The vertical structure of summertime local winds in the Wright Valley,Antarctica

Intensive observations of summertime up- and down-valley winds in a dry valley utilising airsondes, pilot balloons and a monostatic acoustic sounder are described. Both circulations show a distinctive layered vertical wind and temperature structure. Westerly down-valley flow is typically neutral and is characterised by strong surface winds overlain by light variable winds extending to an inversion between 2000 and 4000m in depth. Above this inversion, gradient winds prevail. This structure is similar to that of downslope winds observed elsewhere. The thermally-induced up-valley easterly flow is shown to be extremely well-developed in terms of its strength, depth and persistence. The strong surface easterly may reach 800 m in depth and usually undercuts the warmer westerly. The boundary between the two regimes is marked by an inversion. During easterly flow a surface-based, super-adiabatic layer of 100–200 m in depth is evident and is associated with weak convective activity. An intriguing aspect of the wind regime is the interaction between the easterly and westerly circulations in the valley. These are separated at the surface by a frontal zone which migrates up and down the valley. Further observational and modelling studies are recommended.     

Micrometeorological study of an urban valley

Winds, temperatures, and carbon monoxide concentrations were measured in a cross-section of the North Saskatchewan River Valley in central Edmonton on a clear October evening with cross-valley winds. The evolution of a complex asymmetrical valley inversion with vertical temperature gradients up to 12C (100 m)^–1 on the north-facing slope and 6C (100 m)^–1 on the south-facing slope is described. The inversion is accompanied by downslope winds of about 0.4 m s^–1 and a reversal from upvalley to downvalley winds. Carbon monoxide concentrations on the south-facing slope exhibit a well-defined evening maximum and an immediate response to the reversal from upslope to downslope winds.     

Observations of flow structure in a small forested valley system

Summary Features of the mean flow structure in a small valley system in the Rosalian mountain range are discussed using data from a wind measurement network. Tethered balloon measurements during periods of clear sky form the basic dataset for the analysis of drainage winds and temperature inversions. During periods of weak ambient winds the existence of a pure thermally driven nocturnal valley wind system is shown. With strong ambient winds opposing the drainage flow, a reduced drainage height but the same jet maximum as with weak ambient winds is found. On the other hand with aiding flow the drainage winds are suppressed and flow reversal can occur. This strong valley flow interaction with the ambient wind indicates considerable dynamic influence on the evolution of drainage winds and on the breakup of temperature inversion structure for small valleys.With 15 Figures     

Orographic and stability effects on valley-side drainage flows

The effects of orography and stability on valley-side drainage winds were investigated with the aid of a numerical model. The model is three-dimensional, non-hydrostatic, cast in terrain-following co-ordinates, has a surface energy budget and a 1.5 order TKE closure scheme. Experiments were conducted over a schematic three-dimensional valley to assess the influences on airflow of valley-side slope magnitude, valley cross-section shape, tilt of the valley floor and stability.In drainage flow, magnitudes of horizontal and vertical velocities and heights of their maxima are directly related to slope angle. The velocities are either insensitive to, or slightly inversely related to stability. The cooling which drives the flows is strongest over steep slopes and in large stabilities. The depth of the cooled layer, whilst increasing over steeper slopes, is inversely related to the stability. TKE increases with slope angle and decreases with increasing stability. In the downslope direction, the near-surface cooled layer significantly increases whereas the inversion intensity decreases by about 20%. These two features are due to mixing between the drainage flow and the overlying air. Tha drainage flow accelerates down the slope until it reaches the accumulated pool of cold air in the valley bottom, whereupon it slows down markedly and is accompanied by uplift over the centre of the valley.The cross-valley circulation is influenced by valley-side slope angle, valley cross-section shape and tilt of the valley floor, in addition to the effects of stability. For a given shape, the circulation is a direct function of the valley-side slope and an inverse function of the ambient stability. This relationship is described mathematically.V-shaped valleys generate stronger flows than doU-shaped valleys and a tilted valley floor also leads to a significant increase in velocities.     

The effects of ambient winds on valley drainage flows

The interaction of katabatic winds with ambient winds has been investigated for an idealized valley using Clark's nonhydrostatic model. Ambient ridgetop wind speeds ranged from 0.5 to 6 m/s, and made angles with the valley axis ranging from 0 ° to 90 °: cooling of the valley was based on measured values of sensible heat fluxes taken from observations in Colorado's Brush Creek Valley. The depth and strength of the down-valley winds decreased with increasing ambient wind speeds but showed relatively little sensitivity to wind directions in the range of 10 ° to 60 ° from the valley axis. An observed inverse linear decrease of drainage depth with wind speed in a 100 m thick layer above the ridgetops was also found in the simulations for parts of the valley but not near the valley mouth. Vertical motions over the valley showed marked patchiness, and implications of this structure on valley flow dynamics are discussed.This work was supported by the U.S. Department of Energy (DOE) under Contract DE-AC06-76RLO 1830.     

Valley winds and slope winds — Observations and elementary thoughts

Summary This paper presents a state-of-the-art account of valley wind research, with a bias towards a typical large Alpine valley and towards weak-gradient synoptic conditions. At the center of our attention is the quasiperiodic thermal forcing mechanism which drives the local wind system, in particular the role of slope winds and of topographic relief.Slope winds are at the small-scale end of a whole spectrum of thermally direct circulations which act to transmit the sensible heat input along the slopes to the valley atmosphere via compensating vertical motions. We surmise that the dynamics of slope winds, which react instantly to changes of the insolation or radiation balance, is characterized by local, instantaneous equilibria, rather than by conventional entrainment and boundary layer concepts.As described by Steinacker, the area-height distribution of a valley segment is a fundamental geometric factor which affords a quantitative measure of the slope area available for heat exchange, and of the air volume which must be heated or cooled. Using this concept, one can easily explain why the daily range of the valley mean temperature is, on average, more than twice as large as that of the atmosphere over the adjacent plain. This horizontal temperature contrast between plain and valley, reversing sign twice daily, builds up a corresponding pressure contrast hydrostatically, thereby causing up- and downvalley winds.

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Berg- und Tal- bzw. Hangwinde — Beobachtungen und grundsätzliche Überlegungen

Zusammenfassung Wir berichten über den aktuellen Stand der Talwindforschung mit besonderer Betonung der Verhältnisse in den Alpen und vorzugsweise gradientschwache Wetterlagen betreffend. Einen Schwerpunkt unserer Darstellung bilden die quasiperiodischen thermischen Antriebskräfte der lokalen Windsysteme, vor allem die Rolle der Hangwinde und des Reliefs.Am kleinräumigen Ende eines Spektrums thermisch getriebener direkter Zirkulationsformen stehen die Hangwinde. Sie vermitteln der Talatmosphäre die an den Hängen umgesetzte fühlbare Wärme mittels kompensierender vertikaler Strömungen. Es wird vermutet, daß die Dynamik der Hangwinde eher durch lokale und spontane Gleichgewichtszustände beschrieben werden kann als durch die üblichen Entrainment- und Grenzschichtkonzepte.Steinacker hat gezeigt, daß die Flächen-Höhen-Verteilung von Talabschnitten ein quantitatives Maß der für die Wärmeumsätze zur Verfügung stehenden Hangflächen liefert, und gleichzeitig der abzukühlenden oder zu erwärmenden Luftvolumina. Die in einem Tal im Vergleich zum Vorland mehr als doppelt so große Tagesschwankung der vertikalen Mitteltemperatur kann damit leicht erklärt werden. Dieser horizontale Temperaturunterschied zwischen Ebene und Gebirge mit seinem täglich zweimaligen Vorzeichenwechsel baut hydrostatisch die entsprechenden Druckunterschiede auf, welche die Talein- und Talauswinde antreiben.

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With 14 Figures     

Analysis of the wind field and heat budget in an alpine lake basin during summertime fair weather conditions

Summary ?Observational data collected in the Lake Tekapo hydro catchment of the Southern Alps in New Zealand are used to analyse the wind and temperature fields in the alpine lake basin during summertime fair weather conditions. Measurements from surface stations, pilot balloon and tethersonde soundings, Doppler sodar and an instrumented light aircraft provide evidence of multi-scale interacting wind systems, ranging from microscale slope winds to mesoscale coast-to-basin flows. Thermal forcing of the winds occurred due to differential heating as a consequence of orography and heterogeneous surface features, which is quantified by heat budget and pressure field analysis. The daytime vertical temperature structure was characterised by distinct layering. Features of particular interest are the formation of thermal internal boundary layers due to the lake-land discontinuity and the development of elevated mixed layers. The latter were generated by advective heating from the basin and valley sidewalls by slope winds and by a superimposed valley wind blowing from the basin over Lake Tekapo and up the tributary Godley Valley. Daytime heating in the basin and its tributary valleys caused the development of a strong horizontal temperature gradient between the basin atmosphere and that over the surrounding landscape, and hence the development of a mesoscale heat low over the basin. After noon, air from outside the basin started flowing over mountain saddles into the basin causing cooling in the lowest layers, whereas at ridge top height the horizontal air temperature gradient between inside and outside the basin continued to increase. In the early evening, a more massive intrusion of cold air caused rapid cooling and a transition to a rather uniform slightly stable stratification up to about 2000 m agl. The onset time of this rapid cooling varied about 1–2 h between observation sites and was probably triggered by the decay of up-slope winds inside the basin, which previously countered the intrusion of air over the surrounding ridges. The intrusion of air from outside the basin continued until about mid-night, when a northerly mountain wind from the Godley Valley became dominant. The results illustrate the extreme complexity that can be caused by the operation of thermal forcing processes at a wide range of spatial scales. Received June 25, 2001; Revised December 21, 2001     

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.

     

Characterization of Oscillatory Motions in the Stable Atmosphere of a Deep Valley

In a valley sheltered from strong synoptic effects, the dynamics of the valley atmosphere at night is dominated by katabatic winds. In a stably stratified atmosphere, these winds undergo temporal oscillations, whose frequency is given by $N \sin {\alpha }$ N sin α for an infinitely long slope of constant slope angle $\alpha $ α , $N$ N being the buoyancy frequency. Such an unsteady flow in a stably stratified atmosphere may also generate internal gravity waves (IGWs). The numerical study by Chemel et al. (Meteorol Atmos Phys 203:187–194, 2009) showed that, in the stable atmosphere of a deep valley, the oscillatory motions associated with the IGWs generated by katabatic winds are distinct from those of the katabatic winds. The IGW frequency was found to be independent of $\alpha $ α and about $0.8N$ 0.8 N . Their study did not consider the effects of the background stratification and valley geometry on these results. The present work extends this study by investigating those effects for a wide range of stratifications and slope angles, through numerical simulations for a deep valley. The two oscillatory systems are reproduced in the simulations. The frequency of the oscillations of the katabatic winds is found to be equal to $N$ N times the sine of the maximum slope angle. Remarkably, the IGW frequency is found to also vary as $C_\mathrm{w}N$ C w N , with $C_\mathrm{w}$ C w in the range $0.7$ 0.7 – $0.95$ 0.95 . These values for $C_\mathrm{w}$ C w are similar to those reported for IGWs radiated by any turbulent field with no dominant frequency component. Results suggest that the IGW wavelength is controlled by the valley depth.     

Temperature And Wind Velocity Oscillations Along a Gentle Slope During Sea-Breeze Events

The flow structure on a gentle slope at Vallon dOl in the northern suburbs of Marseille in southern France has been documented by means of surface wind and temperature measurements collected from 7 June to 14 July 2001 during the ESCOMPTE experiment. The analysis of the time series reveals temperature and wind speed oscillations during several nights (about 60--90 min oscillation period) and several days (about 120–180 min oscillation period) during the whole observing period. Oscillating katabatic winds have been reported in the literature from theoretical, experimental and numerical studies. In the present study, the dynamics of the observed oscillating katabatic winds are in good agreement with the theory.In contrast to katabatic winds, no daytime observations of oscillating anabatic upslope flows have ever been published to our knowledge, probably because of temperature inversion break-up that inhibits upslope winds. The present paper shows that cold air advection by a sea breeze generates a mesoscale horizontal temperature gradient, and hence baroclinicity in the atmosphere, which then allows low-frequency oscillations, similar to a katabatic flow. An expression for the oscillation period is derived that accounts for the contribution of the sea-breeze induced mesoscale horizontal temperature gradient. The theoretical prediction of the oscillation period is compared to the measurements, and good agreement is found. The statistical analysis of the wind flow at Vallon dOl shows a dominant north-easterly to easterly flow pattern for nighttime oscillations and a dominant south-westerly flow pattern for daytime oscillations. These results are consistent with published numerical simulation results that show that the air drains off the mountain along the maximum slope direction, which in the studied case is oriented south-west to north-east.     

Numerical modelling of Bora winds

Summary The Bora winds are produced by cold stable air which pours over the Dinaric Alps, often producing intense winds along the Adriatic Coast. Although the flow appears qualitatively similar to the hydraulic flow described by the shallow-water equations, there are certain significant differences: the cold low-level air is continuously stratified and a critical layer in the winds typically occurs near the inversion which caps the cold pool of air. Through two-dimensional numerical mountain wave simulations, we investigate the extent to which hydraulic theory can be used to describe the Bora winds. We analyze the structure of the Bora flow derived from aircraft observations collected during the ALPEX field phase on 15 April 1982 and compare it with a numerical simulation initialized from upstream sounding data. By varying the environmental sounding in our simulations, we find that for this case, neither the critical layer nor the inversion layer play a fundamental dynamical role in generating the strong winds along the lee slope. Instead, the wave overturning which occurs beneath the inversion appears to be the most important factor in producing the strong response. This overturning produces shooting flow over the lee slope and strongly resembles the hydraulic flow which occurs both in shallow water theory and in simulations in which over-turning is suppressed. We believe the hydraulic jump-like mechanism producing the strong Bora slope winds is fundamentally similar to the underlying mechanism which produces the intense winds along the lee slope of the Rocky Mountains. This occurs despite significant differences in the character of the larger scale flow in these two situations.

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Numerische Modellierung der Bora

Zusammenfassung Die Borawinde entstehen durch kalte, stabile Luft, die über die Dinarischen Alpen fließt und dabei oft heftige Winde entlang der adriatischen Küste erzeugt. Obwohl die Strömung der mit Hilfe der Seichtwassergleichungen beschriebenen hydraulischen Strömung qualitativ ähnlich ist, gibt es bestimmte, signifikante Unterschiede: die kalte, bodennahe Luft ist kontinuierlich geschichtet und charakteristischerweise befindet sich eine kritische Windschicht nahe der Inversion, die den Kältesee abschließt. Mittels zweidimensionaler, numerischer Gebirgswellensimulationen untersuchen wir, in welchem Ausmaß die hydraulische Theorie zur Beschreibung von Borawinden herangezogen werden kann. Wir analysieren die Struktur der Boraströmung, die während der ALPEX-Meßphase am 15. April 1982 vom Flugzeug aus beobachtet wurde, und vergleichen sie mit einer numerischen Situation, die mit Daten aus einer Sondierung im Anströmgebiet initialisiert wird. Durch Variieren der Sondierung in den Simulationen haben wir herausgefunden, daß in diesem Fall weder die kritische noch die Inversionsschicht eine fundamentale dynamische Rolle bei der Entstehung der heftigen Winde entlang des leeseitigen Hanges spielen. Stattdessen scheint die umschlagende Welle unterhalb der Inversion der wichtigste Faktor bei der Erzeugung dieser heftigen Reaktion zu sein. Dieses Umschlagen erzeugt eine sehr schnelle Strömung über dem leeseitigen Hang und gleicht damit stark der hydraulischen Strömung, die sowohl in der Seichtwassertheorie vorkommt, als auch in Simulationen, in denen das Umschlagen unterdrückt wird. Wir glauben, daß der dem hydraulic jump ähnliche Mechanismus, der die heftigen Borahangwinde hervorruft, grundsätzlich dem Mechanismus gleicht, der die heftigen Winde entlang der leeseitigen Hänge der Rocky Mountains erzeugt. Und das, obwohl signifikante Unterschiede in den Eigenschaften der großräumigen Strömung in diesen beiden Situationen bestehen.

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With 12 Figures     

Climatological features of glacier and valley winds at the Hintereisferner (Ötztal Alps,Austria)

Summary Homogeneous wind measurements during summer 1971 and the 2 years 1977/78 were analysed at 3 sites of Hintereisferner (HEF) which is a valley-type glacier of 9 km length and northeasterly exposition in the Austrian Ötztal Alps. Some manifestations of glacier winds were found to verify a mesoscale circulation driven by gravity and differential heating of the air above ice surfaces and their ice free moraine surroundings. Modifications are mainly due to local topography and gradient winds.Throughout the year the wind regime at the glacier, esp. at the tongue, is clearly dominated by downsloping winds, reflecting the great potential of snow and ice areas in generating cold air downflow. Undisturbed glacier winds were found to occur most likely on sunny days with weak upper air winds. An influence of katabatic winds down from surrounding moraine slopes is indicated during night time hours. During sunlit hours the occurrence and strength of glacier winds is clearly correlated to the seasonal and daily solar cycle. The development of a regular diurnal variation of wind speeds with a single maximum about 5 m/s during afternoon hours is typical for the glacial wind regime and is most pronounced during the melting season. The observed wind speeds correspond with the diurnal development of vertical and horizontal temperature gradients of the air above the glacier.Clear day northerly winds penetrate most frequently in spring and autumn as far as to the tongue of HEF and are likely to represent thermally driven upvalley winds. They characterize fine weather in alpine valleys, when even signs of a local slope circulation above excessively heated moraine surfaces are indicated too.With 14 Figures     

Shallow Drainage Flows

Two-dimensional sonic anemometers and slowresponse thermistors were deployedacross a shallow gully during CASES99. Weak gully flow of a few tenths of m s^-1 anda depth of a few metres develops in the earlyevening on most nights with clear skies.Flow down the gully developed sometimes evenwhen the opposing ambient wind exceeded10 m s^-1 at the top of the60–m tower. Cold air drainage fromlarger-scale slopes flows over the top ofthe colder gully flow. The gully flowand other drainage flows are generally eliminated in the middle of the night in conjunctionwith flow acceleration abovethe surface inversion layer and downwardmixing of warmer air and highermomentum. As the flow decelerates later inthe night, the gully flow may re-form.The thin drainage flows decouple standard observational levels of3–10 m from the surface.Under such common conditions, eddy correlationflux measurements cannot be used toestimate surface fluxes nor even detect thethin gully and drainage flows. The gentlegully system in this field program is typical ofmuch of the Earths land surface.     

Thermally induced flow in valleys with tributaries part I: Response to heating

Summary Flow in long and deep main valleys with tributaries is studied for constant surface heating switched on att=0. The valley flows are obtained from a numerical model which combines slope wind layer equations with equations for the valley flow off the slopes. Much simpler linear models are used for the intepretation of the model results. If there are no sidevalleys an up-valley wind regime evolves in the main valley after the switch-on of the heating which protrudes towards the head. It is shown that the topographic amplification factor which captures the geometry of the valley and stratification are important factors in determining the intensity of the along-valley flow. However the up-valley winds are also quite sensitive to the specification of the boundary conditions at the upper end of the slope wind layers. If sidevalleys are added strong inflow to these tributaries is found only if their topographic amplification factors are larger than that of the main valley. This flow into the tributaries is mainly balanced by downward motion on top of the main valley but flow entering through the mounth of the main valley can contribute as well. Tributaries can induce flow in the main valley long before the main valley's own up-valley wind regime has reached the location of the tributary.With 10 Figures     

Evening and Morning Transition of Katabatic Flows

An experimental investigation of the evening and morning transition phases of katabatic slope flows has been conducted to identify the mechanisms for their development and destruction over an isolated slope. The momentum and energy equations of the flow have been used to describe these mechanisms for the particular topographic features of the studied slope, and to outline the differences from the dynamics of well-developed simple slope flows. In the lowest portion of the slope, frontal characteristics have been identified in early evening periods when the local pre-existing near-surface thermal structure does not impose a katabatic acceleration. The frontal shape is determined by the near-surface thermal stability and ambient wind. The flow initiation is distinctly different when it is linked to the local surface cooling, in which case it develops gradually and produces a slight local warming.The erosion of the katabatic layer at mid-slope precedes that at the foot and is closely linked to dilution of the local surface inversion. The flow erosion at the foot is often delayed, as the warming of air proceeds uniformly at all heights near the ground, so maintaining the inversion due to warming produced by mixing and advective processes linked to the upslope flow development. The latter initiates first at mid-slope and then at the foot, where for a non-negligible time period it flows over the persistent katabatic flow. The prerequisite for the development of this structure is the maintenance of a shallow inversion in the first 2–3 m above the ground surface.The morning dilution of the katabatic flow is apparently different from common experience over simple slopes and may be attributed to the steep upper portion of the slope in association with its easterly orientation, which results in strong non-uniformity of the solar heating along the slope.     

A numerical investigation of mechanisms affecting drainage flows in highly Complex Terrain

Summary Numerical simulations of increasing complexity are conducted to investigate topographic controls and ambient wind effects upon drainage flows along a portion of the Colorado Front Range in the central Rocky Mountains. A series of two-dimensional simulations show the effects upon the drainage flow of changing slope gradient at the mountain-plain interface. For a given mountain slope, a decrease in the slope of the plain decelerates the mountain drainage jet as it approaches the plain and causes the jet to elevate. The integrated effects of slope and valley drainage are presented with particle trajectories for a particular drainage basin along the Front Range. A nested grid simulation of drainage flow from multiple basins along the Front Range shows that basin area is an important factor in the strength of the drainage flow and that canyon topography variations greatly affect the behavior of the drainage jet as it flows through the canyon mouth onto the plain. Strong drainage winds developed on each of four case night simulations due to the presence of only weak ambient wind below mountaintop. The weak winds represent a decoupling of the near-surface from stronger winds above mountaintop. The canyon drainage exhibited substantial temporal variability in wind speed with the inclusion of ambient winds, due to interactions between ambient and drainage winds.With 11 Figures     

Representativeness of wind measurements on a mesoscale grid with station separations of 312 m to 10 km

A field experiment was carried out in which wind speed and direction were measured over flat terrain at a height of 10 m using 13 identical instruments spaced logarithmically along two perpendicular 10 km lines. Station separations ranged from 312 m to 10 km. One-minute data from 11 sampling periods of duration 6 to 10 h were studied. p ]The statistics showed little dependence on whether the line of instruments was oriented along the wind or across the wind. The correlation coefficients between wind fluctuations at two stations separated by distance x were found to vary exponentially with x, with an integral distance scale on the order of 1 km. The integral time scale derived from the variation of the single station variances with averaging time was found to equal several minutes. At a station separation of 10 km, the correlation coefficients between the wind components at the two sites were calculated to be 0.24, 0.37, and 0.47 for averaging times of 1, 10, and 60 min, respectively. These values for the correlation coefficients correspond to root-mean-square differences in wind speed at the two stations of about 1.3, 1.0, and 0.7 m/s, respectively.Exponential formulas based on dimensional analysis are suggested for fitting these observations. It is found that the observations of spatial correlations are best fit if two independent integral distance scales are used — a boundary-layer distance scale of about 300 m that best applies to small station separations and a mesoscale distance scale of about 10 km that applies to larger station separations.     

Daytime Turbulence Statistics above a Steep Forested Slope

Six levels of simultaneously sampled ultrasonic data are used to analyse the turbulence structure within a mixed forest of 13 m height on a steep slope (35°) in an alpine valley. The data set is compared to other studies carried out over forests in more ideal, flat terrain. The analysis is carried out for 30-min mean data, joint probability distributions, length scales and spectral characteristics.Thermally induced upslope winds and cold air drainage lead to a wind speed maximum within the trunk space. Slope winds are superimposed on valley winds and the valley-wind component becomes stronger with increasing height. Slope and valley winds are thus interacting on different spatial and time scales leading to a quite complex pattern in momentum transport that differs significantly from surface-layer characteristics. Directional shear causes lateral momentum transports that are in the same order or even larger than the longitudinal ones. In the canopy, however, a sharp attenuation of turbulence is observed. Skewed distributions of velocity components indicate that intermittent turbulent transport plays an important role in the energy distribution.Even though large-scale pressure fields lead to characteristic features in the turbulent structure that are superimposed on the canopy flow, it is found that many statistical properties typical of both mixing layers and canopy flow are observed in the data set.     

Concentration of trace gases in the lower troposphere,simultaneously recorded at neighboring alpine stations Part III: Sulfur dioxide,nitrogen dioxide,nitric oxide

Summary The importance of representative and long-term recordings of the trace gases SO₂, NO _x , and NO is explained. Recordings taken under different background conditions and, moreover, simultaneously at neighboring mountain stations, together with other meteorological parameters, are of special interest.The recording stations for the determination of the mentioned gases (a valley station at 740 m a.s.l., a nearby mountain station at 1780 m a.s.l.), the measuring methods, calibration procedures, and zero-air supply are described.The main part deals with the representation of consistent data of trace gases obtained at the two stations (NO only in the valley floor). Special attention was given not only to longterm trends but also to the seasonal and diurnal variations, and to the dependence of the gas concentrations on meteorological parameters. Only on the basis of such a parameterization, the time variations become understandable and the causes can be explained as well as possible. Finally, correlations between the concentrations of the different gas components are shown.With 15 Figures     

Mesoscale nocturnal jetlike winds within the planetary boundary layer over a flat,open coast

Mesoscale nocturnal jetlike winds have been observed over a flat, open coast. They occur within the planetary boundary layer between 100 and 600 m. At times the wind shear may reach 15 m s^-1 per 100 m. Unlike the common low-level jet that occurs most often at the top of the nocturnal inversion and only with a wind from the southerly quadrant, this second kind of jet exists between nocturnal ground-based inversion layers formed by the cool pool, or mesohigh, and the elevated mesoscale inversion layer over the coast. It occurs mostly when light % MathType!MTEF!2!1!+-% feaafiart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXatLxBI9gBaerbd9wDYLwzYbItLDharqqtubsr% 4rNCHbGeaGqiVu0Je9sqqrpepC0xbbL8F4rqqrFfpeea0xe9Lq-Jc9% vqaqpepm0xbba9pwe9Q8fs0-yqaqpepae9pg0FirpepeKkFr0xfr-x% fr-xb9adbaqaaeGaciGaaiaabeqaamaabaabaaGcbaGaaiikaiabgs% MiJkaaiwdacqGHsislcaaI2aGaaeyBaiaabccacaqGZbWaaWbaaSqa% beaacqGHsislcaaIXaaaaOGaaiykaaaa!3FCF!\[( \leqslant 5 - 6{\text{m s}}^{ - 1} )\] geostrophic winds blow from land to sea and when the air temperature over adjacent seas is more than 5 °C warmer than that over the coast. This phenomenon may be explained by combined Venturi and gravity-wind effects existing in a region from just above the area a few kilometres offshore to 100–600 m in height approximately 40–50 km inland because this region is sandwiched between the aforementioned two inversion layers.