Jost von Hardenberg

Ben-Gurion University of the Negev, Be'er Sheva`, Southern District, Israel

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Publications (80)173.57 Total impact

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    ABSTRACT: The mysterious ‘fairy circles’ are vegetation-free discs that cover vast areas along the pro-Namib Desert. Despite 30 yr of research their origin remains unknown. Here we adopt a novel approach that focuses on analysis of the spatial patterns of fairy circles obtained from representative 25-ha aerial images of north-west Namibia. We use spatial point pattern analysis to quantify different features of their spatial structures and then critically inspect existing hypotheses with respect to their ability to generate the observed circle patterns. Our working hypothesis is that fairy circles are a self-organized vegetation pattern. Finally, we test if an existing partial-differential-equation model, that was designed to describe vegetation pattern formation, is able to reproduce the characteristic features of the observed fairy circle patterns. The model is based on key-processes in arid areas such as plant competition for water and local resource-biomass feedbacks.The fairy circles showed at all three study areas the same regular spatial distribution patterns, characterized by Voronoi cells with mostly six corners, negative correlations in their size up to a distance of 13 m, and remarkable homogeneity over large spatial scales. These results cast doubts on abiotic gas-leakage along geological lines or social insects as causal agents of their origin. However, our mathematical model was able to generate spatial patterns that agreed quantitatively in all of these features with the observed patterns. This supports the hypothesis that fairy circles are self-organized vegetation patterns that emerge from positive biomass-water feedbacks involving water transport by extended root systems and soil-water diffusion. Future research should search for mechanisms that explain how the different hypotheses can generate the patterns observed here and test the ability of self-organization to match the birth- and death dynamics of fairy circles and their regional patterns in the density and size with respect to environmental gradients.
    Ecography 05/2014; · 5.12 Impact Factor
  • 03/2014; 15(2).
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    Climatic Change 01/2014; · 3.63 Impact Factor
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  • Mara Baudena, Jost von Hardenberg, Antonello Provenzale
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    ABSTRACT: In arid and semi-arid ecosystems, local vegetation-soil moisture feedback mechanisms are important for vegetation self-organization. Vegetation spatial patterns, such as spots, stripes and gaps, are frequently observed. At larger scales vegetation exerts a direct role on water and energy fluxes, which influence directly local precipitation recycling, particularly in drylands. To evaluate the interactions between vegetation self organization, on the small scale, and land-atmosphere feedbacks, on larger scales, it is crucial to model accurately the evapotranspiration fluxes. In this contribution, we discuss a new explicit-space model for vegetation dynamics in water-limited ecosystems, including two soil layers. The model is suitable for evaluating how evapotranspiration fluxes are modified by the presence of patterns, and to establish the climatological importance of related modifications in local soil-vegetation feedbacks.
    Advances in Water Resources 09/2013; 53:131-138. · 2.41 Impact Factor
  • Jonathan Nathan, Ehud Meron, Jost von Hardenberg
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    ABSTRACT: Using a spatially explicit mathematical model for water-limited vegetation we show that spatial instabilities of uniform states can lead to species coexistence under conditions where uniformly distributed species competitively exclude one another. Coexistence is made possible when water-rich patches formed by a pattern forming species provide habitats for a highly dispersive species that is a better competitor in uniform settings.
    Journal of Theoretical Biology 06/2013; · 2.35 Impact Factor
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    Journal of limnology 05/2013; 72(3):2. · 1.47 Impact Factor
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    ABSTRACT: Initial efforts toward developing a combined organic‐inorganic sea spray source function parameterization for large‐scale models made use of chlorophyll‐a (Chl‐a) and wind speed as input parameters to combine oceanic biology and atmospheric dynamics. These studies reported a modest correlation coefficient (0.55) between chlorophyll‐a and organic matter (OM) enrichment in sea spray, suggesting that chlorophyll‐a is only partially suitable for predicting organic enrichment. A reconstructed chlorophyll‐a field of the North Atlantic Ocean from GlobColour reveals an improved correlation of 0.72 between the fractional mass contribution of organics in sea spray and chlorophyll‐a concentration. A similar analysis, using colored dissolved and detrital organic material absorption and particulate organic carbon concentration, revealed slightly lower correlation coefficients (0.65 and 0.68). These results indicate that to date, chlorophyll‐a is the best biological surrogate for predicting sea spray organic enrichment. In fact, considering the minimal difference between the correlation coefficients obtained with the three ocean color products, there is no reason to substitute chlorophyll‐a, which is the most accurate parameter obtained from ocean color data, with other biological surrogates being generally affected by larger and less known errors. The observed time lag between chlorophyll‐a concentration and organic matter enrichment in aerosol suggests that biological processes in oceanic surface waters and their timescales should be considered when modeling the production of primary marine organic aerosol.
    Journal of Geophysical Research: Atmospheres. 05/2013; 118(10).
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    ABSTRACT: The mountain regions of the Hindu Kush, Karakoram and Himalaya (HKKH) are the "third pole" of our planet, and the glaciers in this area play the role of "water towers", delivering significant amounts of melt water, especially in the dry season, essential for agriculture, drinking purposes, and hydropower production. The recent dynamics of glaciers in the Karakoram area is also called the "Karakoram anomaly", characterized by substantially unchanged ice cover during the last decade, against noticeable area loss worldwide, possibly leading to slightly decreasing stream fluxes. Yet, recent major floods occurring in Pakistan and the Karakoram area, may represent an effect of modified climate in the area, carrying heavier precipitation in the Monsoon season. Therefore, and notwithstanding the uncertainty embedded in measuring and modelling the hydrological behaviour of this area, there is a great need for assessment of future water resources and hydrological variability in this area. We present here results obtained at year two of the SHARE-Paprika project of the EvK2CNR Committee of Italy, aiming at evaluating the impact of recent and prospective climate change on the hydrology of the upper Indus river. We focus here on a particular watershed, the Shigar river close to Shigar, with an area of about 7000 km2, nested within the upper Indus basin, and fed by seasonal melt from two major glaciers (Baltoro and Biafo), at the toe of the K2 peak. We illustrate data gathered during three field campaigns during 2011-2012, aimed at investigating ice ablation dynamics, seasonal accumulation, and hydrological fluxes from the Baltoro-Biafo glaciers area and Shigar river. Based upon these data, topographic information, historical climate data and remote sensing data of ice and snow cover, we set up a semi-distributed, altitude belt based hydrological model, providing acceptable depiction of in stream flows, and snow and ice cover dynamics. We then project the future (until 2050) hydrological cycle in the area by feeding the hydrological model with future precipitation and temperature (plus downscaling, whenever necessary) from two climate models, one global (EC-Earth), and one regional (RegCM), the latter specifically set up for SHARE-Paprika project. The projected flow duration curves, some selected flow descriptors, and the significance of modified flow regimes in the Shigar river are then evaluated. We comment upon modified snow cover, ice ablation regime and implications for future water resources and flood regime in the area. The uncertainty of the results is addressed, and future research questions are discussed. Keywords: Upper Indus basin; hydrological models; climate models; future water resources.
    04/2013;
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    ABSTRACT: We discuss the emergence and temporal evolution of large-scale spatial-temporal oscillating modes in deep moist convection, for an atmosphere in radiative-convective equilibrium. To this end, we use cloud-resolving numerical simulations of the convective atmosphere at very high resolution and on domain sizes spanning from 500 km to a few thousands of kilometers. The results of the simulations indicate the presence of a self-organization process of the tropospheric water vapour field, where convective cells cluster together and generate almost-periodic temporal oscillations of the domain-averaged rainfall field. While the overall picture is robust to changes in cloud microphysics, the details of the clustering process and the amplitude of the oscillations depend on the microphysical parameterizations adopted and on the domain size.
    04/2013;
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    ABSTRACT: In this work we study western weather patterns (WWP), westerly perturbations responsible for most of the precipitation falling over the Hindu-Kush Karakoram (HKK) during winter, and the mechanisms responsible for their regulation. WWP originate from the northeastern Atlantic and the Mediterranean, move eastward and often intensify east of about 40°E before they reach the HKK region. Particular attention is given to the analysis of the link between the North Atlantic Oscillation (NAO) and these systems. To this end, we use 1) an ensemble of precipitation datasets, including satellite TRMM observations, three raingauge-based datasets (APHRODITE, CRU and GPCC), the ERA40 reanalyses and the global climate model EC-Earth, 2) evaporation, specific humidity, geopotential and wind data from ERA40 and EC-Earth, 3) a NAO index computed for ERA40 and EC-Earth from sea level pressure data. Our analysis shows that winter precipitation over the HKK exhibits a high interannual variability and above (below) than normal precipitation is found in correspondence of the positive (negative) NAO phase. The Persian Gulf, the northern Arabian Sea and the Red Sea are important moisture sources for winter precipitation in the HKK and enhanced evaporation from these reservoirs occurs during the positive NAO phase. We investigate the association between enhanced evaporation, changes in surface wind intensity and humidity transport towards the HKK. EC-Earth is able to capture the NAO-precipitation signal over the HKK and the mechanisms associated with the WWP described above. Further investigations will include the possibility to repeat the WWP analysis with EC-Earth for the last century (from 1850) and for the future (until 2100) under different emission scenarios, in order to investigate possible changes that occurred and might occur in the WWP activity and the consequences for precipitation in the Karakoram.
    04/2013;
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    ABSTRACT: Estimating current and future water resources in high mountain regions with complex orography is a difficult but crucial task. In particular, the French-Italian project PAPRIKA is focused on two specific regions in the Hindu-Kush -- Himalaya -- Karakorum (HKKH)region: the Shigar basin in Pakistan, at the feet of K2, and the Khumbu valley in Nepal, at the feet of Mount Everest. In this framework, we use the WRF model to simulate precipitation and meteorological conditions with high resolution in areas with extreme orographic slopes, comparing the model output with station and satellite data. Once validated the model, we shall run a set of three future time-slices at very high spatial resolution, in the periods 2046-2050, 2071-2075 and 2096-2100, nested in different climate change scenarios (EXtreme PREcipitation and Hydrological climate Scenario Simulations -EXPRESS-Hydro project). As a prelude to this study, here we discuss the simulation of specific, high-intensity rainfall events in this area. In this paper we focus on the 2010 Pakistan floods which began in late July 2010, producing heavy monsoon rains in the Khyber Pakhtunkhwa, Sindh, Punjab and Balochistan regions of Pakistan and affecting the Indus River basin. Approximately one-fifth of Pakistan's total land area was underwater, with a death toll of about 2000 people. This event has been simulated with the WRF model (version 3.3.) in cloud-permitting mode (d01 14 km and d02 3.5 km): different convective closures and microphysics parameterization have been used. A deeper understanding of the processes responsible for this event has been gained through comparison with rainfall depth observations, radiosounding data and geostationary/polar satellite images.
    04/2013;
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    ABSTRACT: Snow cover plays a key role in high-altitude environments, and changes in the snow spatial/temporal distribution and thickness affect energy, radiation and water budgets at the Earth's surface. In particular, a reduction in the snow amount has a direct effect on the availability and seasonal distribution of water resources. This is especially true in areas such as the Hindu-Kush Karakoram Himalaya (HKKH) region, which provides water to about 1.5 Billion peoples in India, Nepal, Pakistan and China. Despite its importance, knowledge on snow dynamics in the HKKH region is still incomplete, owing also to sparse and sporadic surface observations. In this work, we used simulations from Global Climate Models (GCMs) to gain information on snowpack characteristics and climatology in the HKKH region. We selected a set of GCM snow depth datasets from the CMIP5 ensemble, esploring snow abundance and distribution at monthly scale. In order to investigate how well Global Climate Models represent the snow climatology, we compared the results with the ERA-Interim reanalysis, used as an approximation to the real conditions. After exploring the average snow conditions in the last decades, we analyzed the effects of climate change in the HKKH region by using an ensemble of future snow projections obtained from different GCMs and in different climate change scenarios.
    04/2013;
  • E. Palazzi, J. von Hardenberg, A. Provenzale
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    ABSTRACT: We study the properties of precipitation in the Hindu-Kush Karakoram Himalaya (HKKH) region using currently available data sets. We consider satellite rainfall estimates (Tropical Rainfall Measuring Mission), reanalyses (ERA-Interim), gridded in situ rain gauge data (Asian Precipitation Highly Resolved Observational Data Integration Towards Evaluation of Water Resources, Climate Research Unit, and Global Precipitation Climatology Centre), and a merged satellite and rain gauge climatology (Global Precipitation Climatology Project). The data are compared with simulation results from the global climate model EC-Earth. All data sets, despite having different resolutions, coherently reproduce the mean annual cycle of precipitation in the western and eastern stretches of the HKKH. While for the Himalaya only a strong summer precipitation signal is present, associated with the monsoon, the data indicate that the Hindu-Kush Karakoram, which is exposed to midlatitude "western weather patterns", receives water inputs in winter. Time series of seasonal precipitation confirm that the various data sets provide a consistent measurement of interannual variability for the HKKH. The longest observational data sets indicate a statistically significant decreasing trend in Himalaya during summer. None of the data sets gives statistically significant precipitation trends in Hindu-Kush Karakoram during winter. Precipitation data from EC-Earth are in good agreement with the climatology of the observations (rainfall distribution and seasonality). The evolution of precipitation under two different future scenarios (RCP 4.5 and RCP 8.5) reveals an increasing trend over the Himalaya during summer, associated with an increase in wet extremes and daily intensity and a decrease in the number of rainy days. Unlike the observations, the model shows an increasing precipitation trend also in the period 1950-2009, possibly as a result of the poor representation of aerosols in this type of GCMs.
    Journal of Geophysical Research 01/2013; 118(1):85-100. · 3.17 Impact Factor
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    ABSTRACT: Periodic and irregular vegetation pattern formation in drylands have been extensively investigated in recent years, showing the importance of local soil water-vegetation feedbacks. Nevertheless, the effect of vegetation spatial patterning on atmospheric-soil water fluxes has yet to be determined in detail. This issue is crucial to understand how much detail is needed when representing vegetation-atmosphere dynamics in arid and semiarid region. Do the evapotranspiration fluxes depend only on bulk vegetation characteristics, such as biomass density or vegetated fraction, or is it necessary to include the vegetation spatial dynamics to represent evapotranspiration fluxes adequately for climatic purposes? We discuss a new explicit-space model for vegetation dynamics in water-limited ecosystems. To include the effects of rainfall intermittency, we study separately soil moisture dynamics of the surface and deep soil layers. The model shows vegetation spatial self-organization. We study the variation of soil-atmospheric evapotranspiration fluxes as a function of precipitation climatology, when different vegetation pattern states occur. The model outcomes show that evaporation flux depends only on the extent of bare soil, but its contribution to the total evapotranspiration flux is minor. Instead, in this model, transpiration fluxes do not depend only on vegetation bulk property. Due to root action, vegetation patterns have an effect on transpiration fluxes (in the days following a rainfall event), even when biomass density and fraction of vegetation cover are the same. The differences are not very large, but they show that the dynamics of different vegetation patterns is intrinsically different. Therefore, it is not possible to catch completely the vegetation-atmosphere dynamics without representing vegetation spatial mechanisms. The model outcomes also suggest that the fluxes above cultivated (i.e. fixed) vegetation can be completely different from those above natural (i.e. dynamic) vegetation, even when the two vegetation types cover the same fraction of space and have the same biomass density.
    Advances in Water Resources 01/2013; 53:131-138. · 2.41 Impact Factor
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    ABSTRACT: In this article, we develop a unifying framework for the understanding of spatial vegetation patterns in heterogeneous landscapes. While much recent research has focused on self-organised vegetation the prevailing view is still that biological patchiness is mostly due to top-down control by the physical landscape template, disturbances or predators. We suggest that vegetation patchiness in real landscapes is controlled both by the physical template and by self-organisation simultaneously, and introduce a conceptual model for the relative roles of the two mechanisms. The model considers four factors that control whether vegetation patchiness is emerged or imposed: soil patch size, plant size, resource input and resource availability. The last three factors determine the plant-patch size, and the plant-to-soil patch size ratio determines the impact of self-organisation, which becomes important when this ratio is sufficiently small. A field study and numerical simulations of a mathematical model support the conceptual model and give further insight by providing examples of self-organised and template-controlled vegetation patterns co-occurring in the same landscape. We conclude that real landscapes are generally mixtures of template-induced and self-organised patchiness. Patchiness variability increases due to source-sink resource relations, and decreases for species of larger patch sizes.
    Ecology Letters 11/2012; · 17.95 Impact Factor
  • Francesco Paparella, Jost von Hardenberg
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    ABSTRACT: We report on high-resolution, three-dimensional, high Rayleigh number, and low density ratio numerical simulations of fingering convection. We observe a previously unreported phenomenon of self-organization of fingers that cluster together to form larger-scale coherent structures. The flow ultimately forms density staircases, alternating well-mixed regions with fingering convective zones. We give evidence that the mechanical mixing induced by the clusters forms the staircases with a mechanism analogous to staircase formation in a stably stratified, nonconvective, stirred fluid.
    Physical Review Letters 07/2012; 109(1):014502. · 7.73 Impact Factor
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    ABSTRACT: We analyse the impact of climate interannual variability on summer forest fires in Catalonia (northeastern Iberian Peninsula). The study period covers 25 years, from 1983 to 2007. During this period more than 16000 fire events were recorded and the total burned area was more than 240 kha, i.e. around 7.5% of whole Catalonia. We show that the interannual variability of summer fires is significantly correlated with summer precipitation and summer maximum temperature. In addition, fires are significantly related to antecedent climate conditions, showing positive correlation with lagged precipitation and negative correlation with lagged temperatures, both with a time lag of two years, and negative correlation with the minimum temperature in the spring of the same year. The interaction between antecedent climate conditions and fire variability highlights the importance of climate not only in regulating fuel flammability, but also fuel structure. On the basis of these results, we discuss a simple regression model that explains up to 76% of the variance of the Burned Area and up to 91% of the variance of the number of fires. This simple regression model produces reliable out-of-sample predictions of the impact of climate variability on summer forest fires and it could be used to estimate fire response to different climate change scenarios, assuming that climate-vegetation-humans-fire interactions will not change significantly.
    Climatic Change 06/2012; 116(3-4):665-678. · 3.63 Impact Factor
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    ABSTRACT: We discuss a simplified mathematical description of internally cooled convection that includes a constant adiabatic lapse rate and an internal energy sink. The latter provides a representation of radiative cooling and, in combination, these two effects break the up-down symmetry of the vertical motions by making the convection penetrative in the upper portion of the fluid layer. At large enough turbulent intensity of the motion, the dynamics is dominated by intense convective updrafts that generate a strongly skewed distribution of vertical velocities. The numerical exploration of this model system exhibits a qualitatively useful description of atmospheric convection.
    Communications in Nonlinear Science and Numerical Simulation 05/2012; · 2.77 Impact Factor
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    ABSTRACT: Regional climate models have a temporal resolution which is often adequate for the application in climate change impact studies, but a spatial resolution which can be insufficient to resolve precipitation extremes and small-scale variability, particularly in the presence of complex terrain and heterogeneous orography. In the absence of fully deterministic models of small-scale rainfall, this scale gap can be bridged using stochastic downscaling techniques to generate ensembles of high-resolution scenarios of rainfall patterns. The aim of this work is to investigate whether precipitation produced by a regional climate model, and downscaled stochastically, is able to reproduce the main properties of precipitation observed by a network of rain gauges. We use a version the stochastic downscaling procedure RainFarm (Rainfall Filtered Auto Regressive Model), optimized for climatic applications, to downscale the rainfall field produced by the atmospheric-ocean regional climate model PROTHEUS. The statistics of the downscaled rainfall fields are compared with rainfall data from a network of 122 rain gauges located in the Piemonte region, North-West of Italy, for the time period from 1958 to 2001. We find that the high-resolution precipitation fields obtained downscaling the PROTHEUS model outputs reproduce well the seasonality and the amplitude distributions of observed rain gauge precipitation during most of the year. Of course, a stochastic downscaling procedure cannot correct the model outputs at large-scales, as evidenced by a the presence of a bias in average precipitation and a disagreement in the frequency of precipitation events, particularly during the winter season.
    04/2012;

Publication Stats

807 Citations
173.57 Total Impact Points

Institutions

  • 2000–2013
    • Ben-Gurion University of the Negev
      • Department of Physics
      Be'er Sheva`, Southern District, Israel
  • 2012
    • Hebrew University of Jerusalem
      • Robert H. Smith Institute of Plant Sciences and Genetics in Agriculture
      Jerusalem, Jerusalem District, Israel
  • 2010–2012
    • Università del Salento
      • Department of Mathematics and Physics "Ennio De Giorgi"
      Lecce, Apulia, Italy
  • 2008–2009
    • National Research Council
      • Institute of Atmospheric Sciences and Climate ISAC
      Roma, Latium, Italy
    • Columbia University
      New York City, New York, United States
  • 2003–2008
    • Università degli Studi di Genova
      Genova, Liguria, Italy
    • Macalester College
      Saint Paul, Minnesota, United States
    • University of Oxford
      Oxford, England, United Kingdom
  • 2004–2007
    • CIMA Research Foundation
      Savona, Liguria, Italy
  • 2005
    • Faculté des Sciences de Tunis
      Tunis-Ville, Tūnis, Tunisia
    • The London School of Economics and Political Science
      • Centre for the Analysis of Time Series (CATS)
      Londinium, England, United Kingdom