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     

Boundary layer evolution over a large and broad mountain basin

Summary In this study observations of the vertical structure of the Atmospheric Boundary Layer (ABL), recorded at a broad mountainous valley are presented. The vertical profiles of temperature, wind speed and direction up to a height of about 800 meters over the valley bottom have been measured and the temporal evolution of ABL structure of the area has been studied. Specifically, the mechanism of nocturnal inversion destruction during morning hours has been studied, which is of major importance in the study of the dispersion of air pollutants over the area. These observations suggest that the break up of nocturnal inversion during morning hours is mainly caused by a combined mechanism, the build up of the Convective Boundary Layer (CBL) and the presence of upslope winds, resulting to a continuous descent of the top of the nocturnal inversion.With 5 Figures     

A Comparative Study and Evaluation of Mixing-Height Estimation Based on Sodar-RASS, Ceilometer Data and Numerical Model Simulations

The unmanned aerial system SUMO (Small Unmanned Meteorological Observer) has been used for the observation of the structure and behaviour of the atmospheric boundary layer above the Advent Valley, Svalbard during a two-week period in early spring 2009. Temperature, humidity and wind profiles measured by the SUMO system have been compared with measurements of a small tethered balloon system that was operated simultaneously. It is shown that both systems complement each other. Above 200?m, the SUMO system outperforms the tethered balloon in terms of flexibility and the ability to penetrate strong inversion layers of the Arctic boundary layer. Below that level, the tethered balloon system provides atmospheric profiles with higher accuracy, mainly due to its ability to operate at very low vertical velocities. For the observational period, a numerical mesoscale model has been run at high resolution and evaluated with SUMO profiles reaching up to a height of 1500?m above the ground. The sensitivity to the choice of atmospheric boundary-layer schemes and horizontal resolution has been investigated. A new scheme especially suited for stable conditions slightly improves the temperature forecast in stable conditions, although all schemes show a warm bias close to the surface and a cold bias above the atmospheric boundary layer. During one cold and cloudless night, the SUMO system could be operated nearly continuously (every 30?C45?minutes). This allowed for a detailed case study addressing the structure and behaviour of the air column within and above Advent Valley and its interaction with the local topography. The SUMO measurements in conjunction with a 10-m meteorological mast enabled the identification of a very stable nocturnal surface layer adjacent to the valley bottom, a stable air column in the valley and a strong inversion layer above the summit height. The results indicate the presence of inertial-gravity waves during the night, a feature not captured by the model.     

复杂地形城市冬季边界层对气溶胶辐射效应的响应

作者着眼于城市气溶胶辐射效应与大气边界层的相互作用问题,针对地形复杂的兰州市及周边地区,开发应用了WRF(Weather Research and Forecasting,天气研究和预报)模式,使之与包含了大气气溶胶辐射效应和气溶胶粒子扩散的综合大气边界层数值模式嵌套起来.通过个例分析,揭示了冬季气溶胶辐射效应对边界层结构的定量影响.主要特征为夜间气溶胶的长波辐射效应使地面附近的气温增高,增温幅度为0.1～0.3 K/h,使低空(25～300 m)大气层冷却,降温幅度为0.08～0.15 K/h,风速在150 m以下减小;白天气溶胶的短波辐射效应使地面层内明显增温,1 h内升温约0.5 K,增温最大值在混合层顶500～600 m高度.受增温影响,垂直风场和水平风场随之调整,风速在450 m以下增大约0.1 m/s左右,而在450 m以上风速减小0.1 m/s左右.     

Evolution of the atmospheric boundary-layer structure of an arid Andes Valley

Summary The boundary-layer structure of the Elqui Valley is investigated, which is situated in the arid north of Chile and extends from the Pacific Ocean in the west to the Andes in the east. The climate is dominated by the south-eastern Pacific subtropical anticyclone and the cold Humboldt Current. This combination leads to considerable temperature and moisture gradients between the coast and the valley and results in the evolution of sea and valley wind systems. The contribution of these mesoscale wind systems to the heat and moisture budget of the valley atmosphere is estimated, based on radiosoundings performed near the coast and in the valley. Near the coast, a well-mixed cloud-topped boundary layer exists. Both, the temperature and the specific humidity do not change considerably during the day. In the stratus layer the potential temperature increases, while the specific humidity decreases slightly with height. The top of the thin stratus layer, about 300 m in depth, is marked by an inversion. Moderate sea breeze winds of 3–4 m s⁻¹ prevail in the sub-cloud and cloud layer during daytime, but no land breeze develops during the night. The nocturnal valley atmosphere is characterized by a strong and 900 m deep stably stratified boundary layer. During the day, no pronounced well-mixed layer with a capping inversion develops in the valley. Above a super-adiabatic surface layer of about 150 m depth, a stably stratified layer prevails throughout the day. However, heating can be observed within a layer above the surface 800 m deep. Heat and moisture budget estimations show that sensible heat flux convergence exceeds cold air advection in the morning, while both processes compensate each other around noon, such that the temperature evolution stagnates. In the afternoon, cold air advection predominates and leads to net cooling of the boundary layer. Furthermore, the advection of moist air results in the accumulation of moisture during the noon and afternoon period, while latent heat flux convergence is of minor relevance to the moisture budget of the boundary layer. Correspondence: Norbert Kalthoff, Institut für Meteorologie und Klimaforschung, Universit?t Karlsruhe/Forschungszentrum Karlsruhe, Postfach 3640, 76021 Karlsruhe, Germany     

Vertical structure and microphysical characteristics of Typhoon Kompasu (2010) at landfall

The vertical structure and microphysics of Typhoon Kompasu that caused a lot of damage associated with strong winds and heavy rainfall over the Seoul metropolitan area on 1～2 September 2010 were examined primarily from wind profiler measurements. Four different periods that represent a stratiform, outer rainband, inner rainband, and eyewall region during passage of Typhoon Kompasu from 1200 to 2300 UTC 1 September were selected based on bright band intensities and vertical profiles of radar reflectivities and Doppler velocities. The bright band signatures observed in all of these periods indicated that the structure of Kompasu was basically stratiform in a weakening phase. Maximum rainfall rates up to 50 mm hr^?1 at the surface and mean wind speeds greater than 30 m s^?1 in the 2–4 km layer were observed in the eyewall region. Unlike the other regions that showed nearly zero vertical air motions or weak downdrafts below a melting layer, a mean updraft of ～1 m s^?1 was analyzed only in the eyewall region, which suggests that the updrafts may have enhanced drop growth that led to increasing surface rainfall rates. For each region, the vertical mean characteristics of rainfall parameters retrieved from wind profiler spectra below the melting layer were also examined. The rain properties between the inner and outer rainband were similar although they were apart with a distance of more than 100 km (> 2 hrs in time). The averaged mass-weighted mean diameters within the rainbands were larger than those in the stratiform and eyewall regions. A weaker bright band in the eyewall region suggests the presence of a relatively larger number of rimed particles associated with the updrafts around the melting layer. A stronger bright band was present in the rainbands, which indicates more active aggregation right above the melting layer.     

Observations of atmospheric boundary layer evolution above the Gulf Stream frontal zone

Ship borne measurements of atmospheric boundary layer (ABL) parameters, sea-surface temperature and radar signals are analyzed to reveal the effects of the ABL transformation above the Gulf Stream temperature frontal zone. It was found that local changes in vertical gradients of wind speed and air temperature are well correlated with sub-mesoscale (~ 10 km) sea surface temperature variations. These effects are accompanied by appropriate variations in surface wind stresses that were identified from microwave backscatter.For steady atmospheric conditions the same effects were observed on spatial scales of 100 km, demonstrating positive radar signal contrast of the Gulf Stream warm waters with respect to surrounding Sargasso sea and shelf water areas. A simplified model of the ABL, accounting for an effect of spatial inhomogeneity by introducing an internal boundary layer, is used to analyze field observations. The model is able to reproduce both sub-mesoscale and mesoscale ABL evolution.     

苏州东山冬季大气边界层结构特征及其对污染物浓度的影响

利用2015年1月15—27日在苏州东山气象观测站系留气艇观测数据以及细颗粒物浓度观测资料,对东山大气边界层结构特征及其对污染物垂直结构分布的影响进行分析研究。结果表明:苏州东山地区冬季空气污染过程的边界层结构演变比较典型,夜间稳定边界层高度约为200 m,白天最大边界层高度可达1 000 m。边界层内污染物垂直结构分布易受边界层高度的影响,较低的大气边界层高度可使细颗粒物在近地层持续累积;反之,边界层高度较高,湍流发展旺盛,颗粒物垂直分布均匀。夜间大气边界层稳定,逆温结构多发,导致近地面出现细颗粒物堆积。风的垂直结构对细颗粒物空间分布也存在显著影响,在风速较小的低空层细颗粒分布较多,而风速较大的中高层的分布较少。      

The vertical structure of summertime local winds in the Wright Valley,Antarctica

Intensive observations of summertime up- and down-valley winds in a dry valley utilising airsondes, pilot balloons and a monostatic acoustic sounder are described. Both circulations show a distinctive layered vertical wind and temperature structure. Westerly down-valley flow is typically neutral and is characterised by strong surface winds overlain by light variable winds extending to an inversion between 2000 and 4000m in depth. Above this inversion, gradient winds prevail. This structure is similar to that of downslope winds observed elsewhere. The thermally-induced up-valley easterly flow is shown to be extremely well-developed in terms of its strength, depth and persistence. The strong surface easterly may reach 800 m in depth and usually undercuts the warmer westerly. The boundary between the two regimes is marked by an inversion. During easterly flow a surface-based, super-adiabatic layer of 100–200 m in depth is evident and is associated with weak convective activity. An intriguing aspect of the wind regime is the interaction between the easterly and westerly circulations in the valley. These are separated at the surface by a frontal zone which migrates up and down the valley. Further observational and modelling studies are recommended.     

农林复合带一维非静力大气边界层模式及其模拟分析

建立了一个农林复合带地区一维非静力大气边界层能量闭合模式，对1000m以下的大气边界层内的风、温、湿作了24h的预报，并对下垫面3种不同参数化方案(农作物、森林、无植被)的输出结果与实测值进行了分析和比较；同时通过敏感性试验，突出比较了农作物和森林下垫面对大气边界层垂直流场，湍流垂直交换和湍能的影响。结果表明，本模式能改善边界层风速、位温和湿度预报的模拟效果，下垫面植被对边界层气象要素大小和分布有显著的作用，对湍能垂直分布有一定影响。     

Summertime atmospheric circulation in the vicinity of a blue ice area in Queen Maud Land,Antarctica

The surface wind field is an important factor controlling the surface mass balance of Antarctica. This paper focuses on the observed atmospheric circulation during summer of an Antarctic blue ice area in Queen Maud Land. Blue ice areas are characterised by a negative surface mass balance and henceforth provide an interesting location to study the influence of meteorological processes on large local mass balance gradients. During lapse conditions, synoptic forcing determines the surface-layer flow. No significant horizontal temperature gradient with coastal stations could be detected along isobaric surfaces, indicating weak or absent thermal wind. Observations performed at the coastal stations Halley and Georg von Neumayer show the pronounced effects of synoptic forcing. The surface winds in the valley of the blue ice area could be divided into two distinct flow patterns, occurring with about equal frequency during the experiment. Flow type I is associated with cyclonic activity at the coast, resulting in strong easterly winds, precipitation and drifting snow. Flow characteristics inside and outside of the valley are similar during these conditions. Flow type II occurs when a high pressure system develops in the Weddell Sea, weakening the free atmosphere geostrophic winds. A local circulation is able to develop inside the valley of the blue ice area during these tranquil conditions. The transition from flow type II to flow type I is associated with front-like phenomena inside the valley. Some simple theoretical considerations show that surface-layer stability and the upper air geostrophic wind determine the surface flow direction in the valley. Finally, the influence of the observed circulation on the energy and mass balance of the blue ice area is discussed.     

利用RAMS模式对山谷城市冬季局地风场的数值模拟

利用美国科罗拉多州立大学和MRC/ASTER发展的中尺度数值模式RAMS, 采用三重嵌套的方法, 模拟研究了兰州山谷地区局地环流特征。结果表明： （1）兰州市近地面流场以偏东风为主, 在城市东西部之间的狭窄地带, 风速相对较大些, 在东西部山谷城市中心区域有大片的静风区; 冬季兰州市山谷夜间是辐合流场, 白天是辐散流场。受城市热岛环流的影响, 白天热岛环流抑制谷风环流, 夜间增大山风环流, 夜间的山风风速大于白天的谷风风速。（2）白天, 兰州市区山顶和山谷之间上空气柱以下沉气流为主, 这主要是由于地形作用使得白天盛行谷风环流和山峰加热作用的共同影响。夜间, 兰州市区山顶和山谷之间上空以上升气流为主, 这主要是由于地形作用使得市区和山谷在夜间盛行山风环流, 但是冬天夜间兰州市区和山谷上空有较厚的逆温层存在, 抑制了气流的上升运动。（3）在午后13：00左右, 兰州市区山谷从近地面到400 m高度, 辐散场在逐渐减弱, 在510 m左右的高度转变为辐合场; 皋兰山顶上空从近地面到400 m高度, 辐合场在逐渐减弱, 在510 m左右的高度转变为辐散场。在凌晨01：00左右, 兰州市区山谷从近地面到400 m高度, 辐合场在逐渐增强, 到400 m高度达到最强, 从400 m到510 m高度又逐渐减弱; 皋兰山顶上空从近地面到220 m左右的高度, 辐散场在逐渐减弱, 在400 m左右的高度辐散场转变为辐合场, 从400 m到510 m左右的高度, 皋兰山顶的辐合场逐渐增强。     

Influence of weather conditions and spatial variability on glacier surface melt in Chilean Patagonia

In order to clarify how differences in weather conditions affect the surface heat balance of a large maritime glacier, meteorological observations were carried out in the ablation area of Glaciar Exploradores in the Chilean Patagonia during the austral summer of 2006/2007. Under cloudy/rainy weather, when the air temperature and wind speed were high due to advection, the average melting heat was 18.8 MJ m^?2 day^?1 and the turbulent heat fluxes contributed 35% of the total melt. During clear weather, the average melting heat was 16.9 MJ m^?2 day^?1 and 13% of the total was the turbulent heat fluxes. A decrease in air temperature due to the development of the glacier boundary layer on clear days will lead to an overestimation of the melt using the air temperature at a weather station outside of the glacier.     

广汉机场春季地面风的变化特征及其对飞行训练的影响

利用广汉机场2010~2014年遥测地面风场资料,分析了春季地面风的年变化、月变化以及日变化特征,讨论了地面风对飞行训练的影响。结果表明:广汉机场春季盛行偏北风,此外主要还受到北西北、北东北风的影响,飞行训练易遭遇左侧风;4~5m/s以及6m/s以上的地面风日数年变化不大,但春季最大风速的年变化差异较大;3~5月月平均地面风速呈递增特征,从3月到5月主导风向由北风顺时针变化为东北风,东东南、东南、南东南风频率逐渐增加,左侧风以及逆风影响增大。地面风速的日变化呈现出“一峰一谷”的大陆型变化特征,即白天风速大,夜间风速小,午后风速最大,4~5m/s的风受日变化影响大,6m/s以上的风主要受天气系统的影响。      

Observations of Coherent Turbulence Structures in the Near-Neutral Atmospheric Boundary Layer

Turbulence structures of high Reynolds number flow in the near-neutral atmospheric boundary layer (ABL) are investigated based on observations at Shionomisaki and Shigaraki, Japan. A Doppler sodar measured the vertical profiles of winds in the ABL. Using the integral wavelet transform for the time series of surface wind data, the pattern of a descending high-speed structure with large vertical extent (from the surface to more than 200-m level) is depicted from the Doppler sodar data. Essentially this structure is a specific type of coherent structure that has been previously shown in experiments on turbulent boundary-layer flows. Large-scale high-speed structures in the ABL are extracted using a long time scale (240 s) for the wavelet transform. The non-dimensional interval of time between structures is evaluated as 3.0–6.2 in most cases. These structures make a large contribution to downward momentum transfer in the surface layer. Quadrant analyses of the turbulent motion measured by the sonic anemometer (20-m height) suggest that the sweep motion (high-speed downward motion) plays a substantial role in the downward momentum transfer. In general, the contribution of sweep motions to the momentum flux is nearly equal to that of ejection motions (low-speed upward motions). This contribution of sweep motions is related to the large-scale high-speed structures.     

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     

Modelling the Interaction Between a River Surface and the Atmosphere at the Bottom of a Valley

The role of a river of small dimensions in driving the surface exchange of sensible and latent heat fluxes at the bottom of a valley is investigated using large-eddy simulation (LES). Simulations were performed using different valley topographies, river widths and large-scale wind speed and direction. In all cases, the river acted as a sink of both sensible and latent heat during daytime. Despite the general agreement concerning the flux direction above the river surface, specific differences exist between the simulations. The topography enhances the wind divergence caused by the river, and the larger negative surface fluxes above the river occur when there are no slopes, a consequence of larger wind speeds above the river. For large-scale winds aligned with the valley axis, the surface fluxes depend on the large-scale wind speed, but this dependence is reduced if the large-scale wind is perpendicular to the valley axis. There is a minimum of temperature and a maximum of specific humidity above the river surface. The scalar budgets show that sensible heat flux converges above the river, being balanced by the warm air subsidence at the centre of the valley. Latent heat fluxes, on the other hand, converge above the river surface, and they are balanced by the horizontal advection of humidity towards the river margins.     

Momentum flux in the subcloud layer of a microburst-producing thunderstorm determined from JAWS Dual-Doppler data

Dual-Doppler winds at 1647 MDT for the 14 July, 1982 convective storm collected during the Joint Airport Weather Studies (JAWS) project at Denver's Stapleton International Airport were objectively analyzed to produce a three-dimensional wind field. The domain of interest had dimensions of 10 × 10 × 8.5 km centered on the microburst. Vertical velocities were computed by integrating the anelastic continuity equation downward from the storm's top. A variational approach was then employed to adjust the derived three-dimensional wind field. Subsequently, fields of deviation perturbation pressure and virtual temperature were retrieved from a detailed wind field using the three momentum equations. These retrieved fields were subjected to internal consistency checks to determine the level of confidence before interpetation. The fields were then used to calculate the generation of the vertical transport of horizontal momentum in the subcloud layer of a microburst-producing storm during the quasi-steady mature stage. Results show that the microburst occurrence in the atmospheric boundary layer (ABL) enhances eddy transfer of momentum. Direct calculation of the vertical transport of u- and v-momentum reveals that momentum was being transferred downward from the mid-levels of the storm to the microburst. The dominant processes contributing to the generation/dissipation of horizontal momentum flux were the total buoyancy production, pressure effects, vertical mean wind shear and vertical transport of momentum. The above processes play an important role in maintaining the strength of the microburst outflow in the ABL during the quasi-steady mature stage of the microburst life cycle.     

Evolutionary characteristics of a dust storm over Oman on 2 February 2008

The evolutionary characteristics of a dust storm over Oman on 2 February 2008 were studied by analyzing the weather associated with it. The National Center for Environmental Prediction/National Centre for Atmospheric Research (NCEP/NCAR) reanalysis wind flow pattern at 1,000?hPa showed a clockwise and outward wind flow pattern over the study region, a manifestation of a high-pressure cold air mass. In addition, ground truth observations for surface temperature and surface winds showed cold northerly winds until the early morning of 2 February 2008. A strong wind shear resulted from differences in wind speed between warm air and trapped cold air. This vertical wind shear enhanced instability. Furthermore, the weakening of the inversion in the lower troposphere and the formation of a mixed layer due to transfer of horizontal momentum from upper air towards the surface led to strong surface winds. These strong winds lifted a large amount of dust particles off the ground, resulting in the dust event of 2 February 2008.