A.F.G. Jacobs

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).

A simple model for potential dewfall in an arid region

It is not always easy to know, post-facto, whether both dewfall and fog may have occurred over a given evening period. Instrumentation limitations make it difficult to quantify dew deposition since they rely on artificial sensing surfaces that are either visually examined on a daily basis or recorded. In arid to Mediterranean regions, both dew and fog can play significant ecological roles as suppliers of moisture. Long-term observation records of dew and fog in such regions tend to be limited, however, due partly to a lack of interest and limited distribution of well-instrumented meteorological stations. Simple meteorological criteria are suggested here to calculate potential dewfall and to indicate whether fog was likely to have occurred over a given evening. A field campaign was carried out in the NW Negev desert, Israel, in September and October 1997, to collect meteorological data and carry out dewfall measurements.

CHANGES OF THE DISPLACEMENT HEIGHT AND ROUGHNESS LENGTH OF MAIZE DURING A GROWING SEASON

Variations in the displacement height (d) and roughness length (z0) of a maize crop were investigated through a growing season. A programme of measurement from which the wind profile, the Richardson gradient number and the turbulent fluxes of heat and momentum were estimated, was carried out. Two techniques were combined to obtain unique values for d and z0: the log-profile fitting technique and the eddy correlation technique. Throughout the growing season, the displacement height appeared to correlate very well (r = 0.98) with the height (H) of the canopy. A mean value for d/H was 0.75. The roughness length was strongly correlated with the difference between the canopy height and the displacement height. A mean value of 0.26 for z0/(H − d) was found (r = 0.86). If the ratio d/H was fixed, the roughness length did not show any clear dependence on wind speed or thermal stability.

Similarity moisture dew profiles within a corn canopy.

Abstract The amount of dewfall and dewrise to a corn canopy has been estimated over 7 nights by using the Bowen ratio energy balance technique and the soil diffusivity technique, respectively. In addition, the distribution of free liquid water within the plant community has been measured by using Leick plates. By combining the Leick plate results with the foliage area distribution of the plant canopy, the total amount of dew deposition within the plant canopy can also be estimated. On average, both techniques did not show a systematic difference and agreed to within 10%. Moreover, it appeared that the nondimensionized dew profiles within the stand showed a more or less similar shape for all nights. The potential dew formation was calculated for all nights. On average, the potential dew was 30% higher than the actual dew. This offers the opportunity to make reliable total dew estimates, and also, due to similarity, dew profile estimates for a corn canopy from simple micrometeorological data. Model calculat...

The conductance of a maize crop and the underlying soil to ozone under various environmental conditions.

Flux measurements of ozone and water vapour employing the eddy correlation technique were used to determine the surface conductance and canopy conductance to ozone. In the surface conductance to ozone, all surfaces at which ozone is destroyed and the transport process to these surfaces are included. The canopy conductance to ozone represents the ozone uptake of transpiring plant parts. The surface conductance to ozone of the maize crop and the underlying soil was generally larger than the canopy conductance to ozone. This means that beside the uptake by stomata, there was another important ozone sink. Under wet soil surface conditions, the surface conductance and the canopy conductance to ozone coincided. This indicates that the resistance of wet soil and the remaining plant parts (cuticle) to ozone was much larger than the stomatal or soil resistance. On the other hand, under dry soil conditions the conductances differ, largely caused by a variation in the transport process to the soil. The transport of ozone to soil increased with increasing friction velocity (u*) and decreased with increasing atmospheric stability, leaf area index (LAI) or crop height (h). These effects for midday (unstable) conditions were parameterized with an “in-crop” aerodynamic resistance,rinc in a very straightforward way;rinc=13.9 LAIh/u*+67 (cc.=0.77). If the ozone flux in air pollution models is described with a simple resistance model (Big Leaf model), the extra destruction at the soil should be modelled using an “in-crop” aerodynamic resistance. For these measurements the ozone flux to the soil was 0–65% of the total ozone flux measured above the crop. Under wet soil conditions, this was less than 20%; under dry soil conditions, this was 30–65%.

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.

An automated microlysimeter to study dew formation and evaporation in arid and semiarid regions

Abstract The development of a simple and low-cost portable weighing microlysimeter that makes use of a load cell for automated recording and for studying daily dew formation, rate of accumulation, and subsequent evaporation in arid or semiarid regions during rainless seasons is presented. The sampling cup is 3.5 cm deep, with the load cell itself situated at 20-cm depth to minimize temperature effects. The device was tested in a sand dune experimental station situated near Nizzana, northwest Negev Desert, Israel, during which extensive micrometeorological measurements were collected. One microlysimeter was placed in a playa and a second was installed on the stabilized midslope of an adjacent linear sand dune. To assess the performance of the load cell microlysimeters (LCM), one pair of manual microlysimeters was installed next to each LCM. A third pair was installed at a point between the LCMs and a fourth pair above the midslope LCM. Sixteen overnight measurements were carried out within a 6-week period....

Temperature variation in a class A evaporation pan

Abstract The thermal behaviour of a class A evaporation pan has been studied by doing an outdoor experiment and by modelling the energy budget of the pan in detail. The observations showed that the temperature of the water remains more or less homogeneous throughout the daily cycle. This means that the water inside the pan is well mixed. This characteristic thermal behaviour was observed even under low wind ( u (0.5 m) −1 . The one-dimensional model simulations agreed well with the experimental evidence. However, in the model simulations corrections had to be carried out for the turbulence generated by the pan itself and in particular by the rim of the pan.

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.

Seasonal changes in the albedo of a maize crop during two seasons

Abstract The albedo of a maize crop during the growing seasons of 1985 and 1986 was monitored from mid-May (bare soil condition) until mid-October (harvest). The precipitation and irradiation regimes were quite different during both years, giving rise to different growth and albedo patterns. A simple model was used to calculate mean daily albedos from plant and soil characteristics, and the results were validated with the measured data. On average, the calculated values agreed to within 3% of the mean measured daily albedos during both years. The mean standard error of the estimate was 0.02 during both seasons. For individual days however the simple model calculations could show considerable deviation from the experimental data. If the course of the diurnal albedo is required, a more detailed model can be used in which the direct and diffuse irradiation fluxes are treated separately, and the water content of the underlying soil must be known. On average, the calculated values agree to within 4 and 2% for a fine day and an overcast day, respectively.