Periodic Reporting for period 4 - DarkMix (Illuminating the dark side of surface meteorology: creating a novel framework to explain atmospheric transport and turbulent mixing in the weak-wind boundary layer)
Reporting period: 2021-11-01 to 2023-01-31
Surface meteorology impacts the abundance and quality of life on Earth through the transfer and mixing of light, heat, water, carbon dioxide, and other substances controlling the resources for humans, plants, and animals. However, current theories and models fail when airflows and turbulence are weak during calm cloudless nights leaving weather, climate, and air quality forecasts uncertain. DarkMix aims at a quantum leap in our understanding of atmospheric transport in weak-wind condition relevant for urban air quality, biogeochemical cycling of greenhouse gases in forests, and agricultural frost damage. The main objective is to combine the technological innovation of the first fiber-optics-based high-resolution environmental sensor for temperature and wind with an innovative theory of a novel conceptual framework to answer the above-mentioned environmental processes.
At completion of the project, we conclude that the project was successful in achieving the technological innovation of constructing and validating the first-ever field-scale fiber-optic distributed sensing system able to resolve the spatial structure of fast-lived turbulent atmospheric motions. We name the novel observational technique 'Large Eddy Observation'. LEO is capable of measuring the fast-changing direction of the airflow as well as the heat transported by the air current without any ancillary measurements from classical atmospheric sensors in certain environments. From the results we gained from deploying LEO in combination with classic sensor networks above grasslands in valley bottoms, forests, and an urban area we conclude that the atmospheric motions generating transport and mixing are different across land covers while all occur in weak winds. Motions above the agricultural grassland in the valley favor horizontal transport and longer-lived wind directional changes, while vertical scales are suppressed by the strong vertical temperature differences. In contrast, in forests and urban environments the weak-wind motions are more three dimensional since the vertical temperature differences are weaker and the airflow interacting with trees and built structures creates additional mixing. Despite their differences, we conclude that results start to converge into the theoretical innovation of the new framework. While its construction continues, we formulated a novel universally applicable quantity, which explains vertical mixing and decoupling irrespective of the environment. We have successfully used it to separate mixing regimes in conditions of the Arctic polar night over snow, agricultural lands in mid-latitude, boreal and temperate forests, as well a mid-European city. While the theoretical innovation is not complete yet, we will continue our research in this promising direction.
At completion of the project, we conclude that the project was successful in achieving the technological innovation of constructing and validating the first-ever field-scale fiber-optic distributed sensing system able to resolve the spatial structure of fast-lived turbulent atmospheric motions. We name the novel observational technique 'Large Eddy Observation'. LEO is capable of measuring the fast-changing direction of the airflow as well as the heat transported by the air current without any ancillary measurements from classical atmospheric sensors in certain environments. From the results we gained from deploying LEO in combination with classic sensor networks above grasslands in valley bottoms, forests, and an urban area we conclude that the atmospheric motions generating transport and mixing are different across land covers while all occur in weak winds. Motions above the agricultural grassland in the valley favor horizontal transport and longer-lived wind directional changes, while vertical scales are suppressed by the strong vertical temperature differences. In contrast, in forests and urban environments the weak-wind motions are more three dimensional since the vertical temperature differences are weaker and the airflow interacting with trees and built structures creates additional mixing. Despite their differences, we conclude that results start to converge into the theoretical innovation of the new framework. While its construction continues, we formulated a novel universally applicable quantity, which explains vertical mixing and decoupling irrespective of the environment. We have successfully used it to separate mixing regimes in conditions of the Arctic polar night over snow, agricultural lands in mid-latitude, boreal and temperate forests, as well a mid-European city. While the theoretical innovation is not complete yet, we will continue our research in this promising direction.
We constructed and validated the LEO technique capable of observing the fast-changing wind direction in field conditions, as well as directly quantifying the heat transported by the moving air in a forest environment. The latter result can be considered an observational breakthrough in near-surface meteorology. The proposed fundamental microscopic approach which attaches centimeter-scale pointy cones on to electrically heated fiber-optic cables in opposite directions was thoroughly validated in laboratory, field, and simulated airflows. The performance of the LEO technique would improve if even higher performing commercially available Distributed Temperature Sensing (DTS) instruments were available. Constructing this desired new generation of DTS instruments was outside of the project's scope.
From the analyzed field experiments in valley locations we learned that even gentle topography with moderate slopes of a few degrees generates and ducts cold-air currents we call thermal submeso-fronts. These cold-air currents create microclimates with air temperatures much cooler than the surrounding creating frost hazards on one hand, but also cool microrefugia in a warming climate. We were surprised to find that the thermal submeso-fronts are not specifically tied to weak-wind and cloud-free conditions, but occur almost always. In more mountainous terrain, the heat and mixing transported along the valley and down from the mountain slopes accelerates the erosion of the cold-air pools and currents.
From the urban experiment we learned that the stark contrasting land covers (concrete, grass) create systematic differences in how heat is distributed and transported at the surface. Over concrete the nocturnal release of the heat stored during the day is significantly larger which leads to more vertical mixing in the urban heat island. However, an impact on urban air quality in gas and particulate concentrations could not be observed as horizontal mixing is strong as a result of the land cover heterogeneity despite limited vertical mixing.
The results from the forest experiment demonstrate that the horizontal transport is mostly decoupled from that in the vertical for the weak-wind regime in the tree trunk space. With the help of a trace gas sampling network and the LEO array, we detected strong spatial differences of trace gas concentrations between carbon dioxide, methane, water vapor and air temperature. Differences in the forest architecture can partly explaine these findings. To date we complete the trace gas and heat budgets from the forest observations.
In computer simulations of the turbulent airflow using the LES technique, we were able to reproduce the cold-air density currents and thermal submerse-fronts, but the simulated mixing strength was overpredicted in comparison to the observations. This finding may be explained by the sharp edges in the digital elevation model even when it is as fine as 1m creating artificially enhanced mixing and wash out typical small-scale phenomena characteristic for nighttime weak-wind conditions.
Despite the differences in atmospheric motions across the investigated weak-wind environments, the vertical coupling is well explained by a novel statistical quantity Omega across all experiments to date. Omega is defined as the ratio of the energy arising from the vertical of moving air to the opposing forces arising from the stable density stratification. Omega lends itself to separating transport regimes with suppressed, limited, and unlimited mixing much better compared to statistical quantities contained in existing classic theories used in state-of-the-art land surface models. We will continue to develop this framework using Omega as the central quantity in modeling heat, trace gas and energy exchange from vertical gradients.
We disseminated our research achievements to the Earth sciences community including meteorologists, hydrologists, geologists, and biologists by offering the first fiber-optic sensing workshop in Earth sciences in Europe, and by numerous conference presentations to scientific audiences. DarkMix created a total of 29 publications, 15 of which are peer-reviewed journal publications and 6 scientific theses. At open-house science days at our university, showcasing the fiber-optic distributed sensing technique attracted significant interest and curiosity of the general public.
From the analyzed field experiments in valley locations we learned that even gentle topography with moderate slopes of a few degrees generates and ducts cold-air currents we call thermal submeso-fronts. These cold-air currents create microclimates with air temperatures much cooler than the surrounding creating frost hazards on one hand, but also cool microrefugia in a warming climate. We were surprised to find that the thermal submeso-fronts are not specifically tied to weak-wind and cloud-free conditions, but occur almost always. In more mountainous terrain, the heat and mixing transported along the valley and down from the mountain slopes accelerates the erosion of the cold-air pools and currents.
From the urban experiment we learned that the stark contrasting land covers (concrete, grass) create systematic differences in how heat is distributed and transported at the surface. Over concrete the nocturnal release of the heat stored during the day is significantly larger which leads to more vertical mixing in the urban heat island. However, an impact on urban air quality in gas and particulate concentrations could not be observed as horizontal mixing is strong as a result of the land cover heterogeneity despite limited vertical mixing.
The results from the forest experiment demonstrate that the horizontal transport is mostly decoupled from that in the vertical for the weak-wind regime in the tree trunk space. With the help of a trace gas sampling network and the LEO array, we detected strong spatial differences of trace gas concentrations between carbon dioxide, methane, water vapor and air temperature. Differences in the forest architecture can partly explaine these findings. To date we complete the trace gas and heat budgets from the forest observations.
In computer simulations of the turbulent airflow using the LES technique, we were able to reproduce the cold-air density currents and thermal submerse-fronts, but the simulated mixing strength was overpredicted in comparison to the observations. This finding may be explained by the sharp edges in the digital elevation model even when it is as fine as 1m creating artificially enhanced mixing and wash out typical small-scale phenomena characteristic for nighttime weak-wind conditions.
Despite the differences in atmospheric motions across the investigated weak-wind environments, the vertical coupling is well explained by a novel statistical quantity Omega across all experiments to date. Omega is defined as the ratio of the energy arising from the vertical of moving air to the opposing forces arising from the stable density stratification. Omega lends itself to separating transport regimes with suppressed, limited, and unlimited mixing much better compared to statistical quantities contained in existing classic theories used in state-of-the-art land surface models. We will continue to develop this framework using Omega as the central quantity in modeling heat, trace gas and energy exchange from vertical gradients.
We disseminated our research achievements to the Earth sciences community including meteorologists, hydrologists, geologists, and biologists by offering the first fiber-optic sensing workshop in Earth sciences in Europe, and by numerous conference presentations to scientific audiences. DarkMix created a total of 29 publications, 15 of which are peer-reviewed journal publications and 6 scientific theses. At open-house science days at our university, showcasing the fiber-optic distributed sensing technique attracted significant interest and curiosity of the general public.
We consider establishing the LEO technique as the most prominent finding which goes beyond the state of the art in observational near-surface meteorology.