arXiv subjects

Ivana Stiperski

Publications and source records attributed to Ivana Stiperski.

10 recordsLinked to original sources

arXiv2026

The TEAMx Observational Campaign

As part of the international research programme TEAMx (multi-scale transport and exchange processes in the atmosphere over mountains - programme and experiment) a one-year long measurement campaign, the TEAMx Observational Campaign (TOC), was conducted between 2024 and 2025 in a north-south transect through the Alps. Building on the dense operational measurement network in the Alps, the TOC was designed to collect long-term atmospheric observations over the highly complex Alpine terrain. During two six-week long Extended Observational Periods, more than 40 research institutions came together to instrument about 30 sites in the four target areas of the TEAMx domain and study different transport processes, from gravity waves to orographic convection, thermally driven flows, and turbulent exchange. In addition to a suite of ground-based in-situ and remote-sensing instruments, observational activities included airborne measurements with up to three research aircraft and multiple UAS. This paper gives an overview of the science goals and the TOC design, together with preliminary analyses that highlight the potential of the collected dataset.

physics.ao-ph↗

arXiv2025

Stratification, turbulence organization, and pressure-strain effects on surface-layer turbulence anisotropy

At large scales, the Reynolds stress tensor exhibits notable anisotropy, a key feature of all wall-bounded turbulent flows. Yet, how the drivers of this anisotropy evolve with shearing and thermal stratification in the atmospheric surface layer (ASL) remains a daunting challenge for theory and models alike. Here, the velocity variance budgets are used to explore the evolution of anisotropy in the daytime ASL close to the surface, region known to be problematic for large eddy simulations. A special focus is placed on the importance of slow and rapid pressure-strain correlations and the role of transport on partitioning the turbulent kinetic energy among the velocity components. Results obtained from near-surface observations of four datasets over flat and horizontally homogeneous terrain show persistent anisotropy over a wide range of flux Richardson numbers $R_{if}$ and wall-normal distances, and highlight the importance of different processes in three distinct flow regimes, roughly related to dynamic ($|R_{if}|\ll1$), dynamic-convective ($|R_{if}|\sim1$) and convective ($|R_{if}|\gg1$) regimes of the ASL. In particular, close to the surface in the dynamic-convective regime, a drop in wall-normal velocity variance and a substantial increase of spanwise velocity variance are shown to result from the increasing role of pressure transport and rapid distortion, related to turbulence organization. This behaviour is not captured by the classic Rotta closure but requires the inclusion of both rapid pressure-strain and transport terms. In all regimes wall blocking is found to influence turbulence close to the surface, thus requiring the adoption of an anisotropic Rotta model to accommodate its effects.

physics.flu-dyn↗

arXiv2025

Impact of Heterogeneity on Scalar Flux Variance Relations Across Diverse Ecosystems

Monin-Obukhov Similarity Theory (MOST), the traditional surface layer theory used to understand the behavior, scaling and exchange of heat, water vapor and carbon dioxide between the land surface and atmosphere relies on a number of commonly broken assumptions. In particular, traditional theory breaks down under three different forms of heterogeneity highlighted in this work: spatial heterogeneity in the sources of the scalars, heterogeneity in the Reynolds stress tensor (turbulence anisotropy), and temporal heterogeneity (non-stationarity). The work explores the relationship between the idealized flux-variance relations and these three forms of heterogeneity across a diverse network of 47 flux towers representing a broad range of ecosystems including forests, agricultural land, grasslands, tundra, tropical and arid: the National Ecological Observation Network (NEON). Results use high resolution spatial data (1 meter resolution) to show a direct relationship between spatial heterogeneity and deviation from traditional scaling relations. The study indicates an interplay between stationarity and anisotropy, with the non-dimensionalized scalar variance scaling more strongly with anisotropy under more non-stationary turbulence conditions. Updated flux-variance relations that leverage turbulence anisotropy for the scaling of heat are introduced, as are novel anisotropy-generalized scalings for water vapor and carbon dioxide. The work also explores in detail how the scaling relations, and their relationship with heterogeneity, vary across the diverse sites in the NEON network. Deviations from traditional theory in carbon dioxide scaling in particular are well correlated with the bioactivity of the site. Results have important implications for development of improved surface layer parameterizations in large scale atmospheric models and flux-variance based flux measurements.

physics.ao-ph↗

arXiv2025

Revealing the drivers of turbulence anisotropy over flat and complex terrain: an interpretable machine learning approach

Turbulence anisotropy was recently integrated into Monin-Obukhov Similarity Theory (MOST), extending its applicability to complex terrain and diverse surface conditions. Implementing this generalized MOST in numerical models, however, requires understanding the key drivers of turbulence anisotropy across various terrain conditions. This study therefore employs random forest models trained on measurement data from both flat and complex terrain and including upstream terrain features, to predict turbulence anisotropy. Two approaches were compared: using dimensional variables directly or employing non-dimensional groups as model input. To address cross-correlation among features, we developed a new selection method, Recursive Effect Elimination. Finally, interpretability methods were used to identify the most influential variables. Contrary to expectations, variables related to terrain influence were not found to significantly impact turbulence anisotropy. Instead, non-dimensional groups of common turbulence length, time and velocity scales proved more robust than dimensional variables in isolating anisotropy drivers, enhancing model performance over complex terrain and reducing location dependence. A ratio of integral and turbulence memory length scales was found to correlate well with turbulence anisotropy in both daytime and nighttime conditions, both over flat and complex terrain. During the day, a refined stability parameter incorporating both the surface and mixed layer scaling emerged as the dominant driver of anisotropy, while at night, parameters related to rapid distortion were strong predictors.

physics.flu-dyn↗

arXiv2025

Evaluating Anisotropy-based Monin-Obukhov Similarity Theory over Canopies and Complex Terrain

Monin Obukhov Similarity Theory (MOST) has long served as the basis for parameterizations of turbulence exchange between the surface and the atmospheric boundary layer in models for weather and climate prediction. Decades of research, however, has illuminated some of the limitations of MOST based surface layer parameterizations, particularly when MOST's foundational assumptions of flat and horizontally homogeneous terrain are violated. Recent work has leveraged the anisotropy of turbulence as an additional non-dimensional term to extend and generalize MOST to complex terrain. In this work, we examine the performance of this generalized MOST for the scaling of velocity variances, refit these scalings, and study key characteristics of turbulence anisotropy across the 47 towers in the wide ranging National Ecological Observation Network (NEON). NEON in particular covers a diverse selection of ecosystems, from the arctic circle to tropical islands, and as such expands the previous generalized MOST to vegetated canopies and other environments not examined in previous studies. The work finds that anisotropy generalized MOST readily extends to these new environments, with robust performance across seasons over a wide range of canopy and terrain configurations. Results also further illuminate velocity variance scaling across degrees and forms (one-component and two-component) of anisotropy, relate the degree of anisotropy to key environmental characteristics, and evaluate scaling differences between canopies and non-vegetated surfaces across atmospheric conditions, seasons and the diurnal cycle. Overall, this study expands our understanding of the complexity of turbulence while paving the way for improved surface layer parameterizations.

physics.ao-ph↗

arXiv2023

Spectral scaling of unstably-stratified atmospheric flows: understanding the low frequency spread of Kaimal model

For more than five decades, a notable spread in the low frequencies of velocity and temperature spectra, scaled using inertial subrange properties and Monin-Obukhov similarity theory, has been observed in unstable stratification. A large ensemble of 14 datasets, over relatively simple terrain (from flat and homogeneous to gentle slope or valley floor) and very complex terrain (steep slope, crater rim, mountain top), are used to assess the reasons of this low-frequency behaviour. Turbulence anisotropy is shown to be the primary factor influencing the spectral energy at the largest scales and the spectral peak position of streamwise and spanwise velocity spectra, with stability acting as a secondary factor. On the contrary, the low-frequency behaviour of surface-normal and temperature spectra is dominated by stability effects, while turbulence anisotropy plays a secondary role. These observations are valid over both simple and complex terrain, even though variability of the largest scales of complex terrain datasets integrate other processes not included in turbulence anisotropy or stability conditions. Using a combination of non-dimensional turbulence parameters - temperature and velocity variances as well as dissipation of turbulence kinetic energy and of temperature variance - and of a semi-empirical model provided in the literature, we are able to describe the behaviour of the velocity and temperature spectra with only turbulence anisotropy and stability as input parameters. Finally, the paper provides some insights into the nature of anisotropic eddies, as well as the relationship between the boundary layer height and the spectral behaviour.

physics.flu-dyn↗

arXiv2023

Flux gradient relations and their dependence on turbulence anisotropy

Monin-Obukhov similarity theory (MOST) is used in virtually every Earth System Model (ESM) to parameterize the near-surface turbulent exchanges, however there is high uncertainty in the literature about the appropriate parameterizations to be used. In addition, MOST has limitations in very stable and unstable regimes, over heterogeneous terrain and complex orography, and has been found to incorrectly represent the surface fluxes. A new approach including turbulence anisotropy as a scaling parameter has recently been developed, allowing to overcome these limitations and generalize the flux-variance relations to complex terrain. In this paper we analyze the flux-gradient relations for five well known datasets. The scaling relations show substantial scatter and highlight the uncertainty in the choice of parameterization even over canonical conditions. We show that by including information on turbulence anisotropy as an additional scaling parameter, the original scatter becomes well bounded and new formulations can be developed, that drastically improve the accuracy of the flux-gradient relations for wind shear ($ϕ_M$) in unstable conditions, and for temperature gradient ($ϕ_H$) both in unstable and stable regime. This analysis shows that both $ϕ_M$ and $ϕ_H$ are strongly dependent on turbulence anisotropy and allows to finally settle the longly discussed free convection regime for $ϕ_M$, which clearly exhibits a $-{1/3}$ power law when anisotropy is accounted for. Furthermore we show that the eddy diffusivities for momentum and heat and the turbulent Prandtl number are strongly dependent on anisotropy and that the latter goes to zero in the free convection limit. These results highlight the necessity to include anisotropy in the study of near surface atmospheric turbulence and lead the way for theoretically more robust simulations of the boundary layer over complex terrain.

physics.ao-ph↗

arXiv2022

Generalizing Monin-Obukhov similarity theory (1954) for complex atmospheric turbulence

Monin-Obukhov similarity theory (MOST) stands at the center of understanding of atmospheric surface-layer turbulence. Based on the hypothesis that a single length scale determined by surface fluxes governs the turbulent exchange of momentum, mass, and energy between the Earth's surface and the atmosphere, MOST has been widely used in parametrizations of turbulent processes in virtually all numerical models of atmospheric flows. However, its inherent limitations to flat and horizontally homogeneous terrain, the non-scaling of horizontal velocity variances, and stable turbulence have plagued the theory since its inception. Thus, the limitations of MOST, and its use beyond its intended range of validity contribute to large uncertainty in weather, climate, and air-pollution models, particularly in polar regions and over complex terrain. Here we present a first generalized extension of MOST encompassing a wide range of realistic surface and flow conditions. The novel theory incorporates information on the directionality of turbulence exchange (anisotropy of the Reynolds stress tensor) as a key missing variable. The constants in the standard empirical MOST relations are shown to be functions of anisotropy, thus allowing a seamless transition between traditional MOST and its novel generalization. The new scaling relations, based on measurements from 13 well-known datasets ranging from flat to highly complex mountainous terrain, show substantial improvements to scaling under all stratifications. The results also highlight the role of anisotropy in explaining general characteristics of complex terrain and stable turbulence, adding to the mounting evidence that anisotropy fully encodes the information on the complexity of the boundary conditions.

physics.ao-ph↗

arXiv2021

Large-eddy Simulations of the Atmospheric Boundary Layer over an Alpine Glacier: Impact of Synoptic Flow Direction and Governing Processes

The mass balance of mountain glaciers is of interest for several applications (local hydrology or climate projections), and turbulent fluxes can be an important contributor to glacier surface mass balance during strong melting events. The underlying complex terrain leads to spatial heterogeneity and non-stationarity of turbulent fluxes. Due to the contribution of thermally-induced flows and gravity waves, exchange mechanisms are fully three-dimensional, instead of only vertical. Additionally, glaciers have their own distinct microclimate, governed by a down-glacier katabatic wind, which protects the glacier ice and interacts with the surrounding flows on multiple scales. In this study, we perform large-eddy simulations with the WRF model with dx=48 m to gain insight on the boundary-layer processes over an Alpine valley glacier, the Hintereisferner (HEF). We choose two case studies from a measurement campaign (August 2018) with different synoptic wind directions (South-West and North-West). Model evaluation with an array of eddy-covariance stations on the glacier tongue and surroundings reveals that WRF is able to simulate the general glacier boundary-layer structure. Under southwesterly airflow, the down-glacier wind is supported by the South-Western synoptic wind direction, a stable boundary layer is present over the ice surface, and local processes govern the turbulence kinetic energy production. Under northwesterly airflow, a cross-glacier valley flow and a breaking gravity wave lead strong turbulent mixing and to the subsequent erosion of the glacier boundary layer. Stationarity analyses of the sensible heat flux suggest non-stationary behaviour for both case study days, while non-stationarity is highest on the NW day during the gravity-wave event. These results suggest that the synoptic wind direction has, in addition to upstream topography and the atmospheric stability, a strong impact on whether a local glacier boundary layer can form or not, influencing whether a glacier is able to maintain its own microclimate.

physics.ao-ph↗

arXiv2019

Scale interactions and anisotropy in stable boundary layers

Regimes of interactions between motions on different time-scales are investigated in the FLOSSII dataset for nocturnal near-surface stable boundary layer (SBL) turbulence. The non-stationary response of turbulent vertical velocity variance to non-turbulent, sub-mesoscale wind velocity variability is analysed using the bounded variation, finite element, vector autoregressive factor models (FEM-BV-VARX) clustering method. Several locally stationary flow regimes are identified with different influences of sub-meso wind velocity on the turbulent vertical velocity variance. In each flow regime, we analyse multiple scale interactions and quantify the amount of turbulent variability which can be statistically explained by external forcing by the sub-meso wind velocity. The state of anisotropy of the Reynolds stress tensor in the different flow regimes is shown to relate to these different signatures of scale interactions. In flow regimes under considerable influence of the sub-mesoscale wind variability, the Reynolds stresses show a clear preference for strongly anisotropic, one-component states. These periods additionally show stronger persistence in their dynamics, compared to periods of more isotropic stresses. The analyses give insights on how the different topologies relate to non-stationary turbulence triggering by sub-mesoscale motions.

physics.flu-dyn↗