen transported upwards from canopy height increased with increasing sw. In addition, Quadrant– Hole analysis applied to the turbulence data sets of 2 days suggested that low values of ejection duration fractions were associated with low values of pollen concentration, while high values were associated with high concentration values. The diurnal pattern of ejection duration fractions was similar to the pattern of the fraction of pollen that reached twice the ting the important effect of large-scale coherent ejections on pollen # 2008 Elsevier B.V. All rights reserved. canopy height, indica

On the Nature of the Transition Between Roll and Cellular Organization in the Convective Boundary Layer

Both observational and numerical studies of the convective boundary layer (CBL) have demonstrated that when surface heat fluxes are small and mean wind shear is strong, convective updrafts tend to organize into horizontal rolls aligned within 10–20

Persistence of velocity fluctuations in non-Gaussian turbulence within and above plant canopies

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The Local Structure of Atmospheric Turbulence and Its Effect on the Smagorinsky Model for Large Eddy Simulation

∘ of the geostrophic wind direction. However, under large surface heat fluxes and weak to negligible shear, convection tends to organize into open cells, similar to turbulent Rayleigh-Bénard convection. Using a suite of 14 large-eddy simulations (LES) spanning a range of