Options to correct local turbulent flux measurements for large-scale fluxes using an approach based on large-eddy simulation

The eddy-covariance method provides the most direct estimates for fluxes between ecosystems and the atmosphere. However, dispersive fluxes can occur in the presence of secondary circulations, which can inherently not be captured by such single-tower measurements. In this study, we present options to correct local flux measurements for such large-scale transport based on a non-local parametric model that has been developed from a set of idealized large-eddy simulations. This method is tested for three real-world sites (DK-Sor, DE-Fen, and DE-Gwg), representing typical conditions in the mid-latitudes with different measurement heights, different terrain complexities, and different landscape-scale heterogeneities. Two ways to determine the boundary-layer height, which is a necessary input variable for modelling the dispersive fluxes, are applied, which are either based on operational radio soundings and local in situ measurements for the flat sites or from backscatter-intensity profiles obtained from co-located ceilometers for the two sites in complex terrain. The adjusted total fluxes are evaluated by assessing the improvement in energy balance closure and by comparing the resulting latent heat fluxes with evapotranspiration rates from nearby lysimeters. The results show that not only the accuracy of the flux estimates is improved but also the precision, which is indicated by RMSE values that are reduced by approximately 50 %. Nevertheless, it needs to be clear that this method is intended to correct for a bias in eddy-covariance measurements due to the presence of large-scale dispersive fluxes. Other reasons potentially causing a systematic underestimated or overestimation, such as low-pass filtering effects and missing storage terms, still need to be considered and minimized as much as possible. Moreover, additional transport induced by surface heterogeneities is not considered.

How Does the Choice of the Lower Boundary Conditions in Large-Eddy Simulations Affect the Development of Dispersive Fluxes Near the Surface?

Large-eddy simulations (LES) are an important tool for investigating the longstanding energy-balance-closure problem, as they provide continuous, spatially-distributed information about turbulent flow at a high temporal resolution. Former LES studies reproduced an energy-balance gap similar to the observations in the field typically amounting to 10–30% for heights on the order of 100 m in convective boundary layers even above homogeneous surfaces. The underestimation is caused by dispersive fluxes associated with large-scale turbulent organized structures that are not captured by single-tower measurements. However, the gap typically vanishes near the surface, i.e. at typical eddy-covariance measurement heights below 20 m, contrary to the findings from field measurements. In this study, we aim to find a LES set-up that can represent the correct magnitude of the energy-balance gap close to the surface. Therefore, we use a nested two-way coupled LES, with a fine grid that allows us to resolve fluxes and atmospheric structures at typical eddy-covariance measurement heights of 20 m. Under different stability regimes we compare three different options for lower boundary conditions featuring grassland and forest surfaces, i.e. (1) prescribed surface fluxes, (2) a land-surface model, and (3) a land-surface model in combination with a resolved canopy. We show that the use of prescribed surface fluxes and a land-surface model yields similar dispersive heat fluxes that are very small near the vegetation top for both grassland and forest surfaces. However, with the resolved forest canopy, dispersive heat fluxes are clearly larger, which we explain by a clear impact of the resolved canopy on the relationship between variance and flux–variance similarity functions.

Options to correct local turbulent flux measurements for large-scale fluxes using a LES-based approach

The eddy-covariance method provides the most direct estimates for fluxes between ecosystems and the atmosphere. However, dispersive fluxes can occur in the presence of secondary circulations, which can inherently not be captured by such single-tower measurements. In this study, we present options to correct local flux measurements for such large-scale transport based on a non-local parametric model that has been developed from a set of idealized LES runs for three real-world sites. The test sites DK-Sor, DE-Fen, and DE-Gwg, represent typical conditions in the mid-latitudes with different measurement height, different terrain complexity and different landscape-scale heterogeneity. Different ways to determine the boundary-layer height, which is a necessary input variable for modelling the dispersive fluxes, are applied, either from operational radio-soundings and local in-situ measurements for the flat site or from backscatter-intensity profile obtained from collocated ceilometers for the two sites in complex terrain. The adjusted total fluxes are evaluated by assessing the improvement in energy balance closure and by comparing the resulting latent heat fluxes with evapotranspiration rates from nearby lysimeters. The results show that not only the accuracy of the flux estimates is improved but also the precision, which is indicated by RMSE values that are reduced by approximately 50 %. Nevertheless, it needs to be clear that this method is intended to correct for a bias in eddy-covariance measurements due to the presence of large-scale dispersive fluxes. Other reasons potentially causing a systematic under- or overestimation, such as low-pass filtering effects and missing storage terms, still need to be considered and minimized as much as possible. Moreover, additional transport induced by surface heterogeneities is not considered.

Surface energy balance closure – the role dispersive fluxes induced by submesoscale secondary circulations

<p>Quantitative knowledge of the surface energy balance is essential for the prediction of weather and climate. However, a multitude of studies from around the world indicates that the turbulent heat fluxes are generally underestimated using eddy-covariance measurements, and hence, the surface energy balance is not closed. This energy balance closure problem has been heavily covered in the literature for more than 25 years, and as a result, several instrumental and methodological aspects have been reconsidered and partially revised. Nevertheless, a non-negligible energy imbalance remains, and we demonstrate that a major portion of this imbalance can be explained by dispersive fluxes in the surface layer, which are associated with submesoscale secondary circulations. Such large-scale organized structures are a very common phenomenon in the convective boundary layer, and depending on static stability, they can either be roll-like or cell-like and occur even over homogeneous surfaces. Over heterogeneous surfaces, thermally-induced mesoscale circulations can occur in addition to those. Either way, the associated dispersive heat fluxes can inherently not be captured by single-tower measurements, since the ergodicity assumption is violated. As a consequence, energy transported non-turbulently will not be sensed by eddy-covariance systems and a bias towards lower energy fluxes will result. The objective of this research is to develop a model that can be used to correct single-tower eddy-covariance data. As a first step towards this goal, we will present a parametrisation for dispersive fluxes, which was developed based on an idealized high-resolution LES study for homogeneous surfaces, as a function of non-local scaling variables. Secondly, we explore how well this parametrisation works for a number of real-world eddy-covariance sites.</p>

Surface thermal heterogeneities, dispersive fluxes and the conundrum of unaccounted statistical spatial inhomogeneities

<p>The use of Numerical Weather Prediction (NWP) models is ubiquitous in our daily lives, whether to decide what to wear, to plan for the weekend, invest on wind turbines, decide strategies for food security or to forecast atmosphere-driven natural disasters, to name a few. Currently, intrinsic to most NWP models is the assumption of spatial homogeneity at kilometer to sub-kilometer scales when, for example, classic similarity scaling relationships are applied to account for unresolved near-surface momentum, heat and mass exchanges. While advances in computation (and computing) are enabling finer grid resolutions in NWP, representing land-atmosphere exchange processes at the lower boundary remains a challenge (regardless of the numerical resolution but not independent from it). This is partially a result of the fact that land-surface heterogeneity exists at all spatial scales and its variability does not ‘average’ out with decreasing scales. Such variability need not rapidly blend away from the boundary and thereby impacts the spatial distribution of fluxes throughout the near-surface region of the atmosphere.</p><p>     While, the effects of spatial surface heterogeneities have long been minimized under the assumption of an existing blending length-scale, in this work evidence is presented of the consequential effect of such surface heterogeneities. Specifically, canonical experiments based on in-situ measurements and high-resolution numerical simulations quantify the effect of surface thermal heterogeneities on an otherwise homogeneous planar surface. Therefore, such near-canonical case describes inhomogeneous scalar transport in an otherwise planar homogeneous flow when thermal stratification is weak or absent. In this work, the interaction between the characteristic length scales of the surface heterogeneities, and the scales of resolved fluid dynamics transport is further unraveled. Dispersive fluxes naturally appear as a means to account for unresolved, and time-lasting advection fluxes generated by a-priori unresolved spatial thermal heterogeneities. Results illustrate that dispersive fluxes can represent as much as 40% of the total resolved advection flux under weak wind conditions, and remain relevant under strong winds. Furthermore, results of this work appear not to only be relevant in the treatment of unresolved heterogeneities in NWP models, but also in understanding the unresolved problem of surface energy budget closure.</p>

Stories of Blogs, E-books and Time: Slowdown, Participation and Resistance in Wu Ming’s Giap and L’archivio e la strada

This paper analyses the relationship between Wu Ming’s blog Giap and the e-book Giap: L’archivio e la strada, where the writing collective gathered a selection of blog posts published between 2010 and 2012. Focusing on the temporal dimension of on-line and off-line reading practices, this paper demonstrates that the transmedia transfer of content from the blog to the e-book—a hybrid medium available both on-line and off-line—affects the experience of the reader offering an alternative reading rhythm, slowing it down and engendering a more thoughtful and critical approach to its content. The focus on the temporal dimension of reading practices must be framed into Wu Ming’s cultural project, primarily aimed at sabotaging and undermining the rhythm of life, production, and consumption imposed by Capitalism. As part of this project, the blog Giap promotes practices of resistance to the commodification of cultural products and impulsive forms of content consumption. In particular, the content available on Giap is protected by copy-left policies, while the comments to its most thoughtful posts are blocked until seventy-two hours after their publication. The transmedia transfer of posts from the blog to the e-book reorders their sequence, establishes a different relationship between reader and content and encourages the latter to reflect on the texts within a restructured temporal dimension. The publication of the e-book therefore represents a further form of slowdown that brings about the possibility of an alternative to on-line impulsive reading practices, bridges on-line and off-line dimensions, tackles issues of accessibility and preserves its content from on-line dispersive fluxes.

Contrasts between momentum and scalar transport over very rough surfaces

Large-eddy simulations are conducted to contrast momentum and passive scalar transport over large, three-dimensional roughness elements in a turbulent channel flow. Special attention is given to the dispersive fluxes, which are shown to be a significant fraction of the total flux within the roughness sublayer. Based on pointwise quadrant analysis, the turbulent components of the transport of momentum and scalars are found to be similar in general, albeit with increasing dissimilarity for roughnesses with low frontal blockage. However, strong dissimilarity is noted between the dispersive momentum and scalar fluxes, especially below the top of the roughness elements. In general, turbulence is found to transport momentum more efficiently than scalars, while the reverse applies to the dispersive contributions. The effects of varying surface geometries, measured by the frontal density, are pronounced on turbulent fluxes and even more so on dispersive fluxes. Increasing frontal density induces a general transition in the flow from a wall bounded type to a mixing layer type. This transition results in an increase in the efficiency of turbulent momentum transport, but the reverse occurs for scalars due to reduced contributions from large-scale motions in the roughness sublayer. This study highlights the need for distinct parameterizations of the turbulent and dispersive fluxes, as well as the importance of considering the contrasts between momentum and scalar transport for flows over very rough surfaces.