A Collaborative Effort to Better Understand, Measure, and Model Atmospheric Exchange Processes over Mountains

Abstract

In this essay, we highlight some challenges the atmospheric community is facing concerning adequate treatment of flows over mountains and their implications for numerical weather prediction (NWP), climate simulations and impact modeling. With recent increases in computing power (and hence model resolution) numerical models start to face new limitations (such as numerical instability over steep terrain). At the same time there is a growing need for sufficiently reliable NWP model output to drive various impact models (for hydrology, air pollution, agriculture, …). The input information for these impact models is largely produced by the boundary layer (BL) parameterizations of NWP models. All known BL parameterizations assume flat and horizontally homogeneous surface conditions, and their performance and interaction with resolved flows is massively understudied over mountains – hence their output may be accidentally acceptable at best. We therefore advocate the systematic investigation of the so-called ‘Mountain Boundary Layer’ (MoBL), introduced to emphasize its many differences to the BL over flat and horizontally homogeneous terrain. An international consortium of scientists has launched a research program (TEAMx: ‘Multi-scale transport and exchange processes in the atmosphere over mountains – programme and experiment’) to address some of the most pressing scientific challenges. TEAMx is endorsed by World Weather Research Programme (WWRP), and the Global Energy and Water Exchanges (GEWEX) project as a ‘cross-cutting project’. A program coordination office was established at the University of Innsbruck (Austria). This essay introduces the background to and content of a recently published White Paper outlining the key research questions of TEAMx.

Exploring the daytime boundary layer evolution based on Doppler spectrum width from multiple coplanar wind lidars during CROSSINN

“…On the other hand, although we have been able to provide highly resolved cross-valley transects of z i and w L , our conclusions were nonetheless restricted to having just three discrete sites offering H. To truly explore the CBL growth framework, similar highly resolved transects of H are necessary, a demand potentially fulfilled with an array of carefully sited scintilometers (Ward, 2017). We are confident that the upcoming Multi-scale Transport and Exchange Processes in the Atmosphere over Mountains (TEAMx) programme and experiment (Serafin et al, 2020;Rotach et al, 2022) will offer the necessary means and resources to address these remaining challenges.…”

“…Here we focus on dry rather than on moist convection, as the latter often results in deep, precipitous convective cells (Kirshbaum et al, 2018), thereby perturbing the gentle balance of quiescent conditions necessary for fairweather development of CBL and MoBL. While the assumption of vertical exchange being responsible for the majority of convectively-driven turbulence in the CBL is acceptable over HHF terrain, this cannot be expected to hold over complex terrain, where horizontal motions and associated effects become increasingly more important to consider (Rotach et al, 2015(Rotach et al, , 2022Lehner and Rotach, 2018). In addition, a multitude of non-turbulent transport mechanisms are made possible by the presence of mountain chains (Whiteman, 2000;Zardi and Whiteman, 2013;De Wekker and Kossmann, 2015;Serafin et al, 2018).…”

The Performance of GRAMM-SCI and WRF in Simulating the Surface-Energy Budget and Thermally Driven Winds in an Alpine Valley

“…2022 ), and the upcoming TEAMx program (Multi-Scale Transport and Exchange Processes in the Atmosphere over Mountains-Program and Experiment; Rotach et al. 2022 ) in highly complex, mountainous terrain will provide important observational data for future model evaluation studies. Already ongoing model intercomparison studies in the framework of TEAMx, in which both WRF and GRAMM-SCI are participating, will also shed further light on the performance of mesoscale models in mountainous areas and help to identify and hopefully improve existing deficiencies.…”

Energy and mass exchange at an urban site in mountainous terrain – the Alpine city of Innsbruck

“…Besides basic meteorological variables and in situ fast-response measurements of wind, temperature, water vapour and carbon dioxide, many trace gases and aerosols are also observed (e.g. Karl et al, 2017;Deventer et al, 2018;Karl et al, 2018), in addition to vertical profiles of wind using a Doppler lidar (Haid et al, 2020(Haid et al, , 2021 and temperature and humidity using a microwave radiometer (Rotach et al, 2017). The focus of this study is on the surface-atmosphere exchange of momentum, heat and mass observed using the eddy covariance (EC) technique.…”

“…a cool sea breeze versus warm foehn; Hirsch et al, 2021). There is a real need for turbulence observations to develop process understanding, evaluate model performance and improve predictive capabilities in complex terrain, particularly when meteorologically based tools and expertise are used to inform planning or policy decisions that have direct consequences for human and environmental health (Rotach et al, 2022). Measures that have been successfully applied to other cities may have inadvertent effects when applied to a different city, especially one with very different surroundings.…”