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.
Highlights
The study of earth–atmosphere exchange processes plays a prominent role in meteorology and in many related disciplines, e.g., climate science, hydrology, cryospheric science, and agricultural science.Exchange is the transfer of fluid properties through a surface, and it may occur either at the boundary of Atmosphere 2018, 9, 102; doi:10.3390/atmos9030102 www.mdpi.com/journal/atmosphereAtmosphere 2018, 9, 102 a fluid or across an imaginary surface within the fluid
The ambiguities posed by data post-processing (e.g., turbulence averaging scale (TAS) determination) reflect a fundamental lack of knowledge about the properties of the flow structures that are responsible for mixing in complex terrain: for instance, how isotropic they are, how their time and length scales are most appropriately defined, how these scales change in time and space, and how they are affected by the terrain geometry
The exchange of heat, mass and momentum in the atmosphere over mountainous terrain is affected by turbulence, thermally driven mesoscale circulations, and synoptic-scale dynamics
Summary
The study of earth–atmosphere exchange processes plays a prominent role in meteorology and in many related disciplines, e.g., climate science, hydrology, cryospheric science, and agricultural science. While these research programmes addressed a broad range of scientific questions, a number of other campaigns targeted specific phenomena, generally in mountain areas with relatively limited geographical extent (ASCOT [26], BLLAST [27], COLPEX [28], COPS [29], CUPIDO [30], DOMEX [31], MATERHORN [32], METCRAX [33], METCRAX II [34], PAP [35], PCAPS [36], Pic 2005 [37], TRACT [38], T-REX [39], VERTIKATOR [40], VOTALP [41], and VTMX [42]) They did not focus primarily on exchange in the ABL over mountains, all of these experimental efforts indirectly led to significant progress in the understanding of the relevant processes. Driven processes and moist convective processes [51] are covered elsewhere in this journal issue
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