Exchange Processes in the Atmospheric Boundary Layer Over Mountainous Terrain

Abstract

Abstract:The exchange of heat, momentum, and mass in the atmosphere over mountainous terrain is controlled by synoptic-scale dynamics, thermally driven mesoscale circulations, and turbulence. This article reviews the key challenges relevant to the understanding of exchange processes in the mountain boundary layer and outlines possible research priorities for the future. The review describes the limitations of the experimental study of turbulent exchange over complex terrain, the impact of slope and valley breezes on the structure of the convective boundary layer, and the role of intermittent mixing and wave-turbulence interaction in the stable boundary layer. The interplay between exchange processes at different spatial scales is discussed in depth, emphasizing the role of elevated and ground-based stable layers in controlling multi-scale interactions in the atmosphere over and near mountains. Implications of the current understanding of exchange processes over mountains towards the improvement of numerical weather prediction and climate models are discussed, considering in particular the representation of surface boundary conditions, the parameterization of sub-grid-scale exchange, and the development of stochastic perturbation schemes.

Prediction of Air Temperature in the Polish Western Carpathian Mountains with the ALADIN-HIRLAM Numerical Weather Prediction System

“…Measurements of air temperature at the points administered by JU were realized in accordance with WMO guidelines [16], and the technical details can be found in Bokwa et al [17]. The stations/measurement points were divided into two groups: (1) representing valley bottoms (Nos. 1, 2, 6, 7, 11, 13, 16, 18, and 19), and (2) representing hill and mountain tops (Nos.…”

“…Tests of new model configurations show that weather prediction for regions with complex topography constitutes a challenging task for the development of numerical weather forecasting (NWF) systems. One internationally coordinated activity is the Transport and Exchange Processes in the Atmosphere over Mountains Experiment (TEAMx) [1]. Regions with highly complex relief are important, not only because of the impact on weather formation, but also the occurrence of processes like katabatic flows and strong temperature inversions caused by stable stratification of the air volume within the valleys.…”

Semi‐idealized simulations of wintertime flows and pollutant transport in an Alpine valley. Part II: Passive tracer tracking

“…The thermally driven flows developed at night may recirculate the pollutants toward their emission sources (Steyn et al, 2013), but they may also represent a source of mixing that favours the plume dispersion (Gudiksen and Shearer, 1989;O'Steen, 2000). As a result, the pollutant dispersion in mountainous terrain is complex because of topographic effects which generate multi-scale transport processes (Adler and Kalthoff, 2014;Lehner and Rotach, 2018;Serafin et al, 2018). These local dynamics may induce a more degraded air quality than would be expected over flat terrain considering the same emissions (Steyn et al, 2013).…”

Semi‐idealized simulations of wintertime flows and pollutant transport in an Alpine valley: Origins of local circulations (Part I)

“…Under quiescent synoptic conditions, mountainous terrains are associated with local dynamics which respond to topographic forcing (Whiteman, 2000;Lehner and Rotach, 2018;Serafin et al, 2018). The first concepts concerning local thermally driven circulations were introduced in the middle of the 20th century by Wagner (1938) and Defant (1949).…”