Field study of the dynamics and modelling of subgrid-scale turbulence in a stable atmospheric surface layer over a glacier

A field experiment - the Snow Horizontal Array Turbulence Study (SnoHATS) - has been performed over an extensive glacier in Switzerland in order to study small-scale turbulence in the stable atmospheric surface layer, and to investigate the role, dynamics and modelling of the subgrid scales (SGSs) in the context of large-eddy simulations. The a priori data analysis aims at comparing the role and behaviour of the SGSs under stable conditions with previous studies under neutral or unstable conditions. It is found that the SGSs in a stable surface layer remain an important sink of temperature variance and turbulent kinetic energy from the resolved scales and carry a significant portion of the fluxes when the filter scale is larger than the distance to the wall. The fraction of SGS fluxes (out of the total fluxes) is found to be independent of stability. In addition, the stress-strain alignment is similar to the alignment under neutral and unstable conditions. The model coefficients vary considerably with stability but in a manner consistent with previous findings, which also showed that scale-dependent dynamic models can capture this variation. Furthermore, the variation of the coefficients for both momentum and heat SGS fluxes can be shown to be better explained by stability parameters based on vertical gradients, rather than vertical fluxes. These findings suggest that small-scale turbulence dynamics and SGS modelling under stable conditions share many important properties with neutral and convective conditions, and that a unified approach is thus possible. This paper concludes with a discussion of some other challenges for stable boundary-layer simulations that are not encountered in the neutral or unstable case

Fliessgewässertemperatur - zwei Modellansätze für zukünftige Prognosen

Seit Längerem steigt die globale Durchschnittstemperatur der Atmosphäre und führt damit auch in der Schweiz zu einem Anstieg der Gewässertemperaturen. Verlässliche längerfristige Prognosen hierfür sind in der Schweiz auf Bundesebene zum jetzigen Zeitpunkt nicht erstellbar. Nun wurden an der Ecole Polytechnique Fédérale de Lausanne (EPFL) zwei Modellvarianten entwickelt, um längerfristig Antworten auf Fragestellungen bezüglich Temperaturentwicklung von Fliessgewässern in der Schweiz zu ermöglichen. Parallel dazu wurde ein modulares deterministisches Modell auf der Basis des räumlich verteilten Schneedeckenmodells Alpine3D der WSL/SLF erstellt

Subgrid-Scale Dynamics of Water Vapour, Heat, and Momentum over a Lake

We examine the dynamics of turbulence subgrid (or sub-filter) scales over a lake surface and the implications for large-eddy simulations (LES) of the atmospheric boundary layer. The analysis is based on measurements obtained during the Lake-Atmosphere Turbulent EXchange (LATEX) field campaign (August-October, 2006) over Lake Geneva, Switzerland. Wind velocity, temperature and humidity profiles were measured at 20Hz using a vertical array of four sonic anemometers and open-path gas analyzers. The results indicate that the observed subgrid-scale statistics are very similar to those observed over land surfaces, suggesting that the effect of the lake waves on surface-layer turbulence during LATEX is small. The measurements allowed, for the first time, the study of subgrid-scale turbulent transport of water vapour, which is found to be well correlated with the transport of heat, suggesting that the subgrid-scale modelling of the two scalars may be coupled to save computational resources during LE

Influence of slope-scale snowmelt on catchment response simulated with the Alpine3D model

Snow and hydrological modeling in alpine environments remains challenging because of the complexity of the processes affecting the mass and energy balance. This study examines the influence of snowmelt on the hydrological response of a high-alpine catchment of 43.2 km2 in the Swiss Alps during the water year 2014–2015. Based on recent advances in Alpine3D, we examine how snow distributions and liquid water transport within the snowpack influence runoff dynamics. By combining these results with multiscale observations (snow lysimeter, distributed snow depths, and streamflow), we demonstrate the added value of a more realistic snow distribution at the onset of melt season. At the site scale, snowpack runoff is well simulated when the mass balance errors are corrected (R2 5 0.95 versus R2 5 0.61). At the subbasin scale, a more heterogeneous snowpack leads to a more rapid runoff pulse originating in the shallower areas while an extended melting period (by a month) is caused by snowmelt from deeper areas. This is a marked improvement over results obtained using a traditional precipitation interpolation method. Hydrological response is also improved by the more realistic snowpack (NSE of 0.85 versus 0.74), even though calibration processes smoothen out the differences. The added value of a more complex liquid water transport scheme is obvious at the site scale but decreases at larger scales. Our results highlight not only the importance but also the difficulty of getting a realistic snowpack distribution even in a well-instrumented area and present a model validation from multiscale experimental data sets

Thermal diffusivity of seasonal snow determined from temperature profiles

Thermal diffusivity of snow is an important thermodynamic property associated with key hydrological phenomena such as snow melt and heat and water vapor exchange with the atmosphere. Direct determination of snow thermal diffusivity requires coupled point measurements of thermal conductivity and density, which continually change due to snow metamorphism. Traditional methods for determining these two quantities are generally limited by temporal resolution. In this study we present a method to determine the thermal diffusivity of snow with high temporal resolution using snow temperature profile measurements. High resolution (between 2.5 and 10 cm at 1 min) temperature measurements from the seasonal snow pack at the Plaine-Morte glacier in Switzerland are used as initial conditions and Neumann (heat flux) boundary conditions to numerically solve the one-dimensional heat equation and iteratively optimize for thermal diffusivity. The implementation of Neumann boundary conditions and a ttest, ensuring statistical significance between solutions of varied thermal diffusivity, are important to help constrain thermal diffusivity such that spurious high and low values as seen with Dirichlet (temperature) boundary conditions are reduced. The results show that time resolved thermal diffusivity can be determined from temperature measurements of seasonal snow and support density-based empirical parameterizations for thermal conductivity

Field study of the dynamics and modelling of subgrid-scale turbulence in a stable atmospheric surface layer over a glacier

A field experiment – the Snow Horizontal Array Turbulence Study (SnoHATS) – has been performed over an extensive glacier in Switzerland in order to study small-scale turbulence in the stable atmospheric surface layer, and to investigate the role, dynamics and modelling of the subgrid scales (SGSs) in the context of large-eddy simulations. The a priori data analysis aims at comparing the role and behaviour of the SGSs under stable conditions with previous studies under neutral or unstable conditions. It is found that the SGSs in a stable surface layer remain an important sink of temperature variance and turbulent kinetic energy from the resolved scales and carry a significant portion of the fluxes when the filter scale is larger than the distance to the wall. The fraction of SGS fluxes (out of the total fluxes) is found to be independent of stability. In addition, the stress–strain alignment is similar to the alignment under neutral and unstable conditions. The model coefficients vary considerably with stability but in a manner consistent with previous findings, which also showed that scale-dependent dynamic models can capture this variation. Furthermore, the variation of the coefficients for both momentum and heat SGS fluxes can be shown to be better explained by stability parameters based on vertical gradients, rather than vertical fluxes. These findings suggest that small-scale turbulence dynamics and SGS modelling under stable conditions share many important properties with neutral and convective conditions, and that a unified approach is thus possible. This paper concludes with a discussion of some other challenges for stable boundary-layer simulations that are not encountered in the neutral or unstable cases

Adapting tilt corrections and the governing flow equations for steep, fully three-dimensional, mountainous terrain

In recent studies of atmospheric turbulent surface exchange in complex terrain, questions arise concerning velocity-sensor tilt corrections and the governing flow equations for coordinate systems aligned with steep slopes. The standard planar-fit method, a popular tilt-correction technique, must be modified when applied to complex mountainous terrain. The ramifications of these adaptations have not previously been fully explored. Here, we carefully evaluate the impacts of the selection of sector size (the range of flow angles admitted for analysis) and planar-fit averaging time. We offer a methodology for determining an optimized sector-wise planar fit (SPF), and evaluate the sensitivity of momentum fluxes to varying these SPF input parameters. Additionally, we clarify discrepancies in the governing flow equations for slope-aligned coordinate systems that arise in the buoyancy terms due to the gravitational vector no longer acting along a coordinate axis. New adaptions to the momentum equations and turbulence kinetic energy budget equation allow for the proper treatment of the buoyancy terms for purely upslope or downslope flows, and for slope flows having a cross-slope component. Field data show that new terms in the slope-aligned forms of the governing flow equations can be significant and should not be omitted. Since the optimized SPF and the proper alignment of buoyancy terms in the governing flow equations both affect turbulent fluxes, these results hold implications for similarity theory or budget analyses for which accurate flux estimates are important

Citizens observatories for effective Earth observations: the WeSenseIt approach

Tese de doutoramento em Biociências, no ramo de Ecologia Marinha, apresentada ao Departamento de Ciências da Vida da Faculdade de Ciências e Tecnologia da Universidade de CoimbraIn the face of the current non-linear and abrupt changes in ecosystems around the world, a systematic impact has arisen to the environmental dynamics of the coastal systems. At their essential intermediate trophic level in the marine system, sardines are recognized for being highly susceptible to the deregulation of their stocks when subjected to intensive exploitation. Given this, there are significant difficulties of management and participatory coastal management of the species of sardines in the Atlantic Ocean. In this way, local ecological knowledge (LEK) emerges as an auxiliary tool that seeks to extract specific information about an individual about the environment as well as to understand the attitudes of local individuals regarding a resource as a way to improve biodiversity conservation practices. Therefore, the present thesis sought to evaluate and share local ecological knowledge (LEK) and attitudes for the conservation of European pilchard (Sardina pilchardus) and Brazilian sardine (Sardinella brasiliensis) in two fishing villages (Peniche, District of Leiria, Portugal and Arraial do Cabo, State of Rio de Janeiro, Brazil). This study was based on 221 semi-structured interviews conducted with fishermen from Peniche (N = 87) about European sardines, and with fishers from Arraial do Cabo (N = 134) about Brazilian sardine. The data extracted from the interviews were qualitatively and quantitatively analysed, and the LEK made available was compared with the literature following an emic-etic approach. Additives knowledge indicators and attitudes about sardines were created for comparison purposes. The classification adopted to evaluate the LEK, and the attitudes respected the premises of the three-point Likert scale. It was also sought to investigate possible correlations between LEK, attitudes and the profile of fishers from both communities. The statistical treatments performed were calculated using program R version 3.3.2. The profiles of the interviewees of the studied communities were investigated and measured succinctly. Informants shared detailed informal knowledge of the significant biological and ecological aspects of the clupeoids in question. Taboos and food aversions were present inexpressively and were not related to conservation measures. Fishers from both Atlantic communities presented moderate local ecological knowledge about sardines when compared to the biological data already published of this small pelagic species. Attitudes towards conservation of sardines were classified as positive in Peniche, while in the Brazilian community analyzed, they were moderate. In Peniche, attitudes showed to be a predictor of LEK, age and educational level of informants. Otherwise, in Arraial do Cabo the variables of the fisher´s profile (source of income, educational level, boat ownership, association with fishing colony and occupation) showed a correlation with LEK and with conservationist attitudes. Encouraging the continuation of regular educational training of informants and the inclusion of environmental education programs with the sharing of information on sardine conservation. This ethnobiological study reported comparable results with other studies examining LEK and local community attitudes about a natural resource, which reaffirms the importance of this socio-ecological tool for environmental management. Informal knowledge not compatible with the literature should not be discarded. This type of information can be further analysed and used in the formulation of testable hypotheses for future investigations of the species studied. The next step would be to include the LEK provided by the surveyed communities for analysis at meetings between all actors directly involved with the fishing resource. This procedure can collaborate and promote greater social inclusion of the less favored in the fisheries management decision-making in the Portuguese and Brazilian communities verified. A discussion with the participation of all the interested parties, without preconceived privileges to any of those involved, becomes of extreme importance because it enables reliability among the participants of these assemblies. This process, if well conducted, still can generate a better understanding of the predisposition of these communities to conserve the environmental resource. Also, this participatory management strategy may also provide an opportunity for local populations to acquire reliable and more scientific knowledge about this depleting fishing resource. Finally, a continuous exchange of information between ecologists and ethnobiologists is suggested on possible gaps in knowledge that may arise about fish stocks in both Atlantic systems.Em virtude das actuais mudanças não-lineares e abruptas nos ecossistemas em todo o planeta, a dinâmica ambiental dos sistemas costeiros tem sido bastante modificada. Por ocuparem o nível trófico intermediário essencial no sistema marinho, as sardinhas são reconhecidas por serem altamente susceptíveis a grandes alterações dos seus efectivos populacionais quando submetidas à exploração intensiva. Diante de tal cenário, observam-se grandes dificuldades na gestão costeira (que se quer participativa) das espécies de sardinhas no oceano Atlântico. Desse modo, o conhecimento ecológico local (CEL) surge como uma ferramenta auxiliar que busca extrair informações específicas de um indivíduo em relação ao meio ambiente como também compreender as atitudes de indivíduos locais no que tange a um recurso em particular, como forma de melhorar as práticas de conservação da biodiversidade. Por conseguinte, a presente tese tentou avaliar o conhecimento ecológico local (CEL), bem como as atitudes para a conservação da sardinha europeia (Sardina pilchardus) e da sardinha brasileira (Sardinella brasiliensis) em duas aldeias piscatórias (Peniche, Distrito de Leiria, Portugal; e Arraial do Cabo, Estado do Rio de Janeiro, Brasil). Este estudo baseou-se em 221 entrevistas semiestruturadas realizadas aleatoriamente com pescadores de Peniche (N = 87) acerca da sardinha europeia, e com pescadores de Arraial do Cabo (N = 134) sobre a sardinha brasileira. Os dados extraídos das entrevistas foram analisados de modo qualitativo e quantitativo e o CEL disponibilizado foi comparado com a literatura através da utilização das abordagens êmica e ética. Foram criados índices para medir o CEL e as atitudes dos pescadores em relação à conservação das espécies de sardinhas. A classificação adotada para avaliar o CEL e as atitudes respeitou as premissas da escala de três pontos de Likert. Por último, também foram investigadas as possíveis correlações entre CEL, atitudes e o perfil dos pescadores de ambas as comunidades. Os tratamentos estatísticos realizados foram calculados por meio do programa R versão 3.3.2. Os perfis dos entrevistados das comunidades estudadas foram investigados e analisados sucintamente. Os entrevistados compartilharam conhecimentos informais detalhados dos principais aspectos biológicos e ecológicos dos clupeóides em questão. Os tabus e aversões alimentares estiveram presentes de forma inexpressiva e não foram relacionados com medidas conservacionistas. Os pescadores de ambas as comunidades Atlânticas apresentaram um conhecimento ecológico local moderado acerca das sardinhas quando comparados aos dados biológicos já publicados dessas pequenas espécies pelágicas. Em Peniche, houve a correlação entre as atitudes e o CEL, a idade e o nível educacional dos informantes respectivamente. Por outro lado, em Arraial do Cabo as variáveis do perfil dos pescadores (fonte de renda, nível educacional, propriedade de embarcação, associação à colónia de pescadores e ocupação) mostraram correlação com o CEL e com as atitudes conservacionistas. O incentivo à continuação da formação educacional convencional dos informantes (pescadores) e a inserção de programas de educação ambiental acompanhados de informações acerca da conservação da sardinha nas aldeias de pescadores investigadas são essenciais para estabelecer e melhorar as atitudes conservacionistas. Este estudo etnobiológico reportou resultados comparáveis com outros estudos que investigam o LEK e atitudes de comunidades locais acerca de um determinado recurso natural, o que reafirma a importância desta ferramenta sócio-ecológica para a gestão ambiental. O conhecimento informal não compatível com a literatura não deve ser totalmente descartado. Esse tipo de informação pode ainda ser analisado minuciosamente e utilizado na formulação de hipóteses para futuras investigações acerca da espécie estudada. O próximo passo seria incluir o CEL fornecido pelas comunidades estudadas para análises em reuniões entre todos os atores envolvidos directamente com o recurso pesqueiro. Esse procedimento pode promover uma maior inclusão social dos menos favorecidos nas decisões relativas à gestão dos stocks pesqueiros nas comunidades portuguesa e brasileira. Uma discussão com a participação de todos os interessados, sem privilégios preconcebidos a nenhum dos envolvidos, torna- se de extrema importância pois aumenta a confiança entre todos os envolvidos. Este processo, se bem conduzido, ainda tem a capacidade de gerar um melhor entendimento da predisposição dessas comunidades em conservar o recurso ambiental. Além disso, esta estratégia de gestão participativa também pode proporcionar uma oportunidade das populações locais adquirirem conhecimentos mais científicos sobre esse recurso pesqueiro em questão. Por fim, sugere-se ainda uma contínua troca de informações entre ecólogos e etnobiólogos sobre as possíveis lacunas de informações que possam surgir relativamente aos stocks pesqueiros em ambos os sistemas Atlânticos.Centre for Functional Ecology - CFE, Department of life Sciences, University of Coimbra, PortugalProject ReNATURE - Valorization of the Natural Endogenous Resources of the Centro Region (Centro 2020, Centro-01-0145-FEDER-000007)CAPES Foundation – Ministry of Education of Brazil for financial support (BEX: 8926/13-1

How do Stability Corrections Perform in the Stable Boundary Layer Over Snow?

We assess sensible heat-flux parametrizations in stable conditions over snow surfaces by testing and developing stability correction functions for two alpine and two polar test sites. Five turbulence datasets are analyzed with respect to, (a) the validity of the Monin–Obukhov similarity theory, (b) the model performance of well-established stability corrections, and (c) the development of new univariate and multivariate stability corrections. Using a wide range of stability corrections reveals an overestimation of the turbulent sensible heat flux for high wind speeds and a generally poor performance of all investigated functions for large temperature differences between snowand the atmosphere above (>10 K).Applying the Monin–Obukhov bulk formulation introduces a mean absolute error in the sensible heat flux of 6W m-2 (compared with heat fluxes calculated directly from eddy covariance). The stability corrections produce an additional error between 1 and 5W m-2, with the smallest error for published stability corrections found for the Holtslag scheme. We confirm from previous studies that stability corrections need improvements for large temperature differences and wind speeds, where sensible heat fluxes are distinctly overestimated. Under these atmospheric conditions our newly developed stability corrections slightly improve the model performance. However, the differences between stability corrections are typically small when compared to the residual error, which stems from the Monin–Obukhov bulk formulation