AN EVALUATION OF DISPERSION COEFFICIENTS FOR USE IN AIR QUALITY MODELS

Standard deviations of concentration in horizontal andvertical directions i.e. y andz have been estimated by using fivedifferent schemes based on empirical(due to Pasquill and Briggs)schemes and sophisticated methods(due to Irwin, Draxler, Taylor, Hanna et al.). The fiveschemes are discussed atlength. The purpose of this study is to make use ofmeteorological observations whichare routinely available, to test all the above methods andintercompare the resultswith one another and observations so that the sensitivityof each schemeunder various atmospheric stability conditions could beassessed. It has beenfound that the existing schemes are good enough to providereasonable estimates ofdispersion coefficient (y) during highly unstableconditions (Pasquill stability classes A and B). However, thesame is not true for the case when the stability increasesfrom C to F and turbulencedecreases, specifically during stable atmospheric conditions,when the observedvalues were found to be much higher than all the existingschemes. This suggests thatwhile we continue to use the current methods of estimatingthe dispersion parameters,a rigorous search is required for methods which give predictionswhich are close-to-realityduring such conditions which are represented by lowlevels (in terms of magnitude)of atmospheric turbulence leading to higher levelsof pollution.As one of the sophisticated methods requiresthe use of v and w (standard deviationsof wind velocity fluctuation in y and z directions),these have been estimated andvalidated with observed data (field experiments conductedby EPRI at Kincaid).Statistical evaluation of v and wbased on performance measures indicate a goodperformance of the parameterisations adopted in thisstudy. In the case of w duringunstable conditions a comparison of three differentschemes with observations is made.     

Short-Term Variability of Atmospheric DMS and Its Oxidation Products at Amsterdam Island during Summer Time

A one-month experiment was performed at Amsterdam Island in January 1998, to investigate the factors controlling the short-term variations of atmospheric dimethylsulfide (DMS) and its oxidation products in the mid-latitudes remote marine atmosphere. High mixing ratios of DMS, sulfur dioxide (SO₂) and dimethylsulfoxide (DMSO) have been observed during this experiment, with mean concentrations of 395 parts per trillion by volume (pptv) (standard deviation, = 285, n = 500), 114 pptv ( = 125, n = 12) and 3 pptv ( = 1.2, n = 167), respectively. Wind speed and direction were identified as the major factors controlling atmospheric DMS levels. Changes in air temperature/air masses origin were found to strongly influence the dimethylsulfoxide (DMSO)/DMS and SO₂/DMS molar ratios, in line with recent laboratory data. Methanesulfonic acid (MSA) and non-sea-salt sulfate (nss-SO₄ ^2–) mean concentrations in aerosols during this experiment were 12.2± 6.5 pptv (1, n=47) and 59 ± 33 pptv (1, n=47), respectively. Evidence of vertical entrainment was reported following frontal passages, with injection of moisture-poor, ozone-rich air. High MSA/ nss-SO₄ ^2– molar ratios (mean 0.44) were calculated during these events. Finally following frontal passages, few spots in condensation nuclei (CN) concentration were also observed.     

Absolute Rate Coefficients for the Gas-Phase Reactions of NO3 Radical with a Series of Monoterpenes at T = 298 to 433 K

The aim of this work is to study the reactivity of some naturally emitted terpenes, 2-carene, sabinene, myrcene, -phellandrene, d-limonene, terpinolene and -terpinene, towards NO3 radical to evaluate the importance of these reactions in the atmosphere and their atmospheric impact. The experiments with these monoterpenes have been carried out under second-order kinetic conditions over the range of temperature 298–433 K, using a discharge flow system and monitoring the NO3 radical by Laser Induced Fluorescence (LIF). This work is the first temperature dependence study for the reactions of the nitrate radical with the above-mentioned monoterpenes. The measured rate constants at 298 K for the reaction of NO3 with such terpenes are as follows: 2-carene, 16.6 ± 1.8, sabinene 10.7 ± 1.6, myrcene 12.8 ± 1.1, -phellandrene 42 ± 10, d-limonene 9.4 ± 0.9, terpinolene 52 ± 9 and -terpinene 24 ± 7, in units of 10-12 cm3 molecule-1 s-1. The proposed Arrhenius expressions, for the reactions of NO3 with 2-carene, sabinene, myrcene and -phellandrene are, respectively k1 = (1.4 ± 0.7) × 10-12 exp[(741 ± 190/T)] (cm3 molecule-1 s-1), k2=(2.3 ± 1.3) × 10-10 exp[–(940 ± 200/T)] (cm3 molecule-1 s-1), k3 = (2.2 ± 0.2) × 10-12 exp[(523 ± 35/T)] (cm3 molecule1 s-1) and k4 = (1.9 ± 1.3) × 10-9 exp[–(1158 ± 270/T)] (cm3 molecule-1 s-1). A decrease in the rate constants when raising the temperature has also been found for the reaction of d-limonene with NO3 while an increase in the rate constant with temperature has been observed for the reactions of terpinolene and -terpinene with NO3. Tropospheric half-lives for these terpenes have been calculated at night and during the day for typical NO3 and OH concentrations showing that both radicals provide an effective tropospheric sink for these compounds and that the night-time reaction with NO3 radical can be an important, if not dominant, loss process for these naturally emitted organics and for NO3 radicals.     

Thermal comfort of man in different urban environments

Summary On July 29, 1985, a hot summer day, biometeorological measurements were performed simultaneously in three different urban structures within the city of Munich and in the trunk space of a nearby tall spruce forest. Based on the results of these experiments the following thermophysiologically relevant biometeorological indices were calculated: Predicted mean vote, skin wettedness and physiologically equivalent temperature. These three indices are derived from different models for the human energy balance. They allow the assessment of the thermal components of the microclimates at the selected sites with regard to application in urban planning. The results quantitatively show the great heat stress in the urban structure street canyon, exposed to south, whereas in the trunk space of the tall spruce forest there is nearly an optimal climate even on hot summer days. Between these extremes the results for street canyon, exposed to north show a little higher heat load than for backyard with trees.

---

Zusammenfassung An einem heißen Sommertag, dem 29. Juli 1985, wurden in drei Stadtstrukturen in München und im Stammraum eines nahegelegenen Fichtenhochwaldes zeitgleich biometeorologische Messungen durchgeführt. Mit den Meßergebnissen wurden folgende thermophysiologisch relevante biometeorologische Indizes berechnet: Predicted mean vote, Hautbenetzungsgrad und physiologisch äquivalente Temperatur. Diese drei Indizes beruhen auf verschiedenen Modellen zur menschlichen Energiebilanz. Mit den drei Indizes wurden die thermischen Komponenten der Mikroklimate an den ausgewählten Meßplätzen im Hinblick auf Stadtplanungsaufgaben bewertet. Die Ergebnisse zeigen quantitativ die relativ große Hitzebelastung bei der Stadtstruktur Straßenschlucht, nach Süd exponiert, während im Stammraum des Fichtenhochwaldes selbst an heißen Sommertagen nahezu optimale Bedingungen herrschen. Zwischen diesen Extremen liegen die Ergebnisse für die anderen Meßplätze, wobei für Straßenschlucht, nach Nord exponiert die Wärmebelastung etwas höher als für Innenhof mit Bäumen ist.

---

---

With 6 Figures     

Measurements of solar radiation and estimation of optical depth in the high Andes of Peru

Summary During an expedition to the high Andes of Southern Peru in June–July 1977, measurements of direct solar radiation in four spectral bands (0.270–0.530–0.630–0.695–2.900 ) were conducted at six sites in elevations ranging from sea level to 5645 m. These measurements were evaluated in Langley plots to determine total optical depths () and irradiances at the top of the atmosphere. In addition, water vapor optical depths (_wv) were calculated from the mean radiosounding over Lima during the expedition, and Rayleigh (_ray) and ozone (_oz) optical depths were obtained from published tabulations. Subtracting _ray, _oz, and _wv from yielded estimates of aerosol optical depth _aer. The components _ray and _oz decrease from the shorter towards the longer wavelength bands and from the lower towards the higher elevation sites; _aer also decreases towards the higher elevations. Particularly pronounced is the decrease of _aer and from the lowlands of the Pacific coast to the highlands of the interior, reflecting the effect of a persistent lower-tropospheric inversion and the contrast from the marine boundary layer to the clear atmosphere of the high Andes.With 4 Figures     

Turbulence Characteristics Over Complex Terrain In West China

Meteorological data of velocity components and temperature have been measured on a mast of height 4.9 m at one site in the Heihe River Basin Field Experiment (HEIFE) conducted in west China. Mean and individual turbulence parameters, power spectra/cospectra, phase angles and their changes withfetch downwind of a change in surface roughness were analyzed. The turbulence characteristics depend strongly on the prevailing wind direction, which in turn is associated with changes in the upwind surface roughness pattern. The results show that values of horizontal velocity standard deviations sigma;_u,v scaled with local friction velocity u under different stratifications are larger than those over flat terrain, while the values of _w/u have the same values as over flat terrain. The differences between variance values of the horizontal velocity components, u and v, over inhomogeneous terrain were found to be significantly smaller than those over flat terrain. Since energy densities of the w spectra, uw and wT cospectra at low frequencies are relatively lower than those of longitudinal velocity spectra, total energies of w spectra, uw and wT cospectra tend to be in equilibrium with the local terrain. The values of phase angles at the low frequency end of the frequency showed obvious differences associated with changes of roughness.     

Diurnal variation of horizontal wind direction fluctuations in complex terrain at Geysers,Cal.

Horizontal diffusion in the surface layer is dependent on the standard deviation of wind direction fluctuations . Diurnal variation of this parameter in complex terrain was studied for the July 1979 Geysers, Cal., experiment using data from a network of 11 short meteorological towers in the 25 km² Anderson Creek watershed Valley side slopes are roughly 20 ° and maximum terrain difference is about 1 km.Values of for wind directions sampled for one hour at a height of 10 m are about 35 ° during the daytime. They slowly decrease to about 20 ° by 8 to 10 p.m. as stability increases but wind speeds are still relatively high. After 10 p.m. the drainage flow sets in at most stations, with speeds of 1 to 2 m s^-1, and average increases to about 30° during the period 11 p.m. to 6 a.m. In general, highest values of at night are associated with lowest values of wind speed and greatest static stability. This enhancement of by the terrain suggests that horizontal diffusion at night always conforms to that expected during nearly neutral stabilities. That is, Pasquill class D diffusion applies to the horizontal component all night in complex terrain.     

Final results of the CONDORS convective diffusion experiment

TheConvectiveDiffusionObserved byRemoteSensors (CONDORS) field experiment conducted at the Boulder Atmospheric Observatory used innovative techniques to obtain three-dimensional mappings of plume concentration fields, /Q, of oil fog detected by lidar and chaff detected by Doppler radar. It included extensive meteorological measurements and, in 1983, tracer gases measured at a single sampling arc. Final results from ten hours of elevated and surface release data are summarized here. Many intercomparisons were made. Oil fog /Q measured 40m above the arc are mostly in good agreement withSF ₆ values, except in a few instances with large spacial inhomogeneities over short distances. After a correction scheme was applied to compensate for the effect of its settling speed, chaff dy/Q agreed well with those of oil except in two cases of oil fog hot spots. Mass or frequency distribution vs. azimuth or elevation angle comparisons were made for chaff, oil, and wind, with mostly good agreements. Spacial standard deviations, _y and _z, of chaff and oil agree overall and are consistent at short range with velocity standard deviations _vand _w 0.6w^* (the convective scale velocity), as measured atz>100m. Surface release _y is enhanced up to 60% at smallx, consistent with the Prairie Grass measurements and with larger _v and reduced wind speed measured near the surface. Decreased _y at small dimensionless average times is also noted. Finally, convectively scaled dy, C _y, were plotted versus dimensionlessx andz for oil, chaff, and corrected chaff for each 30–60 min period. Aggregated CONDORSC _y fields compare well with laboratory tank and LES numerical simulations; surface-released oil fog compares expecially well with the tank experiments. However, large deviations from the norm occurred in individual averaging periods; these deviations correlated strongly with anomalies in measured distributions.On assignment to the US Environmental Protection Agency, Atmospheric Research and Exposure Assessment Laboratory, RTP, NC.     

On the cumulative frequency distributions ofk-day period amounts of precipitation for any station in The Netherlands,while k≧30

Summary A simple method of representing cumulative frequency distributions ofk-day period amounts of precipitation (30k1800), deduced from observations at Hoofddorp during 87 years and from observations at Winterswijk during 73 years, by means of cumulative Poisson distributions, had been developed. Poisson's constant appeared to depend linearly onk only, while the Poisson unitq appeared to be only proportional to the annual variation.In this paper it is shown that the representation of observed cumulative frequency distributions with the aid of Poisson distributions is true for any station in The Netherlands in the interval 30k1800. A map with isopleths of -values fork=360 presents a simple picture, so that for any station an -value fork=360 can be estimated. For the process of representation the meank-day period amount of precipitation is also necessary. This value may be obtained from a map for each of the 12 months, each map covered with isohyetal lines, which give a distribution of the monthly precipitation.

---

Zusammenfassung Es wurde eine Methode entwickelt, um in einfacher Weise Summenfunktionen vonk-tägigen Niederschlagsmengen (30k1800), welche aus Beobachtungen in Hoofddorp während 87 Jahren und in Winterswijk während 73 Jahren angestellt worden sind, durch kumulative Poissonsche Häufigkeitsverteilungen darzustellen. Dabei ergab sich, daß die Poissonsche Konstante nur linear vonk abhängig ist, während die Poissonsche Einheitq sich als proportional zum Jahresgang erwies.In der vorliegenden Untersuchung wird nun gezeigt, daß die Darstellung der beobachteten kumulativen Häufigkeitsverteilungen mit Hilfe von Poisson-Verteilungen für irgendeine Station in den Niederlanden im Bereich von 30k1800 gültig ist. Eine Karte mit Isoplethen der -Werte fürk=360 ergibt ein einfaches Bild, aus dem für jede beliebige Station ein -Wert fürk=360 abgelesen werden kann. Für eine solche Darstellung muß auch die Niederschlagsmenge derk-tägigen Perioden bekannt sein; diese Werte erhält man aus monatlichen Isohyeten-Karten, die die monatliche Niederschlagsverteilung wiedergeben.

---

Résumé On avait développé une méthode pour représenter d'une façon simple les distributions des sommes cumulées dek jours (30k1800) des quantités de précipitations déduites des observations faites à Hoofddorp pendant 87 ans et à Winterswijk pendant 73 ans. On avait démontré que la constante de Poisson ne dépend dek que linéairement et que l'unité de Poissonq est proportionnelle à la variation annuelle.De la présente étude ressort que, dans l'intervalle de 30k1800, la représentation des distributions des fréquences cumulées observées au moyen des formules de Poisson est valable pour une station quelconque aux Pays-Bas. Une carte d'isoplèthes de la valeur pourk=360 donne une image suffisamment simple pour que la valeur d'une station quelconque aux Pays-Bas puisse être déterminée sans difficulté. Pour une telle représentation il faut connaître également les moyennes des quantités des périodes dek jours de la précipitation. Ces valeurs peuvent se déduire de cartes mensuelles des isohyètes moyennes.

---

---

With 1 Figure

---

Dedicated to Dr.Anders K. Ångström on the occasion of his 70th birthday.     

Estimation of the Monin-Obukhov similarity functions from a spectral model

From measured one-dimensional spectra of velocity and temperature variance, the universal functions of the Monin-Obukhov similarity theory are calculated for the range –2 z/L + 2. The calculations show good agreement with observations with the exception of a range –1 z/L 0 in which the function _m , i.e., the nondimensional mean shear, is overestimated. This overestimation is shown to be caused by neglecting the spectral divergence of a vertical transport of turbulent kinetic energy. The integral of the spectral divergence over the entire wave number space is suggested to be negligibly small in comparison with production and dissipation of turbulent kinetic energy.Notation a,b,c contants (see Equations (–4)) - C_i constants i=u, v, w, (see Equation (5) - k_me,k_mT peak wave numbers of 3-d moel spectra of turbulent kinetic energy and of temperature variance, respectively - k_mi peak wave numbers of 1-d spectra of velocity components i=u, v, w and of temperature fluctuations i= - k_sb, k_c characteristics wave numbers of energy-feeding by mechanical effects being modified by mean buoyancy, and of convective energy feeding, respectively - L Monin-Obukhov length - % MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% Gabeivayaaraaaaa!3C5B!\[{\rm{\bar T}}\] difference of mean temperature and mean potential temperature - T* Monin-Obukhov temperature scale - velocity of mean flow in positive x-direction - u* friction velocity - u, v, w components of velocity fluctuations - z height above ground - von Kármanán constant - temperature fluctuation - _m nondimensional mean shear - _H nondimensional mean temperature gradient - nondimensional rate of lolecular dissipation of turbulent kinetic energy - _D nondimensional divergence of vertical transports of turbulent linetic energy     

Size-Resolved Characterisation of Soluble Ions in the Particles in the Tropospheric Plume of Masaya Volcano, Nicaragua: Origins and Plume Processing

We present the first application of a multi-stage impactor to study volcanic particle emissions to the troposphere from Masaya volcano, Nicaragua. Concentrations of soluble SO₄ ^2–,Cl^–, F^–, NO₃ ^–, K⁺, Na⁺,NH₄ ⁺, Ca²⁺ and Mg²⁺ were determined in 11 size bins from 0.07 m to >25.5 m. The near-source size distributions showed major modes at 0.5m (SO₄ ^2–, H⁺,NH₄ ⁺); 0.2 m and 5.0 m (Cl^–) and 2.0–5.0 m(F^–). K⁺ and Na⁺ mirrored the SO₄ ^2– size-resolvedconcentrations closely, suggesting that these were transported primarily asK₂SO₄ and Na₂SO₄ in acidic solution, while Mg²⁺ andCa²⁺ presented modes in both <1 m and >1 m particles. Changes in relative humidity were studied by comparing daytime (transparent plume) and night-time (condensed plume) results. Enhanced particle growth rates were observed in the night-time plume as well as preferential scavenging of soluble gases, such as HCl, by condensed water. Neutralisation of the acidic aerosol by background ammonia was observed at the crater rim and to a greater extent approximately 15 km downwind of the active crater. We report measurements of re-suspended near-source volcanic dust, which may form a component of the plume downwind. Elevated levels ofSO₄ ^2–, Cl^–, F^–,H⁺, Na⁺, K⁺ and Mg²⁺ were observed around the 10 m particle diameter in this dust. The volcanic SO₄ ^2– flux leaving the craterwas 0.07 kg s^–1.     

Re-analysis of the nuclear winter phenomenon

Summary An analysis of the report of the (U.S.) National Academy of Sciences (NAS) on atmospheric effects of a nuclear exchange leads to conclusions that differ from those of the NAS and of the earlier TTAPS and AMBIO studies. Any cooling of the earth's surface is likely to beshort-lived because of rapid removal of the smoke clouds originating from nuclear burst-initiated fires, andminor because of appreciable green-house effects due to several distinct physical causes. (One of these, neglected in prior analyses, is the infrared absorption from cirrus clouds produced directly by the nuclear bursts.) Taken together, these effects may even induce slight surface warming (nuclear summer) instead of cooling (nuclear winter). The consequences to atmospheric ozone are similarly ambiguous; depending on the detailed nuclear scenario, the net ozone content may increase-rather than decrease as argued by TTAPS. Experiments could settle some uncertainties.With 2 Figures     

Flux-profile Relationships for Wind Speed and Temperature in the Stable Atmospheric Boundary Layer

Wind and temperature profiles in the stable boundary layer were analyzed in the context of MoninObukhov similarity. The measurements were made on a 60-m tower in Kansas during October 1999 (CASES-99). Fluxprofile relationships, obtained from these measurements in their integral forms, were established for wind speed and temperature. Use of the integral forms eliminates the uncertainty and accuracy issues resulting from gradient computations. The corresponding stability functions, which were nearly the same for momentum and virtual sensible heat, were found to exhibit different features under weakly stable conditions compared to those under strongly stable conditions. The gradient stability functions were found to be linear, namely _m = 1+ 5.8 and _h = 1 + 5.4 up to a limit of the MoninObukhov stability parameter = 0.8; this is consistent with earlier findings. However, for stronger stabilities beyond a transition range, both functions were observed gradually to approach a constant, with a value of approximately 7. To link these two distinct regimes, a general but pliable functional form with only two parameters is proposed for the stability functions, covering the entire stability range from neutral to very stable conditions.     

Experiments On Buoyant Plume Dispersion In A Laboratory Convection Tank

Plume dispersion in the convective boundary layer (CBL) is investigated experimentally in a laboratory convection tank. The focusis on highly-buoyant plumes that loft near or become trapped in the CBL capping inversion and resistdownward mixing. Such plumes are defined by dimensionless buoyancy fluxes F_* 0.1, where F_* = F_b/(U w_* ² z_i), F_b is the stack buoyancy flux,U is the mean wind speed, w_* is the convective velocity scale, and z_i is the CBL depth. The aim is to obtain statistically-reliable mean (C) and root-mean-square (rms, _c) concentration fields as a function of F_* and the dimensionless distance X = w_*x/(U z_i), where x is the distance downstream of the source.The experiments reveal the following mainresults: (1) For 3 X 4and F_* 0.1, the crosswind-integrated concentration (CWIC) fields exhibit distinctly uniform profiles below z_i with a CWIC maximum aloft, in contrast to the nonuniform profiles obtained earlier by Willis and Deardorff. (2) The lateral dispersion (_y) variation with X is consistent with Taylor's theory for _* 0.1 and a buoyancy-enhanced dispersion, _y/z_i F_* ^1/3X^2/3, forF_* = 0.2 and 0.4. (3) The entrapment, the plume fraction above z_i, has a mean (E) that follows a systematic variationwith X and F_*, and a variability (_e/E) that is broad ( 0.3 to 2) near the source but subsides to 0.25 far downstream. (4) Vertical profiles of the concentration fluctuation intensity (c/C) are uniform for z < z_i and X > 1.5, but exhibit significant increases: (a) at the surface and close to the source (X 1.5), and(b) in the entrainment zone. (5) The cumulative distribution functions (CDFs) of the scaled concentration fluctuations (c/_c) separate into mixed-layer and entrainment-layer CDFs for X 2, with the mixed-layer group collapsing to a single distribution independent of z.These are the first experiments to obtain all components of the lateral and vertical dispersion parameters (rms meander, relative dispersion, total dispersion) for continuous buoyant releases in a convection tank. They also are the first tank experiments to demonstrate agreement with field observations of: (1) the scaled ground-level concentration along the plume centreline, and (2) the dimensionless lateral dispersion _y/z_i of buoyant plumes.     

High resolution spectral features of a series of aromatic hydrocarbons and BrO: Potential interferences in atmospheric OH-measurements

Naphthalene (C₁₀H₈), several other hydrocarbons, mostly derivates of naphthalene, and bromine oxide (BrO) were analyzed for narrow band (0.01 nm) absorption lines in the wavelength range between 307.7 and 308.3 nm to study their potential impact on OH radical measurements by differential absorption spectroscopy.Only naphthalene showed narrow band absorption lines in this wavelength region. From nine naphthalene lines the differential absorption cross-section was determined.The strongest naphthalene line at 308.002 nm is close to the Q ₁(2) OH line, but about a factor of 200 weaker (=(65.2±15.3)×10^-20 cm²/molec). The corresponding detection limit for naphthalene is about 15 ppt. We re-evaluated some spectra of our OH measurement campaign in July 1987 with respect to naphthalene and obtained an upper limit of 30 ppt for its concentration.BrO was recorded in the larger wavelength interval between 307.7 and 308.7 nm. Structured absorptions were only observed at wavelengths above 308.2 nm and no significant structures were found in the vicinity of the Q ₁(2) and Q ₁(3) OH lines.     

Wind and wind forces in a plantation spruce forest

Observations have been made of the tubulent structure within and above a dense (LAI=10.2) plantation spruce forest along with measurements of the movement of individual trees. The mean statistics of the turbulence and the turbulence spectra are compared with observations in other crops and complementary wind-tunnel studies using 175 scale plastic trees. The measurements show that momentum transport and the subsequent motion of the trees is dominated by intermittent sweep/ejection events associated with honami waves moving across the forest. The trees themselves act as forced damped harmonic oscillators and appear to short circuit the normal turbulent energy dissipation process by efficiently absorbing energy at their resonant frequencies. It is argued that understanding the nature and formation of honami waves over forests and crops is a crucial problem in agricultural and forest meteorology because of their important role both in turbulent transport and in causing wind damage.     

Effect of land management on ecosystem carbon fluxes at a subalpine grassland site in the Swiss Alps

Summary The influence of agricultural management on the CO₂ budget of a typical subalpine grassland was investigated at the Swiss CARBOMONT site at Rigi-Seebodenalp (1025m a.s.l.) in Central Switzerland. Eddy covariance flux measurements obtained during the first growing season from the mid of spring until the first snow fall (17 Mai to 25 September 2002) are reported. With respect to the 10-year average 1992–2001, we found that this growing season had started 10 days earlier than normal, but was close to average temperature with above-normal precipitation (100–255% depending on month). Using a footprint model we found that a simple approach using wind direction sectors was adequate to classify our CO₂ fluxes as being controlled by either meadow or pasture. Two significantly different light response curves could be determined: one for periods with external interventions (grass cutting, cattle grazing) and the other for periods without external interventions. Other than this, meadow and pasture were similar, with a net carbon gain of –128±17g Cm^–2 on the undisturbed meadow, and a net carbon loss of 79±17g Cm^–2 on the managed meadow, and 270±24g Cm^–2 on the pasture during 131 days of the growing season, respectively. The grass cut in June reduced the gross CO₂ uptake of the meadow by 50±2% until regrowth of the vegetation. Cattle grazing reduced gross uptake over the whole vegetation period (37±2%), but left respiration at a similar level as observed in the meadow.     

Local Similarity Relationships In The Urban Boundary Layer

To investigate turbulent structures in an urban boundary layer (UBL) with many tallbuildings, a number of non-dimensional variable groups based on turbulent observationsfrom a 325-m meteorological tower in the urban area of Beijing, China, are analyzedin the framework of local similarity. The extension of surface-layer similarity to localsimilarity in the stable and unstable boundary layer is also discussed. According to localsimilarity, dimensionless quantities of variables: e.g., velocity and temperature standarddeviations _i/u_*l (i=u,v,w) and_T/T_*l,correlation coefficients of uw and wT covariance, gradients of wind and temperature_m and _h, and dissipation rates of turbulent kinetic energy (TKE) andtemperature variance and _N can be represented as a functiononly of a local stability parameter z/, where is the local Obukhovlength and z is the height above ground. The average dissipation rates of TKE andtemperature variance are computed by using the u spectrum, and the uw and wTcospectra in the inertial subrange. The functions above were found to be in a goodagreement with observational behaviour of turbulence under unstable conditions, butthere were obvious differences in the stable air.     

Turbulence mechanisms at an agricultural site

An extensive set of turbulence data from the 3- and 12-m heights taken over an agricultural site (Marsta, Sweden) are analyzed and compared with data from ideal sites.In unstable air, Monin-Obukhov similarity is found to be valid for the non-dimensional gradients of wind, _m , temperature, _h , and humidity, _e , for (only a few data), for _T /|T _*|,/ _E /|E _*| and for the non-dimensionalized inertial subrange spectra of temperature and humidity. Where comparison is possible, the unstable data also agree with those found in the Kansas study, with one remarkable exception, the inertial subrange constant of the temperature spectrum, ₁, being only 0.39, compared to the value 0.80 found at the Kansas site.On the stable side, most similarity predictions break down, with most of the data differing systematically from the corresponding Kansas results, the only exception being . The inertial subrange constants for temperature, ₁, and for humidity, ₁ are found to have the same values, 0.39 and 0.30, respectively, as they do on the unstable side. Remarkable similarity is found for the shape of the stable u- and - and e-spectra. In addition, this shape is found to be identical with that found in Kansas. The peak wavelength of the stable u-, and -spectra is found to be about four times larger than it is for the corresponding Kansas spectra. This is interpreted to be a result of the increased macro-roughness at the Marsta site as compared with that at the Kansas site. A possible explanation for the low ₁-value is discussed, suggesting that ₁ is not a universal constant, but instead dependent on the turbulent structure.     

Spatial Variability of Turbulent Fluxes in the Roughness Sublayer of an Even-Aged Pine Forest

The spatial variability of turbulent flow statistics in the roughness sublayer (RSL) of a uniform even-aged 14 m (= h) tall loblolly pine forest was investigated experimentally. Using seven existing walkup towers at this stand, high frequency velocity, temperature, water vapour and carbon dioxide concentrations were measured at 15.5 m above the ground surface from October 6 to 10 in 1997. These seven towers were separated by at least 100m from each other. The objective of this study was to examine whether single tower turbulence statistics measurements represent the flow properties of RSL turbulence above a uniform even-aged managed loblolly pine forest as a best-case scenario for natural forested ecosystems. From the intensive space-time series measurements, it was demonstrated that standard deviations of longitudinal and vertical velocities (_u, _w) and temperature (_T) are more planar homogeneous than their vertical flux of momentum (u_* ²) and sensible heat (H) counterparts. Also, the measured H is more horizontally homogeneous when compared to fluxes of other scalar entities such as CO₂ and water vapour. While the spatial variability in fluxes was significant (>15 %), this unique data set confirmed that single tower measurements represent the canonical structure of single-point RSL turbulence statistics, especially flux-variance relationships. Implications to extending the moving-equilibrium hypothesis for RSL flows are discussed. The spatial variability in all RSL flow variables was not constant in time and varied strongly with spatially averaged friction velocity u_*, especially when u_* was small. It is shown that flow properties derived from two-point temporal statistics such as correlation functions are more sensitive to local variability in leaf area density when compared to single point flow statistics. Specifically, that the local relationship between the reciprocal of the vertical velocity integral time scale (I_w) and the arrival frequency of organized structures (/h) predicted from a mixing-layer theory exhibited dependence on the local leaf area index. The broader implications of these findings to the measurement and modelling of RSL flows are also discussed.