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.     

An observational study of instability and turbulence in nighttime drainage winds

A detailed analysis of two nighttime drainage wind events that commenced on the evenings of 7 and 8 October, 1980 is presented. Data on wind velocity and temperature (10-s values), obtained from each of the eight instrumented levels of the Boulder Atmospheric Observatory, are used to construct 10-min means and root-mean-square values of all the variables. Additional information is provided by acoustic sounder data for 8 October.The analyses reveal that the passage of the drainage front occurs abruptly, between two 10-s observations, on both days. Relatively intense root-mean-square variability in the velocity and temperature fields accompanies the passage of the drainage front. In addition, the undercutting cold drainage air initiates significant variability in the vertical velocity field that extends above the 300 m level of the tower. The most significant variability in the other meteorological fields is primarily restricted to the lowest 150 m with the passage of the drainage front.A principal feature of the analysis is the delineation of Kelvin-Helmholtz instability and billow development, breakdown into turbulence and ultimate decay to a less turbulent state that occurs intermittently behind the drainage front. These features are interpreted in light of Thorpe's (1973a, b) experimental work on stability and turbulence in stratified shear flow and the predictions of linear instability theory. The interpretations are carried out by considering the distributions of the Richardson number, the peak shears of the mean flow and the vertical fluxes of horizontal momentum associated with the unstable growth of the disturbances. Additional comparisons are made between the turbulent structures in Thorpe's laboratory experiments and the turbulence exhibited in traces of the 10-s vertical velocity data measured at various levels both above and below the interface.The relevance of the present results to the design of future field programs, and to the observational data requirements that should be met to incorporate turbulent entrainment processes in models of pollutant dispersal is discussed.     

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.

---

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.

---

---

With 14 Figures     

Variationally adjusted surface winds

This paper describes a one-level variational adjustment process for producing mass-consistent surface winds in the Barrow Strait area, N.W.T. To achieve this result, the continuity equation is employed as a physical constraint. The variational technique adjusts mean winds (vertically averaged through the planetary boundary layer); therefore, a relationship is required between surface and mean winds. Use is made of existing velocity profiles, but interpolation may be used between surface and geostrophic winds. The model was found to be strongly dependent upon specification of boundary-layer height. Channeling effects are not readily seen until topography begins to protrude through the boundary layer. The model might thus be better suited to areas where shallow inversion layers are well defined. By application of the variational adjustment, errors in continuity are reduced by six orders of magnitude. Upon comparison of the variational technique with a diagnostic, one-level, primitive equation model, median errors between computed and observed surface winds were found to be comparable.     

Topographic influence on surface winds

Using data collected during 1975–1976 from a meteorological network operating in the vicinity of the Columbia Generating Site approximately 8 km south of Portage, Wis., the influence of the Baraboo Hills on the surface wind field is determined. Half-hour means of wind speed and direction measured at 9 m at three sites were used to compute divergence and vorticity using Bellamy's method. The data were grouped into 18 sectors each 20 deg wide and averages computed for each quantity. Results indicate that for wind directions perpendicular to the eastern edge of the Baraboo Hills, the surface (9m) wind field is significantly perturbed up to 4 km from the bluffs. The largest convergence of 2.1 × 10^–4 s^–1 occurs with 160 deg wind direction and the largest divergence of 1.2 × 10^–4 s^–1 with 290 deg wind direction. The maximum anticyclonic vorticity was 1.6 × 10^–4 s^–1 at 210 deg and the maximum cyclonic vorticity was 1.6 × 10^–4 s^–1 at 330 deg.     

QuikSCAT卫星遥感与MM5模拟海表面风场的综合分析

利用2000—2009年中国近海海域(105~135°E,0°~45°N)Quik SCAT卫星遥感风场资料和2007年MM5模拟风速结果,运用对比验证、经验正交函数(empirical orthogonal functions,EOF)和奇异值分解(singular value decomposition,SVD)方法,分析了Quik SCAT卫星遥感风场资料和MM5数值模拟风速结果的特征。结果表明:两者风速的空间分布形态相似,且风速变化密切相关;QuikSCAT卫星遥感风场的空间分布形态更加接近实测结果,而模拟风速结果的量值更加接近实测结果。采用PS(Pattern-Scaling)方法分别提取了Quik SCAT卫星遥感风场的空间分布形态和模拟结果的量值,得到的风速空间分布形态与Quik SCAT卫星遥感风场一致,且风速量值与模拟风速结果相同。通过初步检验发现,PS方法改善了卫星遥感风速在中国近海海域风速偏大的问题。     

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.

---

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.

---

---

With 12 Figures     

A comparison of satellite winds and surface buoy winds in the northeast Pacific: Research note

Abstract

Cloud‐motion winds measured from organized and disorganized cumulus cloud fields are compared with winds measured at collocated buoys in the northeast Pacific Ocean. Findings suggest that an automated tracking algorithm using GOES satellite imagery can measure cloud‐level winds at these latitudes. Comparisons with buoy wind measurements show that the influence of boundary‐layer stability should be included in estimates of surface winds from cloud‐motion data.     

On the measurement of atmospheric winds

The measurement of atmospheric winds using a cup anemometer to measure speed and a wind vane to measure direction, recording the data on paper charts is commonplace. Standard Meteorological Service criteria stipulate that the wind charts so recorded are read (averaged over one hour) by taking the dominant wind direction over an hour and the wind run then gives the speed over that hour. However, fluctuations of wind direction can lead to erroneous results. A vector average wind obtained using two orthogonally mounted propeller anemometers is described here, and comparisons are drawn between this and the first-mentioned technique. Prevailing winds are shown to be approximately the same for the two systems, but minor components can differ considerably. It is also shown that the integration time of the wind will have a marked effect on results.     

Persistence of light surface winds in Canada

The persistence of light surface winds (less than or equal to 3 m s^?1 or 7 mi h^?1) is one meteorological factor in air pollution potential. Surface wind data were obtained from 111 Canadian synoptic and aviation weather stations for the period 1957–66. Generally speaking, persistent light winds occur most frequently in British Columbia, the Yukon and northern Alberta. In the ten provinces of Canada, the frequency of occurrence of light winds is a minimum in the spring and a maximum in the winter. In the Yukon and the Northwest Territories it is a minimum in the summer and a maximum in the winter. The seasonal variation is least in the mountain valleys and greatest elsewhere. The spatial and seasonal variations in persistent light winds suggest that, in the mountain valleys, topography is the major factor, while in other regions synoptic weather patterns are relatively important.     

Seasonal prevailing surface winds in Northern Serbia

Theoretical and Applied Climatology - Seasonal prevailing surface winds are analyzed in the territory of Northern Serbia, using observational data from 12 meteorological stations over several...     

Hurricane winds on the territory of Georgia

The statistical structure of hurricane winds is studied using the data of observation at 50 meteorological stations in Georgia for the period of 1961–2008. Determined are the number of days and the duration of hurricane winds in different regions of the country. Studied are the empirical functions of their distribution and the areal limits.     

Summer winds around the southern Red Sea

Theoretical and Applied Climatology - Daily synoptic maps for May–September 1966 have shown the presence of a persistent wind shift or convergence zone, the Afar Convergence Zone (ACZ),...     

Valley winds in the mount Rainier area

Summary Four summer seasons of field work near Mt. Rainier have shown that a well-developed valley wind system tends to have the following features: Airflow within a valley is up the valley during the day and down it at night and is compensated by a return flow (anti-wind) at a higher level. The layers occupied by the two flows are of approximately equal thickness, and the boundary between them is generally at, or somewhat below, ridgeheight. Above the anti-wind, the flow depends on the large-scale synoptic situation.Horizontal wind speed in these two layers is greatest slightly below the center of each layer. Speeds reach a maximum in early afternoon and just beforce sunrise. The reversal between day and nighttime flows is almost simultaneous everywhere in the valley, about an hour after sunnet and sunrise.Vertical transport of air between the two layers appears to be localized, mainly in the neighborhood of the ridges. Slope winds apparently feed the vertical currents. Speed fluctuations, having a period of about 20 minutes, were observed in drainage winds near the surface at night.When a well-developed wind system occured in one valley, well-developed systems tended to occur in other valleys in the same area.

---

Zusammenfassung Beobachtungen im Verlaufe von vier Sommern im Mount-Rainier-Gebiet zeigten ein gutentwickletes Talwind-System mit folgenden Eigenschaften: Die Luft im Tale bewegt sich tags talaufwärts und nachts talabwärts; diese Strömung wird kompensiert durch ein Gegenwind-System in größerer Höhe. Die vertikale Erstreckung der übereinander liegenden Strömungen ist etwa gleich, und ihre Grenzfläche liegt, im allgemeinen, in oder etwas unter der Höhe der Bergkämme. Oberhalb des Gegenwindes beherrscht die weiträumige synoptische Situation die Strömung.Die Horizontalgeschwindigkeit ist für beide Windsysteme am größten etwas unter dem Zentrum der betreffenden Schicht. Geschwindigkeitsmaxima werden am frühen Nachmittag und kurz vor Sonnenaufgang erreicht. Der Umschlag von Tag- zu Nachtströmungen erfolgt nahezu gleichzeitig in allen Teilen des Tales, und zwar je etwa eine Stunde nach Sonnenaufgang bzw. Sonnenuntergang.Der vertikale Luftmassenaustausch zwischen beiden Schichten erfolgt im wesentlichen oberhalb der Bergkämme. Dieser vertikale Kammwind wird von unten durch den Hangwind ernährt. Im nächtlichen Fallwind wurden Geschwindigkeits-Variationen mit einer Periode von etwa 20 Minuten beobachtet.Wenn gut entwickelte Wind-Systeme in einem Tal vorkommen, kann man auch in anderen Tälern gut entwickelte Systeme erwarten.

---

Résumé Des observations faites durant quatre étés dans la région du Mount Rainier ont montré la présence d'un système bien développé de brises de vallée et de montagne. Ce système a les particularités suivants: Le courant est dirigé vers l'amont durant la journée, vers l'aval durant la nuit. Ces courants sont compensés par un système de vents contraires à grande altitude. Le développement vertical des deux courants est à peu près identique et la surface qui les sépare est située, en général, à l'altitude des crêtes ou légèrement au-dessous. Le courant situé au-dessus du ven contraire est déterminé par la situation synoptique générale.La vitesse horizontale du vent est maximum pour les deux systèmes un peu au-dessous du centre de la couche correspondante. Les plus grandes vitesses se mesurent au début de l'après-midi et peu avent le lever du soleil. Le passage de la brise de vallée à la brise de montagne ou vice versa s'opère presque simultanément en tous les points de la vallée et cela approximativement une heure après le lever, respectivement le coucher du soleil.L'échange vertical des masses d'air entre les deux couches se fait principalement au-dessus des crêtes. Ce vent vertical de crêtes est alimenté d'en bas par le courant remontant les pentes. Dans le cas du vent nocturne descendant, on a observé des variations de vitesse ayant une périodicité de 20 minutes environ.Si l'on observe dans une vallée déterminée un système bien développé de brises de vallée et de montagne, on peut s'attendre à ce que des systèmes semblables se retrouvent également dans d'autres vallées de la même région.

---

---

With 9 Figures

---

Contribution No. 97, department of Atmospheric Sciences, University of Washington, Seattle, USA.

---

This research was aided by the U.S. Air Force Cambridge Research Laboratories under AF Contract 19 (604)-7201, Project 7655, Task 7655.     

The behavior of vertically integrated boundary-layer winds

This research provides a study of the behavior of vertically integrated boundary-layer winds (IBLWs). This information should be helpful for both theoretical and practical applications, e.g., boundary-layer parameterization in general circulation models, air pollution models, and low-level parachuting operations. The study concerned itself with winds integrated up to a height of 300 m in the United States. The only data suitable for studying the behavior of IBLWs for such heights were measurements taken on tall towers. An extensive search indicated that data were readily available from only four towers instrumented to or above 300 m. These were located in rural locations in Texas, Oklahoma, Nevada, and South Carolina. At each location, means and coefficients of variation of IBLWs were computed, plotted as a function of time since sunrise/sunset, and compared. The research shows that IBLWs may differ appreciably in magnitude at different locations; may have considerable universality in behavior, and may also be ‘regionally’ dependent; and that IBLWs have a pronounced diurnal variation linked to sunrise/sunset time.     

贵州的夜雨特征

选用贵州省86个气象站10 a逐日逐时的降水资料,计算了全省年平均、各月平均夜雨和白天降水百分比,分析了全省夜雨分布,逐时的降水分布,对夜雨产生的原因进行分析,还重点对避暑之都——贵阳的夜雨特征进行分析。结果表明:贵州省夜雨较多,以春季最为明显,贵州夜雨较多的原因除了大气环流系统如锋面活动、南支槽活动、低空急流日变化等天气因素外,与地形条件造成的局地大气动力、热力条件变化密切相关,夜雨对空气清洁、睡眠以及户外旅游等十分有利。     

Estimation of Winds at Different Isobaric Levels Based on the Observed Winds at 850 hPa Level Using Double Fourier Series

ＥｓｔｉｍａｔｉｏｎｏｆＷｉｎｄｓａｔＤｉｆｆｅｒｅｎｔlｓｏｂａｒｉｃＬｅｖｅｌｓＢａｓｅｄｏｎｔｈｅＯｂｓｅｒｖｅｄ　Ｗｉｎｄｓａｔ８５０ｈＰａＬｅｖｅｌＵｓｉｎｇＤｏｕｂｌｅＦｏｕｒｉｅｒＳｅｒｉｅｓＳ．Ｎ．ＢａｖａｄｅｋａｒａｎｄＲ．Ｍ．Ｋｈ?..     

Forecasting hurricane winds in extratropical cyclones in Russia

In contrast to the common opinion, hurricane winds in extratropical cyclones are a quite frequent phenomenon followed by huge damage, especially in densely populated areas. This phenomenon has been poorly studied and is hardly predictable so far. The features of hurricane winds in extratropical cyclones, and the similarity and difference in their structure as compared to those in tropical cyclones are revealed.     

Theoretical considerations of meandering winds in simplified conditions

The influence of turbulence on the meandering phenomenon is investigated. The study, based on the three-dimensional Navier–Stokes equations, shows that when the turbulent fluxes can be neglected an asymptotic solution results. This solution reproduces a horizontal wind oscillation with an infinite relaxation time. When there is turbulent forcing, on the other hand, a transition occurs to a new order, characterized by a spatial reorganization, leading to a wind field with a well-defined direction.