Numerical simulations of wind and temperature structure within an Alpine lake basin, Lake Tekapo, New Zealand

Summary High-resolution numerical model simulations have been used to study the local and mesoscale thermal circulations in an Alpine lake basin. The lake (87km²) is situated in the Southern Alps, New Zealand and is located in a glacially excavated rock basin surrounded by mountain ranges that reach 3000m in height. The mesoscale model used (RAMS) is a three-dimensional non-hydrostatic model with a level 2.5 turbulence closure scheme. The model demonstrates that thermal forcing at local (within the basin) and regional (coast-to-basin inflow) scales drive the observed boundary-layer airflow in the lake basin during clear anticyclonic summertime conditions. The results show that the lake can modify (perturb) both the local and regional wind systems. Following sunrise, local thermal circulations dominate, including a lake breeze component that becomes embedded within the background valley wind system. This results in a more divergent flow in the basin extending across the lake shoreline. However, a closed lake breeze circulation is neither observed nor modelled. Modelling results indicate that in the latter part of the day when the mesoscale (coast-to-basin) inflow occurs, the relatively cold pool of lake air in the basin can cause the intrusion to decouple from the surface. Measured data provide qualitative and quantitative support for the model results.     

Interacting multi-scale wind systems within an Alpine basin,Lake Tekapo,New Zealand

Summary The wind regime of the Lake Tekapo Basin is examined with reference to the interaction of multi-scale local, regional and synoptic circulations. Analysis of the historical wind direction record from Mt John identifies airflow from three principal directions to most frequently affect the study area. Both seasonal and diurnal trends in the frequency of each directional category are identified, which reflect the influence of local thermal forcings and seasonal changes in synoptic circulation on the Lake Tekapo windfield. Meteorological observations from a network of automatic weather stations and anemographs within the study area identified Lake Tekapo to generate its own circulation system, a lake/land breeze. This combines with the larger scale valley wind, which during ideal conditions continues after sunset in the upper reaches of the lake catchment. During light to moderate foehn northwesterly conditions, the combined lake breeze/valley wind system remains decoupled from the prevailing synoptic airstream. Towards evening when local thermal circulations weakened, a channelised foehn airstream often becomes dominant over the entire field area. Observations made during this investigation have a number of applied implications with respect to air pollution dispersion modelling and forecasting within alpine lake basins.With 7 Figures     

Observations and numerical modelling of Lake Ontario breezes

Abstract

Analysis of two years of land‐based data shows that the Lake Ontario breeze develops on 30% of the days during summer. It typically develops in mid‐morning and persists until the late evening when it is replaced by a well developed land‐breeze regime. Simulations of 4 cases with the Colorado State University mesoscale model show good agreement with observations and suggest that local lake breezes are strongly influenced by adjacent water bodies (e.g. Lake Erie), the elongated shape of the lake, and the large‐scale wind direction. With gradient flows across the long axis of the lake, a broad band of along‐lake flow develops during the afternoon (easterly winds during southerly gradient flows and westerly winds during northerly gradient flows). Furthermore, during west‐to‐northwesterly gradient flow a nocturnal cyclonic eddy is predicted at the western end of the lake. These results imply that wind‐field models applied in the vicinity of Lake Ontario should incorporate the entire lake in their modelling domain.     

Numerical simulation of sea breezes over the Auckland region,New Zealand — Air quality implications

The air pollution meteorology of a typical sea breeze day is investigated using the Colorado State University Mcsoscale Model. Results are qualitatively compared with observations and reveal a complex wind field characterised by migratory sea breeze convergence zones. Associated with these features, the model predicts enhanced upward vertical velocities and doming of the planetary boundary layer (PBL). The diurnal variation in PBL depth is shown to vary markedly at different locations and is dependent on position in relation to the migratory convergence zones. These complex spatial and temporal variations in the wind and PBL depth have important implications for air quality in Auckland and confirm that simple Gaussian or box trajectory approaches are inappropriate for air quality assessment in such environments. The inclusion in the model of variable surface properties, a dynamic synoptic state and improved PBL parameterisations, as well as coupling with a Lagrangian particle model, are recommended if the model is to be used as a tool for further air quality studies in the Auckland area.     

The effect of water surface temperature on lake breezes and thermal internal boundary layers

A two-dimensional prognostic numerical model has been used to study a lake breeze event reported by Keen and Lyons (1978). Model predictions showed fair to good agreement with the observations. For the mature lake breeze, the model predicted inflow at the coast within about 1.5 m s^–1 of the observed value, lake breeze depth within 50–90 m of the observed, and inland penetration within about 6 km of the observed. The top of the thermal internal boundary layer (TIBL) was associated with a minimum in the predicted turbulent kinetic energy profile. This may be of consequence for attempts to evaluate pollutant dispersion using numerical models.Predicted lake breeze characteristics showed little sensitivity to temperature of the water surface, except when the water surface temperature was increased to a value exceeding the inland maximum temperature. The most sensitive lake breeze characteristic was the TIBL, which grew more slowly with inland distance and persisted for a greater distance inland as the lake surface became colder.     

Barrier jets around the Southern Alps of New Zealand and their potential to enhance alpine rainfall

We use new data from the SALPEX'96 campaign to examine the evolution of orographically modified flow over and around New Zealand's Southern Alps. We consider prefrontal situations for which the larger-scale synoptic pattern generates an incoming northwesterly flow perpendicular to the mountain axis, calculate the nondimensional parameters which control the wind flow and determine how long it takes orographic modifications to become established. We place flows over the New Zealand Alps in a Rossby number/Blocking number parameter space and compare them with flows over other mountain ranges on the globe. Using model and aircraft data, we show the region of modified flow extends over 100 km upstream of the Southern Alps.We show 2D model simulations represent many aspects of the flow modification satisfactorily, although they tend to overdevelop the final state of the Barrier Jet flow. In 3D, using real data to force the boundaries, the migratory nature of the weather systems in the area would generally limit the time available for a modified steady state to develop. In addition, in situations where there is significant lateral variation of initial fields, the 2D results may only be applicable for a few hours.From the point of view of potential to influence alpine rainfall processes, for the case studied, the gradual uplift associated with the modified flow between 1 and 2 km above sea level extended far enough upstream in saturated air to enable cloud drops to grow to drizzle drops before reaching the much more substantial updrafts at the foot of the Alps. This allows vigorous prefrontal precipitation to occur over the foothills and mountains due largely to warm rain processes.     

Transverse upwelling in a long narrow lake,with applications to Babine Lake and Lake Michigan

Abstract

The wind generation of transverse thermocline motions in an infinitely long two‐layer channel is studied theoretically. Winds both along and across the channel are considered. Horizontal momentum transfer is parameterized by a constant coefficient of eddy viscosity. For a channel narrow compared with its internal Rossby radius of deformation, the transverse motions are uni‐modal and generated most efficiently by long‐shore winds. Theoretical results in this narrow channel limit agree well with observations made at Babine Lake, British Columbia. For a channel wide compared with its Rossby radius, the response is multi‐modal, especially for cross‐channel winds. For a model Lake Michigan, computed interfacial displacements due to a steady wind either across or along the channel are small compared with the observed displacements. However, a semi‐diurnal wind (in either direction) is in near resonance with the seventh transverse mode. Thus multi‐modal displacements as large as those observed could possibly be generated by semi‐diurnal winds across or along the channel.     

Economic and ecosystem costs and benefits of alternative land use and management scenarios in the Lake Rotorua,New Zealand,catchment

A significant challenge in resource management is addressed: the perceived trade-off between economic growth and ecosystem restoration and conservation. Traditional approaches to management regard restoration as a potential cost to economic productivity. In this study we show that by considering a broader range of economic values, including ecosystem services values, an argument can be made that restoration of lake ecosystems also leads to favourable economic outcomes when commonly disregarded values are considered. Our case study analyses the ecological outcomes of different catchment mitigation and land use scenarios in terms of water quality results in a lake, assessing changes in land use values based on opportunity costs, and ecosystem services values. We show that when considering the value of ecosystem services, intensive agricultural land use is not necessarily the most economically valuable form of land use within a lake catchment. Indeed, a shift towards alternative land uses within a catchment can lead to both ongoing economic benefits and improvements in water quality. In this context, land-use change offers an option for water quality improvement that minimises lake and land mitigation costs, while adding value to catchment land use. An argument is made supporting land use change towards indigenous forest types, which can sustain alternative sources of income such as a range of recreational values, while supporting important ecosystem functions for the region.     

Acoustic sounding of land and sea breezes

Sodar measurements have been made at La Spezia, Italy during land- and sea-breeze conditions. The backscatter returns are discussed qualitatively, including their relation to the vertical structure of the boundary layer as revealed by vertical soundings of wind and temperature. During inversion conditions, the sodar signals may be difficult to interpret especially when there is a land breeze flowing over irregular terrain.     

青岛流亭机场海风特征分析及对飞行的影响

通过分析2008—2016年青岛流亭机场(简称青岛机场)各季节地面风向日变化规律,发现有两支海风环流影响青岛机场：一支是西支海风,风向210°～230°,一般上午影响机场,午后发展至最强,下午消失;另一支为南支海风,风向150°～170°,午后影响机场,下午取代西支海风,傍晚发展至最强,夜间逐渐减弱消失。两支海风夏季最为明显,南支海风强度季节性差异强于西支海风。海风对机场飞行的影响主要体现在两支海风引起的三种海风锋型低空风切变,分别为西支海风锋引起的侧风切变、两支海风环流相互作用引起的侧风切变以及南支海风引起的顺风切变,其季节及日变化规律为：夏季出现概率最大,春、秋季次之,冬季几乎不会发生;一天中最可能发生时段分别为08：00—11：00、12：00—15：00和15：00—17：00,具体时段在各季节略有差异。     

Fuzzy logic model of lake water level fluctuations in Lake Van, Turkey

Summary Lake Van is one of the largest terminal lakes in the world. In recent years, significant lake level fluctuations have occurred and can be related to global climatic change. This fluctuation sometimes exhibits abrupt shifts. Floods originating from the lake can cause considerable damage and loss in agriculture and urban areas. Therefore, water level forecasting plays a significant role in planning and design. This study is aimed at predicting future lake levels from past rainfall amounts and water level records. A dynamical change of the lake level is evaluated by the fuzzy approach. The fuzzy inference system has the ability to use fuzzy membership functions that include the uncertainties of the concerned event. This method is applied for Lake Van, in east Turkey. Furthermore, model capabilities are compared with ARMAX model. It is shown that lower absolute errors are obtained with the Takagi-Sugeno fuzzy approach than with the ARMAX model.     

Thermal asymmetry and cross-valley circulation in a small alpine valley

Wind observations from a small Alpine valley are used to investigate the problem of cross-valley winds. The observed daytime windfield is a superposition of dynamically and thermally forced cross-winds. Prevailing cross-winds above the valley result in a recirculation cell above the lee slope. The return flow is strengthened or weakened by thermal effects which induce a wind that blows from the shaded to the sunny side of the valley. The reaction time of the thermally induced cross-winds is only 4 min. The horizontal and vertical motions of the cross-valley circulation transport heat in such a way that the insolation differences between the two sides of the valley are nearly equalized.     

Regional and local scale characteristics of foehn wind events over the South Island of New Zealand

Summary Regional and local scale windfield and air mass characteristics during two distinct synoptic foehn wind events over southern New Zealand are examined. The Southern Alps were observed to effectively block low level onshore gradient northwesterly airflow and to channel it through both Cook and Foveaux Straits. Blocking of the onshore synoptic northwesterly airstream also resulted in barrier jet formation along the western slopes of the Southern Alps. This feature of the regional windfield has not previously been documented and develops during favourable conditions to a height of between 1500 to 1800 m above sea level. In the immediate lee of the Southern Alps at Lake Tekapo, classic foehn conditions such as warm ambient air temperatures, low relative humidities and gusty winds were monitored throughout both foehn events examined. Differences in the local windfield were however observed, which reflect the importance of local topography on lee side windfield dynamics during foehn events. Spillover of precipitation to the lee of the mountains was monitored in the latter stages of each case study and appeared to be associated with the passage of the cold front over the Southern Alps. Observations made by this investigation have a number of applied and theoretical implications with respect to meso-scale modelling, orographic rainfall distribution and forecasting.With 12 Figures     

A case of offshore convective initiation by interacting land breezes near Darwin, Australia

A case of offshore convective initiation by interacting land breezes near Darwin, Australia is investigated using convection-permitting model simulations, radar-derived precipitation observations, thermodynamic profiles from radiosonde soundings and surface measurements. These analyses elucidate the convergence of two land breezes in the Van Diemen Gulf, one originating from the Tiwi Islands and the other from mainland Australia; the convergence is sufficient to initiate a line of convection that forms parallel to the mainland coast in the early morning. While differing in small-scale features, the modeled system shows reasonably good agreement with the observed precipitation accumulations. However, using simulations with different initialization times and examining a second case, it is shown that the representation of the land-breeze system and subsequent convective initiation is very sensitive to the upstream wind and thermodynamic conditions, making correct simulation of these processes challenging.     

Observations of foehn onset in the Southern Alps, New Zealand

Summary Local scale windfield and air mass characteristics during the onset of two foehn wind events in an alpine hydro-catchment are presented. Grounding of the topographically modified foehn was found to be dependent on daytime surface heating and topographic channelling of flow. The foehn front was observed to advance down-valley until the valley widened significantly. The foehn wind appeared to decouple from the surface downstream of the accelerated flow associated with the valley constriction, and to be lifted above local thermally generated circulations including a lake breeze. Towards evening, the foehn front retreated up valley in response to reduced surface heating and the intrusion into the study area of a deep and cool air mass associated with a regional scale mountain-plain circulation. Differences in the local windfield observed during both case study events reflect the importance of different thermal and dynamic forcings on airflow in complex terrain. These are the result of variation in surface energy exchanges, channelling and blocking of airflow. Observations presented here have both theoretical and applied implications with regard to forecasting foehn onset, wind hazard management, recreational activities and air quality management in alpine settings. Received January 23, 2001 Revised October 17, 2001     

Numerical simulation of sea and bay breezes in a weak shear environment

Summary A numerical mesoscale model (COAMPS) is used to study some of the features associated with the evolution of the kinematic, thermodynamic, and physical structure of the Alabama sea and bay breeze circulations and convections in weak shear environments based on five cases from Medlin and Croft (1998). The general and expected features and evolution of sea and bay breeze circulations are captured by the model simulations, including horizontal and vertical wind shifts, thermal contrast between land and water surface, vertical stability over water and land, return currents and moisture increase. The relationship of the circulations to specific synoptic flow regimes and local physiographic features was investigated. The sea breeze triggered convective cells are confirmed to have a preferred location according to the flow regime and local conditions. This result can assist the forecasters in understanding the anticipated convective cell initiation and development on a given day as related to sea and bay breeze cells as well as improve the short-term forecast accuracy of the location of thunderstorm initiation based on routine observations and subsequent convective activity. If local NWS office model a selective subset of cases then they can better visualize and forecast those cases operationally.     

A study of interacting multi-scale wind systems, Canterbury plains, New Zealand

Summary The wind regime of the Canterbury region, New Zealand, is composed of several interacting multi-scale wind systems all of which show strong diurnal periodicity. The dynamic orographic effect of the Southern Alps on the prevailing westerly flow results in perturbations to the pressure field and localized antitriptic airflow. Superimposed on this larger scale process are thermotopographic effects resulting from both regional and local land-sea thermal contrasts and slope heating. These processes act within an hierarchy of scales to produce a complex wind regime characterized by marked temporal variability, a layered vertical structure and the frequent occurrence of convergence lines and shear zones. The synergistic nature of the forcing mechanisms and the tendency for nocturnal decoupling of the boundary layer due to stability variations makes it difficult to differentiate and label discrete wind components.Attempts to simulate this regime using the Colorado State University mesoscale model showed that the model was unable to adequately resolve both the dynamic orographic effect and the local thermotopographic effect because of their differing scales of influence. These results suggest that a more holistic approach to both empirical and theoretical studies in such environments is required if more accurate wind field forecasting is to be achieved.

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Eine Studie zusammenwirkender Windsysteme verschiedener Größenordnungen, Canterbury Plains, Neuseeland

Zusammenfassung Das Windregime des Gebiets von Canterbury, Neuseeland, setzt sich aus verschiedenen zusammenwirkenden Windsystemen verschiedener Größenordnungen zusammen, die alle einen starken Tagesgang aufweisen. Der dynamisch-orographische Effekt der neuseeländischen Alpen auf die vorherrschende Westströmung führt zu Störungen im Druckfeld und lokalen Luftbewegungen im Lee. Diesem großräumigen Prozeß sind thermisch-topographische Effekte überlagert, die sowohl durch regionale als auch lokale thermische Unterschiede zwischen Land und Meer und die Erwärmung der Hangregion hervorgerufen werden. Die Vorgänge spielen sich in einer Hierarchie von Größenordnungen ab. Sie erzeugen ein kompliziertes Windsystem, das durch hohe zeitliche Variabilität, eine schichtweise thermische Struktur und häufige Konvergenz- und Scherungszonen gekennzeichnet ist. Die synergetische Natur der Antriebe und die Tendenz zum nächtlichen Entkoppeln der planetaren Grenzschicht aufgrund von Stabilitätsschwankungen macht es schwer, die einzelnen Windkomponenten zu trennen und zuzuordnen.Die Versuche, dieses Regime mit Hilfe des Mesoscale-Modells der Colorado State University zu simulieren, zeigten, daß es aufgrund der verschiedenen Größenordnungen des Einflusses nicht geeignet war, gleichzeitig den dynamisch-orographischen und den thermo-topographischen Effekt zu reproduzieren. Diese Ergebnisse legen sowohl für empirische wie für theoretische Untersuchungen einen holistischeren Ansatz nahe, um eine genauere Prognose des Windfeldes zu ermöglichen.

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With 11 Figures     

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

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

太湖及与湖岸城市间大气颗粒物分布特征分析

利用冬季连续2 d在两种气象条件下分别环绕太湖对湖泊近无锡区域、湖心区域、近西山岛区域和近苏州区域近湖层的PM0.5、PM2.5和PM10进行质量浓度观测,并通过分析同期相应气象资料(包括高空和地面风场、流场以及湖面站点的水平垂直风向风速等气象数据),探讨湖泊区域内外颗粒物浓度变化的时空特征以及受气象条件变化的影响。研究表明:第1天当天气为晴转阴且无雾时,由于湖面本身缺乏密集强排放源,以及因湖面大气上升运动较强使颗粒物容易被扩散稀释,且受偏东转东南气流所带来的气团较干净影响,湖面湖心区域和离岛区域颗粒物浓度明显低于上游城市;而在第2天当天气为雾时情况则相反,由于湖面大气较弱的上升运动使得大气颗粒物不容易被扩散稀释,且湖泊强大的水汽源影响有助于气溶胶和颗粒物的生成进而使得湖面颗粒物浓度大于周围城市。此外还发现在两种天气下,颗粒物浓度都存在无锡区域最高,湖心区域次之,苏州区域较低的特征。并且,在雾天不同粒径颗粒物间变化的相关系数更高。