Characterizing Turbulence and Transport Processes in Thermally-Driven Slope Winds

Abstract: The atmospheric boundary layer (ABL) in mountainous regions is characterised by a variety of airflows, originating from complex landform forcing, which encompass a range of scales of motion, from synoptic scale flows to very local phenomena, such as the daily-periodic thermally-driven circulations developing over inclines and in the valleys under clear sky and in the absence of major synoptic forcing. These airflows, and turbulence generated therein, affect a variety of processes, including surface-atmosphere exchanges of momentum, energy and mass, and transport across a variety of scales. They may also contribute to the initiation of orographic convection. 
This contribution focuses on the simplest of these flows, namely slope winds, outlines the state of our present understanding, from measurements as well as from numerical model simulations, and highlights still open questions concerning their structure and their representation in terms of similarity.

In particular, the application of classical Monin-Obukhov similarity theory (MOST), original developed for flat horizontal terrain, has been questioned in the literature, both from the theoretical viewpoint, and on the ground of evidence from measurements showing disagreement of observed slope-normal structure of turbulence properties from MOST predictions.

Hence, starting from the same basic grounds on which MOST is built, an alternative theory is proposed for the surface-layer scaling including contributions of along-slope buoyancy force in the momentum equation and of the along-slope advection of warmer/colder air associated with the background stratification, in the energy equation.
It turns out that (1) turbulent surfaces fluxes of momentum and heat are not independent quantities, but rather closely connected, and (2) Obukhov length is still the relevant similarity scale, but the mathematical representation of the slope-normal structure of turbulent properties is quite different from that envisaged by MOST.

Ongoing efforts to investigate these flows under the umbrella of the current research initiative TEAMx - Multi-scale transport and exchange processes in the atmosphere over mountains – programme and experiment (http://www.teamx-programme.org/), in particular under the connected research project “DECIPHER - Disentangling mechanisms controlling atmospheric transport and mixing processes over mountain areas at different space- and timescales”, are also presented.

References

Farina, S., Zardi, D. 2023: Understanding Thermally Driven Slope Winds: Recent Advances and Open Questions. Boundary-Layer Meteorol., 189, 5–52. https://doi.org/10.1007/s10546-023-00821-1 
Farina, S., Marchio, M., Barbano, F., Di Sabatino, S., and D. Zardi, 2023: Characterization of the morning transition over the gentle slope of a semi-isolated massif, J. Appl. Meteor. Climatol. 62, 449–466. https://doi.org/10.1175/JAMC-D-22-0011.1
 

Biography: Dino Zardi is full professor of Atmospheric Physics at the University of Trento (Italy). He got a MSc in Physics cum laude from the University of Bologna (1991) and a PhD in Hydrodynamics from the University of Genova (1995). His research interests focus on boundary layer processes over mountainous terrain and their implications on air quality, agriculture, renewable energy resources, and climate change impacts. He is also Director of the double-degree international MSc programme in Environmental Meteorology, Vice-President of the Italian Association of Atmospheric Sciences and Meteorology (AISAM) and Co-Chief Editor of the Wiley and Royal Meteorological Society journal Meteorological Applications.