Local Winds In A Valley City

Local winds were studied around a valley city, by using a high resolution two-dimensional mesoscale model forced by surface temperatures from a measurement campaign around Lanzhou City, China, during stagnant conditions. In the simulations nighttime winds are purely katabatic downslope winds without urban effects, despite the fact that the city is 6–7°C warmer than its surroundings all night. In contrast, daytime near-surface winds result from upslope flow resisted by an opposing simultaneous urban heat-island circulation (UHIC). Hence winds remain weak and variable around a city in a narrow valley during daytime. These conditions may lead to severe air quality problems day and night.The local circulations are sensitive to the widths of the valley and/or city,and also latitude, as is demonstrated by model experiments. Interestingly, in a flat and calm environment an extratropical daytime UHIC cell may turn into a weak `anti-UHIC' by the morning, due to frictional decoupling after sunset and subsequent inertial oscillation during the night, analogously tothe land breeze and nocturnal low-level jet formation.     

A study of the diurnal behavior of boundary-layer winds at the Boulder Atmospheric Observatory

Diurnal wind variations are examined at the Boulder Atmospheric Observatory which is located 25 km east of the foothills of the Rocky Mountains. Data were obtained from a 300-m tower which was instrumented at eight levels and operated almost continuously for three weeks during September 1978. Observations on clear days, for which the diurnal heating and cooling of the local terrain slopes can be expected to affect the winds, show that daytime winds tend to be easterly (upslope) throughout the 300-m depth. At night, a temperature inversion typically develops to about 100 m. Below this level, the nocturnal flow tends to be downslope; above the inversion, a distinctly different regime of flow develops. A diurnal wind oscillation, characterized by strong southerly flow beginning near sunset and ending near midnight, occurred in the upper layer on 25% of the days during the study period. Rapid clockwise rotation of the wind vector occurred during the period of increased wind speed. This oscillation occurred only on days when the synoptic-scale geostrophic wind was southerly. It is suggested that this non-steady state behavior is an inertial oscillation affected by the diurnally varying temperature gradients and local topography.     

The KABEG’97 field experiment: An aircraft-based study of katabatic wind dynamics over the Greenland ice sheet

The aircraft-based experiment KABEG97 (Katabatic wind and boundary-layer front experiment around Greenland) was performed in April/May 1997. During the experiment, surface stations were installed at five positions on the ice sheet and in the tundra near Kangerlussuaq, West Greenland. A total of nine katabatic wind flights were performed during quite different synoptic situations and surface conditions, and low-level jets with wind speeds up to 25m s^-1 were measured under strong synoptic forcing of the katabatic wind system. The KABEG data represent a unique data set for the investigation of katabatic winds. For the first time, high-resolution and accurate aircraft measurements can be used to investigate the three-dimensional structure of the katabatic wind system for a variety of synoptic situations.Surface station data show that a pronounced daily cycle of the near-surface wind is present for almost all days due to the nighttime development of the katabatic wind. In a detailed case study the stably-stratified boundary layer over the ice and the complex boundary-layer structure in the transition zone ice/tundra are investigated. The katabatic wind system is found to extend about 10 km over the tundra area and is associated with strong wind convergence and gravity waves. The investigation of the boundary-layer dynamics using the concept of a two-layer katabatic wind model yields the results that the katabatic flow is always a shooting flow and that the pure katabatic force is the main driving mechanism for the flow regime, although a considerable influence of the large-scale synoptic forcing is found as well.     

Analytical investigations of horizontal meso-scale momentum equations with Newtonian nudging

Summary This study presents an analytical investigation of the local behaviour of the solution to a mesoscale model with Newtonian nudging when observed winds are time varying. The analysis examines each Fourier component of the time series of observed winds. Unlike the case with a constant observed wind, the nudged wind vector does not asymptotically approach the observed wind. In response to sinusoidal oscillation of the observed wind, the nudged wind vector is always on a half circle connecting the vector ends of the observed and un-nudged modelled winds. When nudging parameter 0, the nudged wind vector approaches the un-nudged wind; when , the nudged wind vector approaches the observed wind. For commonly used values of nudging parameter , the modelled wind field always carries errors.A target nudging scheme is devised in this study in order to ensure the model result is identical to observed winds with sinusoidal oscillation. Investigation shows that such a target wind exists for a finite value of , and the magnitude of the target-nudging term is about the same as that of a normal nudging term if f, wheref is the Coriolis parameter and is the frequency of the wind oscillation.With 7 Figures     

Indications of topographically-induced eddies in stratified flow during a severe air pollution event

During the last week of June 1978, Melbourne experienced a prolonged period of stagnation over the city, resulting in high levels of air pollutants. Internal Froude numbers through the period ranged from approximately 0.1 to 0.3, indicating that flow in the area should be strongly stratified. Wind patterns determined using data from thirteen anemographs within and immediately adjacent to the city revealed a pattern of eddies across the city each afternoon in a cavity in the lee of upstream topography. That pattern evolved with time, and changed between days as the controlling meteorological conditions altered. The available mean sea level pressure data from the region for the same times indicated perturbations in that field which were consistent with the observed wind patterns.The eddy patterns were replaced at the surface overnight by katabatic winds as cooling took place and a ground-based inversion became reestablished. However, mean sea level pressure data and the anemograph records suggest that the eddy pattern may have continued overnight, aloft.Considerable recycling of air occurred, both within individual circulations and by wind reversal between daytime and nocturnal regimes. This aided the persistence of high pollutant concentrations.     

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     

Orographic effects on a conditionally unstable flow over an idealized three-dimensional mesoscale mountain

Summary Idealized numerical simulations using the Weather and Research Forecast (WRF) model indicate that three flow regimes, based on the moist Froude number, can be identified for a conditionally unstable, rotational, horizontally homogeneous, uniformly stratified flow over an idealized, three-dimensional, mesoscale mountain stretched spanwise to the impinging flow: (I) a quasi-stationary upslope convective system and an upstream-propagating convective system, (II) a quasi-stationary upslope convective system, and (III) a stationary upslope convective system and a quasi-stationary downstream convective system. Several major differences from a similar type of flow with no rotation over a two-dimensional mountain range are found. One important finding is that relatively strong mean flow produces a quasi-stationary mesoscale convective system (MCS) and maximum rainfall on the windward slope (upslope rain), instead of on the mountain peak or over the lee side.We found that the Coriolis force helps produce heavy upslope rainfall by making transition from flow-around the eastern part of the upslope to flow-over the western part of the upslope (transits to a higher flow regime) by deflecting the incident southerly flow to become east–southeasterly barrier winds. We found that the addition of the western flank of the arc-shaped mountain helps slow down the barrier wind from east and causes the maximum rainfall to move east of the windward slope. A lower-Froude number flow tends to produce a rainfall maximum near the concave region.Several other important facts can also be found in this study. The ratio of the maximum grid scale rainfall to the sub-grid scale rainfall increases when the moist Froude number increases. When the CAPE decreases, it is found that the upstream moist flow tends to shift to a higher Froude-number regime. Therefore, the Froude number cannot solely be used to define a moist flow regime when different CAPEs are considered. In another word, other parameters, such as CAPE, might play an important role in determining moist flow regimes.     

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.     

Comparison of Different Generation Mechanisms of Free Convection between Two Stations on the Tibetan Plateau

Based on high-quality data from eddy covariance measurements at the Qomolangma Monitoring and Research Station for Atmosphere and Environment(QOMS) and the Southeast Tibet Monitoring and Research Station for Environment(SETS),near-ground free convection conditions(FCCs) and their characteristics are investigated. At QOMS, strong thermal effects accompanied by lower wind speeds can easily trigger the occurrence of FCCs. The change of circulation from prevailing katabatic glacier winds to prevailing upslope winds and the oscillation of upslope winds due to cloud cover are the two main causes of decreases in wind speed at QOMS. The analysis of results from SETS shows that the most important trigger mechanism of FCCs is strong solar heating. Turbulence structural analysis using wavelet transform indicates that lowerfrequency turbulence near the ground emerges from the detected FCCs both at QOMS and at SETS. It should be noted that the heterogeneous underlying surface at SETS creates large-scale turbulence during periods without the occurrence of FCCs. Regarding datasets of all seasons, the distribution of FCCs presents different characteristics during monsoonal and non-monsoonal periods.     

Modification of the local winds due to hypothetical urbanization of the Zagreb surroundings

Summary The aim of this study was to investigate possible effects of two hypothetical scenarios of the urbanization of Zagreb’s surroundings on the local winds, which are established under summertime anticyclonic conditions. For this purpose, the nonhydrostatic mesoscale meteorological model MEMO was applied to the greater Zagreb area. Three simulations were performed. One employed the current land-use distribution, while the other two corresponded to an increase of the densely urbanized area by 12.5% (test 1) and 37.5% (test 2), respectively. Apart from the hypothetically urbanized areas, where average surface wind speed reductions of 8% and 18% were obtained for test 1 and test 2, respectively, the rest of the domain was not significantly affected by hypothetical urbanization. The differences between the wind vectors for the predicted current state and the hypothetical state were more pronounced and found at higher altitudes during the night compared to daytime values. For all three simulations the same diurnal variation of the depth of anabatic/katabatic wind flow generated on south-facing slopes of 1 km high mountain Medvednica was obtained. During the night the depth of well-developed katabatic flow was about 370 m, while during the day the depth of anabatic flow grew from about 550 m in the late morning up to about 1140 m in the late afternoon. Received October 27, 2000 Revised August 4, 2001     

A three-dimensional numerical sensitivity study of mesoscale circulations induced by circular lakes

Summary A three-dimensional mesoscale planetary boundary layer model with theE- turbulence closure is used to simulate airflow over a lake of circular shape. A series of model sensitivity studies are performed to examine the effects of lake-land temperature difference, ambient wind magnitude and direction, lake size, surface roughness, the Coriolis force and baroclinic ambient wind conditions on mesoscale lake circulations.The lake-land temperature difference is essentially the basic energy source driving the mesoscale circulations over the lake on synoptically undisturbed days. A lake-breeze convergence zone is predicted by the model due to the differential heating between the land and the water. It is found that spatial and temporal variations of this convergence zone and associated convection are strongly controlled by the direction and the magnitude of the ambient wind. Under southeasterly and southwesterly ambient winds, the lake-breeze convergence zone and the associated convection occur primarily along up wind and lateral sides of the lake with reference to the general direction of the ambient flow. In contrast to the southeasterly and southwesterly ambient winds, the lake-breeze convergence zone and the convection are predicted all around the coastline of the lake under calm wind.The model also predicts a cloudless region over the lake in all the case studies due to divergent nature of the lake-breeze circulation. The lake size is found to have a significant effect in intensifying convection. Surface roughness over the land surface is found to be important in determining the intensity of the convection. The combined effect of the Coriolis force and the differential surface roughness between land and water appear to be the responsible mechanism for producing the asymmetric shape of the lake-breeze convergence zone around the symmetric circular lake. Finally, it was found that an initial baroclinic flow has different mesoscale lake-breeze circulation patterns as compared to an initial barotropic flow.With 16 Figures     

Patterns of weak,near-surface winds at Melbourne,Australia

Up to 30 months of near-surface anemograph records have been examined from 13 locations in and near Melbourne, Australia, to determine the wind patterns which existed during prolonged periods of light winds (at least 3 hours at 2 m s^-1 or less). A coherent katabatic wind system was found to develop in at least part of the monitored region on approximately 30% of nights. The flow broadly followed the slope of the basin surrounding the city, with a strong flow down the main river valley, and was partly reinforced by a land breeze in bayside areas. Other valleys also acted as channels for these winds. The general tendency of these katabatic winds was to converge towards the central business district and the northern part of Port Phillip Bay adjacent to the city centre. Where winds from different directions interacted, one of the winds dominated or successive replacement occurred causing the wind direction to vary considerably during a period.There were indications that in the presence of low-level stability with a synoptic gradient wind between east and north, the gradient flow may be deflected around the major topographic barrier to the northeast of the city. The existence of such a situation would have major implications in terms of air quality due to the possibility of pollutants being recirculated in conditions when vertical diffusion was very limited.     

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.     

Numerical Simulation of Wind and Temperature Fields over Beijing Area in Summer

1. IntroductionThe basic role of urban-rural boundary layer re-search is to study all kinds of physical process changesin the atmosphere boundary layer over urban and itssurrounding areas. Urban heat island (UHI) is a well-known feature of urban-rural climate. Attempts toincrease the understanding of the causes of the UHIand other urban-rural boundary layer phenomena haveused observational, theoretical and modelling methodssince long before. Seaman (1989) used a hydrostaticmodel, with real …     

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     

Present and future near-surface wind climate of Greenland from high resolution regional climate modelling

The present and twenty-first century near-surface wind climate of Greenland is presented using output from the regional atmospheric climate model RACMO2. The modelled wind variability and wind distribution compare favourably to observations from three automatic weather stations in the ablation zone of southwest Greenland. The Weibull shape parameter is used to classify the wind climate. High values (κ > 4) are found in northern Greenland, indicative of uniform winds and a dominant katabatic forcing, while lower values (κ < 3) are found over the ocean and southern Greenland, where the synoptic forcing dominates. Very high values of the shape parameter are found over concave topography where confluence strengthens the katabatic circulation, while very low values are found in a narrow band along the coast due to barrier winds. To simulate the future (2081–2098) wind climate RACMO2 was forced with the HadGEM2-ES general circulation model using a scenario of mid-range radiative forcing of +4.5 W m^?2 by 2100. For the future simulated climate, the near-surface potential temperature deficit reduces in all seasons in regions where the surface temperature is below the freezing point, indicating a reduction in strength of the near-surface temperature inversion layer. This leads to a wind speed reduction over the central ice sheet where katabatic forcing dominates, and a wind speed increase over steep coastal topography due to counteracting effects of thermal and katabatic forcing. Thermally forced winds over the seasonally sea ice covered region of the Greenland Sea are reduced by up to 2.5 m s^?1.     

Numerical Modelling of Katabatic Flow Over a Melting Outflow Glacier

A realistic simulation of katabatic flows is not a straightforward task for numerical models. One complicating factor is that katabatic flows develop within a stably stratified boundary layer, which is poorly resolved and described in many numerical models. To capture the jet-shaped shallow flow a model set-up with high vertical resolution is also required. In this study, ‘a state of the art’ mesoscale numerical model is applied in a simulation of katabatic flow over a melting glacier. A basic agreement between observations and model results is found. From scale analysis, it is concluded that the simulated flow can be classified as katabatic. Although the background flow varies in strength and direction, the simulated katabatic flow over Breidamerkurjökull is persistent. Two factors vital for this persistence are identified. First, the melting snow maintains the surface temperature close to 0 °C while the air temperature warms adiabatically as it descends the slope. This provides a ‘self enhanced’ negative buoyancy that drives the flow to a balance with local friction. Second, the jet-like shape of the resulting flow gives rise to a large ‘curvature term’ in the Scorer parameter, which becomes negative in the upper jet. This prevents vertical wave propagation and isolates the katabatic layer of the influence from the free troposphere aloft. Our results suggest that the formation of local microclimates dominated by katabatic flow is a general feature over melting glaciers. The modelled turbulence structure illustrates the importance of non-local processes. Neglecting the vertical transport of turbulence in katabatic flows is not a valid assumption. It is also found that the local friction velocity remains larger than zero through the katabatic jet, due to directional shear where the scalar wind speed approaches its maximum.     

Intraseasonal oscillations of the monsoon circulation over South Asia

The space–time structure of the three-dimensional circulation over the South Asian monsoon region has been studied using the ERA-40 reanalysis of the European Centre for Medium-Range Weather Forecasts. Applying multi-channel singular spectrum analysis on combined daily values of horizontal winds and pressure vertical velocity at ten vertical levels for the period 1958–2001, two leading intraseasonal nonlinear oscillations were extracted. The first oscillation has an average period of 50?days and propagates northeastward from the Indian Ocean to the Indian subcontinent. The second oscillation has a period of 30?days and propagates northwestward from the West Pacific to the Indian region. Both the oscillations exhibit the oscillatory and propagation features at all vertical levels from 1,000 to 100?hPa. The two oscillations correspond well with similar oscillations found in outgoing longwave radiation and precipitation in earlier studies. The wind oscillations also account for the active and break phases of the Indian monsoon. The vertical structures and propagation of specific humidity and temperature are found to be consistent with those of the winds in each oscillation. The structure and movement of regional Hadley and Walker circulations have also been described. The analyses provide further strong evidence for the existence of two distinct monsoon intraseasonal oscillations.     

A three-dimensional numerical sensitivity study of convection over the Florida peninsula

The Florida peninsula has the highest annual number of days with thunderstorms in the United States, partly due to sea breeze convergence. A three-dimensional mesoscale planetary boundary layer (PBL) model with the E- turbulence closure is used to investigate the relationship between sea breeze convergence and convection over the peninsula for two ambient wind cases during typical summer days.It is found that the spatial and temporal variation of the sea breeze convergence zones and the associated convective activities depend to a large extent on the direction and magnitude of the ambient wind. For the case of southeasterly ambient winds, a strong convergence zone and hence significant rainfall occur primarily along the west coast of the peninsula. The convergence zone and the associated rainfall shift towards the east coast for the case of southwesterly ambient winds. These are in agreement with the observations. In contrast to the southeasterly and southwesterly ambient winds, an intense convergence zone and rainfall occur near both coastlines of the peninsula under light ambient winds.It is also found that lake Okeechobee has a substantial influence on south Florida's mesoscale weather. A cloudless region is always present over the lake at least until late afternoon due to its own lake breeze circulation. Finally, increased roughness of the land surface appears to influence the temporal and spatial variation of the convection by determining the intensity of the vertical turbulent transport of heat and momentum.     

Influence of soil moisture on simulations of katabatic flow

Summary Simulations of katabatic flow with a two-dimensional dynamic numerical model with a soil parameterization indicate that downslope flow developed over a moist slope is weaker than that over a dry slope. This agrees with earlier findings that daytime anabatic (upslope) flow is weaker over a moister slope. But, whereas the weaker anabatic flow is produced because surface evaporation prevents the moist slope from heating as much as a dry slope, the weaker katabatic flow is produced over moist slopes because (1) the soil thermal conductivity is greater in moist soil, and (2) downward longwave radiation flux from the atmosphere to the surface is greater because of higher humidity in the air near the surface from evaporation. The higher thermal conductivity allows warmer soil temperatures (heat) to diffuse upward to the soil surface and prevents the surface temperature from becoming as cold in the moist run as in the dry run.With 6 Figures