Experiments on scalar dispersion within a model plant canopy part I: The turbulence structure

This is the first of a series of three papers describing experiments on the dispersion of trace heat from elevated line and plane sources within a model plant canopy in a wind tunnel. Here we consider the wind field and turbulence structure. The model canopy consisted of bluff elements 60 mm high and 10 mm wide in a diamond array with frontal area index 0.23; streamwise and vertical velocity components were measured with a special three-hot-wire anemometer designed for optimum performance in flows of high turbulence intensity. We found that:

1. The momentum flux due to spatial correlations between time-averaged streamwise and vertical velocity components (the dispersive flux) was negligible, at heights near and above the top of the canopy.
2. In the turbulent energy budget, turbulent transport was a major loss (of about one-third of local production) near the top of the canopy, and was the principal gain mechanism lower down. Wake production was greater than shear production throughout the canopy. Pressure transport just above the canopy, inferred by difference, appeared to be a gain in approximate balance with the turbulent transport loss.
3. In the shear stress budget, wake production was negligible. The role of turbulent transport was equivalent to that in the turbulent energy budget, though smaller.
4. Velocity spectra above and within the canopy showed the dominance of large eddies occupying much of the boundary layer and moving downstream with a height-independent convection velocity. Within the canopy, much of the vertical but relatively little of the streamwise variance occurred at frequencies characteristic of wake turbulence.
5. Quadrant analysis of the shear stress showed only a slight excess of sweeps over ejections near the top of the canopy, in contrast with previous studies. This is a result of improved measurement techniques; it suggests some reappraisal of inferences previously drawn from quadrant analysis.

     

Vertical cross-spectra of horizontal velocity components at the Boulder observatory

Cross-spectra between horizontal wind components at different levels of the Boulder Atmospheric Observatory (BAO) tower lead to the following conclusions:

1. Davenport's hypothesis is satisfied that coherence decays exponentially with the ratio of vertical separation to horizontal wave length, at least to very small values of coherence.
2. The decay coefficients increase with z/L for z/L < 0.5. For larger stabilities, irregular fluctuations with periods of order 10–20 min have considerable vertical coherence. Results at BAO are quite consistent with those elsewhere.
3. Eddy slopes in vertical planes increase with wind shear up to a point where the slope (horizontal delay over vertical separation) is just above 2. Beyond that point, the systematic increase of slopes with shear ceases. Since wind shear decreases upward, slopes tend to decrease upward. Slopes for lateral components are significantly larger than those for u-components.

     

Atmospheric boundary layer research at Cabauw

At Cabauw, The Netherlands, a 213 m high mast specifically built for meteorological research has been operational since 1973. Its site, construction, instrumentation and observation programs are reviewed. Regarding analysis of the boundary layer at Cabauw, the following subjects are discussed:

- terrain roughness;

- Monin-Obukhov theory in practice;

- the structure of stable boundary layers;

- observed evolution of fog layers;

- inversion rise and early morning entrainment;

- use of the geostrophic wind as a predictor for wind profiles;

- height variation of wind climate statistics;

- air pollution applications: long range transport and short range dispersion;

- dependence of sound wave propagation on boundary-layer structure;

- testing of weather and climate models.

     

Assessing the Consequences of Climate Change for Food and forest Resources: A View from the IPCC

Important findings on the consequences of climate change for agriculture and forestry from the recently completed Third Assessment Report (TAR) of the Intergovernmental Panel on Climate Change (IPCC) are reviewed, with emphasis on new knowledge that emerged since the Second Assessment Report (SAR). The State-Pressure-Response-Adaptation model is used to organize the review. The major findings are:

• Constant or declining food prices are expected for at least the next 25 yr, although food security problems will persist in many developing countries as those countries deal with population increases, political crisis, poor resource endowments, and steady environmental degradation. Most economic model projections suggest that low relative food prices will extend beyond the next 25 yr, although our confidence in these projections erodes farther out into the 21st century.
• Although deforestation rates may have decreased since the early 1990s, degradation with a loss of forest productivity and biomass has occurred at large spatial scales as a result of fragmentation, non-sustainable practices and infrastructure development.
• According to United Nations estimates, approximately 23% of all forest and agricultural lands were classified as degraded over the period since World War II.
• At a worldwide scale, global change pressures (climate change, land-use practices and changes in atmospheric chemistry) are increasingly affecting the supply of goods and services from forests.
• The most realistic experiments to date – free air experiments in an irrigated environment – indicate that C₃ agricultural crops in particular respond favorably to gradually increasing atmospheric CO₂ concentrations (e.g., wheat yield increases by an average of 28%), although extrapolation of experimental results to real world production where several factors (e.g., nutrients, temperature, precipitation, and others) are likely to be limiting at one time or another remains problematic. Moreover, little is known of crop response to elevated CO₂ in the tropics, as most of the research has been conducted in the mid-latitudes.
• Research suggests that for some crops, for example rice, CO₂ benefits may decline quickly as temperatures warm beyond optimum photosynthetic levels. However, crop plant growth may benefit relatively more from CO₂ enrichment in drought conditions than in wet conditions.
• The unambiguous separation of the relative influences of elevated ambient CO₂ levels, climate change responses, and direct human influences (such as present and historical land-use change) on trees at the global and regional scales is still problematic. In some regions such as the temperate and boreal forests, climate change impacts, direct human interventions (including nitrogen-bearing pollution), and the legacy of past human activities (land-use change) appear to be more significant than CO₂ fertilization effects. This subject is, however an area of continuing scientific debate, although there does appear to be consensus that any CO₂ fertilization effect will saturate (disappear) in the coming century.
• Modeling studies suggest that any warming above current temperatures will diminish crop yields in the tropics while up to 2–3 ^°C of warming in the mid-latitudes may be tolerated by crops, especially if accompanied by increasing precipitation. The preponderance of developing countries lies in or near the tropics; this finding does not bode well for food production in those countries.
• Where direct human pressures do not mask them, there is increasing evidence of the impacts of climate change on forests associated with changes in natural disturbance regimes, growing season length, and local climatic extremes.
• Recent advances in modeling of vegetation response suggest that transient effects associated with dynamically responding ecosystems to climate change will increasingly dominate over the next century and that during these changes the global forest resource is likely to be adversely affected.
• The ability of livestock producers to adapt their herds to the physiological stress of climate change appears encouraging due to a variety of techniques for dealing with climate stress, but this issue is not well constrained, in part because of the general lack of experimentation and simulations of livestock adaptation to climate change.
• Crop and livestock farmers who have sufficient access to capital and technologies should be able to adapt their farming systems to climate change. Substantial changes in their mix of crops and livestock production may be necessary, however, as considerable costs could be involved in this process because investments in learning and gaining experience with different crops or irrigation.
• Impacts of climate change on agriculture after adaptation are estimated to result in small percentage changes in overall global income. Nations with large resource endowments (i.e., developed countries) will fare better in adapting to climate change than those with poor resource endowments (i.e., developing countries and countries in transition, especially in the tropics and subtropics) which will fare worse. This, in turn, could worsen income disparities between developed and developing countries.
• Although local forest ecosystems will be highly affected, with potentially significant local economic impacts, it is believed that, at regional and global scales, the global supply of timber and non-wood goods and services will adapt through changes in the global market place. However, there will be regional shifts in market share associated with changes in forest productivity with climate change: in contrast to the findings of the SAR, recent studies suggest that the changes will favor producers in developing countries, possibly at the expense of temperate and boreal suppliers.
• Global agricultural vulnerability is assessed by the anticipated effects of climate change on food prices. Based on the accumulated evidence of modeling studies, a global temperature rise of greater than 2.5 ^°C is likely to reverse the trend of falling real food prices. This would greatly stress food security in many developing countries.

     

Intercomparison of turbulence data measured by SODAR and sonic anemometers

The capability of SODAR to measure the mean wind field in the lower boundary layer is well known and documented. Therefore, mean wind data are easily obtainable by means of the SODAR-technique, and are used to simulate the transport of pollutants after their release into the atmosphere. But when calculating the diffusion of pollutants, information about atmospheric turbulence is needed, too. In principle, a SODAR can measure turbulence data like the standard deviation of the vertical wind speed or horizontal wind direction. But when measuring turbulence data with a SODAR, one is beset by a host of limitations like volume sampling, spatial and temporal separation of sampling volume, attenuation of the acoustic waves and the slow speed of sound. Therefore, successful turbulence measurements with SODAR are not numerous and little is known about the quality of these data. In this context an intercomparison between a REMTECH-SODAR and a sonic anemometer mounted at the 100 m level of our meteorological tower was performed in summer 1990 at the Kernforschungszentrum Karlsruhe. The intercomparison is in two parts:

1. Half hour mean values of the standard deviation of the vertical wind speed are intercompared by scatter plots and by a linear regression and correlation analysis.
2. During 7 periods, 2 hours each, and covering atmospheric stabilities from unstable to slightly stable, the instantaneous vertical wind speeds were measured by both instruments and spectra were calculated.

The intercomparison demonstrates that DOPPLER-SODAR sounding is a reliable technique to determine besides the mean field, also athmospheric turbulence data like Sigma(w).     

Wind profiles at the Boulder Tower

Analysis of wind profiles at the Boulder Tower (BAO) leads to these conclusions:

1. The variation of roughness with wind direction found earlier is confirmed. Roughness lengths measured on the tower are larger than those measured close to the surface.
2. The profiles and measurements of Reynolds stress are consistent with a von-Karman constant of 0.35.
3. The form φ_m=(1?15z/L)^-1/3 fits best in the range -0.6 < z/L < 0. In the range 0 < z/L < 0.5, θ _m ～ 1 + 4.7z/L provides a good fit to the observations. For z/L < 0.1, φ _m also depends on h, the thickness of the PBL. For z/L < -0.6, Φ _m approaches the constant 0.5, in contrast to all previous suggestions. For larger stabilities, the upper level is usually not in the surface layer, and wind ratios become independent of z/L.
4. With snow cover, the effective roughness diminishes to about 1 cm, even for directions for which the roughness length without snow is large.
5. Estimation of winds at 100 or 150 m from information near the surface is best for similarity theory provided that the ratio of height to Monin-Obukhov L is less than 0.1. For larger z/L, simple power laws seem more appropriate.

     

Experiments on scalar dispersion within a model plant canopy,part III: An elevated line source

An experiment is reported in which heat was released as a passive tracer from an elevated lateral line source within a model plant canopy, with h _s = 0.85 h _c (h _s and h _c being the source and canopy heights, respectively). A sensor assembly consisting of three coplanar hot wires and one cold wire was used to measure profiles of mean temperature % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaiikamaana% aabaGaeqiUdehaaiaacMcaaaa!390C!\[(\overline \theta )\], temperature variance (Σ_θ ²), vertical and streamwise turbulent heat fluxes, and third moments of wind and temperature fluctuations. Conclusions were:

1. Despite the very heterogeneous flow within the canopy, the observed dispersive heat flux (due to spatial correlation between time-averaged temperature and vertical velocity) was small. However, there is evidence from the plume centroid (which was lower than h _s at the source) of systematic recirculating motions within the canopy.
2. The ratio % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaeq4Wdm3aaS% baaSqaaiabeI7aXjaab2gacaqGHbGaaeiEaaqabaGccaGGVaWaa0aa% aeaacqaH4oqCaaWaaSbaaSqaaiaab2gacaqGHbGaaeiEaaqabaaaaa!41DF!\[\sigma _{\theta {\text{max}}} /\overline \theta _{{\text{max}}} \] (of maximum values on vertical profiles) decreased from 1 near the source to an asymptotic value of 0.4 far downstream, in good agreement with previous experimental and theoretical work for concentration fluctuations in the surface layer well above the canopy.
3. The eddy diffusivity for heat from the line source (K _HL ) increased, downstream of the source, to a nearly constant ‘far-field’ vertical profile. Within the canopy, the far-field K _HL was an order of magnitude larger than K _HP , the equivalent diffusivity for a plane source; well above the canopy, the two were equal. The time scale defined by (far-field K _HL )/(vertical velocity variance) was independent of height within the canopy.
4. Budgets for temperature variance, vertical heat flux and streamwise heat flux are remarkably similar to the equivalent budgets for an elevated line source in the surface layer well above the canopy, except in the lower part of the canopy in the far field, where vertical transport is much more important than in the surface layer.
5. A random flight simulation of the mean height and depth of the temperature plume was generally in good agreement with experiment. However, details of the temperature and streamwise turbulent heat flux profiles were not correct, suggesting that the model formulation needs to be improved.

     

Characteristics of the sea breeze in Attica,Greece

The characteristics of the sea breeze in the Attica region of Greece, in which Athens is located, have been studied for occasions of weak synoptic-scale pressure gradient. The analysis is based on synoptic observations from six meteorological stations, three on the coast and three inland. The three inland stations and one of the coastal stations lie almost in a straight line at different distances from the coast. For each meteorological station, the basic characteristics of the sea breeze were determined, i.e.,

1. The mean number of sea-breeze days for each calendar month.
2. The monthly mean wind speed for each synoptic hour.
3. The times of onset and cessation of the sea breeze.
4. The monthly vector mean wind, and its constancy ‘Constancy’ is defined as 100{itV{inr}/V{ins}}, where {itV{inr}} is the magnitude of the vector mean wind, and {itV{ins}} is the scalar mean wind speed. See Brooks and Carruthers (1953). (In this paper, the factor 100 is not used.) for each synoptic hour.
5. For days on which there was a sea breeze at Helliniko (the coastal reference station), the percentage number of days on which there was also a sea breeze at the given station.

An attempt was also made to determine further characteristics, such as the inland penetration of the sea breeze, its depth, the spatial and temporal variation of wind speed and direction, and the existence of the return flow. Finally, the properties of the land breeze are briefly outlined.     

Turbulent transport of carbon dioxide over a paddy field

Turbulent fluctuations in CO₂ concentrations over a paddy field are measured by a fastresponse device with an open sensing path. This IR device coupled with a sonic anemometer constitutes an eddy correlation instrument to measure CO₂ fluxes. Three experiments were conducted in the surface layer over paddy 90 cm high. The stability (z – d)/L ranged from -0.14 to 0.20, where L denotes the Monin-Obukhov length.CO₂ power spectra show the range of applicability of the -2/3 power law to be between f = 0.2 and f = 2, where f is the frequency normalized by wind speed and height. The cospectral estimate between CO₂ and vertical component of wind speed ranging from f = 0.005 to f = 2 shows a peak at about f = 0.15 under near-neutral stratification.Hourly means of CO₂ flux measured by the eddy correlation method increase with intensity of net radiation. The maximum value of downward flux of CO₂ rises to 0.6 mg cm^-2 hr^-1 over the paddy field at the stage of ear emergence.Some turbulence statistics relating to the CO₂ transport are evaluated: the correlation coefficient between CO₂ and vertical velocity is about -0.3, and that between CO₂ and humidity attains -0.7 ~ -0.8 under unstable stratification; nondimensional gradients _c for CO₂ and _m for wind speed are 0.89 and 0.99, respectively.     

Economics of climate policy and collective decision making

This paper explores the reasons why economic instruments of climate change are reluctantly applied and stresses the need for interdisciplinary research linking economic theory and empirical testing to deliberative political procedures. It is divided in three parts. The first one recalls the main issues in implementing Cost-Benefit Analysis such as information problems, uncertainties, discounting the future and irreversibilities. The second part shows how these issues can be treated in integrated assessment and techno-economic models and presents a case study, which shows that

• The chosen scenario tends to stabilize atmospheric CO₂ concentration at around 550 ppm in the long run.
• Exclusion of possibility to trade CO₂ emission permits under a cap regime would increase the cost of emission abatement for OECD countries.
• Combining different flexibility instruments might lead to significant gains in the overall cost of climate policy.

The third part presents results of a survey conducted among the main economic and environmental associations in Switzerland. The survey reveals conflicting views on economic instruments. It shows how the social acceptability of these instruments can be improved in taking explicitly into account these opposing views of special interest groups. Therefore, policy scenarios should be selected in combining techno-economic models with empirical studies about their political and normative context.     

Lake Kinneret (Sea of Galilee) and its exceptional wind system

From 1973–1976, research was performed around the Sea of Galilee, aimed at examining the wind regime in the area and whether the area develops a land-sea breeze despite its particular topographical location.

The main conclusions were:
1. During the summer mornings a lake breeze develops, blowing towards the shores of the lake. It ceases at the peak of its development when a westerly wind, originating in the development of a breeze along the Israeli Mediterranean coast, plunges towards the lake.
2. Late at night, a wind flow develops from the land towards the lake, which combines with the katabatic winds that blow along the steep slopes surrounding the Kinneret.
3. The stations at the upper level, at a height of 400–500 m above the Kinneret, are not affected by the lake breeze during the day or by the land breeze at night.
4. In winter, the Kinneret lake breeze is almost as developed as in summer, because the westerly winds, originating in the Mediterranean sea breeze which hardly develops in this season, do not plunge into the Kinneret.

     

The internal boundary layer — A review

A review is given of relevant work on the internal boundary layer (IBL) associated with:

1. Small-scale flow in neutral conditions across an abrupt change in surface roughness,
2. Small-scale flow in non-neutral conditions across an abrupt change in surface roughness, temperature or heat/moisture flux,
3. Mesoscale flow, with emphasis on flow across the coastline for both convective and stably stratified conditions.

The major theme in all cases is on the downstream, modified profile form (wind and temperature), and on the growth relations for IBL depth.     

Eddy correlation measurements of CO2, latent heat,and sensible heat fluxes over a crop surface

Eddy correlation equipment was used to measure mass and energy fluxes over a soybean crop. A rapid response CO₂ sensor, a drag anemometer, a Lyman-alpha hygrometer and a fine wire thermocouple were used to sense the fluctuating quantities.Diurnal fluxes of sensible heat, latent heat and CO₂ were calculated from these data. Energy budget closure was obtained by summing the sensible and latent heat fluxes determined by eddy correlation which balanced the sum of net radiation and soil heat flux. Peak daytime CO₂ fluxes were near 1.0 mg m^–2 (ground area) s^–1.The eddy correlation technique was also employed in this study to measure nocturnal CO₂ fluxes caused by respiration from plants, soil, and roots. These CO₂ fluxes ranged from - 0.1 to - 0.25 mg m^–2s^–1.From the data collected over mature soybeans, a relationship between CO₂ flux and photosynthetically active radiation (PAR) was developed. The crop did not appear to be light-saturated at PAR flux densities < 1800 Ei m^–2 s^–1. The light compensation point was found to be about 160 Ei m^–2 s^–1.Published as Paper No. 7402, Journal Series, Nebraska Agricultural Experiment Station. The work reported here was conducted under Nebraska Agricultural Experiment Station Project 27-003 and Regional Research Project 11–33.Post-doctoral Research Associate, Professor and Professor, respectively. Center for Agricultural Meteorology and Climatology, Institute of Agriculture and Natural Resources, University of Nebraska, Lincoln, NE 68583-0728.     

An Alternative Approach for CO2 Flux Correction Caused by Heat and Water Vapour Transfer

Energy and CO₂ fluxes are commonly measured above plant canopies using an eddy covariance system that consists of a three-dimensional sonic anemometer and an H₂O/CO₂ infrared gas analyzer. By assuming that the dry air is conserved and inducing mean vertical velocity, Webb et al. (Quart. J. Roy. Meteorol. Soc. 106, 85-100, 1980) obtained two equations to account for density effects due to heat and water vapour transfer on H₂O/CO₂ fluxes. In this paper, directly starting with physical consideration of air-parcel expansion/compression, we derive two alternative equations to correct for these effects that do not require the assumption that dry air is conserved and the use of the mean vertical velocity. We then applied these equations to eddy flux observations from a black spruce forest in interior Alaska during the summer of 2002. In this ecosystem, the equations developed here led to increased estimates of CO₂ uptake by the vegetation during the day (up to about 20%), and decreased estimates of CO₂ respiration by the ecosystem during the night (approximately 4%) as compared with estimates obtained using the Webb et al. approach.     

Bulk characteristics of heat transfer in the unstable,baroclinic atmospheric boundary layer

Field data for the unstable, baroclinic, atmospheric boundary layer over land and over the sea are considered in the context of a general similarity theory of vertical heat transfer. The dependence of δθ/θ_* upon logarithmic functions of h _c z _T and stability (through the similarity function C) is clearly demonstrated in the data. The combined data support the conventional formulation for the heat transfer coefficient δθ/θ_* when,

1. the surface scaling length is z _T (« z ₀), the height at which the surface temperature over land is obtained by extrapolation of the temperature profile
2. the height scale is taken as the depth of convective mixing h _c
3. the temperature profile equivalent of the von Karman constant is taken as 0.41
4. areal average, rather than single point, values of δθ are employed in strongly baroclinic conditions. No significant effect of baroclinity or the height scale ratio as proposed in the general theory is found. Variations in C about a linear regression relation against stability are most probably due to uncertainties in the areal surface temperature and to experimental errors in general temperature measurements.

     

Inversions,and fog,stratus and cumulus formation in warm air over cooler water

Two aspects of convection over oceans are discussed and the following conclusions are derived from theoretical considerations.

1. The air layer over the sea will usually convect even when the water surface is ten degrees or more colder than the initial air temperature.
2. An inversion at stratus cloud tops is created by the stratus, and is not a necessary preexisting condition. Such inversions persist after subsidence evaporates the cloud.
3. Radiation heat exchange does not play an essential role in stratus formation or maintenance, and can either heat or cool the cloud.
4. Dry air convection does not erode inversions at the top of the convecting layer. Examples of soundings are discussed.
5. Fogs are most likely to form at sea where the water is coolest, and need no radiation effects to initiate cooling, or a boost from patches of warmer water, to begin convection.
6. Both stratus cloud growth, and the evaporation of clouds by cloud top entrainment, readjust the vertical structure of the air to leave a constant wet-bulb potential temperature with height.

These conclusions are supported by, firstly, a convective model which has been developed and which shows that vapor-driven convection over the ocean will proceed with zero or negative heat fluxes, at rates which saturate the lowest layer of the atmosphere in a few hours to altitudes of many tens of meters. Secondly, the availability of condensed moisture at the top of the surface layer cools the warmer entrained overlying dry air parcels so that when they descend they are no warmer than the sea surface temperature, and this induces downward moving plumes. This occurs if the wet-bulb potential temperature of the overlying air is less than the sea surface temperature, even if it is ten degrees C, or more, warmer in actual temperature.     

Bare-ground surface heat and water exchanges under dry conditions: Observations and parameterization

A simplified land-surface parameterization is tested against bare-soil data collected during the EFEDA experiment conducted in Spain in June 1991. A complete data set, made up of soil properties as well as hydrological and atmospheric measurements, is described and discussed. The 11-day data set is characterized by very dry conditions and high surface temperatures during the day. Large values of sensible and soil heat fluxes and small values of surface evaporation (≈1 mm/day) were observed. This data set was modelled, leading to the following conclusions:

1. In the model, the parameterization provides values of the soil thermal properties and subsequently of the predicted soil heat fluxes which are overestimated when compared with the observations.
2. Following the literature, a value of the ratio between the roughness lengths for momentumZ _oand heatZ _ohof close to 10 for fairly homogeneous areas of bare soil and vegetation is used. This value leads to a fair prediction of the surface temperature. If the roughness lengths were taken to be equal, as is often assumed in atmospheric modelling, a poorer prediction results.
3. Finally, the vapor phase transfer mode is found dominant close to the surface and a modified parameterization including this effect is proposed. It allows a fair prediction of both surface evaporation and near-surface water content.

     

Wind characteristics at the Boulder Atmospheric Observatory

Wind speeds at the 300 m tower at the Boulder Atmospheric Observatory have been analyzed. This tower is located in slightly rolling farmland. The following conclusions have been reached:

1. For west winds, the terrain is sufficiently uniform for simple surface-layer theory to be adequate without modification even though the air has moved up a small slope to reach the tower. For south and southeast winds, ‘effective’ roughness lengths must be introduced, which are significantly larger than the ‘true’ roughness length.
2. Useful wind estimates up to 150 m can be made from winds at 10 m and stability information, provided the ‘effective’ roughness length is known.
3. The observations are consistent with a von Kármán constant of 0.35.

     

The scavenging of nitrate by clouds and precipitation

A model with spectral microphysics was developed to describe the scavenging of nitrate aerosol particles and HNO₃ gas. This model was incorporated into the dynamic framework of an entraining air parcel model with which we computed the uptake of nitrate by cloud drops whose size distribution changes with time because of condensation, collision-coalescence and break-up. Significant differences were found between the scavenging behavior of nitrate and our former results on the scavenging behavior of sulfate. These reflect the following chemical and microphysical differences between the two systems:

1. nitrate particles occur in a larger size range than sulfate particles.
2. HNO₃ has a much greater solubility than SO₂ and is taken up irreversibly inside the drops in contrast to SO₂.
3. nitric acid in the cloud water is formed directly on uptake of HNO₃ gas whereas on uptake of SO₂ sulfuric acid is formed only after the reaction with oxidizing agents such as e.g., H₂O₂ or O₃.
4. nitrate resulting from uptake of HNO₃ is confined mainly to small drops, whereas sulfate resulting from uptake of SO₂ is most concentrated in the largest, oldest drops, which have had the greatest time for reaction.

Sensitivity studies showed that the nitrate concentration of small drops is significantly affected by the mass accommodation coefficient.