Periodic Reporting for period 2 - Unicorn (Developing a novel framework for understanding and scaling near-surface turbulence in complex terrain)
Reporting period: 2022-12-01 to 2024-05-31
Turbulence in the atmospheric boundary layer (lowest layer of the Earth’s atmosphere) plays an essential role in weather and climate, as it is the key process through which the heat, moisture, mass (e.g. various gases, pollutants), and momentum are transported between the Earth’s surface and the atmosphere. Accurately representing the effects of this turbulent exchange in numerical models is therefore crucial for accurate weather predictions and climate projections. Directly modeling turbulence in numerical models is, however, impossible for any practical purpose due to turbulence’s chaotic nature across different scales. Currently, virtually all numerical models of atmospheric and oceanic flows therefore parametrize this exchange though a statistical approach called Monin-Obukhov similarity theory (MOST). Developed in the mid-20th century, MOST is based on assumptions of flat and uniform terrain under specific atmospheric conditions which are not met over the majority of the Earth’s land surface. Over mountains or polar regions, MOST routinely fails. This failure is manifested through large scatter in MOST predictions, as turbulence is deformed by terrain-induced processes and atmospheric stratification in as yet not fully understood way. This mismatch and the use of MOST beyond its limits of applicability adds uncertainty to weather prediction and climate projections. The overarching goal of Unicorn is to increase our understanding of complex terrain turbulence, and modernize its modeling by taking into account the influence of topography and atmospheric stratification. With this knowledge Unicorn aims to develop a novel turbulence theory valid over terrain types ranging from flat and uniform to highly complex, and over a wide range of atmospheric stratification. This will ultimately lead to parametrizations able to correctly represent the effects of complex-terrain turbulence in models of atmospheric and oceanic flows. The new theory and derived parametrizations will improve the accuracy of weather forecasts and climate projections in regions most susceptible to climate change, such as the polar zones and mountainous areas, thereby supporting more effective climate adaptation and mitigation strategies, as well as help in resource management, preparing for extreme weather events, and making informed decisions affecting environmental and public safety.
The Unicorn team has successfully confirmed the central hypothesis of the project: that turbulence anisotropy, or the directional distribution of energy in turbulence, can explain the observed scatter in MOST relations across a range of terrain complexity. By analyzing a unique ensemble of measurement datasets, the team has established new statistical relations for a wide range of MOST predictions that more accurately describe turbulence, in a first successful approach over complex terrain, thus advancing towards a comprehensive theory of atmospheric turbulence in complex environments. The work for the first time convincingly shows significant inaccuracies in the current models for how wind changes with height in unstable atmospheric conditions. The importance of this research was underscored by the publication of the first paper in Physical Review Letters, which the journal recognized as an Editor’s Selection.
The team has been involved in two notable measurement campaigns, HEFEX II and MoHATS. HEFEX II (Second Hintereisferner Experiment), campaign on the Hintereisferner glacier in Austria, in summer 2023, has been the largest deployment of turbulence measurements on a glacier, aiming to study the multiscale turbulent exchange between the glacier and atmosphere and the impact of topography and stratification on turbulence. MoHATS (Mountain Horizontal Array Turbulence Study) evaluated the three-dimensional turbulence budgets and served as a preliminary study for the upcoming TEAMx Campaign in 2025.
Alongside their empirical work, the PI organized a workshop commemorating a century of turbulence study “Workshop on Atmospheric Turbulence: 100 years of turbulence Innsbruck 1922-2022”, which brought together leading international experts to discuss the major challenges and future of atmospheric turbulence research, and showcased Unicorn’s contributions. The project's findings have been shared broadly within the Earth sciences community through publications, press releases, and invited and keynote presentations at various international academic events and seminars.
The team has been involved in two notable measurement campaigns, HEFEX II and MoHATS. HEFEX II (Second Hintereisferner Experiment), campaign on the Hintereisferner glacier in Austria, in summer 2023, has been the largest deployment of turbulence measurements on a glacier, aiming to study the multiscale turbulent exchange between the glacier and atmosphere and the impact of topography and stratification on turbulence. MoHATS (Mountain Horizontal Array Turbulence Study) evaluated the three-dimensional turbulence budgets and served as a preliminary study for the upcoming TEAMx Campaign in 2025.
Alongside their empirical work, the PI organized a workshop commemorating a century of turbulence study “Workshop on Atmospheric Turbulence: 100 years of turbulence Innsbruck 1922-2022”, which brought together leading international experts to discuss the major challenges and future of atmospheric turbulence research, and showcased Unicorn’s contributions. The project's findings have been shared broadly within the Earth sciences community through publications, press releases, and invited and keynote presentations at various international academic events and seminars.
Unicorn has already progressed beyond the state of the art by demonstrating the viability of a turbulence theory over complex mountainous terrain. The work of the team has provided a compelling explanation for the significant scatter observed in the application of Monin-Obukhov Similarity Theory (MOST) over such landscapes, through the variations of anisotropy between the different types of terrain and atmospheric conditions. The results set a new benchmark for the study and characterization of turbulence in complex environments. By the end of the project, it is expected that a robust set of predictive models that account for anisotropic effects will be achieved, based on variables and driving mechanisms readily available in numerical models. This will allow new turbulence parametrizations that account for complexity to be implemented in the numerical models of atmospheric and oceanic flows. In addition, a theoretical understanding on the role of complex topography on turbulence and its anisotropic structure is expected to be achieved.