J.H. van Boxel

The effect of turbulent flow structures on saltation sand transport in the atmospheric boundary layer

The effect of turbulent flow structures on saltation sand transport was studied during two convective storms in Niger, West Africa. Continuous, synchronous measurements of saltation fluxes and turbulent velocity fluctuations were made with a sampling frequency of 1 Hz. The shear stress production was determined from the vertical and streamwise velocity fluctuations. The greatest stress-bearing events were classified as turbulent structures, with sweep, ejection, inward interaction, and outward interaction described according to the quadrant technique. The classified turbulent structures accounted for 63·5 per cent of the average shear stress during the first storm, and 56·0 per cent during the second storm. The percentage of active time was only 20·6 per cent and 15·8 per cent, respectively. High saltation fluxes were associated with sweeps and outward interactions. These two structures contribute positively (sweeps) and negatively (outward interactions) to the shear stress, but have in common that the streamwise velocity component is higher than average. Therefore, the horizontal drag force seems primarily responsible for saltation sand transport, and not the shear stress. This was also reflected by the low correlation coefficients (r) between shear stress and saltation flux (0·12 and 0·14, respectively), while the correlation coefficients between the streamwise velocity component and saltation flux were much higher (0·65 and 0·57, respectively).

Aeolian processes across transverse dunes. II: modelling the sediment transport and profile development

This paper discusses a model which simulates dune development resulting from aeolian saltation transport. The model was developed for application to coastal foredunes, but is also applicable to sandy deserts with transverse dunes. Sediment transport is calculated using published deterministic and empirical relationships, describing the influence of meteorological conditions, topography, sediment characteristics and vegetation. A so-called adaptation length is incorporated to calculate the development of transport equilibrium along the profile. Changes in topography are derived from the predicted transport, using the continuity equation. Vegetation height is incorporated in the model as a dynamic variable. Vegetation can be buried during transport events, which results in important changes in the sediment transport rates. The sediment transport model is dynamically linked to a second-order closure air flow model, which predicts friction velocities over the profile, influenced by topography and surface roughness. Modelling results are shown for (a) the growth and migration of bare, initially sine-shaped dunes, and (b) dune building on a partly vegetated and initially flat surface. Results show that the bare symmetrical dunes change into asymmetric shapes with a slipface on the lee side. This result could only be achieved in combination with the secondorder closure model for the calculation of air flow. The simulations with the partly vegetated surfaces reveal that the resulting dune morphology strongly depends on the value of the adaptation length parameter and on the vegetation height. The latter result implies that the dynamical interaction between aeolian activity and vegetation (reaction to burial, growth rates) is highly relevant in dune geomorphology and deserves much attention in future studies. Copyright

Aeolian processes across transverse dunes. I: Modelling the air flow

This paper discusses a two-dimensional second-order closure model simulating air flow and turbulence across transverse dunes. Input parameters are upwind wind speed, topography of the dune ridge and surface roughness distribution over the ridge. The most important output is the distribution of the friction velocity over the surface. This model is dynamically linked to a model that calculates sand transport rates and the resulting changes in elevation. The sand transport model is discussed in a separate paper. The simulated wind speeds resemble patterns observed during field experiments. Despite the increased wind speed over the crest, the friction velocity at the crest of a bare dune is reduced compared to the upstream value, because of the effect of stream line curvature on turbulence. These curvature effects explain why desert dunes can grow in height. In order to obtain realistic predictions of friction velocity it was essential to include equations for the turbulent variables in the model. In these equations streamline curvature is an important parameter. The main flaw of the model is that it cannot deal with flow separation and the resulting recirculation vortex. As a result, the increase of the wind speed and friction velocity after a steep dune or a slipface will be too close to the dune foot. In the sand transport model this was overcome by defining a separation zone. Copyright # 1999 John Wiley & Sons, Ltd.

Nighttime free convection characteristics within a plant canopy.

An intensive measurement campaign within and above a maize row canopy was carried out to investigate flow characteristics within this vegetation. Attention was given to finding adequate scaling parameters of the within-canopy windspeed and air temperature profiles under above-canopy stable stratification.During clear and calm nights the within-canopy condition differs considerably from the abovecanopy state. In contrast to the daytime, the windspeed and temperature profiles do not scale with the above-canopy friction velocity,u*, and the scaling temperature,T*, respectively. A free convection flow regime is generated, forced by the soil heat flux at the canopy floor and by cooling at the top of the canopy. However, the windspeed and temperature profiles appear to scale well with the free convective velocity scale,w*, and the free convective temperature scale,Tf, respectively. The free convective state within the canopy agrees well with the free convection criterion Gr>16Re2(u*), where Gr is the Grashof number and Re(u*) the Reynolds number, a criterion often used in technical flow problems. Also it is shown that under within-canopy free convection, there is a unique relation between the Grashof number, Gr, and the Reynolds number if the latter is based on the free convective velocity scale.Under within-canopy free convective conditions, it appears that within the canopy the fluxes of heat and water vapour can be estimated well with the relatively simple variance technique. Under these conditions, the Grashof, or Rayleigh number, represents a measure for the kinetic energy of the turbulence within the canopy.

Ecological effects of reactivation of artificially stabilized blowouts in coastal dunes

In an inner dune area in the Dutch coastal dunes several artificially stabilized blowouts were reactivated. The purpose was to investigate whether these reactivated blowouts could remain active despite the increased atmospheric deposition of nutrients, how much area would be affected by sand from the blowouts, whether the vegetation would respond to the deposition of sand, and whether the reactivation of blowouts could be a measure against the effects of acidification and eutrophication.This paper presents the results of the first years of minitoring the changes in the blowout morphology and the response of the vegetation. In the monitoring period (1991–1994) the blowouts remained active and grew slowly in size and depth. The area which receives more than 1 cm of calcareous sand from the blowout in three years was up to six times the area of the blowouts. Moss vegetation responded to the accumulation of sand:Campylopus introflexus was sparse within the deposition area whereasTortula ruralis was found near the contour of 1 cm deposition in 3 yr. No indications were found that shrubs or marram grass were adversely affected by the deposition of sand.The experiment forms part of a programme to test measures aimed at mitigating the effects of air pollution on natural landscapes. From the viewpoint of the programme the experiment is a success.

The dependence of canopy layer turbulence on within-canopy thermal stratification

Transport properties near the Earths surface are strongly influenced by the thermal stratification of the atmosphere. Until now, no distinction has been made between thermal stability parameters within and above a plant canopy, and it has been usual to classify canopy transport processes in terms of above-canopy stability parameters only. The question arises, however, whether such parameters adequately describe within-canopy properties because it is often the case that thermal stratification differs considerably between air layers above and below the top of the canopy. In the present study, two within-canopy thermal stratification parameters have been defined and tested to determine whether they yield additional information about canopy turbulence. It appears that a within-canopy bulk Richardson number provides useful information under low-wind nocturnal conditions. Strongly unstable conditions inside dense canopies commonly occur at night when the air layers above the canopy are very stable, resulting in a decoupling between the above- and within-canopy regions. A local within-canopy Obukhov length proved to be less useful, perhaps because the sensible heat flux within the canopy was nearly always directed upwards, regardless of the temperature gradient. A penetration length scale, defined for daytime conditions only, was of the order of the height of the canopy. This suggests that the height of the canopy is a suitable length scale for within-canopy processes.

Water and solute fluxes in dry coastal dune grasslands: the effects of grazing and increased nitrogen deposition

A five-year monitoring study has been carried out to examine the combined effects of grazing and atmospheric nitrogen deposition on water and solute fluxes in dry coastal dune grasslands. Two vegetation types were studied: (a) a short, species-rich stand on calcareous sand (foredune site) and (b) a short, species-poor stand on partly decalcified soil on calcareous sand (innerdune site). In each stand four experimental plots were created: (1) control, (2) fertilized with nitrogen, (3) excluded from grazing by rabbits and (4) combination of fertilization and exclusion of grazing by rabbits.Due to the large spatial variability of the soil water content, no differences between the treatments could be measured. Average soil water content at 10 cm depth is very small (3–5%) from May until October and does not increase after rainfall. However, measured soil water content at 20 cm and 50 cm depth increased after rainfall. In winter, nearly all measured soil water contents increase upon rainfall, although sometimes one soil water content remained dry till the end of the next summer.In summer it was impossible to sample soil water for the estimation of the solute concentrations due to the very small soil water content. Therefore, only solute concentrations of the winter period could be evaluated.Without fertilization, fluxes of nitrogen out of the soil system are below the incoming flux, due to storage in the biomass and in the soil compartment. When fertilized, 70% of the added NH4+-N was leached from the foredune soil profile as NO3--N, due to nitrification. Conversely, at the grazed innerdune site most of the added nitrogen remained in the system. Here, nitrification rates will be small due to the decalcified topsoil and NH4+-N is not easily leached out of the soil compartment. At the exclosures of the innerdune site, about 15% of the amount of the added fertilizer N was leached, after added NH4+ is taken up by the plants and partly washed out as nitrate after mineralization and nitrification of dead biomass.

Vertical and horizontal distribution of wind speed and air temperature in a dense vegetation canopy

Wind speed and temperature were measured within a corn row canopy to investigate horizontal and vertical variability of the mean wind speed and temperature. It appears that the mean wind speed can vary between 20% and 30% from its horizontal mean value. In the narrow row crop, the horizontal mean air temperature varies between 0.1°C (night-time) and 0.35°C (daytime) from the spatial mean value. Exceptions occur around noon in daytime when direct irradiation dominates and where the direct beam illuminates the within-row space of the canopy. Then, deviations of 1°C or more from the horizontal mean value are observed. Attention was focused on finding adequate scaling parameters of the within-canopy wind speed and air temperature profiles under various atmospheric stratification states. During daytime and night-time, the above-canopy friction velocity appears to be a good scaling parameter. Clear nights, however, are exceptions, when the above-crop wind speed drops to a very low value. Then, the within-canopy free convection velocity scale appears to be an appropriate scaling parameter for within-canopy processes. During daytime, the within-canopy temperature profiles scale well with the above-canopy temperature scale, T∗, for stationary irradiation and wind speed regimes. On calm nights, however, the relative within-canopy temperature profile scales very well with the within-canopy free convection temperature scale, Θ∗.

Nighttime exchange processes near the soil surface of a maize canopy.

Abstract The exchange process in the lower region of a maize canopy is analyzed for two nights. It appears that during calm nights a free convection state develops in the lower region of the canopy. Convective heat is released at the soils surface and transported directly to the higher portion of the canopy. The released sensible heat at the soils surface can be easily calculated by applying the Nusselt number for free convection. At night thermal energy is also released through cooling of the canopy. The released heat from the stored canopy heat is of the same order of magnitude as all other energy terms. Conversely during daytime for most agricultural crops this energy term is of minor importance in comparison to the other energy terms. The formation of dew at night is an important process. A maximal possible estimate of dew accumulation for a particular night can easily be made by using the potential dew. The potential dew can be deduced more accurately by taking into account the released heat stored in the plant canopy.

Wind Velocity and Algal Crusts in Dune Blowouts

Abstract In two saucer-shaped blowouts wind velocity was measured at 0.18 m height and compared to the wind velocity data from the nearest standard meteorological station at Valkenburg about 3.5 km to the northeast. In blowout 1 relative velocities are highest when the wind blows from directions between E and SSE. In blowout 2 winds blowing from directions between S and WSW are relatively stronger. In general the wind speed increases as its moves along the surface inside the blowout, because the aerodynamic roughness of the blowout surface is less than that of the surrounding area. Less important are the influence of the configuration of the terrain surrounding the blowout and the influence of the slopes within the blowout. The frequency distribution of the general wind at Valkenburg is analysed and combined with the horizontal wind velocity pattern within blowouts. A simulation model, producing the spatial distribution of the wind power shows that the prevalence of algal crusts at the SW side of blowouts agrees well with the location which reaches the lowest wind power values. In other parts of the blowouts algal crusts are damaged by moving sand at times of high wind velocity.