Mountain forests, which play an important role in the global carbon cycle, are supposed to be heavily affected by climatic changes and extremes. Therefore, efforts towards the understanding of the physiological processes regulating mountain forest carbon and water cycling are crucial to determine local and global carbon exchanges. However, among the challenges in quantifying land-atmosphere interactions in complex terrain, the unaccounted presence of advective CO₂ fluxes has the potential to bias the daily and longer-term CO₂ flux measurements 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.

We would like to discuss results from a CO₂ advection experiment conducted during summer 2018 at a European larch site in Northern Italy (2100 m asl). The experimental setup consisted of: the main eddy covariance flux tower (20 m), a sub-canopy eddy covariance flux system (2 m), a 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 analyser, and three automatic chambers measuring bare soil respiration (two chambers) and the net ecosystem CO₂ exchange from the vegetated forest floor (one chamber).

The main points raised from this experiment are 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, and iii) approaches for correcting for the missed fraction of the net ecosystem CO₂ exchange carried by advection could be applied.

Awareness about the presence, composition and abundance of primary biological aerosols (PBAs), like bacteria and fungi in the air, has been increasing rapidly in the past decade. However, most studies were conducted ground-based within the planetary boundary layer.

Insights in long-term bioaerosol composition and variation in the free troposphere and bioaerosol scavenging behaviour in free-tropospheric precipitation are obsolete for obvious sampling challenges. Gaining understanding of free-tropospheric PBA composition and abundance dynamics will yield insights in air-mass mixing between supposedly strongly stratified valley air and free troposphere in cold seasons that have implications for long-distance transport with respect to both, sedimentation and aerosolisation. Recently, a range of free-tropospheric studies used precipitation as proxy for free air-mass transport. However, the scavenging of free floating PBAs to snow or other precipitation is unknown.

A question arising from that, is the unexplored correlation and co-occurrence of certain taxa with chemical compounds in air and if paralleled interactions occur in snow. This is especially unexplored for extreme events like Sahara dust storms, occurring twice in our dataset, that are known to express specific chemical and microbiological signatures in air and snow, but with an obsolete microbial scavenging rate.

This study compares for the first time Alpine free-tropospheric bacterial and fungal composition and abundance over an entire winter season derived from aerosol quartz filters with bacterial and fungal composition and occurrences in a late season snow pack with special focus on Saharan dust events. For this, we use a multidisciplinary approach, synthesizing snow-pack modelling, backward trajectories for air mass source identification, snow-pack- and aerosol chemistry monitoring and molecular biology methods.

We will present here some feedbacks from our experience in monitoring and modelling land surface fluxes, with a focus on evapotraspiration (ET), along an elevation gradient in mountain terrain in the Long Term Ecological Research IT25 - Val Mazia/Matschertal using different approaches. Field data from eddy covariance and lysimeters have been compared with distributed hydrological models (i.e. GEOtop, CAHTY, GeoFrame; e.g. Della Chiesa et al, 2014) and remote-sensing based estimations (i.e TSEB; Bertoldi et al., 2014; Castelli et al, 2017). We would like to discuss advantages, disadvantage and major uncertainties of the different approaches.

We would like to help in defining physical and “virtual” experiments to address the main uncertainties in simulating land surface fluxes with respect elevation, slope and aspect in mountain valleys. We would stimulate the need to design and develop common intercomparison experiments among different models and cross-comparison experiments across LTER sites in mountain regions.

The effects of elevation on evapotranspiration (ET) on dry alpine grassland ecosystems were investigated along an elevational transect between 1000 and 2000 m a.s.l. established in the Long Term Ecological Research Area IT25 - Val Mazia/Matschertal, a relatively dry region in the Italian Alps. We compare here different approaches.

(I) Some of the classical hydrological approaches for estimating ET, like Penman-Monteith (PM-FAO) and Priestley-Taylor (PT);

(II) The semi distributed hydrological model GEOframe, which includes the newly developed Schymanski-Or (SO) model, which allows to compute also the sensible heat, the temperature and humidity gaps between leaves and the atmosphere.

(III) The fully distributed hydrological model, GEOtop-DV, which includes a dynamic vegetation model.

(IV) A diagnostic approach which links ET to land surface temperature (LST) measured by thermal remote sensing, based on TSEB ALEXI (Two Source Energy Balance Atmosphere Land EXchange Inverse)

We discuss here advantages and disadvantages of the different approaches. While classical approaches give reasonable estimations of the total yearly ET, a fully distributed hydrological model, if properly parametrized, allows a better representations of the ET feedbacks due to snow and soil moisture dynamics, especially in early spring. The proposed extended SO approach gives a more accurate estimation of specific parameters as leaf temperatures with respect to the GEOtop model, but it tends to be more sensitive to the chosen canopy radiation transfer scheme. The remote sensing TSEB approach results to be the most accurate when compared to eddy covariance measurements, but is limited in its practical application by the availability and spatial accuracy of remote sensing thermal data.

Peru is facing imminent water resource issues as glaciers retreat and demand increases, yet limited observations and model resolution hamper understanding of hydrometerological processes on local to regional scales. Hourly observations from paired weather stations embedded along Andean valley slopes reveal increases in freezing-level height and greater warming at higher altitudes suggestive of valley fluxes that impact precipitation formation and mass balance of glaciers during the wet season. We analyze more than a decade of meteorological data from two ground-based weather stations anchoring an array of six air temperature and humidity sensors spanning over 1200 m elevation to 4700 m in the Llanganuco Valley, Peru (9°S). The freezing level is consistently higher during the afternoon and exhibits an average diurnal range between 150 and 420 meters during the wet season and between 200 and 420 meters during the dry season. Two-km resolution Weather Research and Forecasting (WRF-ARW) modelling of a typical diurnal composite wet season enabled development of a conceptual model linking the observed nocturnal precipitation maximum to a combination of thermally driven valley and slope winds and synoptic flow from the Amazon. We provide evidence of the valley-influenced stable core decoupling from the free troposphere above the peaks and subsequent convective boundary layer development in the late afternoon and evening.

Dynamical and thermal mechanisms of diurnal convection and freezing level variability in the Peruvian Andes derived from a cost-effective embedded sensor network and WRF modeling. Peru is facing imminent water resource issues as glaciers retreat and demand increases, yet limited observations and model resolution hamper understanding of hydrometerological processes on local to regional scales. Hourly observations from paired weather stations embedded along Andean valley slopes reveal increases in freezing-level height and greater warming at higher altitudes suggestive of valley fluxes that impact precipitation formation and mass balance of glaciers during the wet season. We analyze more than a decade of meteorological data from two ground-based weather stations anchoring an array of six air temperature and humidity sensors spanning over 1200 m elevation to 4700 m in the Llanganuco Valley, Peru (9°S). The freezing level is consistently higher during the afternoon and exhibits an average diurnal range between 150 and 420 meters during the wet season and between 200 and 420 meters during the dry season. Two-km resolution Weather Research and Forecasting (WRF-ARW) modelling of a typical diurnal composite wet season enabled development of a conceptual model linking the observed nocturnal precipitation maximum to a combination of thermally driven valley and slope winds and synoptic flow from the Amazon. We provide evidence of the valley-influenced stable core decoupling from the free troposphere above the peaks and subsequent convective boundary layer development in the late afternoon and evening.

The large-scale climates control the necessary environmental conditions in which glaciers exist, while the local mass and energy flows of each glacier are determined by the microclimates. Given the complex mountain topography around the alpine glaciers, it is not trivial to find a direct link between the large-scale atmospheric state and the microclimates at a single glacier. From a theoretical point of view, it is essential to identify the atmospheric processes and to represent them in a suitable resolution in order to understand the evolution of the mountain glaciers. This challenge can only be met with very high-resolution measurements and calculations, which often proves to be very difficult in view of the available resources. This inevitably leads to a dilemma between costs and benefits. In principle, the question arises under which circumstances the use of high-resolution models for the glacier mass balance is justified and on which scales are process-resolving simulations meaningful? How can one infer from the requirements which scales are relevant? Are there time scales where small-scale (subgrid) information become irrelevant? How large are the biases in the quantification of the mass balances of mountain glaciers if small-scale processes are neglected?

The European Alps can be considered at the crossroads of air pollution and climate change. Some of the EU’s most important international road transport flows go over the Alps and recent projections for the European transport sector (OECD, 2013) estimate a substantial increase of traffic along major European transport routes. As one consequence the interplay between anthropogenic emissions along these transport routes (in particular NO_x) in conjunction with biogenic volatile organic compound (BVOC) emissions in an ecological sensitive region has the potential to significantly modify air chemistry. These factors are further exacerbated due to efficient mixing and poor venting in complex topography. Current climate records show that the Alps are warming at a faster rate than the global average. Climate projections also suggest this trend is likely to continue, concomitant with changes in the seasonality of precipitation, increasing the likelihood of droughts and intensive precipitation during the warm and cold parts of the year, respectively. While these general trends are indicative that climate change can lead to a potential deterioration of air pollution in the Alps, meteorological and chemical variations play a major uncertainty for quantitative assessments. This presentation will discuss how we can best assess the current status of air chemistry and how well we understand chemical emission sources in typical Alpine environments. 

Differences in the energy and carbon budget of different land surface types have the potential to impact climate at local to global scales. Here we investigate how two adjacent grasslands experiencing the same environmental forcing but differing in management (harvested and irrigated vs. grazed and rain-fed) differ in terms of their radiation, energy and carbon budget. We show that the management-driven differences in water availability and seasonal dynamics of above-ground plant biomass development profoundly influence the ecosystem-atmosphere energy, water vapour and carbon dioxide exchange and discuss the implications for local climate and the role of these grasslands as carbon sinks/sources.

There is now consensus that inland waters play an active role in the global carbon cycle. However, the actual magnitude of the contributing lake-atmosphere fluxes is still highly uncertain and largely based on indirect and/or small scale measurements. Recently, an increasing number of studies have started to quantify the lake-atmosphere exchange of energy and trace gases like carbon dioxide (CO₂) and methane using the eddy covariance method. However, lakes, in particular smaller ones, pose a number of challenges for the application of this method, which require careful consideration in order to arrive at defensible flux estimates. Here, we report on three years of eddy covariance CO₂, latent and sensible heat flux measurements from Lake Lunz (Austria), a small (68 ha) pre-alpine lake situated in a steep and narrow valley surrounded by coniferous forest. Eddy covariance flux measurements were made from a floating platform and from two places on the shore at opposing sites of the lake. A characteristic feature of the data set is a tremendous variability in CO₂ flux estimates which contradicts our experience and process understanding of the magnitude and drivers of lake-atmosphere exchange processes. Possible causes for this observation like for example CO₂ drainage and advection from the surrounding slopes will be discussed as well as the question how flux calculations and quality control routines can be improved.

Boundary layer turbulence is one of the main mechanisms of land-atmosphere interaction, yet turbulence remains poorly understood, particularly in complex terrain. Influenced by interactions between different scales of motions, the boundary layer in mountainous terrain shows marked differences to its flat terrain counterpart. This is particularly true for turbulence itself, which does not conform to the similarity scaling valid over flat and horizontally homogeneous terrain. In addition, scaling studies over complex terrain, show large between the different complex terrain data sets pointing to a lack of universality of turbulence or missing processes that scaling does not take into account. Can we develop a universal scaling valid over terrain of all complexity? What are the complex terrain processes that modify turbulence in complex terrain? How can we incorporate these processes into a universal similarity scaling? Is anisotropy of turbulence the key to such an endeavor?

At the Andean region, the páramo biome (located above 3200 m a.s.l.) provides water resources for many cities and communities along Central and South America. These resources are used for drinking water, agriculture, hydropower generation and for sustaining aquatic ecosystems. Although mountainous terrains place difficulties for their study, due to its remoteness, and therefore, data scarcity, knowledge about the functioning of the páramo has improved lately. Precipitation and runoff monitoring has increased dramatically, but this was not the case for evapotranspiration (ETa). Quantifying ETa is important for closing the water balance and energy budget, which is crucial in the understanding of ecosystem functioning and for further development of hydrological and climate modelling. Therefore, an effort towards the measurement and estimation of ETa at the páramo was done using the following methods: eddy-covariance, lysimeters, water balance, energy balance, hydrological models (HBV-light and PDM) and the calibration of the Penman-Monteith equation. For measuring evapotranspiration, the eddy-covariance method was found as the most accurate and with the higher resolution. However, due to its high cost, and complex installation, operation and maintenance, the two hydrological models and the calibrated Penman-Monteith equation were found as robust alternative methods for daily estimation of evapotranspiration at the páramo. These alternative methods are accurate, freely available and easy to implement. The commonly used water balance method, on the other hand, was not suitable for estimating evapotranspiration at daily or monthly scale. Challenges with the use of the lysimeters and the energy balance also aroused with poor performance at daily scale. The quantification of ETa lead to specific studies on the controls of evapotranspiration at the Andean páramo and to the partitioning of ETa (quantification of interception and evaporation). This study contributes to the quantification and understanding of the atmosphere-vegetation-soil interaction at this important biome.

Mountain research on hydrology and climatology are of outmost importance for developed and developing countries towards achieving several of the seventeen goals of the 2030 Agenda for Sustainable Development (UN, 2018); especially for goal 6 (clean water and sanitation) and for goal 13 (climate action). In South America, one of the most important mountain environments, for its ecosystem services, is the páramo. The páramo biome provides water resources for main cities in the Andes. Although mountainous terrains place difficulties for their study, due to its remoteness, and therefore, data scarcity, knowledge about the functioning of this biome has improved lately. Precipitation (P) and runoff monitoring has increased dramatically, but this was not the case for evapotranspiration (ETa). In order to understand the components of the hydrological cycle, this study aimed at understanding evapotranspiration process at this important biome through three specific objectives: (1) to quantify interception, transpiration and their contribution to evapotranspiration, (2) to find suitable methods for measuring and estimating evapotranspiration and (3) to investigate the controls on evapotranspiration. The results showed the high contribution of interception to evapotranspiration process. The maximum capacity of tussock grasslands to intercept water was 2 mm. Precipitation was intercepted up to 100% during small events (P < 2 mm) and it decreased to 10% during large events (P > 2 mm). Transpiration was quantified during dry periods. It was on average 1.7 mm/day (range between 0.7 and 2.7 mm/day). Even during these dry periods, fog and dew were retained by vegetation and contributed to evapotranspiration. For measuring evapotranspiration, the eddy-covariance method was found as the most accurate and with the higher resolution. However, due to its high cost, and complex installation, operation and maintenance, two hydrological models (HBV-light and PDM) and the calibrated Penman-Monteith equation were found as robust alternative methods for daily estimation of evapotranspiration. These alternative methods are accurate, freely available and easy to implement. Finally, it was found that the páramo has a relatively low evapotranspiration rate (annual ETa/P = 0.5) and it is an energy-limited site, where net radiation is the primer control on evapotranspiration (annual ETa/Rn = 0.47). The secondary controls found were wind speed, surface conductance and aerodynamic conductance, which were especially important during dry periods.

Climate information depends among other things on eddy covariance measurements to quantify fluxes and therefore exchange between atmosphere and environment. To make sense of eddy covariance measurements one needs to know which part of the surface emits the measured quantity, for example whether the measured C02 flux comes mostly from the grassland surrounding the instrument or from the forest upstream. This is described by the so-called footprint function. So far, many commonly used footprint models are designed for flat, homogeneous terrain, but most regions contain mountains, cities or inhomogeneous vegetation cover. Studies that show footprints for complex terrain and forested areas exist, but almost nothing for urban areas, except for building-resolving LES that is too computationally expensive for routine measurements. Our goal is to fill this gap and investigate footprints over urban areas, because they become more and more common in the eddy covariance community. Since these researchers in urban areas still need footprints, they use footprint models for flat areas over urban surfaces, without knowing how big the discrepancy is.

To determine the footprint function, we use a Lagrangian particle dispersion model in backwards mode. The dispersion model has been validated against tracer experiments. Earlier works in dispersion show that city effects are significant and this means that the footprint over urban areas is presumably also different. We test this hypothesis by comparing our footprint model with and without included building effects and show sensitivity tests.

Southern Africa has been highlighted as a vulnerable area to the added stresses placed on water resources by environmental change. By the year 2025, South Africa is predicted to be a water scarce country. Considered as the ‘water towers’ of South Africa, the Drakensberg mountains are an important area for the generating of the water resources for both Gauteng and KwaZulu-Natal, making them an important strategic water source area for South Africa. Located within the Drakensberg Mountain range are the Cathedral Peak research catchments. The Cathedral Peak research catchments are an important long-term monitoring site, with valuable datasets dating back to the 1950s. The Cathedral Peak research catchments provide an ideal platform for long-term trend detection through observation.

Two of the main drivers of environmental change are climate change and land use/cover change (LUCC). The Cathedral Peak research catchments consist of ten catchments, of which three diverse catchments stand out. A degraded catchment, a woody encroached catchment, and a pristine grassland catchment. Using current techniques, observation of hydrological processes (rainfall, streamflow, evapotranspiration and soil moisture) are being conducted within these three diverse catchments. The objective is to improve our understanding of the response of hydrological processes to environmental change. Gaining an understanding of how hydrological processes respond to different environmental changes currently, will aid in improving our understanding of how hydrological processes may respond to future change. It is also important to provide specific focus on complex and vulnerable systems, such as mountainous areas, to understand how they are affected and evolve with change. The understanding gained is an important step towards improving water resources management in the future, and our ability to model change with a greater certainty, especially in strategic water source areas such as high-altitude mountainous catchments, which are highly sensitive to environmental change.

Air pollution in the alpine region is increasingly problematic because traffic loads along the alpine transit routes as well as population in valleys have significantly increased during the last decades. The reduced air volume and the special meteorological conditions in inner-alpine valleys lead to a regular exceedance of the legal limits of air pollutants like NO2 and others. As these pollutants have a direct impact on human health and an indirect effect through their role as precursors for secondary formed pollutants fundamental knowledge about the sources and sinks is essential. In the alpine city of Innsbruck long-term eddy covariance measurements of various trace gases have been performed since mid of 2018. The aim of these measurements is to gain a more detailed picture of the diurnal and seasonal variability of these gases. Furthermore, we want to improve our understanding in the quantification of the different dominant anthropogenic sources and the effect of vegetation. Therefore we apply and evaluate a new quantity that we term Turbulent Enhancement Ratio (TER). Based on the normalized enhancement ratio, which is often used for single plume studies, the TER has the advantage to separate local and non-local sources which allows a unique view on the emitters within the city. Weighted by a footprint model and merged with a landcover map, the TER can be used to assess emission inventories and long-term trends. On the poster, we show that the TER can be used as a good alternative to classical flux ratios or used for its validation by using CO₂ and NO_x as tracers. Additional, first seasonal and diurnal results will provide an insightful glimpse on weekday/weekend effects and the variable fraction of the sinks and sources. Supplemented by other combustion markers it should be possible to derive the type of fuel of various urban emitters.