On sonic anemometer measurement theory

In this paper a model for the measuring process of sonic anemometers (ultrasound pulse based) is presented. The differential equations that describe the travel of ultrasound pulses are solved in the general case of non-steady, non-uniform atmospheric flow field. The concepts of instantaneous line-average and travelling pulse-referenced average are established and employed to explain and calculate the differences between the measured turbulent speed (travelling pulse-referenced average) and the line-averaged one. The limit k1l=1 established by Kaimal in 1968, as the maximum value which permits the neglect of the influence of the sonic measuring process on the measurement of turbulent components is reviewed here. Three particular measurement cases are analysed: A non-steady, uniform flow speed field, a steady, non-uniform flow speed field and finally an atmospheric flow speed field. In the first case, for a harmonic time-dependent flow field, Mach number, M (flow speed to sound speed ratio) and time delay between pulses have revealed themselves to be important parameters in the behaviour of sonic anemometers, within the range of operation. The second case demonstrates how the spatial non-uniformity of the flow speed field leads to an influence of the finite transit time of the pulses (M≠0) even in the absence of non-steady behaviour of the wind speed. In the last case, a model of the influence of the sonic anemometer processes on the measurement of wind speed spectral characteristics is presented. The new solution is compared to the line-averaging models existing in the literature. Mach number and time delay significantly distort the measurement in the normal operational range. Classical line averaging solutions are recovered when Mach number and time delay between pulses go to zero in the new proposed model. The results obtained from the mathematical model have been applied to the calculation of errors in different configurations of practical interest, such as an anemometer located on a meteorological mast and the transfer function of a sensor in an atmospheric wind. The expressions obtained can be also applied to determine the quality requirements of the flow in a wind tunnel used for ultrasonic anemometer calibrations.

Deviations of cup anemometer rotational speed measurements due to steady state harmonic accelerations of the rotor

The measurement deviations of cup anemometers are studied by analyzing the rotational speed of the rotor at steady state (constant wind speed). The differences of the measured rotational speed with respect to the averaged one based on complete turns of the rotor are produced by the harmonic terms of the rotational speed. Cup anemometer sampling periods include a certain number of complete turns of the rotor, plus one incomplete turn, the residuals from the harmonic terms integration within that incomplete turn (as part of the averaging process) being responsible for the mentioned deviations. The errors on the rotational speed due to the harmonic terms are studied analytically and then experimentally, with data from more than 500 calibrations performed on commercial anemometers.

Investigation of the flow field of a highly heated jet of air

Measurements of mean and fluctuating velocity and temperature and their self- and cross-products to the third-order are presented for a heated axisymmetric air jet. Froude numbers in the range of 3500 13,190, Reynolds numbers in the range of 3470-8500 and non-dimensional streamwise distances. X*, from 0.27 to 1.98 are covered by the data. It was found that turbulence intensity decreases for the heated jet in the region between the inertia dominated and the buoyancy dominated regions which is contrary to findings with helium jets mixing with ambient air to produce density fluctuations. The effects of heating on the turbulent kinetic energy budget and the temperature variance budget show small differences for the inertia dominated region and the intermediate region which help to explain the transition process to the far field plume region. Constants are evaluated for the isotropic eddy diffusivity and generalised gradient hypothesis models as well as the scalar variance model. No significant effect of heating on the dissipation time-scale ratio was found. A novel wire array with an inclined cold wire was used. Measurements obtained with this probe are found to lead to asymmetries in some of the higher-order products. Further investigation suggested that the asymmetries are attributable to an as yet unreported interference effect produced by the leading prong of the inclined temperature wire, The effect may also have implications for inclined velocity wires which contain a temperature component when used in heated flows. (C) 2002 Elsevier Science Inc. All rights reserved.

Adaptable control for electrical generation at irregural wind speeds

The main aims of this work are the development and the validation of one generic algorithm to provide the optimal control of small power wind generators. That means up to 40 kW and blades with fixed pitch angle. This algorithm allows the development of controllers to fetch the wind generators at the desired operational point in variable operating conditions. The problems posed by the variable wind intensity are solved using the proposed algorithm. This is done with no explicit measure of the wind velocity, and so no special equipment or anemometer is required to compute or measure the wind velocity.

Averiguação do potencial de vento em ambiente edificado para aproveitamentos

O objectivo deste projecto consiste em analisar o potencial eólico em ambiente edificado urbano, considerando a utilização de turbinas eólicas de eixo vertical para produção de energia nesse contexto. Pretende-se com este documento demonstrar que, embora os estudos sobre as turbinas de eixo vertical sejam ainda reduzidos quando comparados aos das de eixo horizontal, tal não implica que as mesmas não tenham características que, em determinados cenários, sejam superiores às turbinas de eixo horizontal. Para a análise da intensidade de vento em cenário edificado urbano, seleccionou-se como local de estudo desta tese o Instituto Superior de Engenharia do Porto (ISEP), mais concretamente, o edifício F e o edifício E. Foi escolhido o edifício F, pelo facto de a acessibilidade ao mesmo ser mais fácil e também pelo facto de nesse edifício se ter acesso à parte norte do mesmo, onde os ventos são de intensidade mais forte. O edifício E como já tinha um anemómetro colocado a recolher dados para a estação meteorológica do ISEP foi igualmente objecto de incorporação na tese e utilizado na avaliação geoestatística exemplificativa. Após a extensa recolha de dados nos locais anteriormente mencionados, procedeu-se à análise de diversas turbinas de eixo vertical em termos dos respectivos perfis de produção. De seguida, efectuou-se uma análise estatística e geoestatística de carácter exemplificativo, de modo a caracterizar a intensidade de vento presente na área compreendida entre o edifício E e o edifício F. De forma a finalizar o documento, é apresentada uma conclusão relativa ao potencial eólico para produção de energia eléctrica em ambiente edificado urbano por recurso a turbinas eólicas de eixo vertical.

Reliable tidal volume estimates at the airway opening with an infant monitor during high-frequency oscillatory ventilation.

OBJECTIVE: To assess the suitability of a hot-wire anemometer infant monitoring system (Florian, Acutronic Medical Systems AG, Hirzel, Switzerland) for measuring flow and tidal volume (Vt) proximal to the endotracheal tube during high-frequency oscillatory ventilation. DESIGN: In vitro model study. SETTING: Respiratory research laboratory. SUBJECT: In vitro lung model simulating moderate to severe respiratory distress. INTERVENTION: The lung model was ventilated with a SensorMedics 3100A ventilator. Vt was recorded from the monitor display (Vt-disp) and compared with the gold standard (Vt-adiab), which was calculated using the adiabatic gas equation from pressure changes inside the model. MEASUREMENTS AND MAIN RESULTS: A range of Vt (1-10 mL), frequencies (5-15 Hz), pressure amplitudes (10-90 cm H2O), inspiratory times (30% to 50%), and Fio2 (0.21-1.0) was used. Accuracy was determined by using modified Bland-Altman plots (95% limits of agreement). An exponential decrease in Vt was observed with increasing oscillatory frequency. Mean DeltaVt-disp was 0.6 mL (limits of agreement, -1.0 to 2.1) with a linear frequency dependence. Mean DeltaVt-disp was -0.2 mL (limits of agreement, -0.5 to 0.1) with increasing pressure amplitude and -0.2 mL (limits of agreement, -0.3 to -0.1) with increasing inspiratory time. Humidity and heating did not affect error, whereas increasing Fio2 from 0.21 to 1.0 increased mean error by 6.3% (+/-2.5%). CONCLUSIONS: The Florian infant hot-wire flowmeter and monitoring system provides reliable measurements of Vt at the airway opening during high-frequency oscillatory ventilation when employed at frequencies of 8-13 Hz. The bedside application could improve monitoring of patients receiving high-frequency oscillatory ventilation, favor a better understanding of the physiologic consequences of different high-frequency oscillatory ventilation strategies, and therefore optimize treatment.

Le pergélisol dans les terrains sédimentaires à forte déclivité: distribution, régime thermique et instabilités

Résumé: Dans le contexte d'un climat de plus en plus chaud, la localisation du pergélisol dans les terrains sédimentaires à forte déclivité et l'évaluation des mouvements de terrain qui y ont cours s'avèrent primordiales. S'insérant dans cette problématique, ce travail de thèse s'articule autour de deux axes de recherche différents. D'un point de vue statique, cette recherche propose une étude de la distribution et des caractéristiques du pergélisol dans les éboulis de la zone périglaciaire alpine. D'un point de vue dynamique, une analyse de l'influence des caractéristiques du pergélisol (teneur en glace, température du pergélisol, etc.) et des variations des températures de l'air et du sol sur les vitesses de fluage des corps sédimentaires gelés est effectuée. Afin de répondre à ce double objectif, l'approche "terrain" a été privilégiée. Pour déterminer la répartition et les caractéristiques du pergélisol, les méthodes traditionnelles de prospection du pergélisol ont été utilisées, à savoir la mesure de la température du sol à la base du manteau neigeux (BTS), la mesure de la température du sol en continu ainsi que la méthode géoélectrique. Les mouvements de terrain ont pour leur part été mesurés à l'aide d'un GPS différentiel. L'étude de la distribution du pergélisol a été effectuée dans une quinzaine d'éboulis situés dans les régions du Mont Gelé (Verbier-Nendaz) et d'Arolla principalement. Dans la plupart des cas, un pergélisol a pu être mis en évidence dans la partie inférieure des accumulations sédimentaires, alors que la partie médiane des éboulis n'est, le plus souvent, pas gelée. Si cette absence de pergélisol se prolonge parfois dans les portions sommitales des pentes, les mesures réalisées montrent que dans d'autres cas des sédiments gelés y sont à nouveau présents. Les résistivités électriques mesurées dans les portions gelées des éboulis étudiés sont dans la plupart des cas nettement inférieures à celles mesurées sur les glaciers rocheux. Des études préalables ont montré que des circulations d'air internes sont responsables de l'anomalie thermique négative et, lorsqu'il existe, du pergélisol que l'on trouve dans la partie inférieure d'éboulis situés plus de 1000 m plus bas que la limite inférieure régionale du pergélisol discontinu. L'étude de quatre sites de basse altitude (1400-1900 m), et notamment l'équipement du site de Dreveneuse (Préalpes Valaisannes) avec deux forages, des capteurs de température de surface et un anémomètre a permis de vérifier et de préciser le mécanisme de ventilation actif au sein des éboulis froids de basse altitude. Ce mécanisme fonctionne de la manière suivante: en hiver, l'air contenu dans l'éboulis, plus chaud et plus léger que l'air extérieur, monte à l'intérieur de l'accumulation sédimentaire et est expulsé dans ses parties sommitales. Cet effet de cheminée provoque une aspiration d'air froid à l'intérieur de la partie inférieure de l'éboulis, causant ainsi un sur-refroidissement marqué du terrain. En été, le mécanisme s'inverse, l'éboulis étant plus froid que l'air environnant. De l'air froid est alors expulsé au bas de la pente. Une ventilation ascendante hivernale a pu être mise en évidence dans certains des éboulis de haute altitude étudiés. Elle est probablement en grande partie responsable de la configuration particulière des zones gelées observées. Même si l'existence d'un effet de cheminée n'a pu être démontrée dans tous les cas, du fait notamment de la glace interstitielle qui entrave le cheminement de l'air, des indices laissant présager son possible fonctionnement existent dans la quasi totalité des éboulis étudiés. L'absence de pergélisol à des altitudes qui lui sont favorables pourrait en tous les cas s'expliquer par un réchauffement du terrain lié à des expulsions d'air relativement chaud. L'étude des mouvements de terrain a été effectuée sur une dizaine de sites, principalement sur des glaciers rocheux, mais également sur une moraine de poussée et - II - Résumé ? abstract quelques éboulis. Plusieurs glaciers rocheux présentent des formes de déstabilisation récente (niches d'arrachement, blocs basculés, apparition de la matrice fine à la surface, etc.), ce qui témoigne d'une récente accélération des vitesses de déplacement. Ce phénomène, qui semble général à l'échelle alpine, est probablement à mettre sur le compte du réchauffement du pergélisol depuis une vingtaine d'années. Les vitesses mesurées sur ces formations sont souvent plus élevées que les valeurs habituellement proposées dans la littérature. On note par ailleurs une forte variabilité inter-annuelle des vitesses, qui semblent dépendre de la variation de la température moyenne annuelle de surface. Abstract: In the context of a warmer climate, the localisation of permafrost in steep sedimentary terrain and the measurement of terrain movements that occur in these areas is of great importance. With respect to these problems, this PhD thesis follows two different research axes. From a static point of view, the research presents a study of the permafrost distribution and characteristics in the talus slopes of the alpine periglacial belt. From a dynamic point of view, an analysis of the influence of the permafrost characteristics (ice content, permafrost temperature, etc.) and air and soil temperature variations on the creep velocities of frozen sedimentary bodies is carried out. In order to attain this double objective, the "field" approach was favoured. To determine the distribution and the characteristics of permafrost, the traditional methods of permafrost prospecting were used, i.e. ground surface temperature measurements at the base of the snow cover (BTS), year-round ground temperature measurements and DC-resistivity prospecting. The terrain movements were measured using a differential GPS. The permafrost distribution study was carried out on 15 talus slopes located mainly in the Mont Gelé (Verbier-Nendaz) and Arolla areas (Swiss Alps). In most cases, permafrost was found in the lower part of the talus slope, whereas the medium part was free of ice. In some cases, the upper part of the talus is also free of permafrost, whereas in other cases permafrost is present. Electrical resistivities measured in the frozen parts of the studied talus are in most cases clearly lower than those measured on rock glaciers. Former studies have shown that internal air circulation is responsible for the negative thermal anomaly and, when it exists, the permafrost present in the lower part of talus slopes located more than 1000 m below the regional lower limit of discontinuous permafrost. The study of four low-altitude talus slopes (1400-1900 m), and notably the equipment of Dreveneuse field site (Valais Prealps) with two boreholes, surface temperature sensors and an anemometer permitted to verify and to detail the ventilation mechanism active in low altitude talus slopes. This mechanism works in the following way: in winter, the air contained in the block accumulation is warmer and lighter than the surrounding air and therefore moves upward in the talus and is expelled in its upper part. This chimney effect induces an aspiration of cold air in the interior of the lower part of talus, that causes a strong overcooling of the ground. In summer, the mechanism is reversed because the talus slope is colder than the surrounding air. Cold air is then expelled in the lower part of the slope. Evidence of ascending ventilation in wintertime could also be found in some of the studied high-altitude talus slopes. It is probably mainly responsible for the particular configuration of the observed frozen areas. Even if the existence of a chimney effect could not be demonstrated in all cases, notably because of interstitial ice that obstructs Résumé ? abstract - III - the air circulation, indices of its presence exist in nearly all the studied talus. The absence of permafrost at altitudes favourable to its presence could be explained, for example, by the terrain warming caused by expulsion of relatively warm air. Terrain movements were measured at about ten sites, mainly on rock glaciers, but also on a push moraine and some talus slopes. Field observations reveal that many rock glaciers display recent destabilization features (landslide scars, tilted blocks, presence of fine grained sediments at the surface, etc.) that indicate a probable recent acceleration of the creep velocities. This phenomenon, which seems to be widespread at the alpine scale, is probably linked to the permafrost warming during the last decades. The measured velocities are often higher than values usually proposed in the literature. In addition, strong inter-annual variations of the velocities were observed, which seems to depend on the mean annual ground temperature variations.

Kuumalanka-anemometri virtausmittauksissa

Tässä työssä selostetaan kuumalanka-anemometrin käyttö virtausmittauksissa. Kuumalanka-anemometrilla saadaan mitattua virtausnopeuden ja -suunnan lisäksi nopeusheilahteluja. Mittaustaajuus on tyypillisesti useita kymmeniä tuhansia mittauksia sekunnissa ja signaali on jatkuva. Nykytekniikalla pystytään helposti tallentamaan mittauslaitteistolta saatu viesti tietokoneelle ja muuntamaan se nopeudeksi. Hetkellisten nopeuksien avulla voidaan laskea turbulenttisen virtauksen ominaisuuksia, kuten turbulenssin intensiteetti ja spektri. Kuumalanka-anemometrissa lämmitetään sähköisesti ohutta lankaa, joka on mitattavassa virtauksessa. Langan sähköteho on suunnilleen yhtäsuuri kuin langasta konvektiolla siirtyvä lämpöteho. Tällöin on teoreettisesti mahdollista laskea virtausnopeus lämpötehosta lämmönsiirtokorrelaatioilla. Käytännössä laitteisto joudutaan kuitenkin erikseen kalibroimaan, mutta sähkötehon teoreettista riippuvuutta konvektiosta käytetään hyväksi. Kuumalangan lämmitettävä osuus on tyypillisesti halkaisijaltaan 5 µm ja pituudeltaan noin 1 mm. Sitä käytetään pääasiassa kaasuvirtausten mittaamiseen ja valtaosassa mittauksissa virtausaineena on ilma. Kuumalanka voi olla toteutettu kuumakalvotekniikalla, jossa halkaisijaltaan noin 50 - 70 µm paksuinen kuitu on päällystetty ohuella sähköä johtavalla kalvolla. Kuumakalvoanturin ei tarvitse olla muodoltaan sylinterimäinen, se voi olla mm. kartiomainen tai kiilamainen. Erikoispäällystetyllä kuumakalvoanturilla on mahdollista mitata myös nestevirtauksia. Mitattaessa kaasuvirtauksia kuumakalvon etuna on selvästi parempi kestävyys verrattuna kuumalankaan. Nimitystä kuumalanka-anemometri käytetään yleisesti molemmista anturityypeistä Tämän työn alussa käsitellään sylinterin yli tapahtuvaan virtaukseen liittyvää virtausmekaniikkaa ja lämmönsiirtoa. Anemometrin sähköinen osa, laitteisto ja sen kalibrointi käydään läpi. Langan suuntariippuvuuden laskentaan esitetään tarvittavat yhtälöt. Työssä esitellään kolme laitteistolla tehtyä perusmittausta: anturin kohtauskulman muuttaminen, pyörähdyssymmerisen suihkun nopeuskenttä ja tuulitunnelin rajakerros. Lisäksi esitellään yksi käytännöllinen ja vaativampi mittaus, jossa on mitattu nopeusprofiili radiaalikompressorin diffuusorin loppuosassa.

Mittausmenetelmien kehittaminen kuplakolonnille

Kuplakolonnireaktoreiden CFD-mallinnus on talla hetkella voimakkaasti kehittyva tutkimusalue. Kaksifaasivirtauksen luotettava simulointi ja mallintaminen on haastavaa kuplakolonnireaktorissa tapahtuvien ilmioiden monimutkaisuuden vuoksi. Reaktorin kayttaytymiseen vaikuttavat tekijat, kuten kolonnin hydrodynamiikka ja aineensiirto, tulee tuntea hyvin ennen mallien tekoa. Tassa tyossa on kokeellisesti tutkittu erilaisten mittausmenetelmien soveltuvuutta kuplakolonnin hydrodynamiikan tutkimiseen. Mittausmenetelmissa on keskitytty erityisesti CFD-mallien vaatimiin paikallisiin mittauksiin. Lisaksi tyossa on arvioitu mittausmenetelmien soveltuvuutta j a luotettavuutta CFD-mallien validointiin. Tyon kirjallisuusosassa on perehdytty kuplakolonnireaktorin hydrodynaamiseen kayttaytymiseen ja siihen vaikuttaviin tekijoihin. Naita ovat mm. reaktorityypit, kaasun dispergointi, virtaustyypit ja -alueet, kaasun tilavuusosuus, kaasukuplan koko ja kuplan nousunopeus. Mittauksia tehtiin kahdessa erikokoisessa kuplakolonnissa, joista pienemman halkaisija oli 0,078 m ja suuremman 0,182 m. Molempien kolonnien nestepinnan korkeus oli 4,62 m. Mittaukset tehtiin vesijohtovedella ja epaorgaanisella prosessiliuoksella. Hydrodynaamisista ominaisuuksista mitattiin kaasun tilavuusosuus, kaasukuplan koko seka kaasukuplan nousunopeus. Kaasun tilavuusosuusmittaukset tehtiin paaasiassa paine-eromittauksella ja joissakin tapauksissa pinnanmittausmenetelmalla. Kuplakoko- ja kuplan nousunopeusmittaukset tehtiin suumopeusvideokameralla ja laser Doppler-anemometrilla. Mittauksissa kaytettiin kahdeksaa erilaista kaasunjakolaitetta, joilla selvitettiin kaasunjakolaitteen ominaisuuksien vaikutusta kolonnin hydrodynamiikkaan. Tuloksista havaittiin, etta nestefaasin ominaisuuksilla oli suuri vaikutus kolonnin hydrodynaamiseen kayttaytymiseen. En kaasunjakolaitteilla vesijohtovedella mitatut hydrodynaamiset ominaisuudet eivat poikenneet paljoa toisistaan, kun taas prosessiliuoksella kaasunjakolaitteiden valille saatiin huomattavat erot. Mittausmenetelmista laser Doppler-anemometri ei kaytettavissa olleella optiikalla soveltunut kaasukuplien mittaamiseen. Kuplat olivat menetelmalle liian suuria. Suumopeusvideokamerallaja paine-eromittauksella paastiin hyviin tuloksiin.

Boundary layer structure and decoupling from synoptic scale flow during NAMBLEX

This paper presents an overview of the meteorology and planetary boundary layer structure observed during the NAMBLEX field campaign to aid interpretation of the chemical and aerosol measurements. The campaign has been separated into five periods corresponding to the prevailing synoptic condition. Comparisons between meteorological measurements ( UHF wind profiler, Doppler sodar, sonic aneometers mounted on a tower at varying heights and a standard anemometer) and the ECMWF analysis at 10 m and 1100 m identified days when the internal boundary layer was decoupled from the synoptic flow aloft. Generally the agreement was remarkably good apart from during period one and on a few days during period four when the diurnal swing in wind direction implies a sea/land breeze circulation near the surface. During these periods the origin of air sampled at Mace Head would not be accurately represented by back trajectories following the winds resolved in ECMWF analyses. The wind profiler observations give a detailed record of boundary layer structure including an indication of its depth, average wind speed and direction. Turbulence statistics have been used to assess the height to which the developing internal boundary layer, caused by the increased surface drag at the coast, reaches the sampling location under a wide range of marine conditions. Sampling conducted below 10 m will be impacted by emission sources at the shoreline in all wind directions and tidal conditions, whereas sampling above 15 m is unlikely to be affected in any of the wind directions and tidal heights sampled during the experiment.

Turbulent flow at 190 m height above London during 2006-2008: A climatology and the applicability of similarity theory

Flow and turbulence above urban terrain is more complex than above rural terrain, due to the different momentum and heat transfer characteristics that are affected by the presence of buildings (e.g. pressure variations around buildings). The applicability of similarity theory (as developed over rural terrain) is tested using observations of flow from a sonic anemometer located at 190.3 m height in London, U.K. using about 6500 h of data. Turbulence statistics—dimensionless wind speed and temperature, standard deviations and correlation coefficients for momentum and heat transfer—were analysed in three ways. First, turbulence statistics were plotted as a function only of a local stability parameter z/Λ (where Λ is the local Obukhov length and z is the height above ground); the σ_i/u_* values (i = u, v, w) for neutral conditions are 2.3, 1.85 and 1.35 respectively, similar to canonical values. Second, analysis of urban mixed-layer formulations during daytime convective conditions over London was undertaken, showing that atmospheric turbulence at high altitude over large cities might not behave dissimilarly from that over rural terrain. Third, correlation coefficients for heat and momentum were analyzed with respect to local stability. The results give confidence in using the framework of local similarity for turbulence measured over London, and perhaps other cities. However, the following caveats for our data are worth noting: (i) the terrain is reasonably flat, (ii) building heights vary little over a large area, and (iii) the sensor height is above the mean roughness sublayer depth.

A thermally stable tension meter for atmospheric soundings using kites

Kites offer considerable potential as wind speed sensors—a role distinct from their traditional use as instrument-carrying platforms. In the sensor role, wind speed is measured by kite-line tension. A kite tether line tension meter is described here, using strain gauges mounted on an aluminum ring in a Wheatstone bridge electronic circuit. It exhibits a linear response to tension 19.5 mV N−1 with good thermal stability mean drift of −0.18 N °C−1 over 5–45 °C temperature range and a rapid time response 0.2 s or better. Field comparisons of tether line tension for a Rokkaku kite with a fixed tower sonic anemometer show an approximately linear tension-wind speed relationship over the range 1–6 ms−1. © 2010 American Institute of Physics. doi:10.1063/1.3465560

A method for estimating the turbulent kinetic energy dissipation rate from a vertically pointing Doppler lidar, and independent evaluation from balloon-borne in situ measurements

A method of estimating dissipation rates from a vertically pointing Doppler lidar with high temporal and spatial resolution has been evaluated by comparison with independent measurements derived from a balloon-borne sonic anemometer. This method utilizes the variance of the mean Doppler velocity from a number of sequential samples and requires an estimate of the horizontal wind speed. The noise contribution to the variance can be estimated from the observed signal-to-noise ratio and removed where appropriate. The relative size of the noise variance to the observed variance provides a measure of the confidence in the retrieval. Comparison with in situ dissipation rates derived from the balloon-borne sonic anemometer reveal that this particular Doppler lidar is capable of retrieving dissipation rates over a range of at least three orders of magnitude. This method is most suitable for retrieval of dissipation rates within the convective well-mixed boundary layer where the scales of motion that the Doppler lidar probes remain well within the inertial subrange. Caution must be applied when estimating dissipation rates in more quiescent conditions. For the particular Doppler lidar described here, the selection of suitably short integration times will permit this method to be applicable in such situations but at the expense of accuracy in the Doppler velocity estimates. The two case studies presented here suggest that, with profiles every 4 s, reliable estimates of ϵ can be derived to within at least an order of magnitude throughout almost all of the lowest 2 km and, in the convective boundary layer, to within 50%. Increasing the integration time for individual profiles to 30 s can improve the accuracy substantially but potentially confines retrievals to within the convective boundary layer. Therefore, optimization of certain instrument parameters may be required for specific implementations.

Vertical velocity variance and skewness in clear and cloud-topped boundary layers as revealed by Doppler lidar

In this paper, observations by a ground-based vertically pointing Doppler lidar and sonic anemometer are used to investigate the diurnal evolution of boundary-layer turbulence in cloudless, cumulus and stratocumulus conditions. When turbulence is driven primarily by surface heating, such as in cloudless and cumulus-topped boundary layers, both the vertical velocity variance and skewness follow similar profiles, on average, to previous observational studies of turbulence in convective conditions, with a peak skewness of around 0.8 in the upper third of the mixed layer. When the turbulence is driven primarily by cloud-top radiative cooling, such as in the presence of nocturnal stratocumulus, it is found that the skewness is inverted in both sign and height: its minimum value of around −0.9 occurs in the lower third of the mixed layer. The profile of variance is consistent with a cloud-top cooling rate of around 30Wm−2. This is also consistent with the evolution of the thermodynamic profile and the rate of growth of the mixed layer into the stable nocturnal boundary layer from above. In conditions where surface heating occurs simultaneously with cloud-top cooling, the skewness is found to be useful for diagnosing the source of the turbulence, suggesting that long-term Doppler lidar observations would be valuable for evaluating boundary-layer parametrization schemes. Copyright c 2009 Royal Meteorological Society

Boundary layer dynamics over London, UK, as observed using Doppler lidar during REPARTEE-II

Urban boundary layers (UBLs) can be highly complex due to the heterogeneous roughness and heating of the surface, particularly at night. Due to a general lack of observations, it is not clear whether canonical models of boundary layer mixing are appropriate in modelling air quality in urban areas. This paper reports Doppler lidar observations of turbulence profiles in the centre of London, UK, as part of the second REPARTEE campaign in autumn 2007. Lidar-measured standard deviation of vertical velocity averaged over 30 min intervals generally compared well with in situ sonic anemometer measurements at 190 m on the BT telecommunications Tower. During calm, nocturnal periods, the lidar underestimated turbulent mixing due mainly to limited sampling rate. Mixing height derived from the turbulence, and aerosol layer height from the backscatter profiles, showed similar diurnal cycles ranging from c. 300 to 800 m, increasing to c. 200 to 850 m under clear skies. The aerosol layer height was sometimes significantly different to the mixing height, particularly at night under clear skies. For convective and neutral cases, the scaled turbulence profiles resembled canonical results; this was less clear for the stable case. Lidar observations clearly showed enhanced mixing beneath stratocumulus clouds reaching down on occasion to approximately half daytime boundary layer depth. On one occasion the nocturnal turbulent structure was consistent with a nocturnal jet, suggesting a stable layer. Given the general agreement between observations and canonical turbulence profiles, mixing timescales were calculated for passive scalars released at street level to reach the BT Tower using existing models of turbulent mixing. It was estimated to take c. 10 min to diffuse up to 190 m, rising to between 20 and 50 min at night, depending on stability. Determination of mixing timescales is important when comparing to physico-chemical processes acting on pollutant species measured simultaneously at both the ground and at the BT Tower during the campaign. From the 3 week autumnal data-set there is evidence for occasional stable layers in central London, effectively decoupling surface emissions from air aloft.