Mountainous terrain exerts an important influence on the Earth's atmosphere and affects atmospheric transport and mixing at a wide range of temporal and spatial scales. The vertical scale of this transport and mixing is determined by the height of the atmospheric boundary layer, which is therefore an important parameter in air pollution studies, weather forecasting, climate modeling, and many other applications. It is recognized that the spatio-temporal structure of the daytime convective boundary layer (CBL) height is strongly modified and more complex in hilly and mountainous terrain compared to flat terrain. While the CBL over flat terrain is mostly dominated by turbulent convection, advection from multi-scale thermally driven flows plays an important role for the CBL evolution over mountainous terrain. However, detailed observations of the CBL structure and understanding of the underlying processes are still limited. Characteristics of CBL heights in mountainous terrain are reviewed for dry, convective conditions. CBLs in valleys and basins, where hazardous accumulation of pollutants is of particular concern, are relatively well-understood compared to CBLs over slopes, ridges, or mountain peaks. Interests in the initiation of shallow and deep convection, and of budgets and long-range transport of air pollutants and trace gases, have triggered some recent studies on terrain induced exchange processes between the CBL and the overlying atmosphere. These studies have helped to gain more insight into CBL structure over complex mountainous terrain, but also show that the universal definition of CBL height over mountains remains an unresolved issue. The review summarizes the progress that has been made in documenting and understanding spatio-temporal behavior of CBL heights in mountainous terrain and concludes with a discussion of open research questions and opportunities for future research.

A pragmatic approach to estimate the impact of climate change on the urban environment, here called the cuboid method, is presented. This method allows one to simulate the urban heat load and the frequency of air temperature threshold exceedances using only eight microscale urban climate simulations for each relevant wind direction and time series of daily meteorological parameters either from observations or regional climate projections. Eight representative simulations are designed to encompass all major potential urban heat-stress conditions. From these representative simulations, the urban-heat-load conditions in any weather situation are derived by interpolation. The presented approach is applied to study possible future heat load in Frankfurt, Germany, using the high-resolution Microscale Urban Climate Model in three dimensions (MUKLIMO_3). To estimate future changes in heat-load-related climate indices in Frankfurt, climate projections from the regional climate models Max Planck Institute Regional Model (REMO), Climate Limited-Area Model (CLM), Wetterlagen-basierte Regionalisierungsmethode (WETTREG), and Statistical Regional Model (STAR) are used. These regional climate models are driven by the ''ECHAM5'' general circulation model and Intergovernmental Panel on Climate Change emission scenario A1B. For the mean annual number of days with a maximum daily temperature exceeding 258C, a comparison between the cuboid method results from observed and projected regional climate time series of the period 1971-2000 shows good agreement, except for CLM for which a clear underestimation is found. On the basis of the 90% significance level of all four regional climate models, the mean annual number of days with a maximum daily temperature exceeding 258C in Frankfurt is expected to increase by 5-32 days for 2021-50 as compared with 1971-2000.

No abstract

No abstract

No abstract

No abstract

In Christchurch and other Canterbury towns on the east coast of New Zealand's South Island, poor dispersion conditions and high emissions of particulate matter and carbon monoxide regularly lead to the build-up of smog during anticyclonic weather conditions in wintertime. This study analyses surface wind fields during smog nights using data collected during the Christchurch Air Pollution Study 2000 (CAPS2000). Westerly land breezes and drainage winds from the foothills of the Southern Alps and the Canterbury Plains are evident in all coastal Canterbury towns, but local terrain features often complicate the wind fields. This holds particularly for Christchurch, where Banks Peninsula causes flow splitting of both the drainage winds on the Canterbury Plains and any superimposed larger scale winds. Furthermore, stagnation of airflow over Christchurch is often caused by the convergence of localized southeasterly drainage winds down the western part of the Banks Peninsula (Port Hills) with the regional-scale northwesterly drainage winds from the Canterbury Plains. Results illustrate the interaction of cold air drainage over Christchurch, although the unusually high frequency of synopticscale easterly winds during the study period appeared to reduce the effects of both these sources of cold air drainage on air pollution concentrations. The effects of topographically induced flow splitting and non-stationary drainage wind convergence on urban air pollution dispersion identified here have not previously been studied in detail internationally and should receive greater attention in the future.

This paper reviews the benefits of a better understanding of atmospheric exchange processes over mountains. These processes affect weather and climate variables that are important in meteorological applications related to many scientific disciplines and sectors of the economy. We focus this review on examples of meteorological applications in hydrology, ecology, agriculture, urban planning, wind energy, transportation, air pollution, and climate change. These examples demonstrate the benefits of a more accurate knowledge of atmospheric exchange processes over mountains, including a better understanding of snow redistribution, microclimate, land-cover change, frost hazards, urban ventilation, wind gusts, road temperatures, air pollution, and the impacts of climate change. The examples show that continued research on atmospheric exchange processes over mountains is warranted, and that a recognition of the potential benefits can inspire new research directions. An awareness of the links between basic research topics and applications is important to the success and impact of new efforts that aim at better understanding atmospheric exchange processes over mountains. To maximize the benefits of future research for meteorological applications, coordinated international efforts involving scientists studying atmospheric exchange processes, as well as scientists and stakeholders representing many other scientific disciplines and economic sectors are required.