Small-scale dynamics of the south foehn in the lower Wipp Valley

Summary In this paper, very-high-resolution numerical simulations are presented to analyze the small-scale dynamics of the foehn in the lower Wipp Valley and the adjacent parts of the Inn Valley. This region was one of the target areas for foehn observations during the Mesoscale Alpine Programme (MAP). Our simulations consider two MAP cases that markedly differed in the depth of the foehn flow. To isolate the dynamical effect of the key orographic features in the Wipp Valley region, we performed sensitivity experiments with different topography modifications. These involve lowering or even removing the Nordkette range, which constitutes the northern side wall of the east–west-oriented Inn Valley, and closing the Stubai Valley, which is the northernmost and largest tributary of the Wipp Valley. A comparison with surface and lidar observations indicates that our present model resolution of 467 m significantly improves the realism of the simulations compared to a resolution of 800 m, as used in a previous study. The Nordkette is found to have a twofold impact on the dynamics of foehn breakthrough into the Inn Valley. In reality, this mountain chain deflects part of the southerly foehn current coming from the Wipp Valley into the perpendicularly oriented Inn Valley. Our sensitivity tests indicate that this flow deflection tends to accelerate the foehn breakthrough into the Inn Valley, while upstream blocking effects induced by the Nordkette act to slow down the process of foehn breakthrough. The flow pattern in the Wipp Valley reveals that the upstream effects of the Nordkette are not quite far-reaching. The amplitude of the gravity waves over the lower Wipp Valley gets somewhat reduced by these upstream effects, but the overall flow pattern remains largely unaffected. Closing the Stubai Valley also has a minor effect of the wave structure and tends to reduce the cross-valley variability of the foehn flow in the lower Wipp Valley.     

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.     

North foehn in the Austrian Inn Valley: A case study and idealized numerical simulations

Summary This study presents high-resolution numerical simulations of north foehn in the Austrian Inn Valley which have been performed with the Penn State/NCAR mesoscale model MM5. As the Inn Valley is located north of the Alpine crest, north foehn occurs comparatively rarely in this valley, and there are only sparse observations available for this phenomenon. Simulations of the 24 January 1993 case as well as idealized simulations are performed to get a deeper insight into the dynamics of the north foehn. Moreover, the synoptic conditions leading to the occurrence of north foehn in the Inn Valley are investigated. The simulations indicate that there are at least four different paths for the foehn to penetrate into the valley. Two of them are running along side valleys entering the upper Inn Valley from the west. These flow paths appear to be most important when the large-scale flow has a significant westerly component. The other possible flow paths enter the Inn Valley from the northwest or north and require a strong northerly component of the large-scale flow. From a dynamical point of view, north foehn appears to be similar to the well researched south foehn in that vertically propagating gravity waves force the descent of the ambient flow into the valleys. However, there are also indications that trapped lee waves have a significant impact on the surface wind field, which has not been reported for south foehn so far. Moreover, the model results show that a precondition for the formation of north foehn in the Inn Valley is the absence of significant orographic precipitation. Evaporative cooling induced by precipitation falling into subsaturated air not only reduces the surface temperatures but also inhibits the formation of large-amplitude gravity waves, suppressing the development of stormy surface winds.     

South foehn in the Wipp Valley – Innsbruck region: Numerical simulations of the 24 October 1999 case (MAP-IOP 10)

Summary Numerical simulations of the 24 October 1999 south foehn (MAP-IOP 10) are performed with the Penn State/NCAR mesoscale model MM5 for the Wipp Valley and the adjacent parts of the Inn Valley. The model is run in a multiple-nest configuration, the area of interest being resolved at a mesh size of 800m in most experiments. The study serves to complement an earlier work in which typical flow features of the foehn in the Wipp Valley region were investigated by means of idealized simulations, assessing whether it is possible to reproduce the temporal evolution and the spatial structure of a particular foehn case. A further objective of the paper is to examine the dependence of the model performance on the horizontal resolution, giving some information which resolution will probably be needed for future high-resolution forecasts.An encouragingly large part of the observed flow features could be well reproduced in the simulations. Except for a small region to the east of Innsbruck, the foehn breakthrough is predicted correctly to within an hour. The spatial structure of the so-called pre-foehn, an enhanced westerly wind occurring at Innsbruck prior to the breakthrough of the foehn, also agrees very well with the observations. Moreover, the maximum extent of the foehn in the Inn Valley, the structure of the gravity wave field above the Wipp Valley and the upvalley progression of a shallow cold front in the evening are consistent with the observations. Except for a few places where the airmass boundary between the warm foehn air and the adjacent colder air is not captured correctly throughout the time, the simulated surface temperatures range within 2K of the observed values. Discrepancies between the model results and the observations are found in the vicinity of Innsbruck where a flow-splitting phenomenon induces a very complex flow pattern at low levels. Another source of problems is the surface potential temperature along the Wipp Valley. The observed potential-temperature increase between the Brenner Pass and Innsbruck, which appears to be related to turbulent vertical mixing of stably stratified air, is underestimated by the model. Reducing the horizontal resolution from 800m to 1.4km deteriorates the model performance in marginally resolved side valleys, but the results obtained for the Wipp Valley and the Inn Valley are still of high quality.     

Local foehn effects in the upper Isar Valley, part 1: Observations

Summary An unusually strong nocturnal downvalley wind can be regularly observed in the upper Isar Valley close to Mittenwald (Bavarian Alps) when a high-pressure system is located over Central Europe or when ambient southerly winds are present. Due to the structure of the local topography, this downvalley wind has foehn-like properties in the sense that the breakthrough of the flow into the valley is characterized by a strong increase in temperature and a decrease in relative humidity. Therefore the author called this flow Minifoehn. In fact, wind speeds are low in comparison to deep foehn, but gusts may reach values up to 20ms^–1, even under the influence of high pressure systems with weak atmospheric pressure gradients. To investigate the Minifoehn, surface stations have been installed for collecting temperature, humidity, wind and pressure data. Measurements have shown that the Minifoehn represents the upper part of one of the drainage currents which flows over a mountain ridge into the valley at Mittenwald. Nocturnally cooled air drains from a plateau south of Mittenwald through different valleys which merge again near Mittenwald. It seems that the forcing of the nocturnal currents is dominated by the temperature difference between this plateau and the free atmosphere above Mittenwald at the same level. Strong temperature differences are found during clear winter nights and in case of subsidence inversions. Moreover, the appearance of the Minifoehn in autumn and winter is so frequent that we even may find a climatic effect: the upper Isar Valley is usually free of fog during these seasons and nocturnal temperatures are often considerably higher than in other Bavarian Alpine valleys at comparable altitude.     

Mesoscale surface-wind characteristics and potential gravity-wave formation during cross-Alpine airflow

Summary Mesoscale flow characteristics in the Alpine region are deduced from a set of daily large-scale analyses (1981–1990) by means of statistical-dynamical downscaling. This method utilizes the results of a large number of mesoscale numerical simulations in combination with known statistics of the forcing large-scale conditions. The investigation is restricted to cross-Alpine large-scale flow from 165 to 265 degrees at 500 hPa. Such types of flow are favourable to south foehn.The results provide model-based climatological estimates of surface wind direction and upper-level gravity-wave formation at a horizontal resolution of 20 and 10 km. Simulated surface wind roses agree well with observations and show a dominance of low-level flow around the Alps with bimodal frequency distributions of wind direction north and south of the mountains. The areas where splitted flows preferably merge are identified. Gravity waves are most likely to occur above the western parts of the Alps. A secondary maximum of likelihood was found above Tyrol and Trentino. Surface wind roses and gravity-wave formation are both checked with respect to their sensitivity to season (spring vs. autumn) and large-scale flow direction (south to southwest vs. southwest to west).With 13 Figures     

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.     

The exceptional Alpine south foehn event of 14–16 November 2002: a case study

Summary This study examines the exceptional Alpine south foehn event of 14–16 November 2002 using routine observations and high-resolution numerical simulations. Besides its long duration and an extremely high temperature level related to warm-air advection from the northern Sahara, this foehn event exhibited an unusual spatial structure of the low-level wind and temperature field. Whereas the foehn was largely restricted to the first half of 14 November in the western part of the Alps (Switzerland), it extended over the full period in the inner-Alpine valleys in the eastern Alps. The duration and intensity of the foehn also tended to decrease from the Alpine crest towards the northern rim of the Alps. Most surprisingly, continuous foehn even occurred on the windward side of the Alpine crest, namely in a basin located in the southeastern Alps. The distribution of the orographic precipitation associated with the foehn case was unusual as well. In Switzerland, intense precipitation was not restricted to the windward (southern) side of the Alps but extended to the northern side of the Alpine crest, particularly on 16 November. The results indicate that the spatio-temporal distribution of the foehn in the northern Alps was related to the fact that the western Alps were within a synoptic-scale transition zone between extremely warm air advected from the south and colder air lying over western Europe. The colder air was advected around the western Alps whereas extremely warm air descended from the Alpine crest farther east. Moreover, a small cyclone formed on 14 November north of the Alps and generated a shallow cold front propagating eastward along the northern Alps. Thus, the tendency towards foehn decreased from west to east and from the Alpine crest towards the north. The occurrence of foehn on the windward side of the Alpine crest was made possible by the extreme strength of the large-scale southerly flow, combined with the fact that the upstream precipitation field did not reach the southeastern edge of the Alps. Finally, the pronounced spillover of precipitation to the northern side in the Swiss part of the Alps appears to be related to the colder air present north of the crest. This prevented the formation of orographic gravity waves and downslope air motion, which usually leads to a rapid evaporation of the precipitation on the lee side of the Alpine crest.     

Genesis Conditions for Thunderstorm Growth and the Development of a Squall Line in the Northern Alpine Foreland

Summary  Two numerical models are used to investigate aspects of thunderstorm dynamics and thunderstorm initiation in the northern Alpine foreland. The first, an isentropic model of airflow over and around the Alps, is used to investigate flow patterns favourable for the initiation of deep convection in the region. It is found that a stably-stratified southerly flow towards the Alps leads to a southwesterly flow in the Alpine foreland, a situation most often found during thunderstorm periods, and to the formation of a gravity wave in the lee of the Alps. This wave is accompanied by raised isentropes which, in reality, would lead to a reduction in static stability and convective inhibition as well as an increase in convective available potential energy. The second model, a cloud model, is used to study the development of an observed squall line over southern Bavaria. The model is initialized with wind, temperature and moisture profiles from a radiosonde sounding ahead of the squall line and the squall line is initiated by an array of thermal bubbles. The model simulation is used to interpret the evolution of the squall line. Received March 9, 1999/Revised July 10, 1999     

Local foehn effects in the upper Isar Valley, Part 2: numerical simulations

Summary The local wind system in the upper Isar Valley (Bavarian Alps) near Mittenwald has the peculiarity that regularly strong foehn-like nocturnal flows occur, mainly during clear nights in autumn and winter. We will refer to this phenomenon as “Minifoehn”, as its properties are similar to the classical deep foehn in the sense that its breakthrough into the Isar Valley usually brings a striking increase in temperature and a concomitant decrease in relative humidity. Numerical simulations with the MM5 model reveal that this phenomenon is related to a nocturnal drainage flow originating from a plateau south of Mittenwald. This flow is driven by the temperature difference between this plateau (1180 m) and the free atmosphere above Mittenwald (920 m, 15 km north of the plateau) at the same level. The air masses flow through two different valleys that merge again further downstream. The upper part of one of the two drainage currents goes over a small mountain ridge (1180 m) south-west of Mittenwald and then descends into the Isar Valley, leading to an advection of potentially warm air towards Mittenwald. This branch of the drainage current constitutes the Minifoehn. The remaining part of the drainage current flows through a narrow gap towards the Isar Valley and then joins the drainage flow of this valley. As these air masses are significantly cooler than the Minifoehn branch, large horizontal temperature gradients can be found around Mittenwald. The dynamical behaviour of the cold air flow turns out to be qualitatively consistent with shallow-water theory only in the absence of a forcing by large-scale winds. Otherwise, gravity-wave induced pressure perturbations interact with the drainage flow and modify the low-level flow field. The simulations show that the gravity waves are excited by the mountain range that separates the two valleys mentioned above. Moreover, the simulations indicate that the structure of this nocturnal wind system is not very sensitive to the direction of synoptic-scale winds as long as they come from the southern sector. On the other hand, ambient northerly winds are able to prevent the drainage flow and therefore the local foehn effects in the Isar Valley provided that synoptic winds are strong enough. The results of the MM5 simulations are in good agreement with the measurements and observations described in part 1 of this study.     

Deep and shallow south foehn in the region of Innsbruck: Typical features and semi-idelized numerical simulations

Summary ?Numerical simulations of the south foehn in the region of Innsbruck are presented. They are semi-idealized in the sense that realistic orography but idealized initial and boundary conditions are used. The focus of this study is on typical features of the fully developed foehn, the breakthrough phase of the foehn and the diurnal cycle of the foehn. In addition, the impact of the large-scale wind direction is examined, including conditions leading to shallow foehn. The simulated flow fields have been found to be in very good agreement with observations except for a few minor details. In the lower part of the Sill Valley (the valley going from the Brenner pass down to Innsbruck), the wind speed is significantly higher than in the upper part. The acceleration can be traced back to the three-dimensional propagation of gravity waves excited over the adjacent mountain ridges. The amplitude of the gravity waves over the various mountain ridges depends sensitively on the wind direction, large wave amplitudes occurring only when the angle between the wind direction and the ridge line is not too small. For southwesterly or south–southwesterly large-scale flow, wave amplitudes are significantly larger to the east of Innsbruck than to the west. Foehn breakthrough at Innsbruck is usually preceded by a moderate westerly (downvalley) wind that is restricted to a rather small area around Innsbruck. The simulations reveal that this so-called pre-foehn is mainly a consequence of the gravity wave asymmetry, producing an asymmetric pressure perturbation with lower pressure to the east of Innsbruck. Shallow foehn, defined as a foehn occurring when the large-scale flow at crest height (700 hPa) is approximately westerly, is associated with relatively weak wave activity along the Sill Valley. It is found that at least a weak southerly wind component below crest height is necessary to maintain a significant shallow foehn over a longer time. Received October 10, 2001; accepted June 20, 2002 Published online: February 20, 2003     

The Summertime Great Plains Low Level Jet And The Effect of Its Origin On Moisture Transport

Radiosonde data from six stations in Kansas and Oklahoma for the period of June 16–24, 1993 indicate that a low-level jet (LLJ) occurred almost every day except on the 20th. Major characteristics of these LLJs are documented in this paper. The maximum wind speed (the jet speed) varied from 13 to 32 m s^-1 and heights ranged from 167 to 910 m. All the jets were southerly except the one on June 19 which changed its direction dramatically from a southerly to a northerly direction in about three hours although its intensity did not change appreciably. Thermal stability of the boundary layer during these LLJ occurrences ranged from near-neutral to highly stable. All the low-level jets exhibited significant diurnal variations. Analyses show that relatively weak large-scale forcing existed for the LLJs on June 21 and June 22, while strong forcing was present on other days. Analyses also show that moisture transport by the LLJ from the Gulf of Mexico to the Great Plains depends on the location of the LLJ origin. In the two weeks of June 13–19 and 20–26, 1993, powerful storms swept through the central United States, accompanied by tornadoes, strong wind, large hail and heavy rainfall. The analyses indicate that these weather events could be a result of the interactions of the LLJs with synoptic-scale flow.     

Impact of Surface Potential Vorticity Density Forcing over the Tibetan Plateau on the South China Extreme Precipitation in January 2008. Part Ⅰ:Data Analysis

The external source/sink of potential vorticity (PV) is the original driving force for the atmospheric circulation. The relationship between surface PV generation and surface PV density forcing is discussed in detail in this paper. Moreover, a case study of the extreme winter freezing rain/snow storm over South China in January 2008 is performed, and the surface PV density forcing over the eastern flank of the Tibetan Plateau (TP) has been found to significantly affect the precipitation over South China in this case. The TP generated PV propagated eastward in the middle troposphere. The associated zonal advection of positive absolute vorticity resulted in the increasing of cyclo-nic relative vorticity in the downstream region of the TP. Ascending air and convergence in the lower troposphere developed, which gave rise to the development of the southerly wind. This favored the increasing of negative meridio-nal absolute vorticity advection in the lower troposphere, which provided a large-scale circulation background conducive to ascending motion such that the absolute vorticity advection increased with height. Consequently, the ascending air further strengthened the southerly wind and the vertical gradient of absolute vorticity advection between the lower and middle troposphere in turn. Under such a situation, the enhanced ascending, together with the moist air transported by the southerly wind, formed the extreme winter precipitation in January 2008 over South China.     

双流机场“7.21”暴雨微物理特征及触发维持机制分析

利用天气雷达、地面自动站和微波辐射仪等多种气象探测资料,对2017年7月发生在成都双流机场的一次暴雨过程进行了分析。结果表明:此次暴雨发生在弱天气系统强迫条件下,大气层结呈现弱对流抑制、低抬升凝结高度、中等对流有效位能,湿层深厚,低层较暖且低层无急流影响。短时强降水由中尺度系统直接产生,午夜前的初始对流由高压西北部偏南暖湿气流与山体下滑冷气流相互作用,结合山前强水平温度梯度产生,之后在冷池和边界层暖湿气流作用下生成新的对流。产生强降水的回波结构密实,暖云特征突出,属于热带低质心降水系统。对抬升凝结高度、自由对流高度、湿层厚度等的分析表明,水汽条件较为极端,但由于系统整体属于前向传播,无明显的"列车效应",限制了实际降水效率。     

Cutoff low systems and their relevance to large-scale extreme precipitation in the European Alps

In this paper, we attempt to highlight the relevance of cutoff low systems (CoLs) to large-scale heavy precipitation events within the Alpine region which often lead to catastrophic flooding. The main results of this study are (1) a detailed climatology (1971–1999) of CoLs for the European region, (2) contribution of CoLs to extreme precipitation events in the European Alpine region, (3) identification of regions within the European Alps most affected by extreme precipitation caused by CoLs, and (4) identification of regions where presence of CoLs is related to extreme precipitation in the Alpine region. The findings of this paper suggest that CoLs have a significant correlation with extreme precipitation events and strongly influence the climate of the Alpine region. The total contribution of CoLs to large-scale heavy precipitation events ranges between 20 and 95 % and is most pronounced in the northern and eastern parts of the Alps. More than 80 % of the events occur in the summer season. The area around the Alps and West of Spain (over the Atlantic Ocean) is the most affected region. The location of the center of CoLs that affect the Alpine region most occur on the northern and southern sides of the Alpine ridge.     

Moisture transport from the Atlantic to the Pacific basin and its response to North Atlantic cooling and global warming

Atmospheric moisture transport from the Atlantic to the Pacific basin plays an important role in regulating North Atlantic salinity and thus the strength of the thermohaline circulation. Potential changes in the strength of this moisture transport are investigated for two different climate-change scenarios: North Atlantic cooling representative of Heinrich events, and increased greenhouse gas (GHG) forcing. The effect of North Atlantic cooling is studied using a coupled regional model with comparatively high resolution that successfully simulates Central American gap winds and other important aspects of the region. Cooler North Atlantic sea surface temperature (SST) in this model leads to a regional decrease of atmospheric moisture but also to an increase in wind speed across Central America via an anomalous pressure gradient. The latter effect dominates, resulting in a 0.13 Sv (1 Sv = 10⁶ m³ s^?1) increase in overall moisture transport to the Pacific basin. In fresh water forcing simulations with four different general circulation models, the wind speed effect is also present but not strong enough to completely offset the effect of moisture decrease except in one model. The influence of GHG forcing is studied using simulations from the Intergovernmental Panel on Climate Change archive. In these simulations atmospheric moisture increases globally, resulting in an increase of moisture transport by 0.25 Sv from the Atlantic to Pacific. Thus, in both scenarios, moisture transport changes act to stabilize the thermohaline circulation. The notion that the Andes effectively block moisture transport from the Atlantic to the Pacific basin is not supported by the simulations and atmospheric reanalyses examined here. This indicates that such a blocking effect does not exist or else that higher resolution is needed to adequately represent the steep orography of the Andes.     

Regional climate simulations for the European Alpine Region��sensitivity of precipitation to large-scale flow conditions of driving input data

The MM5 modelling system has been used to perform regional climate simulations over Western Europe on a 45-km grid for the years 1971 to 2000. We focus our analysis on the impact of the driving input data on simulated precipitation in the Alpine area. Using ERA40 reanalysis data, the MM5 climatology of precipitation compares reasonably well with an observational climatology for the Alpine region. Switching to an ECHAM5 climate simulation as driving data induces excessive overprediction by up to 80% in the colder seasons there, primarily over the Alpine slopes. The large-scale flow provided by the global datasets revealed moderate differences indicating an increased number of low-pressure systems travelling from the Atlantic into the Alpine region for ECHAM5 compared with ERA40. Mean seasonal 700-hPa wind speeds correspondingly showed higher values for the ECHAM5 driven simulation in the central Alps. Partitioning three-hourly 700-hPa winds according to direction and speed in the central Alps specifically revealed a distinct shift to stronger westerly and north-westerly winds. Furthermore, aggregating three-hourly rainfall amounts to the same wind direction and wind speed intervals as for the wind statistics revealed strongly intensified precipitation due to the overly intense westerly winds, implying too intense orographic precipitation enhancement.     

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.     

Present and future near-surface wind climate of Greenland from high resolution regional climate modelling

The present and twenty-first century near-surface wind climate of Greenland is presented using output from the regional atmospheric climate model RACMO2. The modelled wind variability and wind distribution compare favourably to observations from three automatic weather stations in the ablation zone of southwest Greenland. The Weibull shape parameter is used to classify the wind climate. High values (κ > 4) are found in northern Greenland, indicative of uniform winds and a dominant katabatic forcing, while lower values (κ < 3) are found over the ocean and southern Greenland, where the synoptic forcing dominates. Very high values of the shape parameter are found over concave topography where confluence strengthens the katabatic circulation, while very low values are found in a narrow band along the coast due to barrier winds. To simulate the future (2081–2098) wind climate RACMO2 was forced with the HadGEM2-ES general circulation model using a scenario of mid-range radiative forcing of +4.5 W m^?2 by 2100. For the future simulated climate, the near-surface potential temperature deficit reduces in all seasons in regions where the surface temperature is below the freezing point, indicating a reduction in strength of the near-surface temperature inversion layer. This leads to a wind speed reduction over the central ice sheet where katabatic forcing dominates, and a wind speed increase over steep coastal topography due to counteracting effects of thermal and katabatic forcing. Thermally forced winds over the seasonally sea ice covered region of the Greenland Sea are reduced by up to 2.5 m s^?1.     

Dynamic and thermal effects on surface airflow associated with southerly changes over the South Island,New Zealand

Summary The Southerly Change Experiment (SOUCHEX) was conducted to examine the influence of the New Zealand Southern Alps on the structure and evolution of cold fronts, locally called southerly changes, as they travel up the east coast. The extensive data obtained by the augmented surface weather station network is used to examine in detail the mesoscale wind field associated with the events observed during the experiment. A comparison of the wind fields observed during the different events illustrates the influence of local dynamic and thermal factors. In particular, lee trough-induced northeasterlies and thermally developed diurnal wind systems are seen to interact with the wind field created by the passage of the front over the Southern Alps.It is apparent that the wind field associated with southerly changes responds to a variety of factors as the cold fronts propagate northwards. For example, there is a tendency for the flow to turn onshore producing a southeast wind during daytime over the Canterbury Plains south of Banks Peninsula probably due to diabatic heating of the mountains and plains. This onshore flow is in direct opposition to pre-frontal foehn northwesterly flow which often continues in the mountain regions and aloft after the front has moved up the coast. The interaction of these air masses over Canterbury creates difficulties for local forecasting. Also, the nocturnal passage of a southerly change is often difficult to detect in surface anemograph traces because of the decoupling of the boundary layer air from that above, producing low level drainage flow over the Canterbury Plains. The overall effect is to create a complex mesoscale wind field resulting from interaction of cold fronts with regional orographic and thermal influences.With 8 Figures