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1.
The daytime boundary-layer heating process and the air-land heat budget were investigated over the coastal sea-breeze region by means of observations over the Sendai plain in Japan during the summer. In this area, the onset of the sea breeze begins at the coast around 0900 LST, intruding about 35 km inland by late afternoon. The cold sea breeze creates a temperature difference of over 10°C between the coastal and inland areas in the afternoon. On the other hand, warm air advection due to the combination of the counter-sea breeze and land-to-sea synoptic wind occurs in the layer above the cold sea breeze in the coastal region. Owing to this local warm air advection, there is no significant difference in the daytime heating rate over the entire atmospheric boundary layer between the coastal and inland areas. The sensible heat flux from the land surface gradually decreases as distance from the coastline increases, being mainly attributed to the cold sea breeze. The daytime mean cold air advection due to the sea breeze is estimated asQ adv local =–29 W m–2 averaged over the sea breeze region (035 km from the coastline). This value is 17% of the surface sensible heat fluxH over the same region. The results of a two-dimensional numerical model show that the value ofQ adv local /H is strongly affected by the upper-level synoptic wind direction. The absolute value ofQ adv local /H becomes smaller when the synoptic wind has the opposite direction of the sea breeze. This condition occurred during the observations used in the present study.  相似文献   

2.
Sonic anemometer measurements are analyzed from two primary field programs and 12 supplementary sites to examine the behaviour of the turbulent heat flux near the surface with high wind speeds in the nocturnal boundary layer. On average, large downward heat flux is found for high wind speeds for most of the sites where some stratification is maintained in spite of relatively intense vertical mixing. The stratification for high wind speeds is found to be dependent on wind direction, suggesting the importance of warm-air advection, even for locally homogenous sites. Warm-air advection is also inferred from a large imbalance of the heat budget of the air for strong winds. Shortcomings of our study are noted.  相似文献   

3.
Summary The modification of the flow structure arising from the removal of large patches of trees in a managed forest plantation near Gainesville, Florida is described. Using wavelet analysis of turbulence measurements taken above a forest canopy hundreds of meters downwind from the forest gap and well outside the footprint, the present paper examines changes in flow characteristics and demonstrates that the presence of the nearby clearcut introduces extraneous coherent events passing by the eddy-covariance flux measurement system.  相似文献   

4.
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  相似文献   

5.
The urbanization of Daegu metropolitan region has progressed rapidly over the last 100 years, but changing land-use resulting from that urbanization has also caused the regional circulations to evolve. The effect of 100 years of urbanization on the regional circulations was simulated for the analysis using an A2C (Atmosphere to Computational Fluid Dynamics) model. Calm synoptic conditions and a stable lapse rate were assumed for the initial model atmosphere, with a horizontal grid resolution of 1 km. The simulation demonstrated that land-use changes have affected the Daegu metropolitan area’s regional circulations by deepening the convective boundary layer (CBL) over the urbanized area, resulting in deviations from mean surface temperature and surface wind fields. The last 100 years have seen the following changes in the region: 1) the urbanized area has increased by around 15 times, while the number of forests and paddy fields has rapidly decreased; 2) surface wind speeds have decreased by about 25% and 35% during the day and at night, respectively, and severe changes in wind direction have been noted over the urbanized areas; 3) regional warming has been around 1.5°C and 4°C in the daytime and early morning hours, respectively (in particular, warming in the early morning hours suggests the presence of overnight heat islands), and 4) sensible heat flux and CBL height have increased by about 40 W m?2 and 500 m for the period, respectively.  相似文献   

6.
为进一步了解平流海雾的形成、发展和消散过程特点和机制,利用风廓线雷达、自动观测、探空、数值产品以及常规气象观测等资料,分析了2014年2月17—18日珠海三灶机场一次平流海雾过程的边界层温湿风三维演变特征及天气学成因。结果表明:边界层内暖湿平流输入、浅层辐合抬升、湍流加强、多层逆温结构以及夜间地面的辐射冷却有利于近地层饱和湿空气的凝结和逆温层的维持,是雾形成的主要物理机制;暖湿平流持续及湍流减弱或停歇是雾维持的原因;近地层偏北风干冷平流入侵并出现下沉气流及日间地面辐射增温使逆温层被破坏是雾消散的主要因素。  相似文献   

7.
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.  相似文献   

8.
The flow structure at the intersection between the Rhine and the Seez valleys nearthe Swiss city of Bad Ragaz has been documented by means of wind and pressuremeasurements collected from 9 September to 10 November 1999 during the MesoscaleAlpine Programme (MAP) experiment. To understand better the dynamics of theageostrophic winds that develop in this part of the Rhine valley, some key questionsare answered in this paper including the following: (i) How does air blow at theintersection of the Rhine and Seez valleys? and (ii) what are the dynamical processes(mechanical or thermal) driving the flow circulations in the valleys? Statistical analysis of the wind and pressure patterns at synoptic scale and at the scaleof the valley shows that five main flow patterns, SE/S, NW/W, NW/N, NW/S, SE/N(wind direction in the Seez valley/wind direction in the Rhine valley) prevail. The SE/S regime is the flow splitting situation. It is mainly driven by a strong pressure gradient across the Alps leading to foehn, even though some nocturnal cases are generated bylocal thermal gradients. The NW/W and NW/N regimes are mechanically forced bythe synoptic pressure gradient (as the flow splitting case). The difference between thetwo regimes is due to the synoptic flow direction [westerly (northerly) synoptic flowfor the NW/W (NW/N) regime], showing that the Rhine valley (particularly from BadRagaz to Lake Constance) is less efficient in channelling the flow than the Seez valley.The NW/S (occurring mainly during nighttime) and SE/N (occurring mainly duringdaytime) regimes are mainly katabatic flows. However, the SE/N regime is also partlyforced at the synoptic scale during the foehn case that occurred between 18 October and 20 October 1999, with a complex layered vertical structure. This analysis also shows that, contrary to what was observed in a broad section of theupper Rhine valley near Mannheim, very few countercurrents were observed near BadRagaz where the valley width is much smaller.  相似文献   

9.
利用2018年1月1日至12月31日在北京国家综合气象观测实验基地获得的风廓线雷达资料和同时期在河北香河的华北香河全大气层野外科学观测研究站获得的多普勒声雷达资料,比较分析北京城区和远郊区的低层(0～600 m)大气风场特征.结果 表明:水平风速随高度增加而增大,同一高度层,远郊区的平均水平风速大于城区,且受湍流活动影...  相似文献   

10.
Turbulence characteristics in the surface layer of the coastal area of Akhtopol (Bulgaria) under conditions of the breeze circulation are measured. The measurements were carried out at the level of 4.5 m by means of three-component ultrasonic anemometer. To estimate the wind regime in the atmospheric boundary layer, the sodar data and synoptic charts were used. All turbulent characteristics except the correlation coefficient of the friction flow have an appreciable daily course in the case of the sea breeze. In the frontal zone, some characteristics demonstrate sometimes short-term variations of their numerical values. The sea-land front of the breeze circulation is effectively detected from the measurements of wind speed, wind direction, and temperature in the surface layer. It is also possible to judge about the breeze circulation type and turbulence structure in this layer on the basis of these measurements.  相似文献   

11.
A complete yearly record (1988) of surface measurements is used to examine the atmospheric diurnal secondary circulations over the entire area of Hong Kong in conjunction with spatial and temporal variations of surface temperature, wind speed and rainfall. Evidence of atmospheric diurnal secondary circulations is found at 10 sites. The occurrence of a summer morning rainfall maximum over the coast results from the interaction of the large-scale summer monsoon and local mesoscale secondary circulations. The afternoon onshore secondary circulation accelerates the advection of warm, humid unstable air and, coupled with the upward orographic lifting, produces enhanced rainfall along windward mountain ridges.Dynamical and scaling considerations suggest that the blocking effect is negligible and the primary forcing mechanism is land-sea temperature difference, but terrain effects are also important. Although the secondary circulation system's strength and timing vary, the circulation behaves like a classic sea-land breeze circulation, complicated by superimposed mountain-valley breezes.  相似文献   

12.
In mountainous lake areas, lake–land and mountain–valley breezes interact with each other, leading to an "extended lake breeze". These extended lake breezes can regulate and control energy and carbon cycles at different scales. Based on meteorological and turbulent fluxes data from an eddy covariance observation site at Erhai Lake in the Dali Basin,southwest China, characteristics of daytime and nighttime extended lake breezes and their impacts on energy and carbon dioxide exchange in 2015 are investigated. Lake breezes dominate during the daytime while, due to different prevailing circulations at night, there are two types of nighttime breezes. The mountain breeze from the Cangshan Mountain range leads to N1 type nighttime breeze events. When a cyclonic circulation forms and maintains in the southern part of Erhai Lake at night, its northern branch contributes to the formation of N2 type nighttime breeze events. The prevailing wind directions for daytime, N1, and N2 breeze events are southeast, west, and southeast, respectively. Daytime breeze events are more intense than N1 events and weaker than N2 events. During daytime breeze events, the lake breeze decreases the sensible heat flux(Hs) and carbon dioxide flux(FCO_2) and increases the latent heat flux(LE). During N1 breeze events, the mountain breeze decreases Hs and LE and increases FCO_2. For N2 breeze events, the southeast wind from the lake surface increases Hs and LE and decreases suppress carbon dioxide exchange.  相似文献   

13.
Large-eddy simulation is used to study secondary circulations in the convective boundary layer modulated as a result of horizontally varying surface properties and surface heat fluxes over flat terrain. The presence of heat flux heterogeneity and its alignment with respect to geostrophic wind influences the formation, strength and orientation of organized thermals. Results show boundary-attached roll formation along heat flux maxima in the streamwise direction. The streamwise organization of the updrafts and downdrafts formed downwind of heterogeneities leads to counter-rotating secondary circulations in the crosswind plane. The distribution of resolved-scale pressure deviations shows large pressure gradients in the crosswind plane. Spanwise and vertical velocity variances and heat flux profiles depict considerable spatial variability compared to a homogeneous forest simulation. Secondary circulations are observed for various ambient wind scenarios parallel and perpendicular to heterogeneities. In the presence of increased wind speed, thermals emerging from the heat flux heterogeneity are elongated, and organize along and downwind of large-scale heterogeneity in the streamwise direction. Simulation with a reduced heat flux shows a shallower circulation with a lower aspect ratio. Point measurements of heat flux inside the roll circulation could be overestimated by up to 15–25% compared to a homogeneous case.  相似文献   

14.
Convective plume patterns, characteristic of clear sky and light wind daytime boundary layers over land, were observed for two nights with a tri-axial Doppler sodar operated in the central area of Rome during the summer of 1994. An urban heat island effect, combined with a continuation of a breeze from the sea late into night during both days, is believed to be responsible for the observed nocturnal convection. Estimates of the surface heat flux and the vertical velocity scaling parameter are obtained from profiles of vertical velocity variance, and the Raman lidar water vapor measurements are used to obtain the humidity scaling parameter. Convective scaling results for vertical wind and humidity fairly agree with the results of other experiments and models. On the basis of available measurements, it appears that mixed-layer similarity formulations used to characterize the daytime convective boundary layer over horizontally homogeneous surfaces can also be applied to the nocturnal urban boundary layer during periods of reasonable convective activity.  相似文献   

15.
Summary Remote sensing and ground measurements were used to evaluate the homogeneity of the Scots pine plantation of Hartheim and the interactions between the forest and its surroundings during HartX.Remote sensing data contribute to characterization of the Scots pine plantation of Hartheim in terms of surface properties and thermal and biological characteristics of the forest. The surroundings of the main experiment site in the forest has homogenous normalized difference vegetation index, microwave specle and brightness temperatures to all directions for at least 500 m. Local sensible heat flux estimates using satellite measured brightness temperatures andin situ measured wind and air temperatures showed good agreement.On some days during HartX the test area was influenced by advection of dry air from the northern Upper Rhine Valley. Agricultural surroundings close to the forest are influenced by breezes flowing out of the forest during radiative nights with weak large-scale motion.With 7 Figures  相似文献   

16.
介绍了国家自然科学基金重点项目 "地表通量参数化与大气边界层过程的基础研究" 在河北省白洋淀地区进行的两次综合观测实验(时间分别是2004年11月16～22日和2005年9月8～27日),这两次实验获得了大量宝贵的资料,全面深入的资料分析正在进行中.此文主要限于白洋淀地区水陆不均匀地表近地面层微气象特征的分析.结果表明: 陆地上近地面层的气温日变化比水域上的大,而风速比水域上的小,其中9月份陆地上白天的气温比水域上的高,夜间比水域上的低,11月份两地白天气温接近,但夜间陆地上的气温明显偏低; 无论是9月还是11月,水域上近地面层都是以下沉气流为主,而陆地则在中午前后存在弱的上升运动; 9月份,两地的近地面层短波射入辐射比较接近,但水域上的短波射出辐射比陆地上的大; 长波射入辐射则是陆地比水域的大,而长波射出辐射则是水域的比陆地的大; 两地的净辐射白天接近,夜间水域地区负的净辐射值明显比陆地的大; 9月份,水陆两地的感热通量相差不大,为150～200 W/m2,而潜热通量比感热通量大,天气晴朗时可达到300 W/m2; 11月份,陆地的感热通量比潜热通量大,为100～120 W/m2,而水域地区的感热通量则与潜热通量相当; 两地的潜热通量相差不大,一般不超过50 W/m2.无论是9月还是11月,水域地区夜间都存在水汽向下输送的逆湿现象,而陆地只在9月份存在逆湿现象.2005年9月份的水温观测结果表明,白洋淀水体平均温度比水表平均温度明显偏高,而且日变化幅度很小,水表平均温度则日变化较大.  相似文献   

17.
In order to characterise the local low-level circulation in the Tyrrhenian Sea coastal area near Rome, the wind field observed by conventional anemometers, Doppler sodar, and rawinsonde has been analysed. The prevailing diurnal behaviour of wind speed and direction as a function of season was highlighted, and the existence of two different patterns of the local circulation, mainly due to land and sea breezes and to the drainage flow from the mouth of the Tiber valley, revealed. The comparison between the low-level circulation and synoptic flow allowed us to determine the influence of the large-scale flow on nocturnal currents that are observed at the Pratica di Mare site and the way that wind direction evolves during the day. Numerical simulations are consistent with experimental data and depict the main features of the low-level wind field in the area.  相似文献   

18.
Ultrasonic wind measurements, sonic temperature and air temperature data at two heights in the advection experiment MORE II were used to establish a complete budget of sensible heat including vertical advection, horizontal advection and horizontal turbulent flux divergence. MORE II took place at the long-term Carbo-Europe IP site in Tharandt, Germany. During the growing period of 2003 three additional towers were established to measure all relevant parameters for an estimation of advective fluxes, primarily of CO2. Additionally, in relation to other advection experiments, a calculation of the horizontal turbulent flux divergence is proposed and the relation of this flux to atmospheric stability and friction velocity is discussed. In order to obtain a complete budget, different scaling heights for horizontal advection and horizontal turbulent flux divergence are tested. It is shown that neglecting advective fluxes may lead to incorrect results. If advective fluxes are taken into account, the sensible heat budget based upon vertical turbulent flux and storage change only, is reduced by approximately 30%. Additional consideration of horizontal turbulent flux divergence would in turn add 5–10% to this sum (i.e., the sum of vertical turbulent flux plus storage change plus horizontal and vertical advection). In comparison with available energy horizontal advection is important at night whilst horizontal turbulent flux divergence is rather insignificant. Obviously, advective fluxes typically improve poor nighttime energy budget closure and might change ecosystem respiration fluxes considerably.  相似文献   

19.
上海世博园上空边界层风垂直变化观测研究   总被引：5，自引：0，他引：5       下载免费PDF全文
利用平矩阵风廓线雷达于2007年7月17日至9月28日对上海世博园规划区上空的风垂直分布进行了观测.通过分析该地上空三维风场的日变化,发现夜间以偏西气流为主,白天风速较小,以偏东气流为主,表明该地区以海陆风为主导的环流特征是这一地区的局地环流日变化的基本特征.逐日变化分析表明,在8月2日以前主要以偏西北气流为主,之后基本是以东南气流为主,并且垂直运动特征以上升气流为主,强烈的垂直运动与偏西气流相关密切,同时偏北气流往往带来较强的上升运动.日夜平均廓线分析表明,夜间风速较大,并且夜间风速的垂直变化与白天相比也有很大不同,白天270 m以下风速随高度基本不变,而夜间从近地面向上风速随高度逐渐增大,低层<90 m的范围内白天风速大于夜间风速.城市冠层以上风向的日夜变化不明显,多为偏北气流控制.城郊风廓线的对比表明受城市下垫面粗糙度的影响,城市风速明显比郊区减小.城市和郊区的水平风速变化在城市冠层以上比较接近,相关系数达到了87%.  相似文献   

20.
In this study the results from a boundary layer experiment,conducted in autumn 1991 over a flat,build-up urban area in Southeast Sofia,together with some models for mixed layer growth rates are used to investigate the layered structure of the vertical atmospheric stability distribution in the Sofia Valley.Lidar measurements of aerosol layer heights and morning boundary layer development are combined with surface eddy correlation measurements of kinematic heat and moisture fluxes,profiles of temperature and humidity,wind speed and wind direction.A diagnostic method is presented for determining vertical lapse rates using surface meteorological measurements and lidar returns observed during the transition from nighttime stable stratification to daytime convective boundary layer after the sunrise.  相似文献   

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