Heating of a small Alpine valley

Summary Two- and three-dimensional fields of meteorological variables are constructed from measurements in a small Alpine end valley during daytime of two fine weather days. These are fields of the surface energy balance, of temperature and wind. With these data the energy budget of the valley atmosphere is calculated. In the morning, the sensible heat which is released on the valley surface is used to heat the valley atmosphere. The advection terms are rather small. In the afternoon, the energy gain is nearly completely exported out of the valley region. The transport is managed by advection with the valley wind and the related upward motion, which are branches of the valley wind system which overlap into the next larger valley. The role of a small valley in the interplay of the thermal circulation systems in the heating of the mountainous boundary layer is discussed.

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Erwärmung eines kleinen Alpentals

Zusammenfassung Messungen in einem kleinen alpinen Endtal gestatten die Berechnung zwei- und dreidimensionaler Felder meteorologischer Größen tagsüber an zwei Schönwettertagen. Dies sind Felder der Terme der Energiebilanz am Untergrund, der Temperatur und des Windes. Daraus läßt sich der Energiehaushalt der Talatmosphäre berechnen. Am Vormittag wird die am Untergrund freigesetzte fühlbare Wärme zur Erwärmung des Tals genutzt, die Advektionsterme sind ziemlich klein. Am Nachmittag wird dieser Wärmegewinn nahezu vollständig aus dem Talbereich exportiert. Dieser Transport wird durch die Advektion mit dem Talwind und dem resultierenden Aufwind bewerkstelligt, die Zweige eines ins nächste Tal übergreifenden Talwindsystems sind. Die Rolle eines kleinen Alpentals im Zusammenspiel der verschiedenen thermischen Zirkulationssysteme bei der Erwärmung der Gebirgsgrenzschicht wird diskutiert.

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With 6 Figures     

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     

Thermal structure and circulation in the great lakes

Abstract

Large enough to include many oceanic phenomena, the Laurentian Great Lakes are more accurately described as inland seas. With the exception of the shallow Western Basin of Lake Erie, the lakes are thermally stratified in summer, homogeneous in winter, with average temperatures passing through the temperature of maximum density of fresh water (4°C) in both the spring and the fall. The circulation is mainly powered by the wind but is strongly modified by thermal stratification and basin geometry. Effects of the earth's rotation are present in all large‐scale flows. Current speeds are typically 10 cm s^?1; they are too small, with rare exceptions, to present difficulties to navigation but a knowledge of the patterns of water movement is essential for interpreting the behaviour of these valuable lakes as complex ecosystems. This paper will review more than a century of physical study of the Great Lakes.     

Characteristics and evolution of atmospheric circulation patterns during Meiyu over the Jianghuai valley

This study focuses on the evolution of large-scale circulations before and after the beginning of the Meiyu and analyzes the formations of the typical vertical circulation pattern associated with Meiyu and its relationship with the Meiyu rainband. Results show that the typical vertical circulation pattern during the Meiyu season is characterized by the ??two-leg??-type pattern in the vorticity field, a typhoon eye-like structure of the equivalent potential temperature field, as well as the sharp gradient of the equivalent potential temperature zone, i.e., the Meiyu front are typically presented in the Meiyu season. Tracking its evolution process, we find that the typical vertical circulation pattern is built at late March and early April along with the rainband locating at the area south to the Yangtze River. This typical pattern and the rainband both advance northward affecting Jianghuai valley since the beginning of Meiyu. Moreover, the typical vertical circulation pattern derived from Meiyu season has been formed in April and corresponds to the reverse of the land-sea thermal contrast between the Eastern Asia and western Pacific Ocean, demonstrating the close relationship of the movement between the rainband and the march of the East Asian subtropical summer monsoon.     

Numerical simulation of pollutant dispersion in a small valley under conditions with supercritical Richardson numbers

The numerical drainage wind model of Wonget al. (1987) is used together with a Lagrangian particle model in the simulation of carbon monoxide (CO) dispersion within a small urban valley in Edmonton, Alberta, Canada. The conditions studied are those of strong static stability when vertical mixing is suppressed. These are conditions with the Richardson number exceeding its critical value (hereafter referred to as supercritical conditions). Observations showed that under such conditions, vertical turbulence is suppressed but horizontal turbulence still exists. The effects of turbulence in the dispersion and transport of pollutants under such conditions are small. However, in the present simulation, a simple turbulence parameterization based on observations is used for supercritical conditions. Some field experiments were performed and the observations are compared with model results. For a location downwind of the CO source, two peaks can be observed during the course of the drainage flow regime. The model results suggest that these represent an initial flux from the drainage flow and a second flux later from drainage wind recirculation. Another main feature of the model-predicted concentration field is zones of maximum concentration at and above the valley floor. There is a drainage wind cell on each side of the valley slope and the cells are effectively decoupled from the prevailing wind above. The present modelling results show that when the prevailing wind exists before the development of the drainage wind, it can be instrumental in transporting CO from one drainage wind cell to the other. Otherwise, the CO released within one drainage wind cell is well contained.     

夏季淮河流域大气环流型在降水趋势预测中的应用

采用NCEP/NCAR逐日海平面气压场资料,利用Lamb-Jenkinson大气环流分型方法对淮河流域夏季环流进行分型,并利用逐步回归方法建立了淮河流域夏季降水趋势的预测模型。结果表明:影响夏季淮河流域的主要环流型有南风型、西南风型、东南风型及气旋性环流,这些环流型都具有显著的年际和年代际变化特征,利用大气环流型建立的夏季淮河流域降水趋势预测模型具有一定的预报能力。     

Relationships between atmospheric circulation patterns and co2 greenhouse-gas concentration levels in the alpine troposphere

Middle tropospheric CO₂ air concentrations, measured during a four year observation period at an Alpine Station (Plateau Rosa, Italian North-West Alps, 3480 m a.s.l.), have been correlated with the relevant synoptic air trajectories crossing the observation site. Meaningful relationships have been found among average curvature, altitude and potential temperature of clusters of homogeneous trajectories and their related CO₂ concentrations measured at Plateau Rosà, allowing an objective identification of weather conditions giving rise to fully mixed air masses corresponding to background atmospheric CO₂ levels. Air trajectories were calculated by using the wind speed fields provided by the ECMWF objective analysis. As during the analysis period a change of the ECMWF model resolution occurred (from T106 to T213), the study was also carried out in the sub-periods respectively preceding and following this change (in September 1991). Even if some features of the wind field turned out to be statistically different in the two sub-periods, nevertheless the differences on the trajectory patterns were small enough to keep almost unchanged all conclusions drawn for the whole four-year period.     

海风与谷风环流对万宁气候的影响

利用海陆风和山谷风的机制和特点来分析万宁温度、降水分布及变化原因。     

江淮夏季降水异常与西印度洋地区大气环流异常的关系

运用NCEP/NCAR再分析资料和中国气候中心整编的160站月平均降水资料应用经验正交函数、线性相关分析等,分析了江淮地区夏季降水异常特征及其与西印度洋区域大气环流年际异常关系的变化及其可能的机理。结果表明:当500 hPa中纬度低槽活动偏多(少),西太平洋副热带高压偏强(弱),东亚夏季风偏强(弱)时,江淮地区降水偏多(少)。进一步分析还发现西印度洋上空的垂直环流与江淮夏季降水存在较好的关系,但这种年际异常之间的联系受到背景场的影响明显:1979—1993年西印度洋垂直上升运动与江淮夏季降水的变化趋势基本相反,两者线性相关系数为-0.43;1994—2010年两者的变化趋势基本一致,相关系数达0.71。即当西印度洋地区存在环流异常下沉(上升)时,西太平洋副热带高压通常异常减弱东退(增强西伸),副热带季风减弱(增强),有利于雨带偏南(北)。因此,在西太平洋副热带高压和副热带季风年代际偏强(弱)阶段,西印度洋环流与江淮夏季降水呈负(正)相关。     

Water-tank experiment on the thermal circulation induced by the bottom heating in an asymmetric valley

Water tank experiments were carried out to investigate the thermal convection due to the bottom heating in an asymmetrical valley under neutral and stably stratified approach flows with the Particle Image Velometry (PIV) visualization technique. In the neutral stratification approach flow, the ascending draft induced by bottom heating is mainly located in the center of the valley in calm ambient wind. However~with ambient wind flow, the thermal convection is shifted leeward, and the descending draft is located on the leeward side of the valley, while the ascending draft is located on the windward side. The descending draft is minorly turbulent and organized, while the ascending draft is highly turbulent. With the increase of the towing speed, the descending and ascending drafts induced by the mechanical elevation begin to play a more dominant role in the valley flow, while the role of the thermal convection in the valley airflow becomes limited. In the stable stratification approach flow, the thermal convection is limited by the stable stratification and no distinct circulation is formed in calm ambient wind. With ambient wind, agravity wave appears in the upper layer in the valley. With the increase of the ambient wind speed, a gravity wave plays an important role in the valley flow, and the location and intensity of the thermal convection are also modulated by the gravity internal waves. The thermal convection has difficulty penetrating the upper stable layer. Its exchange is limited between the air in the upper layer and that in the lower layer in the valley, and it is adverse to the diffusion of pollutants in the valley.     

淮河流域持续性强降水过程的环流变化特征

利用淮河流域4省20：00-次日20：00逐日降水量资料,界定了1961-2006年淮河流域持续性强降水过程,并利用NCEP资料,应用合成分析方法对强降水过程和前期环流特征进行了研究。结果表明,从持续性强降水开始之前到强降水过程中乌拉尔山附近阻高减弱,贝加尔湖以北到鄂霍次克海附近的阻高偏强;西太平洋副热带高压在500hPa上表现为西伸北抬,但在850hPa上西伸明显,基本无北抬;南亚高压范围明显扩大,尤其是淮河流域以北地区200hPa高度明显增大,在中国东部到日本上空200hPa急流中心也有明显变化。850hPa西南急流的建立与西太平洋副热带高压的西伸有关,同时乌拉尔山附近高压减弱,冷空气南下,东西气压梯度增大也有利于西南急流的建立。     

雅鲁藏布江河谷盛夏降水年际变化及其与环流的联系

基于近57 a （1961—2017年）西藏雅鲁藏布江中游河谷地区（简称雅江河谷）4个站（拉萨、日喀则、泽当和江孜）盛夏（7—8月）月平均降水和同期NCEP/NCAR再分析资料,采用合成、相关分析等统计诊断方法,分析了雅江河谷盛夏降水的年际变化特征及其与大气环流的联系。结果表明：1）近57 a雅江河谷盛夏降水无显著线性趋势,降水主要以3～4 a显著周期的年际振荡为主。2）雅江河谷盛夏降水年际波动与区域内水汽收支的变化直接相关,其中印度半岛-东南亚异常反气旋引起的水汽输送通量和水汽在高原腹地辐合上升的动力过程是盛夏降水年际变化的主要原因。3）对流层中低层印度半岛-东南亚异常反气旋环流是该地区盛夏降水年际异常的重要水汽输送通道,该通道将西太平洋、南海和孟加拉湾等地水汽不断输送到高原,期间西太副高和伊朗高压等大尺度系统异常对水汽输送过程起到了重要作用,同时高原盛夏季风低压和南亚高压异常给水汽在高原腹地辐合抬升提供了动力条件。     

Nighttime temperature field in a valley

Summary An analysis of the nighttime near-surface temperature field at 27 stations in a deep mountain valley has been carried out for 5 nights during which drainage winds developed. It was found that a simple exponential function of time and the pressure level of each station accurately represents the temperature evolution during the night. Of particular interest is the fact that above a particular pressure elevation the temperature increases during the night.

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Nächtliches Temperaturfeld in einem Tal

Zusammenfassung Es wurde eine Analyse des nächdichen oberflächennahen Temperaturfeldes an 27 Stationen in einem tiefen Gebirgstal für funf Nächte, in denen Bergwind auftrat, durchgeführt. Es stellte sich heraus, da\ die Temperaturentwicklung während der Nacht exakt durch eine einfache Exponentialfunktion der Zeit und des Druckniveaus jeder Station beschrieben wird. Von besonderem Interesse ist die Tatsache, da\ die Temperatur während der Nacht über einem bestimmten Druckniveau ansteigt.

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With 3 Figures     

Thermal and dynamic characteristics of alpine lake breezes,Lake Tekapo,New Zealand

Thermodynamic characteristics and temporal variation of alpine lake breezes in the eastern Southern Alps are examined. Research was conducted in a large glacially excavated basin dominated by an 87 square kilometre melt-water lake as part of a study of windblown dust dispersion. The surrounding mountain ranges were found to shelter the lake basin from most synoptic winds, thereby allowing local and regional thermally generated circulations to develop to ridge height, approximately 1300m above the surrounding landscape. During favourable synoptic conditions the local lake breeze becomes embedded within the regional valley wind forming an extended lake breeze. Tethersonde flights during these conditions made using a kite based sounding system identified both stable internal (SIBL) and thermal internal boundary layer (TIBL) conditions over the down wind shoreline. Two equations for estimating the height of both boundary-layer types were tested against observations and found to provide good first order predictive estimates of boundary-layer height.     

Results from the MERKUR experiment: Mass budget and vertical motions in a large valley during mountain and valley wind

Summary Based on measurements made in March 1982 in the Inn valley during the MERKUR experiment, an attempt was made to compute the mass budget of a large alpine valley during periods of mountain and valley wind.The computations come from measurements of the alongvalley mass flux and assumptions on the fluxes in the slope layer and tributaries.Vertical motions in the valley's atmosphere have been evaluated from the mass budget computations. These motions, including the subsidence which compensates for daytime upslope winds and the subsidence which compensates for the valley wind flowing into tributaries during the day, are of great importance for the understanding of the thermal circulation.The results allow better estimation of vertical advection, which contributes to the budgets of momentum and energy.With 8 Figures     

春季长江中下游旱涝的环流特征及对前期海温异常的响应

春季长江中下游降水有显着的年际、年代际变化特征,进入21世纪以来长江中下游春季降水偏少现象频繁发生.根据中国国家气候中心160站月平均降水资料和美国国家环境预报中心/国家大气研究中心(NCEP/NCAR)月平均再分析资料,重点探讨春季(3-5月)长江中下游地区降水异常的环流特征、可能成因、机理以及对外强迫的响应.春季长江中下游降水异常偏多(少)的环流主要特征是:高层200hPa风场上东亚副热带西风急流中心位置比气候态偏北(南);中层500hPa亚洲地区的阻塞高压主要发生在乌拉尔山(鄂霍次克海)附近、西太平洋副热带高压位置偏北(南);低层850hPa风场的东亚沿海地区为偏南(北)风距平,有利于(不利于)水汽向长江中下游地区输送.大气环流内部动力过程的分析指出:东亚地区上空Eliassen-Palm(EP)通量散度在40°N为正(负)异常、30°N为负(正)异常,有利于东亚高空西风急流中心位置偏北(南),从而导致春季长江中下游降水偏多(偏少).春季长江中下游降水异常偏多(少)年最显着的前期外强迫信号表现为赤道太平洋海温呈现厄尔尼诺(拉尼娜)型.     

A simple model of drainage flow in a valley

A model of the drainage flow in a valley under calm conditions has been developed on the basis of the conservation laws of mass, momentum, and heat. The inflow of mass and heat from side-slopes is incorporated, and the momentum and sensible heat exchanges between valley drainage flow and valley floor are parameterized.The characteristic velocity of valley drainage flow is expressed in terms of the following parameters: three potential temperature differences representing the temperature field in the valey; topographic parameters of the valley; mean bulk coefficients representing the aerodynamic conditions of the valley floor; and the stability of the ambient atmosphere. The characteristic thickness includes additional parameters of side-slope flow.That the model satisfactorily predicts the characteristic thickness and velocity is shown from comparison with observations from valleys several hundred meters to a few hundred kilometers long.