To showcase their strong thematic connection, the two sessions “Air-Land Interactions (General Session)” and “Understanding and Characterization of Land-Atmosphere Feedback” were merged.

The session is addressed to experimentalists and modellers working on land surface fluxes from local to regional scales. The programme is open to a wide range of new studies in micrometeorology. The topics include the development of new devices, measurement techniques and experimental design methods, as well as novel findings on surface layer theory and parametrization at the local scale. The theoretical parts encompass soil-vegetation-atmosphere transport, internal boundary-layer theories and flux footprint analyses, etc.. Of special interest are comparisons of experimental data, parametrizations and models. This includes energy and trace gas fluxes (inert and reactive) as well as water, carbon dioxide and other GHG fluxes. Specific focus is given to outstanding problems in land surface boundary layer descriptions such as complex terrain, energy balance closure, stable stratification and night time fluxes, as well as to the dynamic interactions with atmosphere, plants (in canopy and above canopy) and soils including the scale problems in atmosphere and soil exchange processes.

The understanding of feedback processes in the land-atmosphere (L-A) system is crucial for advanced modeling and prediction of weather and climate. However, the impact of soil moisture and evapotranspiration on the diurnal cycle of the planetary boundary layer (PBL), clouds, and precipitation remains a sore gap in our understanding of weather processes and climate statistics. For this purpose, the exchange of momentum, water, energy, and carbon at the land surface and at the top of the PBL has to be investigated from the local to regional scales in great detail. In this session, we accept observational and modeling approaches to address these challenges. With respect to the observations, emphasis is put on the application of new sensor synergies for studying L-A exchange processes and entrainment at the PBL top based on long-term data sets or recent field campaigns, e.g., combining multi-tower, scanning lidar, airborne, and satellite observations. With respect to theoretical understanding and modeling, we welcome the study of feedback processes as well as the derivation and application of feedback metrics from the mesoscale to turbulent scales, e.g., derived by large eddy simulations.

Co-organized by BG1/HS13
Convener: Andreas Ibrom | Co-conveners: Christoph Thomas, Natascha Kljun, Volker Wulfmeyer, Linda Schlemmer, Matthias Mauder, Georg Jocher
| Attendance Wed, 06 May, 10:45–12:30 (CEST), Attendance Wed, 06 May, 14:00–15:45 (CEST)

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Chat time: Wednesday, 6 May 2020, 10:45–12:30

Chairperson: Natascha Kljun
D3045 |
Ralf Staebler, Samar Moussa, Yuan You, Hayley Hung, Maryam Moradi, Amy Leithead, Peter Brickell, Katherine Hayden, Richard Mittermeier, and James Beck

Canada’s Oil Sands Region in northern Alberta contains the world’s largest deposits of commercially exploited bitumen. Extraction of synthetic crude oil from these deposits is a water intensive process, requiring large ponds for water recycling and/or final storage of tailings, already covering a total of over 100 km2 of liquid surface area in the Athabasca Oil sands. The primary extraction tailings ponds primarily contain sand, silt, clay and unrecovered bitumen, while a few secondary extraction ponds also receive solvents and inorganic and organic by-products of the extraction process. Fugitive emissions of pollutants from these ponds to the atmosphere may therefore be a concern, but until recently, data on emission rates for many pollutants, other than a few reported under regulatory compliance monitoring, were sparse. We present here the results from a comprehensive field campaign to quantify the emissions from a secondary extraction pond to the atmosphere of 68 volatile organic compounds (VOCs), 22 polycyclic aromatic compounds (PACs), 8 reduced sulfur compounds as well as methane, carbon dioxide and ammonia. Three micrometeorological flux methods (eddy covariance, vertical gradients and inverse dispersion modeling) were evaluated for methane fluxes to ensure their mutual comparability. Methane and carbon dioxide fluxes were similar to previous results based on flux chamber measurements. Emission rates for 12 PACs, alkanes and aromatic VOCs, several sulfur species, and ammonia were found to be significant. PACs were dominated by methyl naphthalenes and phenanthrenes, while diethylsulfide and  and n-heptane were the dominant reduced sulfur and VOC species, respectively. The role of these previously unavailable emission rates in regional pollutant budgets will be discussed.

How to cite: Staebler, R., Moussa, S., You, Y., Hung, H., Moradi, M., Leithead, A., Brickell, P., Hayden, K., Mittermeier, R., and Beck, J.: First Comprehensive Measurements of Emission Rates of Greenhouse Gases, Volatile Organic Compounds and Reduced Sulfur Compounds from a Tailings Pond in the Alberta Oil Sands, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19004, https://doi.org/10.5194/egusphere-egu2020-19004, 2020.

D3046 |
Milan Fischer, Gabriel Katul, Asko Noormets, Gabriela Pozníková, Jean-Christophe Domec, Matěj Orság, Miroslav Trnka, and John King

Eddy covariance (EC) has become the standard method for determining energy fluxes at the soil-plant-atmosphere interface. However, the cost and complexity of EC often limit its widespread deployment, and therefore, alternatives need to be considered. Here, two alternative methods, flux-variance (FV) and surface renewal (SR), are evaluated in quantifying sensible heat flux at three sites representing agricultural (wheat field, straw and bare soil), agroforestry (pine-switchgrass intercroping) and natural forested wetland (mixed conifer-deciduous wetland forest) systems that span a broad range of canopy height and structural complexity. By considering the position of the sensors with respect to canopy, the measurements at these three sites were carried out in the atmospheric surface layer, roughness layer, and roughness to surface transitional layer, respectively. Since the introduction of FV and SR, several versions of these methods have been proposed, with significantly differing perspectives and assumptions. Until now, the differences between the methods have not been fully standardized or clarified. In principle, both methods require the monitoring of high frequency (e.g. 10 Hz) air temperature variation while some approaches require additional wind velocity measurements. This presentation provides an overview of the FV and SR approaches, including new perspectives as well as identifies the common framework of the methods rather than carrying out their mere comparison. We show that the frequently reported need for the calibration (e.g. against EC) cannot be fully overcome. However, it can be put in a more universal framework where the parameters of both methods requiring calibration are represented by joint physically based parameters such as surface aerodynamic properties rather than similarity constants in the case of FV or the mean volume over the area of the air parcels in the case of SR. After the selection of the most reliable approaches, regression analyses against EC shows that both methods can estimate sensible heat flux with slopes within ±10 % from unity and R2 >0.9 across all the three sites. The best performance of both FV and SR was at the agricultural field, where the measurements are well in the surface layer while the worst in the case of the tall forest where the measurements are still in the roughness sublayer and the roughness layer depth (with its inherent uncertainty) needs to be taken into account in the calculations. We conclude there may be opportunities to fill gaps in knowledge of ecosystem energy balance at substantial cost-savings in specialized circumstances where EC may not be appropriate using both FV and SR methods.

Acknowledgement: This study was conducted with support of SustES - Adaptation strategies for sustainable ecosystem services and food security under adverse environmental conditions (CZ.02.1.01/0.0/0.0/16_019/0000797) and USDA NIFA-AFRI Sustainable Bioenergy Program, 2011-67009-20089, Loblolly pine-switch grass intercropping for sustainable timber and biofuels production in the Southeastern United States.  Funding for AmeriFlux core site US-NC4 (natural forested wetland) was provided by the U.S. Department of Energy’s Office of Science.

How to cite: Fischer, M., Katul, G., Noormets, A., Pozníková, G., Domec, J.-C., Orság, M., Trnka, M., and King, J.: Deriving the sensible heat flux from the air temperature time-series through the flux-variance and the surface renewal methods, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-11796, https://doi.org/10.5194/egusphere-egu2020-11796, 2020.

D3047 |
Olli Peltola, Karl Lapo, Christoph Thomas, and Timo Vesala

Air flows above forest canopies are typically governed by large coherent eddies generated mechanically by inflected mean wind velocity profile or thermally by buoyancy in the convective regime. A significant body of research have been devoted to the role of these eddies on ecosystem scalar (gases and heat) exchange since they are likely related to the energy balance closure problem observed at the eddy covariance (EC) stations and turbulent flux divergence under stable stratification. Here we utilize fiber-optic distributed sensing on a tall mast to observe the turbulent fluctuations of air temperature with high spatial (25 cm) and temporal resolution (1 Hz) from the forest floor up to 120 m above the ground. These unique measurements resolved the continuous vertical profile of scalar turbulence and hence enabled us to study the topology (height – time space) of the turbulent eddies in different stability regimes. For example, the inclination angle of the eddies changed with stability and the scalar ramps often observed in canopy flows were evident only close to the canopy top, whereas higher up thermal eddies dominated the flow. Furthermore, the measurements permitted the identification of coupled air layers and hence analysis on the dynamics of below-canopy decoupling. During stable conditions with wind shear large eddies and the related inverted ramps in the temperature time series were observed at the top of the decoupling layer, however when the wind shear decreased the flow switched to submeso regime with canopy waves. These analyses were then combined with concurrent turbulence measurements with 3D sonic anemometers at several heights and EC gas flux measurements at one height to gain new insights on the role of these eddies on gas (e.g. carbon dioxide) transport. The measurements were conducted during summer 2019 at the Hyytiälä SMEAR II station located in central Finland and the permanent ICOS measurements at the site were utilized to the fullest.

How to cite: Peltola, O., Lapo, K., Thomas, C., and Vesala, T.: Spatial observations of large eddies and cross-canopy coupling with joint fiber-optic distributed sensing and eddy covariance flux measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15840, https://doi.org/10.5194/egusphere-egu2020-15840, 2020.

D3048 |
Natalia Kowalska, Georg Jocher, Ladislav Šigut, and Marian Pavelka

Since the eddy covariance (EC) method became a key method for measurements of the energy and greenhouse gas exchange between ecosystems and the atmosphere, a large number of studies was conducted to understand the mechanisms driving the carbon exchange in forest ecosystems. In recent years, case studies further focused on testing and validating the applicability of the EC technique above forest ecosystems, also assessing the spatial and temporal variability of sub canopy fluxes. These studies led to the conclusion that there is a high probability of overestimating the forest carbon sink strength with EC measurements above the forest canopy only, as these measurements may miss respiration components from within and below the canopy due to insufficient mixing across the canopy. Additional below canopy EC measurements were suggested to tackle this problem and to get information about potential decoupling between below and above forest canopy air masses as well as potentially missing respiration components in the above canopy derived signal.

The overall goal of the study here is to derive an as detailed as possible understanding of the carbon exchange in Lanžhot floodplain forest with the help of concurrent EC measurements below and above the forest canopy. Lanžhot floodplain forest is situated 6.5 km north of the confluence of the Morava and Thaya rivers in Czech Republic (48.6815483 N, 16.9463317 E). The long-term average annual precipitation at this site is around 517 mm and the mean annual temperature is 9.5 °C. The average groundwater level is -2.7 m. Since a long time flooding occurs here very rarely, the last flooding event was in 2013. In addition, the site is hydrologically managed. Consequently, the water regime of the site changed over the years and represents nowadays relatively dry conditions for such type of ecosystem.

To reach our research goal we evaluate different single- and two-level filtering strategies of the above canopy derived carbon exchange values and the impact of these filterings on the annual ecosystem carbon exchange rates. Our hypothesis is that conventional single-level EC flux filtering strategies like the u*-filtering might not be sufficient to fully capture the carbon exchange of the studied floodplain forest ecosystem. We further hypothesize that additional below canopy EC measurements are mandatory to achieve unbiased forest carbon exchange values with the EC technique.

How to cite: Kowalska, N., Jocher, G., Šigut, L., and Pavelka, M.: Floodplain forest carbon exchange – a micrometeorological point of view, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7820, https://doi.org/10.5194/egusphere-egu2020-7820, 2020.

D3049 |
Jean-Claude Krapez, Gregoire Ky, and Claire Sarrat

The flux footprint (or so-called source area) is the zone of the surface that contributes to a measured vertical flux (e.g. of water vapor or carbon dioxide) between the ground and the atmosphere: Footprint models are then used to derive location and size of the source area and for interpretation of flux-tower measurements, in particular to estimate the contribution of passive scalar sources to these measured fluxes, and to combine measured fluxes with remotely sensed data.

Existing footprint models are of two types: either they derive from the solution of an advection-diffusion differential equation or they result from a parameterization based on numerical simulations performed with a Lagrangian stochastic particle dispersion model. Models of the first type are essentially based on the hypothesis of power-law profiles of the mean wind speed u(z) and eddy diffusivity K(z). Our objective was to suppress this constraint and to build a footprint model for any type of profile, in particular Monin-Obukhov surface-layer profiles.

The model was developed in the frame of the K-theory. Homogeneous conditions were assumed in the horizontal plane and the alongwind diffusion term was neglected with respect to the advection term. A semi-analytical tool has been developed to cope with any type of atmosphere stratification. Applying a dedicated quadrupole method, the boundary layer is divided into a series of sublayers and an extended power law model is applied in each of them (10 to 15 sublayers are enough to reach an error of less than 0.1%, whatever the atmosphere stability).

In the end, a highly accurate estimation of the footprint can be obtained very quickly for any profile of wind speed and eddy diffusivity.

How to cite: Krapez, J.-C., Ky, G., and Sarrat, C.: Highly accurate analytical footprint model for general stratification of the atmosphere , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3140, https://doi.org/10.5194/egusphere-egu2020-3140, 2020.

D3050 |
Andrew Kowalski, Gerardo Fratini, Gabriela Miranda, Penélope Serrano-Ortiz, and George Burba

Arithmetic averaging procedures are traditionally used in many applications in the field of micrometeorology, but these neglect Osborne Reynolds's specification of turbulence, and thus, strictly speaking, violate the momentum conservation law. Recently, it has been shown  that applying linear momentum conservation to surface exchanges defines an average motion in the surface-normal direction (i.e., a Stefan flow), and thereby describes a non-diffusive transport that is distinct from turbulent transport. Here we examine data from a nearly ideal micrometeorological field site (extensive, flat, and mono specific-reed wetland) to show that traditional flux-tower calculations, including but not limited to the Webb corrections,  generally provide an inadequate approximation of turbulent  fluxes and yet still adequately characterize the net fluxes in most traditional cases. The importance of such conflation of diffusive and non-diffusive transport is greatest for situations with relatively large non-diffusive fluxes, as occurs during particular times of day in general and particularly when considering fluxes in the stream-wise direction. An examination of fluxes calculated using the traditional arithmetic averaging procedure, versus the proposed, more theoretically appropriate calculations that fully obey conservation law, illustrates important implications for the characterization of gas-exchange processes and more generally the discipline of micrometeorology. These implications may become particularly critical in near future as gas flux measurements enter an era of automated operation on massive network scales, including automated gas flux calculations. At the same time, such measurements strive to adequately represent gas exchange of newer species with extremely low fluxes (vs traditionally measured larger fluxes of H2O and CO2). Multiple assumptions, and neglected terms and processes historically deployed for evaluating larger fluxes, may no longer work well when much smaller fluxes are considered, especially when measured by a non-expert using a fully automated flux station. These no-longer-negligible aspects include fundamentals of adequately handling the diffusive and non-diffusive transport mechanisms addressed in this presentation.

How to cite: Kowalski, A., Fratini, G., Miranda, G., Serrano-Ortiz, P., and Burba, G.: Disentangling turbulent and non-diffusive fluxes in the boundary layer, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5506, https://doi.org/10.5194/egusphere-egu2020-5506, 2020.

D3051 |
Marta Galvagno, Georg Wohlfahrt, Peng Zhao, Edoardo Cremonese, and Gianluca Filippa

Mountain forests, which play an important role in the mitigation of anthropogenic CO2 emissions are supposed to be heavily affected by climatic changes and extremes. Efforts towards the understanding of the physiological processes regulating mountain forest carbon and water fluxes are crucial to correctly manage and protect these key ecosystems. However, among the challenges in micrometeorological flux measurements in complex terrain, the unaccounted presence of advective CO2 fluxes has the potential to bias the daily and longer-term CO2 exchange estimates towards unrealistic net uptake, a bias that urgently needs to be accounted for in order to reduce uncertainties related to role of mountain forests in the global carbon cycle. On the other hand, given the typical local bi-directional wind system in mountains, information on advective flows at these sites could be easier to detect compared to other terrains. We present the results of a CO2 advection experiment conducted at a European larch site in Northern Italy (2100 m asl). The setup consisted of: the main eddy covariance flux tower (20 m), a sub-canopy eddy covariance flux system (2 m), a home-assembled system for measuring CO2 concentrations at three heights on the four sides of a 40 x 40 m control volume, composed by a solenoid valve system, multiple sampling inlets and a gas analyzer, and three automatic chambers measuring bare soil respiration (two chambers) and the net ecosystem CO2 exchange from the vegetated forest floor (one chamber). Results show that: i) advection is a not-negligible fraction of the total net ecosystem CO2 exchange of this forest, ii) coupling measurements of above and below canopy eddy covariance in mountain forest sites could emerge essential for detecting/estimating the unaccounted CO2 flux

How to cite: Galvagno, M., Wohlfahrt, G., Zhao, P., Cremonese, E., and Filippa, G.: Challenges and opportunities of quantifying local CO2 advection at a mountain forest in the Alps, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17997, https://doi.org/10.5194/egusphere-egu2020-17997, 2020.

D3052 |
Sandra Grunicke, Max Plorin, Christian Bernhofer, and Ronald Queck

Drainage from the canopy is one of the sub-processes described in conceptual interception models. In theory, this refers to rainfall water that is temporally stored on the canopy and starts to drip down, when the canopy is saturated. In most Rutter-type models, storage in the canopy is described as a single linear storage (in multilayer models as a storage cascade) from which the drainage occurs after a saturation value is reached. For a precise simulation of the timing and amount of canopy drainage, this approach appears to be insufficient. E.g., with large time-steps and highly filled storage, drainage rates can become larger than the precipitation intensity, leading to a negative storage.

In our study, we present a review of different approaches to simulate drainage according to literature. We test those approaches using the interception model CanWat, which allows temporally and spatially resolved simulations of the relevant processes. CanWat relies on a vegetation model derived from terrestrial laser scans for a detailed description of the stand. CanWat is written in R language allowing to process multi-annual time series for a study area up to 1 km² with a spatial resolution of 1 m3 on a PC. For validation purposes, we use rain events that are available from long-term continuous measurements of interception in a spruce stand (mainly Picea abies; a continuous flux site since started within EUROFLUX in 1996) within the Tharandter Wald Southwest of Dresden.

We outline benefits and limitations of each approach and evaluate which one best fits the needs for a precise simulation of the timing and amount of canopy drainage. The implementation and refinement of the most suitable drainage simulation approach into CanWat is part of the overall attempt to improve the model description of the interception process being the focus of a German Science Foundation (DFG BE-1721/23-1) interception project.

How to cite: Grunicke, S., Plorin, M., Bernhofer, C., and Queck, R.: Testing approaches to simulate drainage from the canopy in interception models, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13065, https://doi.org/10.5194/egusphere-egu2020-13065, 2020.

D3053 |
Anne Verhoef, Magna S. B. Moura, and Rodolfo Nóbrega

The Caatinga is a seasonally dry tropical forest, which is the dominant vegetation type in the northeastern region of Brazil. Its many plant species have adapted to the semiarid climate through different biophysical and physiological traits and drought survival strategies. In recent years, this region has endured a number of prolonged droughts that have adversely affected this already severely water-limited region. Despite the relatively small amounts of rainfall (with annual rainfall ranging approximately between 100–800 mm/year), there is an almost perpetual occurrence of clouds due to the regional atmospheric circulation; broadly speaking cumulus or cumulonimbus in the rainy season, and mostly stratocumulus during the transition from wet to dry, and dry seasons.  We studied the effect of cloud cover on the radiation balance, as well on the surface energy- and carbon balance of a pristine Caatinga area from 2011 to 2018.

This study used radiation and weather data obtained from a SONDA BSRN radiation station, as well from a flux tower installed in the study area; both were near the urban areaofPetrolina, Brazil. Furthermore, radio-sounding data collected nearby were employed to obtain column integrated atmospheric water vapour, to estimate atmospheric emissivity.

We derived cloudiness from a number of indirect methods (using shortwave- and longwave incoming radiation) at diurnal, seasonal and multi-year timescales. We also employed observed cloud cover data, including those from sky-cameras, for verification.

Estimates of clear-sky atmospheric emissivity were required to determine cloud cover.  These were obtained from well-known equations (e.g., Brunt, Brutsaert and Prata) using tower air temperature and/or vapour pressure; calibration of the constants in these equations was required and their performance varied considerably. Occasionally, there were large differences between column integrated atmospheric water vapour and near-surface humidity; this had implications for estimates of atmospheric emissivity and hence of cloud cover.

Seasonal variations in turbidity varied by a factor of 2. Clear-sky conditions occurred for between 8-18% of the time, with the lowest percentage occurring for the wettest year (2011).

Despite its considerable effect on the radiation balance, the variation in cloud cover had a relatively modest effect only on the energy- and carbon balance fluxes. This has implications for our understanding of the Caatinga vegetation functioning, as well as for the development and testing of land surface models for this ecosystem.

This work has been supported by The Natural Environment Research Council (NE/N012488/1) and Fundação de Amparo à Ciência e Tecnologia do Estado de Pernambuco (Caatinga-FLUX Phase 2 APQ 0062-1.07/15).

How to cite: Verhoef, A., Moura, M. S. B., and Nóbrega, R.: The effect of cloud cover on the radiation-, energy- and carbon balance of a seasonally dry tropical forest in Brazil (Caatinga), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9210, https://doi.org/10.5194/egusphere-egu2020-9210, 2020.

D3054 |
Brian Butterworth, Ankur Desai, Sreenath Paleri, Stefan Metzger, David Durden, Christopher Florian, Matthias Mauder, Luise Wanner, Matthias Sühring, and Ke Xu

Land surface heterogeneity influences patterns of sensible and latent heat flux, which in turn affect processes in the atmospheric boundary layer. However, gridded atmospheric models often fail to incorporate the influence of land surface heterogeneity due to differences between the temporal and spatial scales of models compared to the local, sub-grid processes. Improving models requires the scaling of surface flux measurements; a process made difficult by the fact that surface measurements usually find an imbalance in the energy budget.

The Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors (CHEESEHEAD19) was an observational experiment designed to investigate how the atmospheric boundary layer responds to scales of spatial heterogeneity in surface-atmosphere heat and water exchanges. The campaign was conducted from June – October 2019, measuring surface energy fluxes over a heterogeneous forest ecosystem as fluxes transitioned from latent heat-dominated summer through sensible heat-dominated fall. Observations were made by ground, airborne, and satellite platforms within the 10 x 10 km study region, which was chosen to match the scale of a typical model grid cell. The spatial distribution of energy fluxes was observed by an array of 20 eddy covariance towers and a low-flying aircraft. Mesoscale atmospheric properties were measured by a suite of LiDAR and sounding instruments, measuring winds, water vapor, temperature, and boundary layer development. Plant phenology was measured in-situ and mapped remotely using hyperspectral imaging.

The dense set of multi-scale observations of land-atmosphere exchange collected during the CHEESEHEAD field campaign permits combining the spatial and temporal distribution of energy fluxes with mesoscale surface and atmospheric properties. This provides an unprecedented data foundation to evaluate theoretical explanations of energy balance non-closure, as well as to evaluate methods for scaling surface energy fluxes for improved model-data comparison. Here we show how fluxes calculated using a spatial eddy covariance technique across the 20-tower network compare to those of standard temporal eddy covariance fluxes in order to characterize of the spatial representativeness of single tower eddy covariance measurements. Additionally, we show how spatial EC fluxes can be used to better understand the energy balance over heterogeneous ecosystems.

How to cite: Butterworth, B., Desai, A., Paleri, S., Metzger, S., Durden, D., Florian, C., Mauder, M., Wanner, L., Sühring, M., and Xu, K.: Using Spatial Eddy Covariance to Investigate Energy Balance Closure over a Heterogeneous Ecosystem, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22480, https://doi.org/10.5194/egusphere-egu2020-22480, 2020.

D3055 |
Jason Simon, Khaled Ghannam, Gabriel Katul, Paul Dirmeyer, Kirsten Findell, Joseph Santanello, and Nathaniel Chaney

Land-surface heterogeneity is known to play an important role in land surface hydrology and thus the boundary conditions for numerical weather prediction (NWP) and climate modeling. For this reason, there have been considerable efforts over the past two decades to improve its representation in large scale models. However, to date, the inclusion of sub-grid heterogeneity in modeling land-atmosphere interactions in regional and global models has been limited to sub-grid spatial means and thus have almost entirely disregarded its multi-scale impact on the simulated atmospheric dynamics. To begin to address this challenge, here we use large-eddy simulations (LES) coupled to a land-surface model to gain a more complete understanding of its role in the coupled land-atmosphere system. In this work, we illustrate its impact over the Southern Great Plains (SGP) site in the United States and present a path forward for using these modeling experiments to guide the development of a complementary coupling parameterization within climate models.

More specifically, over the SGP site, we use high-resolution LES to investigate the impact of SGS land heterogeneity under different atmospheric and surface conditions to inform the development of land-surface and planetary boundary layer (PBL) parameterizations for coarser, operational-scale weather and climate modeling efforts. The experiment methodology uses a high-resolution land-surface model (WRF-Hydro), spun-up over multiple years using reanalysis data, which is then coupled to the Weather Research and Forecasting (WRF) model for high-resolution LES. Cases are considered using both the fully heterogeneous land model as well as using a homogeneous surface with domain-averaged flux values at all grid points, allowing the dynamical effects of land-surface heterogeneity on the atmosphere to be isolated, and the land/atmospheric conditions under which land-surface heterogeneity plays a role to be studied. Results are evaluated primarily by the differences in the development of the planetary boundary layer and the extent, duration and intensity of developing rainfall events.

How to cite: Simon, J., Ghannam, K., Katul, G., Dirmeyer, P., Findell, K., Santanello, J., and Chaney, N.: An investigation into the influence of high-resolution land-surface heterogeneity on atmospheric dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20762, https://doi.org/10.5194/egusphere-egu2020-20762, 2020.

D3056 |
Annu Panwar, Axel Kleidon, and Maik Renner

Diurnal temperature variations are strongly shaped by the absorption of solar radiation, but evaporation, or the latent heat flux, also plays an important role. Generally, evaporation cools, but its relation to diurnal temperature variations is unclear. This study investigates the diurnal response of surface and air temperatures to solar radiation and how evaporation and vegetation modify their response. We used the warming rate of temperature to absorbed solar radiation in the morning under clear-sky conditions and evaluated how the warming rates change for different evaporative fractions. Results for 51 FLUXNET sites show that air temperature carries very weak imprints of evaporation across all vegetation types. However, surface temperature warming rates of short vegetation decrease significantly by ~23 x 10-3 K/W m-2 from dry to wet conditions. Contrarily, warming rates of surface and air temperatures are similar at forest sites and carry literally no imprints of evaporation. We explain these contrasting patterns with a surface energy balance model. The model reveals a strong sensitivity of the warming rates to evaporative fraction and aerodynamic conductance. However, for the large aerodynamic conductance, the sensitivity to the evaporative fraction is strongly reduced. We then show that in addition to the higher aerodynamic conductance of forests, imprints of evaporation in the warming rate of surface temperature are reduced by 50% through an enhanced aerodynamic conductance under dry conditions. This contribution is comparatively weak (28%) for short vegetation. These findings have implications for the interpretation of land-atmosphere interactions and the roles of moisture limitation and vegetation on diurnal maximum temperatures, which is of key importance for ecological functioning. We conclude that surface temperature warming rate is a promising predictor of evaporation for short vegetation. These findings are in agreement with our previous study (Panwar et al., 2019) where the weaker response of air temperature to the evaporative fraction is explained by the larger growth of the boundary layer on drier days. In forests, however, the diurnal variation in temperatures is governed by their aerodynamic properties resulting in no imprint of evaporation in diurnal temperature variations.

Reference: Panwar, A., Kleidon, A. and Renner, M.: Do Surface and Air Temperatures Contain Similar Imprints of Evaporative Conditions?, Geophysical Research Letters, 46(7), 3802–3809, doi:10.1029/2019GL082248, 2019.

How to cite: Panwar, A., Kleidon, A., and Renner, M.: Imprints of evaporation and vegetation type in diurnal temperature variations, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-3371, https://doi.org/10.5194/egusphere-egu2020-3371, 2020.

D3057 |
Emma Barton, Christopher Taylor, Cornelia Klein, and Phil Harris

The Tibetan Plateau is the highest and most extensive plateau in the world, profoundly affecting climate and weather in the region. Due to its average elevation of more than 4000m, provides a strong thermal and dynamical forcing in the mid-troposphere during the summer months, fostering the frequent development of intense storms. Mesoscale convective systems (MCSs) are known to be associated with particularly extreme rainfall events and contribute up to ~60% of rainfall over the Tibetan Plateau (TP) and adjacent areas. In particular, MCSs that form on the TP may move off and bring heavy rain and flooding to downstream parts of China, affecting millions of people. A better understanding of the processes that impact MCS genesis over the TP could contribute to improved forecasting of these extreme events. Furthermore, there is strong evidence for accelerated climate warming on the TP, which may affect convection and makes the identification of factors for MCS development even more important.

Previous work in the Sahel has shown that mesoscale soil moisture patterns can influence the initiation of new MCSs, however the relationship has yet to be investigated for the more hydrologically and topographically complex TP. In this study we investigate the impact of mesoscale soil moisture features on convective initiation over the TP during the monsoon season (May – September) using satellite imagery. Convective clouds are identified using the Fengyun-2 cloud top temperature product. Fengyun-2 is a series of geostationary satellites that provide hourly data, allowing us to track systems as they evolve. Land surface temperature anomalies are used as a proxy to map pre-storm mesoscale soil moisture patterns.

Despite the presence of complex topography, we identify a tendency for MCS initiations to occur in the vicinity of mesoscale soil moisture gradients. Our results suggest that improved representation of land-atmosphere coupling on the TP within weather and climate models could impact the entire region.

How to cite: Barton, E., Taylor, C., Klein, C., and Harris, P.: Influence of Mesoscale Soil Moisture Patterns on Convective Initiation over the Tibetan Plateau, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4943, https://doi.org/10.5194/egusphere-egu2020-4943, 2020.

D3058 |
Marcus Breil

The intensity of latent heat fluxes strongly depends on the available amount of soil water for evapotranspiration, i.e. the water amount stored within the rooted soil depth. But the determination of the root depth itself, and consequently also of the water supply for evapotranspiration, is associated with large uncertainties. The latent heat fluxes are therefore in many cases spuriously simulated, leading to temperature biases especially in soil-moisture limited evapotranspiration regimes like Southern Europe.

To take these uncertainties into account, a new method is introduced, in which the root depths within the Regional Climate Model (RCM) COSMO-CLM are stochastically varied. For this, the root depths in each model grid box are perturbed by uniformly distributed random numbers. The results of this stochastic simulation are compared to the results of an unperturbed reference run. The study reveals that during the winter season, evapotranspiration is virtually not affected by the stochastic root depth perturbation. Changes in the simulated near-surface temperatures are caused by indirectly induced chaotic changes in the atmospheric circulation. But in summer, the latent heat fluxes are considerably increased all over Southern Europe, due to a stochastic increase of the available soil water amount for evapotranspiration. Soil warming is consequently reduced and lower near-surface temperatures are simulated for the whole Mediterranean region. These lower temperatures reduce the strong warm bias in South-Eastern Europe in the reference run, which is also consistently simulated in several other RCMs and General Circulation Models (GCMs). Therefore, stochastic root depth modelling constitutes a simple method that can be implemented in every modelling system, to mitigate systematically simulated warm biases and thus, potentially improving model performances in semi-arid regions.

How to cite: Breil, M.: The reduction of the systematic RCM summertime warm bias in South-Eastern Europe by stochastic root depth variation, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5318, https://doi.org/10.5194/egusphere-egu2020-5318, 2020.

D3059 |
Ivan Bogoev and David Holl

Open-path eddy covariance systems, based on broad-band non-dispersive infrared (NDIR) gas analyzers, are widely used for CO2 and H2O flux measurements in remote locations around the world, because of their low power consumption, fast response and reliable operation. Nevertheless, agreement between open- and closed-path CO2 fluxes has limited inter-site comparability, especially in cold or non-growing seasons and low-flux environments, where physiologically unreasonable CO2 uptake is often observed by the open-path systems. A possible explanation is sensor-surface heating from internal-electronics power dissipation and solar radiation, which causes unaccounted gas density changes in the optical path. Fast-response thermometers, co-located with the gas analyzer, have been used to correct these effects. However, the fragility of the thermometers has prevented the wide adoption of this approach.

A challenge for the open-path sensor design is that in-situ air temperature affects not only the gas density but also the broadened half-width and intensity of the spectral absorption lines. We hypothesize that fast air temperature fluctuations in the optical path of the gas analyzer can change the amount of absorbed light and cause errors in the CO2 concentration measurement. Because of the natural covariance of sensible and CO2 fluxes, such errors are well correlated with the vertical wind and can potentially propagate into flux calculations.

We used spectral-line parameters, obtained from the high-resolution transmission molecular spectroscopic database (HITRAN), to evaluate the temperature effects on the integrated absorption spectra of CO2-air-mixtures across the 4.2 to 4.3 μm infrared active region utilized by NDIR analyzers.  Results show that air temperature strongly influences absorption, and if not properly corrected, potentially introduces biases in the CO2 concentration measurements.  Strong lines exhibit Doppler broadening, where the line peak and width decline with increasing temperatures, causing underestimation of CO2 concentration. Weak lines exhibit the opposite behavior. Based on our simulations, optimizing the optical filter pass-band can balance these opposing effects and greatly reduce the temperature dependence. In practice, manufacturing tolerances, shifts in the center wavelength, and the temperature sensitivity of the optical filters prevent complete elimination of the temperature-line broadening. A 13 nanometer shift in the filter pass band can introduce a 0.008 mmol m-3 K-1 underestimation in the CO2 concentration, which is a 0.67 μmol m-2 s-1 systematic error in CO2 flux per 100 watts of sensible heat flux.

How to cite: Bogoev, I. and Holl, D.: Temperature Induced Spectroscopic Line-broadening Effects in Open-path Eddy Covariance CO2 Flux, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2779, https://doi.org/10.5194/egusphere-egu2020-2779, 2020.

D3060 |
Justus van Ramshorst, Christian Markwitz, Timothy Hill, Robert Clement, Alexander Knohl, and Lukas Siebicke

Agroforestry is a combination of monoculture agriculture and trees. Studies of net ecosystem exchange of CO2 (NEE) of agroforestry systems are rare, in comparison to the extensive studies of NEE of agricultural systems (croplands and grasslands). Agroforestry has been shown to alter the microclimate, productivity, and nutrient and water usage – as compared to standard agricultural practise. The, potentially, higher carbon sequestration of agroforestry, relative to monoculture systems, provides an interesting option for mitigating climate change, highlighting the need for improved study of agroforestry systems. The current study, as part of the SIGNAL (sustainable intensification of agriculture through agroforestry) project, investigates NEE of agroforestry compared to that of monoculture agriculture. The study employs paired comparisons of flux measurements above agroforestry and monoculture agronomy, replicated at five locations across Germany. Fluxes are measured, using innovative low-cost CO2 eddy covariance sensors (slow response Vaisala GMP343 IRGA with custom made housing), which have been successfully used in a study over grassland. Continuous data series from mid-summer until winter 2019 show that both systems acted as a sink with comparable fluxes during summer. The diurnal CO2 cycle and the response to management activities are distinguishable and in autumn preliminary results suggest a small difference in fluxes between the two systems. The low-cost eddy covariance system is able to capture the turbulence and to measure the CO2 flux over the agroforestry and monoculture agricultural system. We aim to further improve the quality of the CO2 fluxes, by adapting post-processing software to better estimate the difference in carbon uptake between the agroforestry and monoculture systems.

How to cite: van Ramshorst, J., Markwitz, C., Hill, T., Clement, R., Knohl, A., and Siebicke, L.: First application of low-cost eddy covariance for CO2 fluxes over agroforestry, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-10329, https://doi.org/10.5194/egusphere-egu2020-10329, 2020.

D3061 |
Anas Emad and Lukas Siebicke

Eddy accumulation is a direct flux measurement technique for trace gas exchange between the land surface and the atmosphere. Eddy accumulation complements the now common eddy covariance method in its ability to measure even small fluxes accurately with slow response gas analyzers and being power efficient. However, the physically most direct way of eddy accumulation, also known as true eddy accumulation (TEA), requires the sampling of air at a rate proportional to the vertical wind velocity at a fast rate of typically 10 Hz or more. Lack of suitable methods for high-speed air sampling has been a primary limitation for the practical application of eddy accumulation in the past. The compressibility of air causes a variation of pressure inside the sampling system, which affects the ability to control the sample flow rate accurately, potentially compromising the derived flux measurements. It is therefore essential to quantify the effect of compressibility on fluxes and understand the parameters which allow for mitigating the effect at the design stage.
In this study, we present successful true eddy accumulation measurements over the old-growth forest at the Fluxnet site Hainich (DE-Hai) and quantify the compressibility effects on fluxes. Performing simulations on high-frequency data of CO2 and vertical wind velocity for a range of system configurations, we are able to quantify the impact of compressibility on fluxes and explain why our measurements were successful. We find that different system configurations lead to flux changes over a representative range of 1 to 25 percent of the flux. Key controlling parameters are the size and arrangement of internal buffer volumes and the appropriate control of the inlet flow rate sampling device as a function of internal and external pressure states. This knowledge allows to mitigate compressibility effects and design accurate true eddy accumulation flux measurements for a range of atmospheric constituents.

How to cite: Emad, A. and Siebicke, L.: Compressibility of air effects on eddy accumulation flux measurements, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17817, https://doi.org/10.5194/egusphere-egu2020-17817, 2020.

D3062 |
Yuanjie Zhang, Dan Li, and Zhiqiu Gao

This study investigates the diurnal variations of the planetary boundary layer (PBL) during heatwaves (HWs) based on a decade-long data record of hourly profiles from the Aircraft Meteorological Data Reports (AMDAR) at 54 major airports over the Contiguous United States. The results are also corroborated by the surface observations from weather station data. Temperature differences between HW and non-HW periods show strong diurnal and vertical variations in the PBL. Under HWs, the daytime convective PBL becomes higher while the nocturnal residual layer becomes excessively hotter, which creates a positive feedback on HW intensity and duration by providing heat for the next day through entrainment. Changes in wind speed in the PBL during HWs are dependent on the relative location between the observational site and the center of the high-pressure system and do not always show decreases. The specific humidity often increases in humid regions mainly due to the higher evaporation demand which causes more surface evaporation, but decreases in arid regions due to the dry air entrainment and subsidence. In addition, moisture advection could also play an important role in modulating the change of PBL humidity. This study improves our scientific understanding of HW-related changes in the PBL structure and highlights their possible connections with the synoptic-scale atmospheric circulation patterns and land-atmosphere feedbacks. However, future research examining the role of PBL in the onset and demise of HWs and examining whether meteorological models can capture the observed response of the PBL to HWs is strongly needed.

How to cite: Zhang, Y., Li, D., and Gao, Z.: Aircraft observed diurnal variations of the planetary boundary layer under heatwaves, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-4352, https://doi.org/10.5194/egusphere-egu2020-4352, 2020.

D3063 |
Jasper Denissen, Hendrik Wouters, René Orth, Diego Miralles, and Ryan Teuling

The land surface can influence near-surface weather. This happens, amongst others, through the impact of soil moisture availability on surface heat fluxes: when soil moisture is unavailable in soil moisture-limited conditions, most of the available energy will be used for heating the air above the land surface (sensible heat flux). But as soil moisture increases, evapotranspiration (latent heat flux) increases, affecting the surface heat flux partitioning. At the point that ample soil moisture is available in energy-limited conditions, the surface heat flux partitioning remains unaffected by soil moisture. The atmospheric boundary layer (ABL) responds to changes in surface heat flux partitioning in particular in terms of its temperature and humidity. Based on these mechanisms, observations of boundary layer dynamics should allow to infer the large-scale land surface state.

The goal of this study is to use atmospheric measurements of temperature and humidity to estimate the surface heat flux partitioning. This is achieved by constraining an ABL model (CLASS4GL) with the vertical temperature and humidity profiles as observed by thousands of soundings of hot air balloons across the globe. In CLASS4GL, the initial soil moisture is adjusted to yield matching modelled versus observed vertical temperature and humidity profiles. By doing so, the resulting surface fluxes are inferred exclusively from atmospheric measurements.

We find that ABL’s tend to higher, warmer and drier in water-limited conditions. This largely results from changes in soil moisture availability, which mainly affects the sensible heat flux and consequently, the surface heat flux partitioning. We determine the critical soil moisture, which distinguishes between soil moisture- and energy- limited conditions, using the ratio between the sensible- and latent heat flux and independent satellite surface soil moisture.

This is the first time that balloon soundings are used globally to assess the critical soil moisture. This research will help to further improve our understanding of land-atmosphere feedbacks and foster a correct representation of land surface characteristics in Land Models and subsequently, Climate Models.

How to cite: Denissen, J., Wouters, H., Orth, R., Miralles, D., and Teuling, R.: What balloon soundings can tell about surface heat flux partitioning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15661, https://doi.org/10.5194/egusphere-egu2020-15661, 2020.

D3064 |
Johannes Speidel, Hannes Vogelmann, Matthias Mauder, Matthias Perfahl, Timothy J. Wagner, and Luise Wanner

Water vapor plays a crucial role for several processes on almost every scale of Earth's atmosphere. However, its turbulent transport throughout the PBL is at the same time particularly important and not very well understood. With the increasing resolution of numerical models arises the need for improved representations of the small-scale and nonlinear turbulent processes inside the PBL. Recent papers have shown that these refinements, predominantly on water vapor transport, are urgently needed as they are a limiting factor in the process of lifting numerical weather prediction to the next level.
 The CHEESEHEAD campaign, carried out in summer 2019, especially addresses the complex, turbulent land-atmosphere interactions over heterogeneous terrain and aims to close the energy balance. Therefore, a dense network of sensors has been installed measuring throughout the scales of the PBL with a multiple set of different measurement techniques.
 Within this proposed contribution, first results from the CHEESEHEAD measurements with a water vapor DIAL in combination with several Doppler wind LiDARs will be presented. The synergy of a virtual tower scanning geometry of the Doppler LiDARs right next to the water vapor DIAL delivers highly resolved data throughout the entire PBL. Therefore, special focus of this work lies on turbulent fluctuations inside vertical water vapor columns during the measuring time, spanning the entire PBL. This gives the additional opportunity to observe important entrainment processes at the very top of the PBL. Furthermore, this work deals with the calculation of vertical fluxes of latent heat. Therefore, the essential question whether this data is suitable for these calculations, which are highly sensitive towards temporal resolution, will be addressed. As a further step, aerosol and temperature data that have been measured with the same LiDAR system shall be integrated as well - aiming towards a comprehensive insight on relevant processes throughout the entire PBL. 

How to cite: Speidel, J., Vogelmann, H., Mauder, M., Perfahl, M., Wagner, T. J., and Wanner, L.: Water vapor transport in the turbulent planetary boundary layer (PBL) measured over heterogeneous terrain using multiple LiDAR systems., EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17200, https://doi.org/10.5194/egusphere-egu2020-17200, 2020.

D3065 |
Josipa Milovac, Klaus Goergen, Jesus Fernandez, Kirsten Warrach-Sagi, Alvaro Lavin-Gullon, Joachim Ingwersen, and Volker Wulfmeyer

The evaluation of the first of its kind multi-model convection permitting - regional climate model (CP-RCM) ensemble, produced within the framework of the international CORDEX - Flagship Pilot Study on Convective phenomena over the Mediterranean (CORDEX-FPS-CEM), showed the improved spatial representation, frequency and extremes of precipitation compared to coarser resolution counterparts (Ban et al., submitted 2019). It is important, though, to keep in mind that the quality of each of such high resolution simulations strongly depend on the quality of the forcing used to drive the model.

In this work we investigate the impact of the land-surface forcing on the model representation of land-atmosphere (LA) feedback. For that we used the 2 Weather Research and Forecasting (WRF) settings from the CORDEX-FPS-CEM ensemble to run 4 simulations for 2 seasons (summer and fall), combining 2 sets of data for the land cover (MODIS and CORINE) and the top soil texture (FAO and HWSD). In such a way we generated two ensembles with 8 simulations for each season. Additionally we run 4 simulations with perturbed starting date for the summer season to obtain an additional 5-member ensemble to investigate the impact of the internal variability on the final results.

The objective of this study is to investigate the model sensitivity to (1) land-surface static forcing, (2) season, and (3) the model configuration. To quantify the strength of LA coupling for each grid, we use coupling metrics appropriate for seasonal time scales. We applied mixing diagram approach to investigate the impact of the land-surface changes on the boundary layer evolution. We also investigate the impact of these changes on the potential for convection triggering, and we calculate correlation matrices to quantify the impact on the strength of the land-atmosphere coupling.

Preliminary results show evident effects of the land surface changes on surface variables, boundary layer evolution, atmospheric stability and humidity in the lower atmosphere. Strength of the sensitivity to a specific change in the land-surface forcing depend on the model configuration: WRF with less sophisticated parameterization schemes is more sensitive to land use changes, while more sophisticated configuration shows higher sensitivity to the soil texture changes in representing boundary layer evolution. Furthermore, the WRF shows stronger coupling and therefore stronger sensitivity to the land surface changes in the summer season.

Acknowledgement: This work is partially funded by the Spanish Government R+D programme through grant INSIGNIA (CGL2016-79210-R) co-funded by the ERDF/FEDER.

References: Ban N., and coauthors: The first multi-model ensemble of regional climate simulations at kilometer-scale resolution, Part I: Evaluation of precipitation, (submitted to Climate Dynamics, 2019)

How to cite: Milovac, J., Goergen, K., Fernandez, J., Warrach-Sagi, K., Lavin-Gullon, A., Ingwersen, J., and Wulfmeyer, V.: Impact of the land surface forcing on land-atmosphere feedback in the convection permitting modeling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-20891, https://doi.org/10.5194/egusphere-egu2020-20891, 2020.

D3066 |
Shikhar Upadhyay, Sarit Das, and Chandra Shekhar Ojha

The spatial variations of ABL depth has wide applications in aeronautical meteorology, urban meteorology, agricultural meteorology and hydrology. In the context of Indian subcontinent, it is more important where air pollution episodes, smog, fog etc. are getting worse over the years. The dispersal of smog and low-level pollutants depends strongly on meteorological conditions. Monitoring and management of air quality is closely associated with the transport and dispersal of atmospheric pollutants, including industrial plumes. Processes of pollutant transport include turbulent mixing in the ABL, particularly the role of convection, photochemistry and dry and wet deposition to the surface. The depth of the ABL determine the extent of thermal and mechanical mixing of pollutants. Further, mean ABL depth can be used to determine the average seasonal air pollution scenarios. Soil surface temperature is one of the major factors which derives the ABL depth. Thus, it is important to know - what is the spatial ABL depth and soil surface temperature variation, in which direction changes in ABL depth and soil surface temperature is more or less consistent, over Indian subcontinent.

To understand the spatial variability of ABL depth and soil surface temperature, a variogram analysis is performed taking 30 stations over Indian sub-continent. Data at 30 stations (Ahmadabad, Bhopal, Gwalior, Aurangabad, Nagpur, Raipur, New Delhi, Gorakhpur, Patna, Lucknow, Patiala, Siliguri, Karaikal, Vishakhapatnam, Machilipatnam, Lhasa, Minfeng, Jodhpur, Agartalla, Bengaluru, Bhubaneshwar, Chennai, Dibrugarh, Hotan, Hyderabad, Jagdalpur, Kolkata, Panjim, Port Blair, Srinagar) are collected for three years 1994, 1997 and 2000. ABL depths are computed using soundings obtained from the Integrated Global Radiosonde Archived (IGRA) by adopting the bulk Richardson method.

Both ABL depth and soil surface temperature are greater in central region, but low near shore and in hilly regions. By using both these parameters, omnidirectional variograms are drawn, which show the spatial distribution of ABL depths and surface soil temperature over India are determined for different years. The particular variogram demonstrates a well-suited spatial relationship for geostatistical analysis as pairs of points are more correlated the closer they are together and the greater the distance between points becomes less correlated. There are certain parameters of variogram (sill and range) that adjust iteratively to get the best fitted model. Then, models are fitted to the experimental variogram using least square approach between the experimental and modelled variogram values. The model with its corresponding parameters based on least square method is selected as the best variogram model. These parameters are finally used in the ordinary kriging analysis. Spherical variograms are fitted and found to have significant correlation for stations within a lags of 19, 18, 18 and 17, 17, 20 degrees latitude/longitude change for ABL depth and soil surface temperature and for the year 1994, 1997 and 2000 respectively. Utilizing variogram parameters, the spatial distributions are plotted using ordinary Kriging. A polynomial curve of order 3 fitted Cubic curve fitting on the scatter plots between soil surface temperature and ABL depth, yield R2 value as 0.44, 0.52 and 0.53 in 1994, 1997, 2000 respectively.


How to cite: Upadhyay, S., Das, S., and Ojha, C. S.: Spatial Distribution of ABL height and Soil Temperature over Indian Subcontinent, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21239, https://doi.org/10.5194/egusphere-egu2020-21239, 2020.

D3067 |
Minttu Havu, Liisa Kulmala, Anu Riikonen, and Leena Järvi

A high proportion of anthropogenic carbon dioxide emissions originate from urban areas, which has led cities to become interested in reducing their own emissions and determining how much carbon could be sequestered by their own vegetation and soil. The challenge with the latter is that our current knowledge on carbon storage is based on data and models from natural and forest ecosystems, whereas the response of vegetation and soil to environmental factors most probably is altered in urban green space where the soil conditions, water availability and temperature are highly variable. Therefore, ecosystem models are required to correctly account for urban vegetation and soil to understand and quantify the biogenic carbon cycle in urban areas.

In this study, urban land surface model SUEWS (the Surface Urban Energy and Water Balance Scheme) and the soil carbon decomposition model Yasso15 are used to simulate urban carbon cycle on two streets in Helsinki, Finland for years 2003-2016. Curbside trees (Alnus glutinosa and Tilia x Vulgaris) were planted while the two test streets were constructed in 2002. Thereafter, carbon and water fluxes and pools with detailed street tree soil compositions were monitored in 2002-2014. SUEWS creates a local spatially variable temperature and specific humidity environment which is used in the model runs. The modelled evaporation is evaluated against sap flow measurements and modelled soil moisture against soil moisture observations. The Yasso15 model is evaluated against loss-on-ignition based soil carbon measurements as it has not been previously evaluated in urban soils. The modelled carbon dioxide flux combined with the changes in the soil carbon stock is used to estimate the carbon cycle of urban street trees and soils.

How to cite: Havu, M., Kulmala, L., Riikonen, A., and Järvi, L.: Modelling carbon sink of urban street trees and soil in Helsinki, Finland, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13833, https://doi.org/10.5194/egusphere-egu2020-13833, 2020.

Chat time: Wednesday, 6 May 2020, 14:00–15:45

Chairperson: Matthias Mauder
D3068 |
Jeongwon Kim, Junhong Lee, Je-Woo Hong, Jinkyu Hong, Ja-Ho Koo, Joo-Hong Kim, Juyeol Yun, Sungjin Nam, Ji Young Jung, Taejin Choi, and Bang Yong Lee

Arctic tundra is changing rapidly under the influence of global warming and it is important to know its impact on local and regional climate. Polar Weather Research and Forecasting (PWRF) model is a regional climate model optimized for the polar region and it is a useful tool for studying the Arctic tundra in high resolution. In this study, we evaluate the performance of the PWRF model over the Arctic tundra on clear summer days, when the transition is taking place the most, based on the surface energy fluxes and PBL observations in Cambridge Bay, Nunavut, Canada.

The PWRF simulates a drier and warmer environment in PBL than the observations. Our analysis shows that it is due to the surface energy imbalance in the model caused by the uncertainties in prescribed initial input data and physical parameters rather than structural flaws in the model physics. The performance of the PWRF model over the Arctic tundra is improved when those values are modified based on the observation data.

How to cite: Kim, J., Lee, J., Hong, J.-W., Hong, J., Koo, J.-H., Kim, J.-H., Yun, J., Nam, S., Jung, J. Y., Choi, T., and Lee, B. Y.: Evaluation of land-atmosphere processes of the Polar WRF in the summertime Arctic tundra, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21140, https://doi.org/10.5194/egusphere-egu2020-21140, 2020.

D3069 |
Jan-Peter Schulz and Gerd Vogel

Land surface processes have a significant impact on near-surface atmospheric phenomena. They determine, among others, near-surface sensible and latent heat fluxes and the radiation budget, and thus influence atmosphere and land characteristics, such as temperature and humidity, the structure of the planetary boundary layer, and even cloud formation processes. It is therefore important to simulate the land surface processes in atmospheric models as realistically as possible.

Verifications have shown that the amplitude of the diurnal cycle of the surface temperature simulated by the land surface scheme TERRA of the COSMO atmospheric model is systematically underestimated. In contrast, the diurnal cycles of the temperatures in the soil are overestimated, instead. This means that the other components of the surface energy balance are biased as well, for instance, the surface turbulent heat fluxes or the ground heat flux.

Data from the Meteorological Observatory Lindenberg of the German Meteorological Service (DWD) were used to analyse this model behaviour. In the standard model configuration of TERRA, there is no representation of the vegetation in the surface energy balance. This means, there is no energy budget including a temperature for the vegetation layer. Furthermore, the insulating effects by the vegetation at the sub-canopy level are missing as well. In this work, a scheme providing both of these missing model characteristics was implemented in TERRA. As a result, the simulated diurnal amplitude of the surface temperature is increased and the one of the soil temperature is reduced, both leading to better agreements with the measurements. These improvements are found in TERRA in offline mode, using Lindenberg observations, as well as in coupled mode in the atmospheric models of DWD, i.e. the limited-area COSMO model and the global ICON model.

How to cite: Schulz, J.-P. and Vogel, G.: An improved representation of the land surface temperature including the effects of vegetation in the COSMO model, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-22029, https://doi.org/10.5194/egusphere-egu2020-22029, 2020.

D3070 |
Luise Wanner, Frederik De Roo, and Matthias Mauder

The eddy-covariance method generally underestimates sensible and latent heat fluxes, resulting in an energy-balance gap from 10 % to even 30 % across sites worldwide. In contrast to single-tower eddy-covariance measurements, large-eddy simulations (LES) provide information on a 3D array of grid points and can capture atmospheric processes such as secondary circulations on all relevant scales, which makes them a powerful tool to investigate this problem. In order to compare LES results to field measurements at 20 m height from the CHEESEHEAD (Chequamegon Heterogeneous Ecosystem Energy-balance Study Enabled by a High-density Extensive Array of Detectors) campaign, a LES-setup that provides comparability to the measurements at these low levels is necessary. However, former LES studies have shown that the energy balance is almost closed near the surface, which does not reflect the energy-balance gap in measurements. One possible reason might be the common use of prescribed surface fluxes that cannot adapt to changes in surface temperature and moisture, which would allow for the self-reinforcement of secondary circulations. Therefore, we set up an idealized study, in which we compare the performance of the land-surface and plant-canopy models implemented in PALM to the use of prescribed surface fluxes above homogeneous forest and grassland ecosystems under different atmospheric conditions with respect to realistic energy-balance closure behavior. Furthermore, we evaluate the performance of a dynamic subgrid-scale model, as well as an alternative to the Monin-Obukhov similarity theory (Banerjee et al. 2015, Q. J. R. Met. Soc.).

How to cite: Wanner, L., De Roo, F., and Mauder, M.: Optimizing large-eddy simulations for investigating the energy-balance closure problem at typical flux measurement heights, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-19678, https://doi.org/10.5194/egusphere-egu2020-19678, 2020.

D3071 |
Dan Li and Liang Wang

While land use and land cover change (LULCC) is often a temporal phenomenon (i.e., a patch transitions from one land cover type to another), many studies use a space-for-time approximation that quantifies the LULCC impact (say on surface temperature or fluxes) by comparing two adjacent patches of different land covers. An important consideration embedded in this space-for-time approximation is the scale, which determines what assumptions we can make when constructing models for studying land-atmosphere interactions over heterogeneous terrain. Most previous studies employ one-dimensional models without considering the appropriate scale associated with these models. In this presentation, the scale issue in studying LULCC-induced surface temperature anomalies will be discussed using a hierarchy of models. Typical one-dimensional models based on the surface energy balance and/or convective boundary layer dynamics will be compared to two-dimensional models where horizontal advection is explicitly considered. The results highlight the importance of scale in determining the sensitivity of land surface temperature to changes in albedo and moisture/vegetation characteristics. 

How to cite: Li, D. and Wang, L.: Land use and land cover change impact on surface temperature: the scale issue, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1634, https://doi.org/10.5194/egusphere-egu2020-1634, 2020.

D3072 |
Thierry Hedde and Michiel De Bode

The land cover defines what is on the earth’s surface. For atmospheric research, land cover is a leading influencer of surface exchange rates, roughness length and the surface heat flux. Nowadays, with finer resolution, the importance of land cover being correct increases. Especially over heterogeneous terrain, as gradients created by short-distance variability can influence local meteorology. For processing purposes, land covers maps group all land covers in classes based on certain conditions. Every land cover map has different classes and conditions to serve their purpose best.

The land cover map USGS is the default map for weather simulations with WRF. Since Pineda et al., 2004 developed a method to convert the Corine Land Cover (CLC) to a WRF readable format, is CLC available for simulations over Europe. CLC is a land cover map that focuses primarily on the EU and a few collaborating countries.

Our objective is to investigate the influence of land cover on valley winds. Our study focuses on the central part of the pre-Alpine Durance valley. Simulations exist out of a domain with three nested domains and verified with the KASCADE 2013 data. In previous simulations with USGS, land cover is in the domain near the Cadarache site almost homogeneous. Results and representation improved with the introduction of the CLC as land cover map.

This CLC study will act as an update and extension of a previous study of the area. In this study, we use an updated version of CLC, and we look at different methods of aggregation of the CLC data to quantify their effects on simulation results. CLC has 44 categories and 3 levels of detail. We compare direct aggregation to an aggregation, which takes into account the available detail levels.

First comparisons of aggregation methods show a mismatch ranging from 1.4 % in at 300 m resolution to values around 11 % for resolutions of 3 km and coarser.

How to cite: Hedde, T. and De Bode, M.: Different aggregations of Corine land cover for WRF simulations of a pre-Alpine valley, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5158, https://doi.org/10.5194/egusphere-egu2020-5158, 2020.

D3073 |
Ji-Qin Zhong, Bing Lu, Wei Wang, Cheng-Cheng Huang, and Yang Yang

 The causes of the underestimated diurnal 2-m temperature range and the overestimated 2-m specific humidity in Northern China’s winter in the Rapid-refresh Multi-scale Analysis and Prediction System - Short Term (RMAPS-ST) system are investigated. Three simulations based on RMAPS-ST are conducted from Nov. 1st, 2016 to Feb. 28th, 2017. Further analyses show that the partitioning of surface upward sensible heat fluxes and downward ground heat fluxes might be the main contributing factor in 2-m temperature forecast biases. In this study, two simulations are conducted to examine the effect of soil moisture initialization and soil hydraulic property on the 2-m temperature and 2-m specific humidity forecast biases. Firstly, the High-Resolution Land Data Assimilation System (HRLDAS) is used to provide an alternative soil moisture initialization, and the result shows that the drier soil moisture leads to noticeable change in energy partition at the land surface, which in turn results in improved prediction of the diurnal 2-m temperature range, although it also enlarges the 2-m specific humidity bias in some parts of the domain. Secondly, a soil texture dataset developed by Beijing Normal University (BNU) and a revised hydraulic parameters are applied to provide a more detailed description of soil properties, which could further improve the 2-m specific humidity biases. In summary, the combination of using optimized soil moisture initialization, updated soil map and revised soil hydraulic parameters can help improve the 2-m temperature and 2-m specific humidity prediction in RMAPS-ST.

How to cite: Zhong, J.-Q., Lu, B., Wang, W., Huang, C.-C., and Yang, Y.: Impact of Soil Moisture on Winter 2-m Temperature Forecast in Northern China, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6521, https://doi.org/10.5194/egusphere-egu2020-6521, 2020.

D3074 |
Verena Rajtschan, Florian Späth, Thilo Streck, and Volker Wulfmeyer

Soil, land cover and the lower atmosphere form the land atmosphere system. The feedbacks in this system are nonlinear, because of two-way coupling between single variables such as soil moisture and precipitation. A detailed characterization of fluxes and feedbacks within and between the different compartments is essential to improve our understanding of the land atmosphere system. To investigate these fluxes and feedbacks the Land Atmosphere Feedback Observatory (LAFO) was established at the experimental station “Heidfeldhof” of the University of Hohenheim in 2018. LAFO applies a novel synergy of eddy covariance stations, vegetation measurements, and innovative scanning lidar systems. The measurements are comprehensive, highly resolved and very precise, so that new parameterizations of land-atmosphere exchange processes between soil/vegetation and the lower troposphere for model systems can be developed, implemented, and tested.

LAFO is situated in heterogeneous cropland with nearby urban areas and forests. The heterogeneous terrain forms a complex land atmosphere system. A footprint analysis of eddy covariance data from 2019 enables us to better characterize our site.  Here we present LAFO, its concept and first results on a footprint analysis of the eddy covariance data.

How to cite: Rajtschan, V., Späth, F., Streck, T., and Wulfmeyer, V.: Land Atmosphere Feedback Observatory (LAFO) and heterogeneous terrain in its footprint, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-21367, https://doi.org/10.5194/egusphere-egu2020-21367, 2020.

D3075 |
| Highlight
Bart Schilperoort, Miriam Coenders-Gerrits, and Hubert Savenije

One of the challenges of flux measurements above tall canopies, is that parts of the canopy can be decoupled from the atmosphere above. This decoupling can, for example, occur when the forest understory is colder than the air above, limiting exchange through convection. While concurrent above and below canopy eddy covariance (EC) measurements help with addressing the decoupling issue, these are still disconnected point measurements and do not show what is happening along the entire vertical profile. For this, Distributed Temperature Sensing (DTS) can give additional insights, as it can perform continuous temperature measurements along a vertically deployed fiber optic cable.

Measurements were performed at the ‘Speulderbos’ forest site in the Netherlands, where a 48 m tall measurement tower is located in a stand of 34 m tall Douglas Fir trees.  We measured a vertical temperature profile through the canopy using DTS (from the surface up to 32 m). The measurement frequency was ~0.5 Hz, with a vertical resolution 0.30 cm, and data was collected for two months. The fiber optic cable used had a diameter of 0.8 mm, allowing a sufficiently quick response to temperature changes. With this data we were able to detect the presence, height, and strength of inversions. The inversions appeared to occur mostly at night. The height of the inversion showed a bistable behavior, either staying around 1 m above the ground, or at approximately 16 m, which is just below the dense branches of the canopy.

By locating and tracking inversions within the canopy, decoupling events can be studied and explained in more detail. If vertical DTS profiles are available at a site, these can be used for filtering EC measurements as well. While more research will be needed before a wide application at flux sites is possible, this study can serve as a ‘proof-of-concept’ and demonstrates how vertical DTS profiles can help understand problematic flux sites.

How to cite: Schilperoort, B., Coenders-Gerrits, M., and Savenije, H.: Measuring and tracking nighttime inversions within a forest canopy, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2670, https://doi.org/10.5194/egusphere-egu2020-2670, 2020.

D3076 |
Georg Jocher, Milan Fischer, Ladislav Šigut, Marian Pavelka, Pavel Sedlák, and Gabriel Katul

Concurrent below (0.14 • canopy height) and above canopy sonic anemometer vertical velocity (w) measurements reveal frequent decoupling events between the air masses below and above the canopy at a dense spruce forest stand in mountainous terrain. Decoupling events occurred predominantly during nighttime but not exclusively. Several single-level approaches based on steady state and integral turbulence characteristic tests as well as u* filtering and two-level CO2 flux filtering methods are tested. These tests aimed at evaluating the filtering schemes to address decoupling and its effect on above canopy derived eddy covariance CO2 fluxes. In addition to the already existing two-level filtering approach based on the correlation of σw above and below canopy, two new filtering methods are introduced based on w raw data below and above the canopy. One is a telegraphic approximation agreement, which assumes coupling when w both above and below canopy are pointing in the same direction. Another one evaluates the cross correlation maximum between below and above canopy w data. This study suggests that none of the single-level approaches can detect decoupling when compared to two-level filtering approaches. It further suggests that the newly introduced two-level approaches based on w raw data may have advantages in comparison to the conventional σw approach regarding their flexibility on shorter time scales than one year. We tested the correlation of the newly introduced filtering approaches with the parameters u*, global radiation, buoyancy forcing across the canopy and wind shear across the canopy. In any case, this correlation was not existing or weakly positive, suggesting that concurrent below and above canopy measurements are mandatory for addressing decoupling sufficiently. Sonic anemometer measurements near the forest floor and above the canopy are sufficient to apply the new procedures and can be implemented in a routine manner at any forest site globally.

How to cite: Jocher, G., Fischer, M., Šigut, L., Pavelka, M., Sedlák, P., and Katul, G.: Assessing decoupling of above and below canopy air masses at a Norway spruce stand in complex terrain , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7739, https://doi.org/10.5194/egusphere-egu2020-7739, 2020.

D3077 |
Jonathan D. Müller, Eyal Rotenberg, Fedor Tatarinov, Itay Oz, Efrat Schwartz, and Dan Yakir

Dry forests are expected to heat up considerably more than adjacent shrubland areas due to lower albedo and reduced latent heat flux. Paradoxically, the forest surface at our research site was observed to be cooler than the non-forested neighbouring areas during drought. This reflected the control over canopy temperature through the sensible heat flux, i.e. a 'convector effect'. Our objective was to examine how the efficient non-evaporative energy management, critical to protect the biological functioning of dryland ecosystems, develops at the small, leaf scale. 

We developed a novel system to continuously measure the energy balance and heat dissipation mechanisms on a leaf scale under field conditions. It allows the measurement of emitted leaf and background longwave radiation, and estimating the incoming, absorbed and reflected shortwave radiation using PAR measurements and full spectrum models.  Latent heat exchange and photosynthetic activity were measured with branch chambers. The system was deployed during the long summer drought (>8 months) in drought-exposed and irrigated plots in our semi-arid research Aleppo Pine forest site in southern Israel (mean daytime temperature of >30°C).

Preliminary results showed that in spite of a x10 higher transpiration rate in the irrigated plot compared with the control plots, leaf temperature remains within 1-2°C of air temperature on average in both plots during direct exposure to sunlight at midday. These results suggest an effective leaf to air heat transfer which prevents overheating independent of the latent heat flux. Under the high radiation load, the midday summer value of incoming shortwave radiation was >800 W·m-2 (mostly absorbed by the low albedo leaves), and background longwave radiation was >500 W·m-2. In turn, the energy dissipation in the drought-exposed trees was dominated by sensible heat flux of >500 W·m-2, while the long-wave radiation balance was near neutral (~50 W m-2), and the residual latent heat flux was <50 W·m-2. We demonstrated a system that provided new insights to leaf and canopy energy management under drought, which is a basis for the evolution of the convector effect.

How to cite: Müller, J. D., Rotenberg, E., Tatarinov, F., Oz, I., Schwartz, E., and Yakir, D.: Energy management in dry canopy starts with efficient leaf-scale heat exchange, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-7789, https://doi.org/10.5194/egusphere-egu2020-7789, 2020.

D3078 |
Jinshu Chi, Mats Nilsson, Natascha Kljun, and Matthias Peichl

Boreal forests cover a large portion of land surface area in the northern hemisphere and greatly affect the global carbon (C) cycle and climate. Since these forests exchange carbon dioxide (CO2) with the atmosphere in different vertical layers, many different CO2 sources and sinks exist within the complex forest stand. The forest floor (soil and understory vegetation) may act as an important component of the C budget in a forest stand, however its contribution may vary from negligible to determining the inter-annual variability of ecosystem C balance. To date, there are a limited number of studies that have directly quantified the CO2 fluxes over a forest floor using the eddy covariance (EC) method primarily due to challenges and potential violation of underlying assumptions when applying this method in the trunk space where turbulence characteristics are complicated, intermittent, and not in accordance with universal theories.

In this study, we installed two identical EC flux systems at two contrasting boreal forests (sparse pine stand vs. a dense mixed pine-spruce stand) in Sweden to measure the forest floor CO2 exchange. We developed site-specific ideal cospectral models for the below-canopy fluxes in the trunk space under the well-mixed condition as defined by the standard deviation of the vertical wind speed. Spectral correction to the half-hourly fluxes was performed based on the newly fitted cospectral models at each site. Chamber measurements of CO2 fluxes during the growing season were conducted to compare with the estimates from the below-canopy EC data.

Our below-canopy cospectral models show that more high-frequency signals (small eddies) occurred in the forest trunk space compared to the ideal above-canopy cospectral model. The high-frequency contribution was greater in the dense pine-spruce forest compared to the open pine stand. The spectral corrected CO2 fluxes measured by the EC method agreed well with the concurrent chamber measurements. The EC results revealed that the forest floor of the two contrasting stands acted as net CO2 sources during the 3-year period (2017-2019). This study highlights that by applying a data correction based on site-specific below-canopy cospectral models, the EC method can be used in the trunk space to accurately measure the net CO2 exchange between the forest floor and the atmosphere and thus to improve our understanding of the role of the forest floor in the ecosystem-scale C budget in the boreal forest region.

How to cite: Chi, J., Nilsson, M., Kljun, N., and Peichl, M.: Eddy covariance measurements of the forest floor CO2 exchange in two contrasting forest stands in boreal Sweden, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-17618, https://doi.org/10.5194/egusphere-egu2020-17618, 2020.

D3079 |
Tjalfe G. Poulsen

Below-surface horizontal pore air velocity profiles in porous media in response to above-surface wind conditions were measured for 16 combinations of wind speed (1.4 – 5.7 ms-1) and main wind gust frequency (0 – 1 Hz) in four granular porous media (particle size: 0.6 – 10 mm) yielding 64 combinations of wind condition and porous medium. Measurements were carried out under controlled (wind tunnel) conditions using a recently developed experimental gas tracer tracking method. Gusty wind conditions were induced by a wind driven propeller capable of variable rotation frequencies.

A recent empirical model for predicting wind-induced horizontal pore velocity in porous media (using one empirical parameter, B together with surface wind speed as input) was validated against the experimental data and found to provide accurate approximations. Further investigations revealed that B could be accurately predicted from porous medium gas permeability. Overall the results indicate, that it is possible to accurately predict below-surface wind-induced pore velocity profiles based on the above-surface wind speed profile and the porous medium gas permeability.


How to cite: Poulsen, T. G.: Predicting below-surface pore velocity profiles in porous media from above-surface wind speed and porous medium gas permeability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1747, https://doi.org/10.5194/egusphere-egu2020-1747, 2020.

D3080 |
Pedro Berliner, Anxia Jiang, Carmel Neuberger, and Agam Nurit

In arid and semiarid environments non-rainfall water inputs (NRWI) are an important source of water. In Israel's Negev desert direct absorption of atmospheric water vapor is the dominant NRWI and is strongly affected by soil properties, in particular clay content. The presence of a surface crust layer, whose physical and physico-chemical properties are substantially different from those of the underlying undisturbed substrate will likely affect the absorption patterns.  The objective of our study was to quantify the effect of soil type (loess vs. sand) and crust cover (crust vs. crust removed) on direct atmospheric water absorption.

The loess soil samples were obtained in an open field adjacent to the Jacob Bluestein Institutes for Desert Research (BIDR), Ben-Gurion University of the Negev (30˚51’ N, 034˚46’ E, 470 m a.s.l); and the sand samples from the Nizzana Sand Dune area (30˚58’N, 034˚24’E, 226 m a.s.l.).  The loess crusts were physically induced while those present on the sand samples were of biological origin.

A field experiment was carried out in the open field adjacent to the BIDR.  Four undisturbed 0.5 m depth soil samples (sand and loess with crust and with crust removed) were placed in micro-lysimeters and automatically weighed at 30 min. intervals.  This field experiment was carried during the dry season of May to October 2016.

The field study was supplemented with a laboratory experiment in which undisturbed samples (1,3, 7 and 10 cm) obtained from the above mentioned sites were used. Oven-dry samples were exposed during 6 days to constant temperature and relative humidity conditions (25±1 oC and  85±5 %, respectively)  in sealed chambers.  Mass changes were recorded at varying time intervals.   

The adsorption process in the field started in the late afternoon with the arrival of the sea breeze and ended with sun rise. On a daily basis the crusted loess sample adsorbed more water than the crusted sand sample, and the crust removed loess soil absorbed more water than the crust removed sand.  The crusted samples generally absorbed less water than the corresponding non-crusted ones.

The results of the laboratory tests showed that loess samples with crust and with crust removed absorbed similar water amounts for all sample depths throughout the study period. The crusted sand samples however absorbed systematically more water than the crust removed samples for all sample depths.

We conclude that the higher resistance of crusts to gaseous flux, a result of their higher bulk density and smaller pores, does not limit water vapor flux into the deeper soil layers and does not explain the field results.  

How to cite: Berliner, P., Jiang, A., Neuberger, C., and Nurit, A.: The effect of soil type and crust cover on the absorption of atmospheric water vapor – laboratory and field trials. , EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-5136, https://doi.org/10.5194/egusphere-egu2020-5136, 2020.

D3081 |
Theodora Bello, Adewale Ajao, and Oluwagbemiga Jegede

The study investigates impact of wind speeds on the turbulent transport of CO2 fluxes for a land-surface atmosphere interface in a low-wind tropical area between May 28th and June 14th, 2010; and May 24th and June 15th, 2015. Eddy covariance technique was used to acquire turbulent mass fluxes of CO2 and wind speed at the study site located inside the main campus of Obafemi Awolowo University, Ile – Ife, Nigeria. The results showed high levels of CO2 fluxes at nighttime attributed to stable boundary layer conditions and low wind speed. Large transport and distribution of CO2 fluxes were observed in the early mornings due to strong wind speeds recorded at the study location. In addition, negative CO2 fluxes were observed during the daytime attributed to prominent convective and photosynthetic activities. The study concludes there was an inverse relationship between turbulent transport of CO2 fluxes and wind speed for daytime period while nighttime CO2 fluxes showed no significant correlation.

Keywords: CO2 fluxes, Wind speed, Turbulent transport, Low-wind tropical area, Stable boundary layer

How to cite: Bello, T., Ajao, A., and Jegede, O.: Impact of wind speeds on turbulent transport of carbon dioxide (CO2) fluxes at a tropical location in Ile - Ife, southwest Nigeria, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-1882, https://doi.org/10.5194/egusphere-egu2020-1882, 2020.

D3082 |
Henriikka Vekuri, Juha-Pekka Tuovinen, Mika Korkiakoski, Laura Heimsch, Liisa Kulmala, Juuso Rainne, Timo Mäkelä, Juha Hatakka, Jari Liski, Tuomas Laurila, and Annalea Lohila

Mitigation of climate change requires – besides reductions in greenhouse gas emissions – actions to increase carbon sinks and storages in terrestrial ecosystems. Agricultural lands have a high potential for increased carbon sequestration through climate-smart land management and agricultural practices. However, in order to make climate-smart farming an accredited solution for climate policy, carbon markets and product footprints, reliable verification of carbon sequestration is needed. Direct measurement of the changes in soil carbon stock is slow, laborious and expensive and has significant uncertainties due to large background stocks and high spatial variability. An alternative is to infer the soil carbon stock change from measurements of the gaseous carbon fluxes between ecosystems and the atmosphere using the micrometeorological eddy covariance (EC) method.

Eddy covariance measures net ecosystem exchange (NEE), which is a small difference between two large components: carbon uptake by photosynthesis and losses due to plant and soil respiration. Therefore, small changes in either of them results in a large change in NEE. This sensitivity is also reflected in uncertainty estimates, which are critical for defining confidence intervals for annual carbon budget estimates and for making statistically valid comparisons of different management practices.  In addition, there are inevitable gaps in the data due to instrument failure, power shortages and non-ideal flow conditions. Therefore, in order to calculate daily and annual sums, the collected data must be temporally upscaled or gap-filled, which constitutes a major additional source of uncertainty. This study compares two different gap-filling methods for CO₂ fluxes: (1) an artificial neural network and (2) non-linear regression, which uses temperature and radiation as drivers. Uncertainties associated with both methods are estimated and discussed. The analysis is based on EC flux measurements conducted at two agricultural grassland sites in Finland.

How to cite: Vekuri, H., Tuovinen, J.-P., Korkiakoski, M., Heimsch, L., Kulmala, L., Rainne, J., Mäkelä, T., Hatakka, J., Liski, J., Laurila, T., and Lohila, A.: Carbon dioxide fluxes on agricultural grasslands – Uncertainty associated with gap-filling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-15930, https://doi.org/10.5194/egusphere-egu2020-15930, 2020.

D3083 |
Florian Kitz, Georg Wohlfahrt, Mathias W Rotach, Erich Tasser, Simon Tscholl, Paulina Bartkowiak, Mariapina Castelli, Claudia Notarnicola, Hetal Dabhi, and Thorsten Simon

Land ecosystems presently sequester around 25% of the carbon dioxide (CO2) that is emitted into the atmosphere by human activity and thus, along with the oceans (absorbing a similar fraction), slow down the increase of atmospheric CO2. Whether land ecosystems will be able to continue to sequester atmospheric CO2 at similar rates in the future or whether carbon cycle-climate feedbacks will cause the land sink to saturate or even turn into a source, is a topic of controversial discussion. While taking up CO2 through the stomata, plants inevitably lose water through transpiration. Terrestrial evapotranspiration (ET) can have a feedback to (local) precipitation and therefore modulate near-surface climate. The terrestrial carbon and water cycles are highly connected and controlled by complex interactions between biological and abiotic drivers. Mountain ecosystems in the European Alps are a hot spot of climate and land-use changes. Over the last century, temperatures have increased in the region with a rate double that of the global average and are expected to rise rapidly. In addition, precipitation changes are highly complex with an increasing and a decreasing trend in the northern and southern Alps, respectively and different seasonal patterns. Socio-economic development in the Alps during the past centuries have caused large-scale changes in land-use and its intensity, which has contributed to the uncertainty about future land-atmosphere interactions. The objective of the CYCLAMEN project is to quantify and project the resilience and vulnerability of carbon and water cycling in North and South Tyrol. We aim at providing information for predicting likely future changes in climate and land-use over the region.

In the study we used a comprehensive and multidisciplinary approach to model biosphere-atmosphere interactions in the Alps. Data from eddy covariance stations spread across the region were chosen to test and calibrate the biosphere model SiB4. The meteorological data from the same stations was used to train a stochastic Weather Generator and simulate weather conditions under climate scenarios RCP8.5 and RCP2.6 until 2100. To account for future land- use/ land- cover (LULC) changes the SPA-LUCC model was used. Both the simulated weather conditions and the expected LULC were fed back to the SiB4 model to calculate ecosystem parameters, including carbon dioxide net ecosystem exchange and evapotranspiration. In parallel, an enhanced thermal remote sensing dataset was produced, specifically adapted for mountainous areas. This dataset will be the main driver for modelling ET with an energy balance model whose output will be cross compared with the one of the biosphere model SiB4.

How to cite: Kitz, F., Wohlfahrt, G., Rotach, M. W., Tasser, E., Tscholl, S., Bartkowiak, P., Castelli, M., Notarnicola, C., Dabhi, H., and Simon, T.: Cycling of carbon and water in mountain ecosystems under changing climate and land use (CYCLAMEN), EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13970, https://doi.org/10.5194/egusphere-egu2020-13970, 2020.

D3084 |
Zuosinan Chen, Zhiqiang Zhang, and Lixin Chen

Nocturnal sap flow (Qn) affect not only forest carbon and water budgets but also their responses to climate change as it consists of two ecohydrological and ecophysiological significant components: nighttime transpiration and water recharge. A vapor pressure deficit (VPD) based sap flow partitioning method has been developed to estimate nighttime transpiration, which is normally quantified through the discretely measured nighttime stomatal conductance, from the widely and continuously measured sap flow. However, given the increasing knowledge of Qn mechanisms, whether Qn could be partitioning simply by VPD and whether this method is valid in semi-arid regions remain unclear. We measured sap flow of Pinus tabuliformis and Acer truncatum in a middle-aged and a young monoculture forest stand, respectively, in a semi-arid mountainous area of northern China. We found the influence of VPD on Qn conditioned by soil moisture. Meanwhile, a considerable impact of wind speed on Qn was observed. In the stands with relatively dry soils, both increased and decreased soil moisture promoted Qn, which might be due to enhanced nighttime water recharge for two distinct purposes, i.e., capacitance refilling and avoiding hydraulic failures. For these three environmental factors (i.e., VPD, wind speed, and soil moisture) that have been considered most in previous studies, their total effect explained less than 55% of the Qn variations. This study highlights that physiological influences of VPD on nighttime stomatal water loss were uncertain. Furthermore, it suggests that there could exist considerable nighttime water loss induced by wind, possible region-specific patterns of nighttime water recharge, and limited concurrent environmental controls on Qn. Our findings are helpful to improve the VPD-based sap flow partitioning method to differentiate nighttime transpiration and water recharge.

How to cite: Chen, Z., Zhang, Z., and Chen, L.: How to differentiate nighttime transpiration and water recharge in nocturnal sap flow?, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-6469, https://doi.org/10.5194/egusphere-egu2020-6469, 2020.

D3085 |
Jun Yin and Amilcare Porporato

By linearizing the saturation water vapor curve, Penman (1948) not only found the famous explicit approximation of wet-surface evaporation but also obtained a less well-known expression of surface temperature. Here the latter has been taken into the slab model of Atmospheric Boundary Layer (ABL) to derive multiple analytical approximations of ABL dynamics, which share the features of the Penman equation with evaporation driven by energy and drying power of the air. Noticing that these two parts of evaporation are proportional to each other within the Priestley-Taylor approximation at sub-daily timescale, a unified framework is obtained that links the Penman approach and Priestley-Taylor method to the diurnal behaviors of ABL. The resulting model is useful for diagnosing the land-atmosphere interactions.

How to cite: Yin, J. and Porporato, A.: Penman and Priestley-Taylor approaches in Atmospheric Boundary-Layer Dynamics, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-9216, https://doi.org/10.5194/egusphere-egu2020-9216, 2020.

D3086 |
Juan Carlos Pastene, Alexander Siegmund, Camilo del Río, and Pablo Osses

The coastal Chilean Atacama Desert comprise some of the driest areas of the world with anual mean precipitation partly less than 1 mm/year, like in the Tarapacá region. It is in these environments, where fog plays a relevant role for local ecosystems, like the so called Tillandsia Lomas. These fog ecosystems contain Tillandsia landbeckii as an endemic species, which covers a vertical range of about 800 to 1,250 m, related to fog availability. The study area “Oyarbide” (20°29’ S, 70°03’ W) is situated inland desert, over a range of 300 m elevation where the advective and orographic fog penetrate far enough to reach the east border of the site at around 1,200 m.

On local level, the understanding of the fog climate characteristics and variability is still poor as well as knowledge about the driving parameters, the temporal dynamics and spatial gradients. For this reason, various parameters of fog climate are analysed and characterised on the basis of a local station network in order to determine the local fog climatology.

From 2016, several high quality climatological stations (Thies Clima) were installed in “Oyarbide”, located in a transect from ca. 1,160 m to ca. 1,350 m in a distance between 10.3 km to 10.7 km from the coast. The local network of climate stations is generating a high temporal and spatial acquisition of climatological data of standard fog water (2 m), air temperature & humidity (2 m), surface temperature (5 cm), wind speed & direction (10 m & 2 m), air pressure, global radiation, leaf wetness and dew every 10 minutes until nowadays. Additionally, ten mini fog collectors (Mini FCs) were installed at the beginning 2019, covering a surface of ca. 3 km2, generating a monthly data of ground fog water collected (50 cm).

First spatio-temporal analyses of different parameters of the local fog climate will be presented. The results of the study show a seasonal, monthly and daily variability, with altitudinal and vertical differences and oscillation. The results will serve as input for the understanding of the fog variability into hyperarid zones.

How to cite: Pastene, J. C., Siegmund, A., del Río, C., and Osses, P.: Fog climatology analyses in coastal fog ecosystem at the Atacama Desert/Chile – spatio-temporal analysis of fog water characteristics and variability, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-2608, https://doi.org/10.5194/egusphere-egu2020-2608, 2020.

D3087 |
Eva Pauli, Hendrik Andersen, Jörg Bendix, Jan Cermak, and Sebastian Egli

In this study the distribution of fog and low stratus (FLS) relative to land cover and meteorological conditions in continental Europe is investigated using a state-of-the-art machine learning technique  and geostationary satellite data. While analyses of the spatial and temporal patterns of FLS exist, the relationships to land cover and meteorological conditions have not been studied explicitly, quantitatively and on a continental scale.
The machine learning model is built using daily means of a FLS dataset from Egli et al. (2017) as the predictand, and different land surface and meteorological parameters as predictors over continental Europe from 2006-2015. The application of a machine learning approach provides the ability to explicitly, synchronously and quantitatively link suspected determinants of FLS to its occurrence.
The model shows good performance with R² values ranging between 0.5 and 0.9, depending on model grid size, season and further settings such as the exclusion of low pressure values and subtraction of seasonality. It is thus able to adequately represent the dynamics that drive FLS development. Based on a systematic analysis of this model, the most important features for FLS prediction are mean surface pressure, wind speed, and FLS on the previous day. High mean surface pressure, high FLS cover on the previous day, low evapotranspiration, wind speed and land surface temperature lead to higher predicted FLS values.
Generally the results show that it is possible to predict FLS occurrence over continental Europe using meteorological as well as land surface parameters with good performance indicating the benefits of using machine learning in the analysis of non-linear, multivariate systems such as the land-atmosphere system. Further studies will integrate the machine learning model into a land surface based model grid and implement fog and low cloud properties as predictands.

How to cite: Pauli, E., Andersen, H., Bendix, J., Cermak, J., and Egli, S.: Drivers of fog and low stratus - a satellite-based evaluation with machine learning, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-13625, https://doi.org/10.5194/egusphere-egu2020-13625, 2020.

D3088 |
Tamara Emmerichs, Huug Ouwersloot, Astrid Kerkweg, Silvano Fares, Ivan Mammarella, and Domenico Taraborrelli

Surface ozone is a harmful air pollutant, heavily influenced by chemical production and loss processes. Dry deposition to vegetation is a relevant loss process responsible for 20 % of the total tropospheric ozone loss. Its parametrization in atmospheric chemistry models represents a major source of uncertainty for the global tropospheric ozone budget and might account for the mismatch with observations. The model used in this study, the Modular Earth Submodel System (MESSy2) linked to ECHAM5 as atmospheric circulation model (EMAC) is no exception. Like many global models, EMAC employs a “resistances in series” scheme with the major surface deposition via plant stomata which is hardly sensitive to meteorology depending only on solar radiation. Unlike many global models, however, EMAC uses a simplified high resistance for non-stomatal deposition which makes this pathway negligible.                             

Hence, a revision of the dry deposition scheme of EMAC is desirable. The scheme has been extended with empirical adjustment factors to predict stomatal responses to temperature and vapour pressure deficit. Furthermore, an explicit formulation of humidity depending non-stomatal deposition at the leaf surface (cuticle) has been implemented based on established schemes. Next, the soil moisture availability function for plants has been critically reviewed and modified in order to avoid a stomatal closure where the model shows a strong soil dry bias, e.g. Amazon basin in dry season.

The last part of the presentation will show comparisons of dry deposition velocities and fluxes comparing simulations with data obtained from four experimental sites where ozone deposition is measured with micrometeorological techniques. The impacts of the changes on daily and seasonal patterns of ozone dry deposition will be discussed with a highlight on surface ozone, global distribution and budget.

How to cite: Emmerichs, T., Ouwersloot, H., Kerkweg, A., Fares, S., Mammarella, I., and Taraborrelli, D.: Surface ozone and land-atmosphere coupling, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-14054, https://doi.org/10.5194/egusphere-egu2020-14054, 2020.

D3089 |
Ting-wei Chang, Motonori Okumura, Ken-hui Chang, Tomonori Kume, Lin-jie Jiao, Si-yu Chen, Ding-kang Xu, Zhi-ning Liu, and Yoshiko Kosugi

Bamboos are naturally distributed in Asia, Africa and America, and intentionally introduced in Europe. It has been reported with expansion of bamboos due to abandon of management in plantations and the niche shift under climate change. Furthermore, certain canopy type bamboo species are reported with high emission of isoprene, which can impact air quality and climate change. However, research about the isoprene emission from understory type species of bamboo, such as Sasa spp and Sasaella spp, are currently absent. This may cause uncertainties when estimating the isoprene emission from forest ecosystems. Thus, this study conducted measurement on isoprene emission flux (I) from leaves of 18 species of bamboo within five genera including understory type (Sasa and Sasaella) and canopy type (Pleioblastus, Semiarundinaria and Phyllostachys) species in a specimen garden in Kyoto, Japan, to compare the isoprene emission trait of the two types. The measurements were conducted monthly in 2nd-5th August, 12th-16th September and 15th-17th October 2019. Isoprene emitted from leaf is collected through an adsorbent tube while measuring factors such as photosynthesis rate and leaf temperature (TL) under a controlled intensity of photosynthetic active radiation (PAR) at 1000 μmol m-2 s-1 with a modified photosynthesis-measuring system equipped with a LED light-source leaf chamber. The isoprene in the adsorbents were then desorbed and quantified respectively with a preconcentrate system and a Gas chromatography - Mass spectrometer system; measured leaves were taken to laboratory for area and dry weight measurement. As the result, most of the species showed the largest I in August (18.8 nmol m-2 s-1), and then gradually decrease or ceased in the following two months (8.1 and 1.3 nmol m-2 s-1 in September and October, respectively), which was consistent with the tendency of monthly temperature; that is, positive correlations of I and TL were found in most of the species. Meanwhile, photosynthesis rate did not show significant variance to month in any species, which can be attributed to the weak or none correlation between photosynthesis rate and TL. On the other hand, the species within the same genus showed similar I and dependence to TL. The understory type genera showed significantly lower I than those from canopy type genera. The dependence of I to TL was also weaker in understory type genera than in canopy type genera. The low isoprene emission in understory type genera may attribute to the potentially lower heat stress for the understory vegetations, where the isoprene is produced in plants for enhancing heat tolerance. The significant difference in I and its seasonal variation between understory type species and canopy type species suggest that these two types of bamboo must be seemed as different groups when in the regional modelling of isoprene fluxes.

How to cite: Chang, T., Okumura, M., Chang, K., Kume, T., Jiao, L., Chen, S., Xu, D., Liu, Z., and Kosugi, Y.: Comparison of Seasonal Response of Isoprene Emission from Understory Type Bamboo and Canopy Type Bamboo Species, EGU General Assembly 2020, Online, 4–8 May 2020, EGU2020-12258, https://doi.org/10.5194/egusphere-egu2020-12258, 2020.