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     

Computational Fluid Dynamics Modelling of the Diurnal Variation of Flow in a Street Canyon

Urban surface and radiation processes are incorporated into a computational fluid dynamics (CFD) model to investigate the diurnal variation of flow in a street canyon with an aspect ratio of 1. The developed CFD model predicts surface and substrate temperatures of the roof, walls, and road. One-day simulations are performed with various ambient wind speeds of 2, 3, 4, 5, and 6 ms⁻¹, with the ambient wind perpendicular to the north–south oriented canyon. During the day, the largest maximum surface temperature for all surfaces is found at the road surface for an ambient wind speed of 3 ms⁻¹ (56.0°C). Two flow regimes are identified by the vortex configuration in the street canyon. Flow regime I is characterized by a primary vortex. Flow regime II is characterized by two counter-rotating vortices, which appears in the presence of strong downwind building-wall heating. Air temperature is relatively low near the downwind building wall in flow regime I and inside the upper vortex in flow regime II. In flow regime II, the upper vortex expands with increasing ambient wind speed, thus enlarging the extent of cool air within the canyon. The canyon wind speed in flow regime II is proportional to the ambient wind speed, but that in flow regime I is not. For weak ambient winds, the dependency of surface sensible heat flux on the ambient wind speed is found to play an essential role in determining the relationship between canyon wind speed and ambient wind speed.     

Local foehn effects in the upper Isar Valley, Part 2: numerical simulations

Summary The local wind system in the upper Isar Valley (Bavarian Alps) near Mittenwald has the peculiarity that regularly strong foehn-like nocturnal flows occur, mainly during clear nights in autumn and winter. We will refer to this phenomenon as “Minifoehn”, as its properties are similar to the classical deep foehn in the sense that its breakthrough into the Isar Valley usually brings a striking increase in temperature and a concomitant decrease in relative humidity. Numerical simulations with the MM5 model reveal that this phenomenon is related to a nocturnal drainage flow originating from a plateau south of Mittenwald. This flow is driven by the temperature difference between this plateau (1180 m) and the free atmosphere above Mittenwald (920 m, 15 km north of the plateau) at the same level. The air masses flow through two different valleys that merge again further downstream. The upper part of one of the two drainage currents goes over a small mountain ridge (1180 m) south-west of Mittenwald and then descends into the Isar Valley, leading to an advection of potentially warm air towards Mittenwald. This branch of the drainage current constitutes the Minifoehn. The remaining part of the drainage current flows through a narrow gap towards the Isar Valley and then joins the drainage flow of this valley. As these air masses are significantly cooler than the Minifoehn branch, large horizontal temperature gradients can be found around Mittenwald. The dynamical behaviour of the cold air flow turns out to be qualitatively consistent with shallow-water theory only in the absence of a forcing by large-scale winds. Otherwise, gravity-wave induced pressure perturbations interact with the drainage flow and modify the low-level flow field. The simulations show that the gravity waves are excited by the mountain range that separates the two valleys mentioned above. Moreover, the simulations indicate that the structure of this nocturnal wind system is not very sensitive to the direction of synoptic-scale winds as long as they come from the southern sector. On the other hand, ambient northerly winds are able to prevent the drainage flow and therefore the local foehn effects in the Isar Valley provided that synoptic winds are strong enough. The results of the MM5 simulations are in good agreement with the measurements and observations described in part 1 of this study.     

Large-Eddy Simulation of Katabatic Flows

A large-eddy simulation model with rotated coordinates and an open boundary is used to simulate the characteristics of katabatic flows over simple terrain. Experiments examine the effects of cross winds on the development of the slope-flow boundary layer for a steep (20°) slope and the role of drainage winds in preventing turbulence collapse on a gentle slope (1°). For the steep flow cases, comparisons between model average boundary-layer velocity, temperature deficit, and turbulence kinetic energy budget terms and tower observations show reasonable agreement. Results for different cross slope winds show that as the cross slope winds increase, the slope flow deepens faster and behaves more like a weakly stratified, sheared boundary layer. Analysis of the momentum budget shows that near the surface the flow is maintained by a balance between downslope buoyancy forcing and vertical turbulence flux from surface drag. Above the downslope jet, the turbulence vertical momentum flux reverses sign and acceleration of the flow by buoyancy is controlled by horizontal advection of slower moving ambient air. The turbulence budget is dominated by a balance between shear production and eddy dissipation, however, buoyancy and pressure transport both are significant in reducing the strength of turbulence above the jet. Results from the gentle slope case show that even a slight terrain variation can lead to significant drainage winds. Comparison of the gentle slope case with a flat terrain simulation indicates that drainage winds can effectively prevent the formation of very stable boundary layers, at least near the top of sloping terrain.     

Effects of stability on the profiles of vertical velocity and its variance in katabatic flow

The atmospheric katabatic flow in the foothills of the Front Range of the Rocky Mountains has been monitored by a network of towers and sodars for several years as part of the Atmospheric Studies in COmplex Terrain (ASCOT) program. We used three years of data from the network to explore the dependence on surface cooling and channeling by winds above the canyon of (1) profiles of the mean and variance of the vertical (perpendicular to the geopotential) component of motion and (2) the mean component of the wind perpendicular to the local terrain of Coal Creek Canyon. Previously we found that the magnitude of the near-surface temperature difference decreases with increasing surface cooling in light winds, apparently because of increasing turbulence caused when increasing drainage winds interact with surface topography. The variance of vertical velocity exhibits three types of vertical profiles, corresponding to different cooling rates and external wind speeds. The mean variance was found to depend strongly on a locally derived Richardson number.     

Influence of canyon-induced flows on flow and dispersion over adjacent plains

Summary The study investigates two effects that a valley or canyon opening onto a plain can have on flow and contaminant dispersion over the downwind plain. The first effect is the channeling of strong ambient flow by the canyon when the wind is nearly aligned with the canyon axis. Two cases showed that these conditions produced a region of focused flow downwind of the canyon mouth. The second effect is the formation of canyon exit jets on nights with weaker ambient flow. In two case studies under these conditions strong exit jets formed that were several hundred meters deep. The jets remained narrow and strong at least 10 km onto the plains, and in one of the cases the jet extended more than 20 km over the plains. These deep jets only lasted 2–3 h, and they had a small but significant effect on surface-released tracer transport as indicated by surface sampling. We hypothesize that the near-surface advection of tracer was accomplished by a thin katabatic layer of flow, and that an elevated release or elevated sampling would have indicated a greater effect of the exit jet on tracer transport.With 18 Figures     

Local foehn effects in the Upper Isar Valley,Part 3: Additional investigations

Summary The present paper is the continuation of two recent studies investigating the foehn-like valley wind system around Mittenwald (Bavarian Isar Valley). We deal with the synoptic/mesoscale conditions causing the local foehn (“Minifoehn”), considering field campaigns from both the mesoscale and the climatological point of view. Furthermore, we describe the structure and further features of the local foehn at smaller scales, using both the results of the VERTIKATOR field campaign and numerical simulations. We obtain as a new result that the foehn-caused local warm air pool around Mittenwald induces slight nocturnal upvalley winds between an adjacent valley basin located some 8 km north of Mittenwald and the basin of Mittenwald. Furthermore, a weak northerly flow may also occur at Mittenwald prior to the onset of the Minifoehn. Numerical simulations indicate that the local pressure gradient responsible for this phenomenon is related to a gravity wave forming over the hill range southwest of Mittenwald. Observations within a five-year period indicate that Minifoehn frequently occurs when ambient winds coming from the southern sector are predominant, but, contrary to deep foehn, weather conditions with northerly synoptic-scale flows do not necessarily exclude the development of the local foehn which comes from the southwest. We also present further evidence that in the presence of southerly synoptic-scale winds, orographic gravity waves interact with the drainage flow. Another new result is that strong synoptic-scale westerly winds are able to suppress the occurrence of Minifoehn. In addition, the possible influence of the Inn Valley wind system as well as dynamical differences between the thermally driven up- and downvalley winds are briefly discussed.     

Numerical simulation of nocturnal drainage flow properties in a rugged canyon

A two-dimensional, time-dependent flow model coupled with a radiative transfer module has been applied to examine the characteristics of nocturnal flow in a steep canyon in the Rocky Mountains in Colorado. The effect of nighttime surface cooling on drainage flow is examined and compared with observations. In a complementary study, tracer data have been analyzed to estimate the mass flux from a tributary canyon and to examine processes of transport and diffusion. Simulations indicate that the strength and structure of the drainage wind are controlled mainly by terrain features, ambient wind conditions, and effective radiative cooling rates. The transport of tracer from a lower secondary vortex to an upper primary vortex is largely controlled by diffusional processes; removal of tracer from the canyon is controlled by the primary vortex and its interaction with the ambient wind. Differences between mass fluxes from model simulations and those calculated from experiments involve uncertainties in both the structure of the model and the analysis of data.     

Convective initiation by topographically induced convergence forcing over the Dabie Mountains on 24 June 2010

The initiation of convective cells in the late morning of 24 June 2010 along the eastward extending ridge of the Dabie Mountains in the Anhui region,China,is studied through numerical simulations that include local data assimilation.A primary convergence line is found over the ridge of the Dabie Mountains,and along the ridge line several locally enhanced convergence centers preferentially initiate convection.Three processes responsible for creating the overall convergence pattern are identified.First,thermally-driven upslope winds induce convergence zones over the main mountain peaks along the ridge,which are shifted slightly downwind in location by the moderate low-level easterly flow found on the north side of a Mei-yu front.Second,flows around the main mountain peaks along the ridge create further convergence on the lee side of the peaks.Third,upslope winds develop along the roughly north–south oriented valleys on both sides of the ridge due to thermal and dynamic channeling effects,and create additional convergence between the peaks along the ridge.The superposition of the above convergence features creates the primary convergence line along the ridge line of the Dabie Mountains.Locally enhanced convergence centers on the primary line cause the initiation of the first convection cells along the ridge.These conclusions are supported by two sensitivity experiments in which the environmental wind(dynamic forcing) or radiative and land surface thermal forcing are removed,respectively.Overall,the thermal forcing effects are stronger than dynamic forcing given the relatively weak environmental flow.     

Mini-Sodar Observations of Drainage Flows in the Rocky Mountains

Summary  Vertical profiles of drainage winds were monitored continuously by a Doppler-Mini-Sodar during case studies in two valleys, on both sides of the U. S. Continental Divide. A tethered balloon provided additional information on the vertical temperature and wind structure up to the Divide level. Ambient wind data were collected by a radar wind profiler on the west side, and a tower on the crest of the Divide. The onset, evolution and breakup of the drainage flow were studied on two nights, when the ridge-top winds were westerly and skies were clear. To study the influence of the ambient flow on drainage winds, changes in drainage wind speed, direction and depth, along with the volume flux were examined. It was found that, on the leeward side, the drainage was strongly influenced by the ambient winds (King, 1995b), which led to interruption and erosion of the locally generated valley flow. The drainage on the windward side of the Divide was almost undisturbed. A comparison of balloon and sodar wind profiles showed very good agreement during steady drainage conditions. Received October 21, 1996 Revised November 30, 1998     

The influence of large-scale forcing on the katabatic wind regime at Adélie Land,Antarctica

Summary The Adélie Land coastal section of East Antarctica is known for strong katabatic winds. Although the primary forcing of these persistent drainage flows has been attributed to the radiative cooling of the sloping ice topography, effects of ambient horizontal pressure gradients can play a central role in shaping the Antarctic surface wind regime as well. Oberrvations of the katabatic wind at the near-coastal Adélie Land station D-10 have been sorted into strong and weak wind classes. Concurrent radiosonde ascents at nearby Dumont D'Urville have been used to depict the timeaveraged large scale conditions accompanying the katabatic wind classes. Results suggest that strong katabatic wind cases are associated with low pressure over the coastal margin and easterly upper level motions. Numerical simulations have been conducted to examine the effect of prescribed large scale forcing on the evolution of the katabatic wind. The model runs indicate that the ambient environment plays a key role in the development and intensity of the katabatic wind regime.With 7 Figures     

A Laboratory Model for the Flow in Urban Street Canyons Induced by Bottom Heating

Water tank experiments are carried out to investigate the convection flow induced by bottom heating and the effects of the ambient wind on the flow in non-symmetrical urban street canyons based on the PIV (Particle Image Visualization) technique. Fluid experiments show that with calm ambient wind,the flows in the street canyon are completely driven by thermal force, and the convection can reach the upper atmosphere of the street canyon. Horizontal and vertical motions also appear above the roofs of the buildings. These are the conditions which favor the exchange of momentum and air mass between the street canyon and its environment. More than two vortices are induced by the convection, and the complex circulation pattern will vary with time in a wider street canyon. However， in a narrow street canyon， just one vortex appears. With a light ambient wind, the bottom heating and the associated convection result in just one main vortex. As the ambient wind speed increases, the vortex becomes more organized and its center shifts closer to the leeward building.     

二维街谷地面加热引起的流场特征的水槽实验研究

利用拖曳式水槽,采用激光粒子成像速度场测量系统(PIV),模拟了街谷存在地面加热时流场特征;讨论了环境风场对其的影响。我们发现在静风条件下,街谷中环流完全由热力驱动,对流活动可伸展至街谷上方;在建筑物层顶以上,也可发现水平和垂直方向的运动。这些对流活动有助于基本风场为零时,街谷内外动量和物质的交换。当街谷较宽时,对流形成的涡旋可能为两个以上,形态较为复杂并随时间变化,当街谷变窄时,涡旋蜕化成只有一个。当有弱环境风场存在时,街谷中的对流呈现为一个主涡旋,随着风速增加,涡旋形状更加规则,其中心并向下风向移动。     

THE IMPACT OF MOUNTAIN TO BASIN WINDS ON THE DIURNAL VARIATION IN FOG OVER THE SICHUAN BASIN, CHINA

There is an increased demand for the accurate prediction of fog events in the Sichuan Basin (SCB) using numerical methods. A dense fog event that occurred over the SCB on 22 December 2016 was investigated. The results show that this dense fog event was influenced by the southwest of a low pressure with a weak horizontal pressure gradient and high relative humidity. This fog event showed typical diurnal variations. The fog began to form at 1800 UTC on 21 December 2016 (0200 local standard time on 22 December 2016) and dissipated at 0600 UTC on 22 December 2016 (1400 local standard time on 22 December 2016). The Weather Research and Forecasting model was able to partially reproduce the main features of this fog event and the diurnal variation in the local mountain to basin winds. The simulated horizontal visibility and liquid water content were used to characterize the fog. The mountain to basin winds had an important role in the diurnal variation of the fog event. The positive feedback mechanism between the fog and mountain to basin winds was good for the formation and maintain of the fog during the night. During the day, the mountain to basin wind displayed a transition from downslope flows to upslope flows. Water vapor evaporated easily from the warm, strong upslope winds, which resulted in the dissipation of fog during the day. The topography surrounding the SCB favored the lifting and condensation of air parcels in the lower troposphere as a result of the low height of the lifting condensation level.     

Summertime Thermally-Induced Circulations over the Lake Nam Co Region of the Tibetan Plateau

Performance of the fifth-generation Pennsylvania State University/National Center for Atmospheric Research Mesoscale Model(MM5) over the Lake Nam Co region of the Tibetan Plateau was evaluated based on the data from five surface observation sites in 2006. The interaction between two thermally-induced circulations(lake breezes and mountain-valley winds) was also investigated. The results show that MM5 could be used to simulate 2-m air temperature; however, MM5 needs improvement in wind field simulation.Two numerical simulations were conducted to study the effect of the lake on the local weather and wind system. The original land cover of the model was used in the control experiment, and the lake was replaced with grassland resembling the area surrounding the lake in the sensitive experiment. The results of the simulations indicate that the lake enhanced the north slope mountain-valley wind and the mountain changed the offshore flow direction at the north shore. During the day, a clear convergent zone and a strong upflow were observed over the north slope of the Nyainq?entanglha Range, which may cause frequent precipitation over the north slope. During the night, the entire area was controlled by a south flow.     

Observations of the Relation Between Upslope Flows and the Convective Boundary Layer in Steep Terrain

Slope flow mechanisms are crucial for the transport of air pollutants in complex terrain. Previous observations in sloping terrain showed upslope flows filling the entire convective boundary layer (CBL) and reducing air pollution concentrations by venting air pollutants out of the CBL into the free atmosphere. During the Pacific 2001 Air Quality Field Study in the Lower Fraser Valley, British Columbia, Canada, we observed slope flows during weak synoptic winds, clear skies, and strong daytime solar heating. With a Doppler sodar we measured the three wind components at the foot of a slope having an average angle of 19° and a ridge height of 780 m. We operated a scanning lidar system and a tethersonde at a nearby site on the adjacent plain to measure backscatter of particulate matter, temperature, wind speed, wind direction, and specific humidity. Strong daytime upslope flows of up to 6 m s⁻¹ through a depth of up to 500 m occurred in the lower CBL, but with often equally strong and deep return flows in the upper part of the CBL. The mass transport of upslope flow and return flow approximately balanced over a 4-h morning period, suggesting a closed slope-flow circulation within the CBL. These observations showed that air pollutants can remain trapped within a CBL rather than being vented from the CBL into the free atmosphere.     

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     

Interactions of nocturnal slope flows with ambient winds

A quasi-one-dimensional numerical model containing a prognostic turbulent kinetic energy parameterization and simplified approximations to horizontal gradients is used to study interactions of thermally induced nocturnal slope flows with following and opposing ambient winds. It is found that a following ambient wind causes the peak perturbation wind to be weaker and to be realized at a greater height, while an opposing ambient wind leads to a stronger perturbation wind at a lower height. The reason for this response lies in the interactions of the shears of the thermal and ambient components through the mechanical production of turbulent kinetic energy.     

Observational Study of Surface Wind along a Sloping Surface over Mountainous Terrain during Winter

The 2018 Winter Olympic and Paralympic Games will be held in Pyeongchang, Korea, during February and March. We examined the near surface winds and wind gusts along the sloping surface at two outdoor venues in Pyeongchang during February and March using surface wind data. The outdoor venues are located in a complex, mountainous terrain, and hence the near-surface winds form intricate patterns due to the interplay between large-scale and locally forced winds. During February and March, the dominant wind at the ridge level is westerly; however, a significant wind direction change is observed along the sloping surface at the venues. The winds on the sloping surface are also influenced by thermal forcing,showing increased upslope flow during daytime. When neutral air flows over the hill, the windward and leeward flows show a significantly different behavior. A higher correlation of the wind speed between upper-and lower-level stations is shown in the windward region compared with the leeward region. The strong synoptic wind, small width of the ridge, and steep leeward ridge slope angle provide favorable conditions for flow separation at the leeward foot of the ridge. The gust factor increases with decreasing surface elevation and is larger during daytime than nighttime. A significantly large gust factor is also observed in the leeward region.     

Determining the Oxygen Isotope Composition of Evapotranspiration Using Eddy Covariance

Impacts of different terrain configurations on the general behaviour of idealised katabatic flows are investigated in a numerical model study. Various simplified terrain models are applied to unveil modifications of the dynamics of nocturnal cold drainage of air as a result of predefined topographical structures. The generated idealised terrain models encompass all major topographical elements of an area in the tropical eastern Andes of southern Ecuador and northern Peru, and the adjacent Amazon. The idealised simulations corroborate that (i) katabatic flows develop over topographical elements (slopes and valleys), that (ii) confluence of katabatic flows in a lowland basin with a concave terrainline occur, and (iii) a complex drainage flow system regime directed into such a basin can sustain the confluence despite varying slope angles and slope distances.