Slope Winds Research Articles

Accounting for the Mesoscale Effects on the Air Pollution in Some Cases of Large Sulfur Pollution in Bulgaria or Northern Greece

The objective of the present work is to demonstrate the influence of the meso-scale dynamic phenomena on the larger scale air pollution characteristics. A limited set of episodes with very large sulphur pollution in Bulgaria or Northern Greece is chosen for the study. A 3D quasi-hydrostatic model of the meso-scale dynamics, based on the Businesque approximation (the formulation of Guthman) is used for the purpose. Some numerical experiments for the Balkan Peninsula are carried out under different background (synoptic) conditions. The simulated flow systems outline the main topography effects, typical for the region, such as slope winds, channelling of the air flows or blocking effects. Numerical simulations of the air pollution transport are also carried out, with and without accounting for the meso-scale deformations of the wind field. The comparison of the air pollution characteristics, obtained in both the cases demonstrates that the underlying surface heterogeneity in some of the synoptic situations may have influence not only on the detailed air pollution field in the region, but also on some larger scale pollution characteristics – the total pollution quantity in the air above the countries, the mean surface concentration, the pollution fluxes trough the country boundaries, dry and wet deposition.

Modeling the Martian dust cycle, 1. Representations of dust transport processes

A dust transport scheme has been developed for a general circulation model of the Martian atmosphere. This enables radiatively active dust transport, with the atmospheric state responding to changes in the dust distribution via atmospheric heating, as well as dust transport being determined by atmospheric conditions. The scheme includes dust lifting, advection by model winds, atmospheric mixing, and gravitational sedimentation. Parameterizations of lifting initiated by (1) near‐surface wind stress and (2) convective vortices known as dust devils are considered. Two parameterizations are defined for each mechanism and are first investigated offline using data previously output from the non‐dust‐transporting model. The threshold‐insensitive parameterizations predict some lifting over most regions, varying smoothly in space and time. The threshold‐sensitive parameterizations predict lifting only during extreme atmospheric conditions (such as exceptionally strong winds), so lifting is rarer and more confined to specific regions and times. Wind stress lifting is predicted to peak during southern summer, largely between latitudes 15° and 35°S, with maxima also in regions of strong slope winds or thermal contrast flows. These areas are consistent with observed storm onset regions and dark streak surface features. Dust devil lifting is also predicted to peak during southern summer, with a moderate peak during northern summer. The greatest dust devil lifting occurs in early afternoon, particularly in the Noachis, Arcadia/Amazonis, Sirenum, and Thaumasia regions. Radiatively active dust transport experiments reveal strong positive feedbacks on lifting by near‐surface wind stress and negative feedbacks on lifting by dust devils.

Gas diffusion over an isolated hill under neutral, stable and unstable conditions

Wind tunnel experiments of gas diffusion were performed over flat terrain and over an isolated three-dimensional hill under neutral, stable and unstable (sea breeze) conditions. Conditions of airflow in the wind tunnel were determined so as to satisfy the similarity rule for the bulk Richardson number, by controlling temperature profiles and wind velocity of the thermally stratified wind tunnel. Typical characteristics were observed under each condition of atmospheric stability; reversed vortex behind the hill in neutral condition, downward slope wind in stable one and convective motion in unstable one. We compared these experiments with the results of a Direct Numerical Simulation (DNS) model for the wind velocity over the hill under neutral conditions. The numerical results showed good agreement with the experimental results.

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

¶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.

Mesoscale Heat Transport Over Complex Terrain By Slope Winds – A Conceptual Model And Numerical Simulations

Vertical heat fluxes induced by mesoscale thermally driven circulations maycontribute significantly to the subgrid-scale fluxes in large-scale models (e.g.,general circulation models). However, they are not considered in these modelsyet. To gain insight into the importance and possible parameterisation of themesoscale flux associated with slope winds, an analytical (conceptual) modelis developed to describe the relationship between the mesoscale heat flux andatmospheric and land-surface characteristics. The analytical model allows usto evaluate the mesoscale flux induced by slope winds from only a few profilemeasurements within a domain. To validate the analytical model the resultingheat flux profiles are compared to profiles of highly resolved wind and temperaturefields obtained by simulations with a mesoscale numerical model.

An Improved Method for Computing Horizontal Diffusion in a Sigma-Coordinate Model and Its Application to Simulations over Mountainous Topography

Abstract A set of modifications is presented to reduce the unphysical impact of horizontal diffusion in numerical models with a terrain-following sigma-coordinate system. At model levels sufficiently far away from the ground, vertical interpolation is used to compute diffusion truly horizontally when the coordinate surfaces are sloping. Close to the ground, where truly horizontal computation of diffusion is not everywhere possible without intersecting the topography, a combination of one-sided truly horizontal diffusion and orography-adjusted diffusion along the sigma surfaces is used for most of the variables. The latter means that the diffusion coefficient is reduced strongly when the grid points involved in the computation of horizontal diffusion are located at greatly different heights. For temperature, one-sided horizontal diffusion is not used because it damps the slope wind circulation in an unphysical way. However, a temperature gradient correction is applied to the terrain-following part of the t...

MANDOP Analysis over Complex Orography in the Context of the MAP Experiment

Abstract The present paper deals with the adaptation of the Multiple Analytical Doppler (MANDOP) analysis to Doppler radar observations over complex orography. The objective, in the context of the Mesoscale Alpine Programme (MAP), is to design a reliable and flexible tool able to process any kind of radar data (airborne/ground based) in order to retrieve mesoscale three-dimensional wind fields over complex terrain with a convective-scale resolution. The various effects of the orography on the wind field (slope winds) and on the radar sampling (ground clutter, shadowing effect) are considered and taken into account in each step of the wind synthesis. In addition to these specific aspects, some numerical improvements are presented that make the method quicker and more objective. The method is validated with quasi-real simulated data, and three different boundary conditions are compared (bottom, top, and both). The analysis is eventually applied to real data gathered during IOP2b of the MAP special observing...

Diurnal momentum budget analysis of thermally induced slope winds

Data measured during the TRACT field campaign at various stations along a 7.5° steep and west-northwest facing slope of the Black Forest mountain range (Germany) are used to analyze the thermal structure and the momentum budget associated with thermally induced slope winds. Acceleration of the air close to the ground is found to be directed nearly vertically downward during the night and nearly vertically upward during the day, rather than parallel to the slope. This means that during the night the airflow is deflected by the slope surface in the down-slope direction, whereas during daytime stable stratification above the heated slope layer is required to establish up-slope flow parallel to the slope. The diurnal cycle of the momentum budget of the along-slope wind component near the surface is analyzed in detail with respect to the driving forces (buoyancy and pressure gradient force) and friction. It is found that a small imbalance between forcing and friction is responsible for the diurnal change in slope flow intensity. The along-slope components of the horizontal pressure gradient force and the buoyancy force are shown to have the same order of magnitude. This means that for small to moderate slope angles the pressure gradient force cannot be neglected as is done in some analytical slope wind models. The reaction time of the slope flow to changes in forcing is estimated to be in the range of 30 to 120 seconds, which confirms the empirically known fact that slope winds react very quickly.

A Formulation of the Continuity Equation of MUSCAT for either Flat or Complex Terrain

Abstract The Mesoscale Alpine Programme (MAP) involved an ensemble of airborne and ground-based Doppler radars dedicated to the observation of precipitating systems over the Alps. The derivation of the three-dimensional wind fields from the multiple-Doppler synthesis and continuity adjustment technique (MUSCAT) requires that orography-induced air circulation, in particular when solving the mass continuity equation, is taken into account. A formulation of this equation in its flux form is proposed, which has the advantage of applying to either flat or complex terrain and thus eliminating the need to explicitly evaluate the vertical velocity associated with the slope wind at the surface. Pseudo-Doppler observations of a pair of ground-based radars that were operated during MAP, deduced from a modeled pre-MAP case, are used to validate the proposed solution and to investigate the performances of the Doppler wind synthesis above mountainous regions.

The VOTALP Mesolcina Valley Campaign 1996 – concept, background and some highlights

The Mesolcina Valley campaign was an important part of the VOTALP project. Its main goals were the study of the effects of thermal wind systems on horizontal and vertical ozone transport over various distances, and on local production of ozone in an Alpine valley. The field measurements took place in the Mesolcina Valley in southern Switzerland in July and August 1996. The Mesolcina Valley is typical for the Alps for its size and its rural character, and contains an important traffic route. Ground-based and airborne instruments were deployed to obtain a four-dimensional chemical and physical picture of the valley atmosphere and the surroundings. Field measurements were complemented by numerical modeling studies. An overview of the campaign, its geographic background, and the measurement plan is given together with selected highlights of the results obtained so far. A synoptic-climatological approach tries to evaluate the representativeness of the observation days. It was found that during the campaign representative data for typical ozone days were collected, while extreme events did not occur. The main results of the campaign are that very high pollutant concentrations at the Alpine crests are the effect of advective, larger-scale transport, mainly from the Alpine forelands. The valley is very effective in pumping air into elevated layers during the day, with the slope winds probably carrying the bulk of the air volume to higher levels. Up to five times the valley volume may be exported during one upwind phase. Net local production is estimated to be approximately 3–8 ppb h −1 of ozone, but seems to be compensated by dry deposition. This indicates that the Alpine valleys may be important net sinks of air pollutants, which may lead to negative impacts on the Alpine ecosystems.

Modelling of the combined late-winter ice cap edge and slope winds in Mars Hellas and Argyre regions

Towards the end of southern hemisphere winter (Ls ≈ 180°) the Martian southern polar cap extends equatorward to 40°S and covers at least, the southern slopes of the Hellas and Argyre impact basins. Subsequently, during retreat of the seasonal ice cap, varying configurations of ice coverage on these slopes occur. Since both sloping topography and ice-edge effects can independently drive mesoscale circulations, the superposition of these two processes may then generate interesting wind patterns. A set of numerical experiments has been performed with the University of Helsinki 2-D Mars Mesoscale Circulation Model (MMCM) in order to study the characteristics of circulations driven by these combined forcings. A model-centre latitude of 57°S and a slope angle of 0.6°, both representative of Hellas southern slope, are used. When compared with the winds arising in the ice-free slope case, ice coverage in the upper extent of the slope results in diminished upslope (daytime) winds, while the down-slope (nighttime) flow is enhanced. Ice coverage in the lower section of the slope in turn causes enhanced upslope (daytime) and attenuated downslope (nocturnal) flows. This arises due to the daytime off-ice near-surface flow induced by the thermal contrast at the ice cap edge. The surface winds are persistently downslope over a fully ice-covered slope. Inclusion of atmospheric dust (τ = 0.3) reduces the ice-edge forcing. In comparison with the dust-free situation, the resulting circulation is almost unchanged in the case of ice-covered upper part of the slope, in the opposite case the daytime flow is attenuated and the nocturnal downslope flow enhanced. When the entire slope is ice-covered, the flow is amplified due to the increased direct atmospheric heating. Inclusion of a large scale circulation component (7 m⧸s southerly wind) in conjunction with an ice-covered slope top results in the generation of a downslope windstorm (föhn, or bora-type of event) with near surface winds exceeding 30 m⧸s. Winds of this magnitude, not realised in any of the other experiments, approach speeds deemed capable of lifting dust from the surface.

Rapid urban growth, land-use changes and air pollution in Santiago, Chile

This paper is a contribution to the understanding of the topoclimatic and environmental geography of the basin where Santiago — one of the most polluted Latin American city – is located. In the first part, land-use change is analysed looking at the climatic transformation caused by the rapid transit from natural semiarid surface to urban areas. In the second part, seasonal weather and daily cycles of slope winds and the available ventilation are described trying to relate those patterns with the spatial distribution of air pollution. A combination of meteorological, geographical and cultural factors explain extreme air pollution events: meteorologically, Santiago is under permanent subsidence inversion layers. Geographically, the city is located in a closed basin surrounded by mountains. Culturally, the urban area has the highest population concentration (40% of the national total), industries (near 70% of the total) and vehicles, which are the main sources of smog. The urban and suburban transport system is based on a large number of buses (diesel) and private cars, both experiencing a rapid growth from the past few years. The city and specially the transport system generates high emissions of pollutant, but the natural semiarid deforested soils and slopes are also important sources. The local wind system can explain the differential spatial distribution on the concentration of air pollutants in the city and its periphery. In winter (rain season) concentrations of particulate matter are higher at the centre and the SW part of the city. The andean piedmont area (E part of the city) shows minimum values, suggesting major ventilation effects of slope and valley winds. Ozone exceeds air quality standards in summer (dry season) at all sites in the centre and periphery. However, the O 3-concentrations are higher on preferred residential areas located at the piedmont area (E part of the city), suggesting air pollution transport effects. Currently, there is no consideration of these local climatic features in the process of urban planning.

221 霧によって可視化された大気中のハイドロリック ジャンプ

This study deals with the mechanism of a local wind called which is a down slope wind observed along the Hijikawa river near Nagahama in Ehime Prefecture. This river is connected to an inland basin (Ohsu basin) where a cold air mass is accu nulated during nighttime by radiation cooling and overflows to the sea (Iyo-nada of Seto Inland Sea) through a valley along the Hijikawa river. Hijikawa Arashi gives a considerable influence to daily life of the people in the area, because the wind speed sometimes exceeds 10m/s and fog associated with the wind reduces the visibility significantly. The mechanism of the strong wind is not known yet, and remains as an interesting problem to meteorologists.In this study, Hijikawa Arashi is studied by means of a shallow water theory, laboratory experiment and field observation. It is shown that the strong wind is caused by a hydraulic jump of the down slope wind which is produced by the effect of topography of the valley. The theory and the laboratory experiments predict that a steady hydraulic jump is produced in the downstream side of the narrowest portion of the valley. The result was confirmed by a videotape record of the fog which visualized the hydraulic jump of the Hijikawa Arashi.

Aerosol climatology at the high‐alpine site Jungfraujoch, Switzerland

Continuous aerosol measurements have been performed at the high‐alpine site Jungfraujoch (3450 m above sea level) since 1988 by means of an epiphaniometer. The instrument, which determines the Fuchs surface area of the aerosol particles, was operated with a time resolution of 30 min. High correlation coefficients (r>0.8) were found between the epiphaniometer signal and other aerosol parameters, which could be attributed to a rather constant size distribution of the Jungfraujoch aerosol in the accumulation range (0.1<d<1 μm). Well‐defined diurnal variations with a peak in the late afternoon were observed on many days during summer, which was not the case during winter. Comparison with black carbon and radon daughter measurements revealed that these diurnal variations are due to vertical transport processes. A statistical analysis showed that the fraction of days with a well‐defined diurnal pattern increased with decreasing stability of the atmosphere; however, late afternoon peaks also occurred during days when the potential temperature profile indicated a stable atmosphere. First simulations with ALPTHERM, a new convection model which takes topography into account, were able to explain the observed aerosol patterns. This indicates that slope winds over a certain catchment area are responsible for the transport to this high‐elevation site. The distinct seasonal variation with summer values, which are about a factor of 10 higher than winter values, could therefore be attributed to seasonally varying transport processes, due to the seasonal variation of radiation. The data show that even sites at very high elevation cannot be assumed to be in the free troposphere all the time.

Diurnal winds around lake Tanganyika

AbstractRegular diurnal variations of coastal winds are observed around Lake Tanganyika during the dry season, when insolation is intense and the prevailing trade winds are weak. the long deep lake is at the bottom of the East African rift valley. A two‐dimensional mesoscale model across the schematic rift‐valley topography is used to study these local tropical winds. the full model gives a good simulation when compared with the observed winds on the east coast. It turns out that about half of the strong diurnal wind variation there, in the form of slope winds, is due to the sloping mountains. Roughly one quarter is due to the thermal lake effect (sea breeze), and one quarter is due to the existence of the south‐easterly trade wind, which enhances the sea‐breeze circulation component considerably during daytime, and adds on to the downslope winds during night‐time. the wind field across the lake is quite variable from hour to hour and from place to place, yet predictable to a good extent during the dry season, when the external conditions are rather similar from day to day.

Low‐level jets in the NASA Ames Mars general circulation model

Previous simulations of the Martian atmosphere have shown how topography acts to confine the low‐level Hadley cell flow into intense jets on the eastern flanks of Tharsis and Syrtis Major. We now conduct detailed studies of these jets using the NASA Ames Mars general circulation model (MGCM). The structure of the flow is found to be sensitive to local topography as well as large‐scale diabatic heating patterns, consistent with terrestrial studies, and MGCM studies carried out with simplified topography. The summer subtropical zonal winds associated with the Hadley circulation also form spatially confined intense jet cores. Diurnal variations in heating affect jet structure in three distinct ways. Global tides interact with the jets, resulting in effects such as the two reinforcing each other at the summer subtropics near midday, leading to high winds and surface stresses at this time. Slope winds act to change the character of the jets during the course of a day, especially at Syrtis Major and the Hellas basin, where slopes are large. Vertical mixing acts to decrease low‐level winds during the late afternoon. The sensitivity of the results to atmospheric dust loading is examined. We finally show how a decrease in boundary layer height due to dust loading actually augments mid‐afternoon jet strength near the surface. The resulting increase in maximum surface stress indicates that this is a positive feedback to dust lifting.

Seasonal variations in low level flow in the NASA-Ames Mars GCM

The Martian Hadley circulation consists of two cells of different sizes at most times of the year, with one cell dominating near the solstices. The lower branch of this flow is strongly topographically controlled, especially east of the Tharsis plateau and Syrtis Major. Simulations using idealized topography show that longitudinal asymmetries in heating also play a role in forming the jets. Nearer the equinoxes, the total cross-equatorial flow intensity is reduced and the effect of the seasonal condensation flow is felt more strongly. Because of effects such as topography, zero cross-equatorial flow is not actually achieved at the equinoxes, but is slightly offset in time. Even when the mean flow is zero, the two jets are still present but flow in opposite directions: the Tharsis jet flows south and the Syrtis jet north. The latter is a result of the time-averaged effect of slope wind circulations. The two jets are still relatively intense at this time. For example, with an optical depth τ of 0.3, and at a height of 250 m, v at L s = 270° is about 9 m s −1 southward, while at 45°W v is 26 m s −1. At L s = 350°, v is 0.5 m s −1, while at 45°W it is still above 8 m s −1. The effect of atmospheric dust loading is also examined.

Ozone distributions over the los angeles basin: Three-dimensional simulations with the smog model

The UCLA Surface Meteorology and Ozone Generation (SMOG) model has been applied to simulate and analyze the distributions of ozone in the Los Angeles basin on 27 and 28 August 1987, during the Southern California Air Quality Study (SCAQS). SMOG is an integrated air pollution modeling system that includes the coupled effects of meteorology, pollutant sources and dispersion, photochemistry and aerosol microphysics, and radiative processes. High surface ozone concentrations are predicted along the slopes of the surrounding mountain barriers and in the eastern basin, as observed. The pattern of surface ozone concentrations is shown to be influenced by the evolution of oxidants within the temperature inversion that typically blankets the Los Angeles basin. In the past, dense layers of ozone have been observed to be embedded within the temperature inversion, but their origin has remained unexplained. The present model simulations reproduce these and other three-dimensional structures in the ozone distribution, as documented by measurements in the basin. The high concentrations of oxidants aloft are explained through a detailed dynamical/chemical mechanism. High mountains surrounding the Los Angeles basin block the dispersion of ozone and other pollutants and generally contain them within the basin. At the same time, vertical circulations associated with the interaction of the sea-breeze and mountain slope winds inject pollutants into the base of the inversion and create high concentrations of ozone, PAN, nitric acid, organic nitrates and a variety of other pollutants, including aerosols. The contrast between ozone concentrations at the surface and in these elevated layers can be enhanced in the evening by NO emissions, which titrate ozone near the surface. The elevated layers act as a reservoir for aged pollutants, which can be mixed downward on subsequent days to enhance surface concentrations. The recirculation of pollutants in the Los Angeles basin is thus reinforced by the presence of these oxidant layers, and may be responsible for the rapid increase in surface ozone concentrations often seen in the morning, particularly in the eastern basin where emissions of primary pollutants are relatively small. The impact on surface ozone concentrations of recirculated photochemically aged air is estimated to be as high as 8 pphmv in the eastern Los Angeles basin. These simulations represent the first quantitative description of the role of pollutants aloft on surface air quality.

Saltation and wind-flow interaction in a variable slope wind tunnel

In a 15 m long horizontal wind tunnel, we studied the internal boundary layer over fixed arrays with roughness (z 0) similar to the roughness of saltation of a sand bed. Utilizing turbulence spires the logarithmic overlap-layer is at least 75 mm thick some 5 m downwind of the entry. This permits a precise determination of u ∗ from wind profile data. Over a sand bed in a variable slope wind tunnel with a 6 m long working section we studied how slope, grain size, and friction speed influence bed roughness. Owen (1964) proposed that z 0 = C · u ∗ 2/2 g where C is a constant, but we found that C depends in a complex manner on grain size and friction speed. C increases from very low values near the threshold to an apparently limiting value at large friction speeds. For an almost uniform 125 μm sand, the limiting value of C is 0.06. The limiting value decreases, however, with particle size: for a uniform 544 μm sand, C only approaches 0.03 for friction speeds as high as 1 m/s. With the measured values of friction speed corrected for the effects of slope, our results appear to show that the limiting values of C do not depend on the slope angle, even though the slope affects the individual shapes of particle trajectory. C is perhaps independent of slope angle because the roughness height, and, therefore, C, is more directly related to the amount of momentum extracted from the air by the particles (i.e. the shear stress) rather than to the manner of extraction (particle trajectory shapes).

Western boundary currents in the Martian atmosphere: Numerical simulations and observational evidence

Western boundary currents (hereafter WBCs) are an intensification of meridional fluid flow along the eastward facing flank of a boundary. They occur in the Earth's oceans, the best known instance being the Gulf Stream, and in the terrestrial atmosphere, an example of which is the East African Jet. We have investigated WBCs in numerical simulations of the Martian atmosphere, where they occur in the presence of large longitudinal topographical gradients, combined with the β effect. We suggest that WBCs have already been simulated by other Martian atmospheric models, although not identified as such. The intensity of these currents is dependent on basic model parameters, notably surface drag. We show that for physically reasonable values of drag, frictional forces dominate the WBCs' behavior. The requirement of zero net cross‐equatorial time‐mean mass flow produces a “return flow” in simulations with relatively low surface drag, which acts in opposition to the Tharsis WBC. We also show that the intense flow associated with WBCs advects potential vorticity across the equator, although the presence of strong radiative damping inhibits low‐level inertial instabilities that might result from this. Slope winds are found to have a profound effect on WBC structure, especially where they have a component parallel to the jet. In these cases, slope winds can cancel out or reinforce WBCs, depending on their direction. Enhanced low‐level winds associated with WBCs may be a factor that influences the locations of dust storm generation. The high vertical shear associated with WBCs may also play a role in the maintenance and subsequent decay of these dust storms. Some observational evidence may exist for WBCs in the form of the alignment of bright depositional dust streaks, wave clouds, and in wind measurements taken during the parachute descent phase of Viking Lander 1 through the planetary boundary layer.