Vertical Air Mass Exchange Driven by the Local Circulation on the Northern Slope of Mount Everest

To better understand vertical air mass exchange driven by local circulation in the Himalayas, the volume flux of air mass is estimated in the Rongbuk Valley on the northern slope of Mount Everest, based on a volume closure method and wind-profiler measurements during the HEST2006 campaign in June 2006. Vertical air mass exchange was found to be dominated by a strong downward mass transfer from the late morning to late night. The average vertical air volume flux was 0.09 m s-1, which could be equivalent to a daily ventilation of 30 times the enclosed valley volume. This vertical air mass exchange process was greatly affected by the evolution of the South Asian summer monsoon (SASM), with a strong downward transfer during the SASM break stage, and a weak transfer during the SASM active stage.     

Preliminary estimation of the moisture exchange in Rongbuk Valley on the northern slope of Mt. Everest

Using observed wind and water vapor data from June 2006,water vapor exchange between the Rongbuk Valley and its above atmosphere is estimated for the first time.The water vapor level shows a high value from 23-29 June and a low from 12-21 June,which co-incide with the South Asian summer monsoon (SASM) active and break stages,respectively.The water vapor can be strongly injected into the closed region of the Rongbuk Valley from the outside atmosphere,with an average strength of 0.4 g s-1 m-2 in June 2006,given that no evaporation occurred.The air moisture exchange proc-esses can be greatly affected by the SASM evolution through changes in local radiation forcing.     

HEST2007珠峰北坡风廓线观测研究

为了研究青藏高原南部喜马拉雅山区局地大气环流系统,继2006年HEST2006大气科学实验之后,2007年中国科学院大气物理研究所和中国科学院青藏高原研究所在珠穆朗玛峰北坡实施HEST2007综合观测.本文使用该观测实验中LAP-3000风廓线仪获得的绒布河谷内三维风场观测资料,并结合地面辐射资料,分析研究了该地区观测期间局地大气环流的日变化和逐日变化过程.研究表明,该地区局地环流系统,特别是沿河谷的轴向风,与其上空西风环流间存在非常紧密的联系,这种联系似乎与不同天气条件下喜马拉雅山区的大气辐射状况有关,即高层西风环流较强的阶段,地面辐射较强,激发出的局地环流也较强,反之亦然.     

珠穆朗玛峰北坡局地环流日变化的观测研究

青藏高原地-气间的物质/能量交换是高原与全球大气系统相联系的重要纽带.陡峭的地形和强烈的地表差异在高原山区形成特殊的局地大气环流系统,在地气交换中起着重要作用.为研究珠峰北坡的局地环流系统,于2006年5～6月间在珠峰北坡绒布河谷实施强化观测实验HEST2006,对该地区的局地环流以及辐射和热力状况进行观测,分析了该地区局地环流的日变化过程,包括:(1)地面风场的分布和变化;(2)风场垂直结构;(3)垂直运动及可能的驱动机制.研究表明,该地区局地大气环流是由地形与地表状态调整的大气辐射加热和冷却所驱动,包含多种不同的山地环流成分,与典型山谷风环流不同,具有很强的特殊性,对地气问的交换有重要影响.     

珠峰绒布河谷近地面层气象要素及湍流通量变化

利用2006年5～6月和2007年5～6月中国科学院HEST大气科学实验在珠峰绒布寺河谷野外观测期间获得的观测资料,分析了珠峰地区河谷近地层风向、风速、温度、湿度和CO2的日变化特征,讨论了珠峰北坡冰川风和山谷风的特点以及高原地表辐射、地表反照率和近地层湍流通量的变化特征.结果表明:在复杂地形和特殊下垫面影响下,珠峰绒布河谷地区近地面层各个气象要素和湍流通量日变化特征显著,并且明显存在冰川风和山谷风复合的局地环流,冰川风对该地区地气间物质能量交换起着重要作用.     

Observed Impact of the South Asian Summer Monsoon on the Local Meteorology in the Himalayas

The South Asian summer monsoon(SASM) is the most important climate system in Asia.Using observational data from the HEST2006(Himalayan Exchange between the Surface and Troposphere 2006) campaign and large-scale grid data,this paper analyzed the SASM impact on local meteorological parameters including radiation,temperature,humidity,and wind in the Himalayas.The SASM experienced one active and one break period during the HEST2006 campaign.The local meteorological parameters exhibit great differences between the active period and the break period of the SASM.The radiation fluxes are greater in the break period than in the active period.The air temperature and specific humidity are lower,but soil temperature and wind speed are higher in the break period than in the active period.Further analysis indicates that the SASM greatly affects the meteorological features of the Himalayan region.     

珠峰北坡绒布河谷大气二氧化碳浓度低值观测

In the summers of 2006 and 2007, the atmospheric CO2 concentration and the wind speed in the Rongbuk Valley on the northern slope of Mt. Everest were measured by an ultrasonic anemometer with an Li-7500 CO2/H2O gas analyzer. The average CO2 concentration was 370.23±0.59 and 367.45±1.91 ppm in June of 2006 and 2007, respectively. The values are much lower than those at sites with similar latitudes and altitudes worldwide. The observed atmospheric CO2 concentration in Rongbuk Valley can be affected by the transportation of prevailing down-valley winds from the up-valley direction to the observation site. Our results suggest that the Mt. Everest region could be ideal for background atmospheric and environmental studies.     

Air Pollution Transport in an Alpine Valley: Results From Airborne and Ground-Based Observations

An observational dataset from a wintertime field campaign in the Inn Valley, Austria, is analysed in order to study mechanisms of air pollution transport in an Alpine valley. The results illustrate three types of mechanisms: transport by a density current, back-and-forth transport by valley winds, and transport by slope winds. The first type is associated with an air mass difference along the valley. Cooler air located in the lower part of the valley behaves like a density current and produces the advection of pollutants by upvalley winds. In the second type, strong horizontal gradients in pollution concentrations exist close to ground. Multiple wind reversals result in a back-and-forth transport of pollutants by weak valley winds. In the third type, upslope winds during daytime decrease low-level pollution concentrations and cause the formation of elevated pollution layers.     

Air flow analysis in the upper Río Negro Valley (Argentina)

The so called Upper Río Negro Valley in Argentina is one of the most important fruit and vegetable production regions of the country. It comprises the lower valleys of the Limay and Neuquén rivers and the upper Negro river valley. Out of the 41,671 cultivated hectares, 84.6% are cultivated with fruit trees, especially apple, pear and stone fruit trees. Late frosts occurring when trees are sensitive to low temperatures have a significant impact on the regional production. This study presents an analysis of air flow characteristics in the Upper Río Negro Valley and its relationship with ambient air flow. To such effect, observations made when synoptic-scale weather patterns were favorable for radiative frosts (light wind and clear sky) or nocturnal temperature inversion in the lower layer were used. In the Negro river valley, both wind channeling and downward horizontal momentum transport from ambient wind were observed; in nighttime, very light wind events occurred, possibly associated with drainage winds from the nearby higher levels of the barda. In the Neuquén river valley, the prevailing effect appeared to be forced channeling, consistent with the results obtained in valleys where the synoptic scale wind crossed the axis of the valley. In the Limay river valley, the flow was observed to blow parallel to the longitudinal valley axis, possibly influenced by pressure gradient and forced channeling.     

珠穆朗玛峰地区气象要素特征观测研究

利用中央气象台奥运火炬传递珠峰气象保障队获取的第一手资料,对2007年4月12日-5月8日珠穆朗玛峰(简称珠峰)大本营地面自动站和79次探空实测资料进行分析,揭示了珠峰地区气象要素变化的观测事实。结果表明:受珠峰北坡绒布冰川沉降风影响,珠峰大本营盛行南风;风速和气温日变化呈单峰分布,风速极小值和日最低气温出现在08:00(北京时,下同),而极大值出现在15:00前后;相对湿度峰值出现在07:00和23:00,正午前后相对湿度最小。6200 m高度以下几乎昼夜恒吹下山风,偏南风在22:00至次日00:00最强;7200-9000 m高度风向主要以偏西风为主,风速随高度也明显增加。5200-8000 m各高度上,日最低气温始终出现在08:00,最高气温出现在16:00,8200 m以上气温呈现出多波动起伏。在6700-9700 m高度能维持相对湿度的高值中心,大值区主要位于6500-8800 m高度。高空西风锋区位于12000-15000 m,对流层顶高度大约在18200 m左右。     

A reexamination of the valley wind system in the Alpine Inn Valley with numerical simulations

Summary This paper presents idealized numerical simulations of the valley wind circulation in the Alpine Inn Valley, which are compared with existing data and are used to improve our dynamical understanding of the valley wind. The simulations have been performed with the Penn State/NCAR mesoscale model MM5. They use a high-resolution realistic topography but idealized large-scale conditions without any synoptic forcing to focus on the thermally induced valley wind system. The comparison with the available observations shows that this simplified set-up is sufficient to reproduce the essential features of the valley wind.The results show that the tributaries of the Inn Valley have a considerable impact on the along-valley mass fluxes associated with the valley wind circulation. The upvalley mass flux is found to increase where tributaries enter the Inn Valley from the north, that is, from the direction where the Alpine foreland is located. On the other hand, the upvalley mass flux is reduced at the junctions with southern tributaries because part of the upvalley flow is deflected into these tributaries. For the downvalley flow, the situation is essentially reversed, but the influence of the valley geometry on the flow structure is larger than for the upvalley flow. The most important feature is a lateral valley contraction near the valley exit into the Alpine foreland. It reduces the downvalley mass flux at low levels, so that the wind maximum in the interior of the valley is shifted to a fairly large distance from the ground. North of the valley contraction, however, the downvalley flow strongly accelerates and forms a pronounced low-level jet. A dynamical analysis indicates that this acceleration can be interpreted as a transition from subcritical to supercritical hydraulic flow. Another interesting feature is that the low-level jet maintains its structure for several tenths of kilometres into the Alpine foreland. This appears to be related to the fact that the lateral wind shear on the flanks of the jet is associated with a strong dipole of potential vorticity (PV). Due to the conservation properties of the PV, the downstream advection of the PV dipole leads to the formation of a band-like feature that decays fairly slowly.     

华南沿海暖海雾过程中的湍流热量交换特征

湍流交换是海雾中的关键物理过程, 在海雾的热量和水汽平衡过程中起重要作用。本文根据2007年3月24～25日一次海雾的外场观测数据, 分析了海雾过程中近海面湍流热量交换特征; 并在区分风切变机械湍流与雾顶长波辐射冷却热力湍流的基础上, 着重分析了两种不同性质的湍流对海雾发展和维持的作用。结果表明: (1) 本次海雾是在西南低压和变性冷高压的控制下, 来自南海东部暖水区的空气平流在近岸冷海面上形成的暖海雾; 暖海雾中的湍流热量交换过程比冷海雾更为复杂; (2) 在暖海雾的形成和消散阶段, 风切变机械湍流的热量输送起主要作用; 而在发展和维持阶段, 既有风切变机械湍流的热量输送作用, 也有雾顶长波辐射冷却热力湍流的热量输送作用; (3) 风切变机械湍流向冷海面输送热量, 对近海面空气起到降温和增湿作用; 热力湍流同样向冷海面输送热量, 但对雾层起到增温和降湿作用; (4) 暖海雾中的湍流热量交换机制与雾层的非充分混合结构有密切关系。     

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     

金沙本底站空气质量与颗粒物浓度变化特征分析

利用华中区域代表性站点金沙国家大气本底站2007—2018年的PM_2.5、PM₁₀颗粒物质量浓度数据,2019年3月—2019年6月反应性气体数据,对华中区域空气质量进行整体评价,并分析了颗粒物浓度的变化特征及其影响因素。结果表明,反应性气体CO、SO₂、NO、NO₂质量浓度其日平均最大值、平均值均达到一级标准,O₃日平均值超标率为10.6%,PM_2.5超标率为18%,PM₁₀超标率为17%。从颗粒物质量浓度多年变化来看,2007—2013年金沙站大气颗粒物质量浓度属于上升趋势,2013年至今质量浓度呈现下降趋势,风向风速也在一定程度上影响颗粒物的来源。     

Local foehn effects in the Upper Isar Valley,Part 3: Additional investigations

Summary The present paper is the continuation of two recent studies investigating the foehn-like valley wind system around Mittenwald (Bavarian Isar Valley). We deal with the synoptic/mesoscale conditions causing the local foehn (“Minifoehn”), considering field campaigns from both the mesoscale and the climatological point of view. Furthermore, we describe the structure and further features of the local foehn at smaller scales, using both the results of the VERTIKATOR field campaign and numerical simulations. We obtain as a new result that the foehn-caused local warm air pool around Mittenwald induces slight nocturnal upvalley winds between an adjacent valley basin located some 8 km north of Mittenwald and the basin of Mittenwald. Furthermore, a weak northerly flow may also occur at Mittenwald prior to the onset of the Minifoehn. Numerical simulations indicate that the local pressure gradient responsible for this phenomenon is related to a gravity wave forming over the hill range southwest of Mittenwald. Observations within a five-year period indicate that Minifoehn frequently occurs when ambient winds coming from the southern sector are predominant, but, contrary to deep foehn, weather conditions with northerly synoptic-scale flows do not necessarily exclude the development of the local foehn which comes from the southwest. We also present further evidence that in the presence of southerly synoptic-scale winds, orographic gravity waves interact with the drainage flow. Another new result is that strong synoptic-scale westerly winds are able to suppress the occurrence of Minifoehn. In addition, the possible influence of the Inn Valley wind system as well as dynamical differences between the thermally driven up- and downvalley winds are briefly discussed.     

2007年6—7月西太平洋副热带高压的异常特征及其成因分析

采用NCEP资料以及NOAA卫星观测的OLR场资料,分析了2007年6—7月淮河流域暴雨及江南、华南异常高温期间西太平洋副热带高压的异常特征,其表现为强度强且南北位置相对稳定,东西方向进退明显于南北方向的移动。通过分析经圈平面上的垂直环流及风矢量场的特征表明,副高南北两侧都存在一定强度的上升气流,北侧上升气流对西太平洋副高中心区700 hPa以上高度的下沉运动有激发作用,而南侧上升气流对西太平洋副高中心区的下沉运动作用不大。中高纬度阻塞高压双阻型的建立,有利于西太平洋副高的加强和发展。强劲而稳定少动的中纬度西风急流和热带地区异常活跃的对流均有利于西太平洋副高的相对稳定。     

Characteristics of nocturnal low-level jets over Ulsan airport

We analyzed wind profiler data collected over Ulsan airport during the period from 2008 to 2009 to examine the characteristics of low level jets (LLJs). The Ulsan airport is located within the narrow valley with north-south axis. The frequency analysis results indicates that the nearly 19% of the total nocturnal periods have the presence of jets and LLJ occurrence rate is high in winter (32%) and low in summer (10%). The mode in the wind speed histogram is 4?C6 m s^?1. A majority of jet occurs below 100 m (about 77.8 m) above ground. The predominant wind direction of jet is northerly. In order to examine the favorable conditions for LLJ formation of Ulsan airport, we investigated temperature difference between valley and plain at the surface and synoptic wind direction and speed at 850 hPa. Our results show that air temperature in the valley is lower than over the plain during the nighttime, indicating the existence of thermal forcing for along-valley wind. Under a significant temperature difference along the valley, westerly wind speed at 850 hPa is slightly weaker on LLJs event night than no event night, indicating weaker north-south large-scale pressure gradient on LLJ event night. The magnitude of northerly wind at 850 hPa is much stronger on event night than no event night, implying higher downward transfer of northerly wind on event night. Our findings suggest that jet formation over Ulsan airport is related to the strong northerly wind at 850 hPa in the presence of thermal forcing due to temperature contrast between valley and plain.     

Early-Morning Flow Transition in a Valley in Low-Mountain Terrain Under Clear-Sky Conditions

We investigate the evolution of the early-morning boundary layer in a low-mountain valley in south-western Germany during COPS (convective and orographically induced precipitation study) in summer 2007. The term low-mountain refers to a mountainous region with a relief of gentle slopes and with an absolute altitude that remains under a specified height (usually 1,500 m a.s.l.). A subset of 23 fair weather days from the campaign was selected to study the transition of the boundary-layer flow in the early morning. The typical valley atmosphere in the morning hours was characterized by a stable temperature stratification and a pronounced valley wind system. During the reversal period—called the low wind period—of the valley wind system (duration of 1–2 h), the horizontal flow was very weak and the conditions for free convection were fulfilled close to the ground. Ground-based sodar observations of the vertical wind show enhanced values of upward motion, and the corresponding statistical properties differ from those observed under windless convective conditions over flat terrain. Large-eddy simulations of the boundary-layer transition in the valley were conducted, and statistical properties of the simulated flow agree with the observed quantities. Spatially coherent turbulence structures are present in the temporal as well as in the ensemble mean analysis. Thus, the complex orography induces coherent convective structures at predictable, specific locations during the early-morning low wind situations. These coherent updrafts, found in both the sodar observations and the simulation, lead to a flux counter to the gradient of the stably stratified valley atmosphere and reach up to the heights of the surrounding ridges. Furthermore, the energy balance in the surface layer during the low wind periods is closed. However, it becomes unclosed after the onset of the valley wind. The partition into the sensible and the latent heat fluxes indicates that missing flux components of sensible heat are the main reason for the unclosed energy balance in the considered situations. This result supports previously published investigations on the energy balance closure.     

大气污染对太行山中部地区地表风的影响

为了研究大气污染对太行山中部地区地表风的影响,我们对阳泉、榆社(高山站),石家庄、邢台(平原站)4个站点1966~2005年间的能见度、近地面温度、近地面风速数据进行了统计计算与趋势分析。结果显示:在平原站能见度相对山坡站下降更加明显的背景下,平原站的近地面温度、近地面风速、850hPa风速都呈下降趋势;而山坡站的近地面温度、近地面风速呈上升趋势。分析表明:(1)由于气溶胶的辐射效应与冷却效应,抑制了垂直通量的上下传输,致使平原站下午的近地面气温呈下降趋势,平原站和高山站的地表风速呈相反的变化趋势。(2)平原站850hPa (与高山站高度相近)风速呈现下降趋势,印证了高山站的近地面风速增加是气溶胶的辐射效应减弱了垂直能量交换造成的。      

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