ORCID Profile
0000-0001-9768-3693
Current Organisation
Duke University
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Publisher: American Physical Society (APS)
Date: 14-10-2020
Publisher: American Society of Civil Engineers (ASCE)
Date: 12-2015
Publisher: Elsevier BV
Date: 09-2018
Publisher: IOP Publishing
Date: 10-1995
Publisher: Wiley
Date: 25-03-2013
DOI: 10.1111/GCB.12173
Abstract: Migration of plant populations is a potential survival response to climate change that depends critically on seed dispersal. Biological and physical factors determine dispersal and migration of wind-dispersed species. Recent field and wind tunnel studies demonstrate biological adaptations that bias seed release toward conditions of higher wind velocity, promoting longer dispersal distances and faster migration. However, another suite of international studies also recently highlighted a global decrease in near-surface wind speeds, or 'global stilling'. This study assessed the implications of both factors on potential plant population migration rates, using a mechanistic modeling framework. Nonrandom abscission was investigated using models of three seed release mechanisms: (i) a simple drag model (ii) a seed deflection model and (iii) a 'wear and tear' model. The models generated a single functional relationship between the frequency of seed release and statistics of the near-surface wind environment, independent of the abscission mechanism. An Inertial-Particle, Coupled Eulerian-Lagrangian Closure model (IP-CELC) was used to investigate abscission effects on seed dispersal kernels and plant population migration rates under contemporary and potential future wind conditions (based on reported global stilling trends). The results confirm that nonrandom seed abscission increased dispersal distances, particularly for light seeds. The increases were mitigated by two physical feedbacks: (i) although nonrandom abscission increased the initial acceleration of seeds from rest, the sensitivity of the seed dispersal to this initial condition declined as the wind speed increased and (ii) while nonrandom abscission increased the mean dispersal length, it reduced the kurtosis of seasonal dispersal kernels, and thus the chance of long-distance dispersal. Wind stilling greatly reduced the modeled migration rates under biased seed release conditions. Thus, species that require high wind velocities for seed abscission could experience threshold-like reductions in dispersal and migration potential if near-surface wind speeds continue to decline.
Publisher: American Geophysical Union (AGU)
Date: 11-1995
DOI: 10.1029/95WR00003
Publisher: American Geophysical Union (AGU)
Date: 06-2010
DOI: 10.1029/2009JG001134
Publisher: American Geophysical Union (AGU)
Date: 11-11-2021
DOI: 10.1029/2021GL093746
Abstract: Taylor’s frozen turbulence hypothesis (FTH) is investigated in the roughness sublayer of a sloped vineyard canopy using a spatial array of fine‐wire thermocouples and ultrasonic anemometers. The Ellipse Approximation (EA) method is applied to the measured space‐time temperature correlation function to delineate sweeping effects from advection velocity. Sweeping effects are explained primarily by the turbulence kinetic energy. Upon the removal of sweeping effects, the advection velocity is found to be commensurate with the mean velocity.
Publisher: American Meteorological Society
Date: 09-1996
Publisher: Cambridge University Press (CUP)
Date: 23-05-2022
DOI: 10.1017/JFM.2022.409
Abstract: The inertial subrange of turbulent scales is commonly reflected by a power law signature in ensemble statistics such as the energy spectrum and structure functions – both in theory and from observations. Despite promising findings on the topic of fractal geometries in turbulence, there is no accepted image for the physical flow features corresponding to this statistical signature in the inertial subrange. The present study uses boundary layer turbulence measurements to evaluate the self-similar geometric properties of velocity isosurfaces and investigate their influence on statistics for the velocity signal. The fractal dimension of streamwise velocity isosurfaces, indicating statistical self-similarity in the size of ‘wrinkles’ along each isosurface, is shown to be constant only within the inertial subrange of scales. For the transition between the inertial subrange and production range, it is inferred that the largest wrinkles become increasingly confined by the overall size of large-scale coherent velocity regions such as uniform momentum zones. The self-similarity of isosurfaces yields power-law trends in subsequent one-dimensional statistics. For instance, the theoretical 2/3 power-law exponent for the structure function can be recovered by considering the collective behaviour of numerous isosurface level sets. The results suggest that the physical presence of inertial subrange eddies is manifested in the self-similar wrinkles of isosurfaces.
Publisher: Springer Science and Business Media LLC
Date: 17-04-2014
Publisher: American Geophysical Union (AGU)
Date: 04-1993
DOI: 10.1029/93WR00094
Publisher: Springer Science and Business Media LLC
Date: 16-02-2011
Publisher: Springer Science and Business Media LLC
Date: 20-12-2022
DOI: 10.1007/S10546-022-00771-0
Abstract: A persistent spatial organization of eddies is identified in the lowest portion of the stably stratified planetary boundary layer. The analysis uses flow realizations from published large-eddy simulations (Sullivan et al. in J Atmos Sci 73(4):1815–1840, 2016) ranging in stability from near-neutral to almost z-less stratification. The coherent turbulent structure is well approximated as a series of uniform momentum zones (UMZs) and uniform temperature zones (UTZs) separated by thin layers of intense gradients that are significantly greater than the mean. This pattern yields stairstep-like instantaneous flow profiles whose shape is distinct from the mean profiles that emerge from long-term averaging. However, the scaling of the stairstep organization is closely related to the resulting mean profiles. The differences in velocity and temperature across the thin gradient layers remain proportional to the surface momentum and heat flux conditions regardless of stratification. The vertical thickness of UMZs and UTZs is proportional to height above the surface for near-neutral and weak stratification, but becomes thinner and less dependent on height as the stability increases. Deviations from the logarithmic mean profiles for velocity and temperature observed under neutral conditions are therefore predominately due to the reduction in eddy size with increasing stratification, which is empirically captured by existing Monin–Obukhov similarity relations for momentum and heat. The zone properties are additionally used to explain trends in the turbulent Prandtl number, thus providing a connection between the eddy organization, mean profiles, and turbulent diffusivity in stably stratified conditions.
Publisher: Elsevier BV
Date: 03-1992
Publisher: Springer Science and Business Media LLC
Date: 22-08-2014
Publisher: American Geophysical Union (AGU)
Date: 04-1993
DOI: 10.1029/92WR02593
Publisher: Springer Science and Business Media LLC
Date: 18-10-2017
Publisher: Springer Science and Business Media LLC
Date: 11-10-2020
Publisher: Elsevier BV
Date: 10-1999
Publisher: Elsevier BV
Date: 2008
Publisher: American Geophysical Union (AGU)
Date: 04-1995
DOI: 10.1029/94WR01949
Publisher: Wiley
Date: 25-09-2021
DOI: 10.1111/GCB.15872
Abstract: Droughts in a warming climate have become more common and more extreme, making understanding forest responses to water stress increasingly pressing. Analysis of water stress in trees has long focused on water potential in xylem and leaves, which influences stomatal closure and water flow through the soil‐plant‐atmosphere continuum. At the same time, changes of vegetation water content (VWC) are linked to a range of tree responses, including fluxes of water and carbon, mortality, flammability, and more. Unlike water potential, which requires demanding in situ measurements, VWC can be retrieved from remote sensing measurements, particularly at microwave frequencies using radar and radiometry. Here, we highlight key frontiers through which VWC has the potential to significantly increase our understanding of forest responses to water stress. To validate remote sensing observations of VWC at landscape scale and to better relate them to data assimilation model parameters, we introduce an ecosystem‐scale analog of the pressure–volume curve, the non‐linear relationship between average leaf or branch water potential and water content commonly used in plant hydraulics. The sources of variability in these ecosystem‐scale pressure‐volume curves and their relationship to forest response to water stress are discussed. We further show to what extent diel, seasonal, and decadal dynamics of VWC reflect variations in different processes relating the tree response to water stress. VWC can also be used for inferring belowground conditions—which are difficult to impossible to observe directly. Lastly, we discuss how a dedicated geostationary spaceborne observational system for VWC, when combined with existing datasets, can capture diel and seasonal water dynamics to advance the science and applications of global forest vulnerability to future droughts.
Publisher: Springer Science and Business Media LLC
Date: 04-07-2019
Publisher: American Geophysical Union (AGU)
Date: 05-2007
DOI: 10.1029/2006WR005356
Publisher: Elsevier BV
Date: 02-1999
Publisher: Wiley
Date: 07-2008
DOI: 10.1002/QJ.276
Publisher: Elsevier BV
Date: 12-1997
Publisher: Elsevier BV
Date: 2013
Publisher: American Geophysical Union (AGU)
Date: 07-2010
DOI: 10.1029/2009WR008835
Publisher: Springer Science and Business Media LLC
Date: 04-2000
Publisher: American Geophysical Union (AGU)
Date: 08-2015
DOI: 10.1002/2015JG003047
Publisher: Elsevier BV
Date: 06-2009
Publisher: Elsevier
Date: 1994
Publisher: Springer Science and Business Media LLC
Date: 2006
Publisher: Wiley
Date: 02-2002
Publisher: American Geophysical Union (AGU)
Date: 1992
DOI: 10.1029/91WR02324
Publisher: Elsevier BV
Date: 07-2008
Publisher: Wiley
Date: 12-2000
Publisher: American Geophysical Union (AGU)
Date: 10-2008
DOI: 10.1029/2008WR006916
Publisher: Springer Science and Business Media LLC
Date: 1995
DOI: 10.1007/BF00712392
Publisher: University of Chicago Press
Date: 04-2008
DOI: 10.1086/528966
Abstract: Scale separation crossing many orders of magnitude is a consistent challenge in the ecological sciences. Wind dispersal of seed that generates plant propagation fronts is a typical case where timescales range from less than a second for fast turbulent processes to interannual timescales governing plant growth and climatic forcing. We show that the scale separation can be overcome by developing mechanistic and statistical links between processes at the different timescales. A mechanistic model is used to scale up from the turbulent regime to hourly timescales, while a superstatistical approach is used to relate the half-hourly timescales to annual vegetation migration speeds. We derive a semianalytical model to predict vegetation front movement as a function of wind-forcing statistics and characteristics of the species being dispersed. This model achieves better than order-of-magnitude agreement in a case study of tree dispersal from the early Holocene, a marked improvement over diffusion models. Plant migration is shown to depend nonlinearly on the wind environment forcing the movement but linearly on most physiological parameters. Applications of these analytical results to parameterizing models of plant dispersion and the implications of the superstatistical approach for addressing other ecological problems plagued by similar "dimensionality curses" are outlined.
Publisher: American Geophysical Union (AGU)
Date: 11-1994
DOI: 10.1029/94WR01673
Publisher: Elsevier BV
Date: 03-2012
Publisher: Elsevier BV
Date: 03-1992
Publisher: American Geophysical Union (AGU)
Date: 12-2015
DOI: 10.1002/2015WR017658
Publisher: Springer Science and Business Media LLC
Date: 2004
DOI: 10.1007/S00442-003-1388-Z
Abstract: Grasslands cover about 40% of the ice-free global terrestrial surface, but their contribution to local and regional water and carbon fluxes and sensitivity to climatic perturbations such as drought remains uncertain. Here, we assess the direction and magnitude of net ecosystem carbon exchange (NEE) and its components, ecosystem carbon assimilation ( A(c)) and ecosystem respiration ( R(E)), in a southeastern United States grassland ecosystem subject to periodic drought and harvest using a combination of eddy-covariance measurements and model calculations. We modeled A(c) and evapotranspiration (ET) using a big-leaf canopy scheme in conjunction with ecophysiological and radiative transfer principles, and applied the model to assess the sensitivity of NEE and ET to soil moisture dynamics and rapid excursions in leaf area index (LAI) following grass harvesting. Model results closely match eddy-covariance flux estimations on daily, and longer, time steps. Both model calculations and eddy-covariance estimates suggest that the grassland became a net source of carbon to the atmosphere immediately following the harvest, but a rapid recovery in LAI maintained a marginal carbon sink during summer. However, when integrated over the year, this grassland ecosystem was a net C source (97 g C m(-2) a(-1)) due to a minor imbalance between large A(c) (-1,202 g C m(-2) a(-1)) and R(E) (1,299 g C m(-2) a(-1)) fluxes. Mild drought conditions during the measurement period resulted in many instances of low soil moisture (theta<0.2 m(3)m(-3)), which influenced A(c) and thereby NEE by decreasing stomatal conductance. For this experiment, low theta had minor impact on R(E). Thus, stomatal limitations to A(c) were the primary reason that this grassland was a net C source. In the absence of soil moisture limitations, model calculations suggest a net C sink of -65 g C m(-2) a(-1) assuming the LAI dynamics and physiological properties are unaltered. These results, and the results of other studies, suggest that perturbations to the hydrologic cycle are key determinants of C cycling in grassland ecosystems.
Publisher: Springer Science and Business Media LLC
Date: 07-05-2009
Publisher: American Geophysical Union (AGU)
Date: 2009
DOI: 10.1029/2008GL036044
Publisher: American Geophysical Union (AGU)
Date: 05-2020
DOI: 10.1029/2020WR027194
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 1993
DOI: 10.1109/36.210446
Publisher: Informa UK Limited
Date: 13-11-2012
Publisher: American Geophysical Union (AGU)
Date: 24-05-2023
DOI: 10.1029/2022GL101801
Abstract: In drylands, runoff during storms redistributes water and nutrients from bare soil areas to vegetated patches, subsidizing vegetation with additional resources. The extent of this redistribution depends on the interplay between surface roughness and permeability greater permeability in vegetated patches promotes run‐on to vegetation, but greater surface roughness erts runoff, producing tortuous flow paths that bypass vegetation. Here, this interplay is examined in virtual experiments using the 2D Saint Venant Equations to measure runoff connectivity. Flowpaths are delineated using tracers advected by the flow. Distances between tracer sources and sinks along flowpaths measure hydrologic connectivity at two lengthscales: connectivity to the hillslope outlet and within‐slope source‐sink connectivity. Differences between these connectivity lengthscales indicate how flow may “by‐pass” vegetated patches within hillslopes. At the hillslope scale, a derived power‐law relation between the runoff coefficient and outlet connectivity describes hillslope water losses, providing a foundation for identifying landscapes likely to shed water.
Publisher: American Geophysical Union (AGU)
Date: 10-2011
DOI: 10.1029/2011GL049182
Publisher: Elsevier BV
Date: 08-2011
Publisher: American Geophysical Union (AGU)
Date: 04-1995
DOI: 10.1029/94WR02978
Publisher: Springer Science and Business Media LLC
Date: 21-07-2006
Publisher: Elsevier BV
Date: 10-2023
Publisher: Springer Science and Business Media LLC
Date: 02-2001
Abstract: Dynamic responses of understory plants to sunflecks have been extensively studied, but how much differences in dynamic light responses affect daily photosynthesis (A
Publisher: American Geophysical Union (AGU)
Date: 09-1992
DOI: 10.1029/92WR01241
Publisher: Wiley
Date: 08-09-2022
DOI: 10.1111/PCE.14427
Publisher: Elsevier BV
Date: 11-2001
Publisher: Elsevier BV
Date: 10-2023
Publisher: American Geophysical Union (AGU)
Date: 02-2012
DOI: 10.1029/2011WR011000
Publisher: American Meteorological Society
Date: 08-1994
Publisher: CRC Press
Date: 25-11-2016
Publisher: Springer Science and Business Media LLC
Date: 12-04-2013
Publisher: Elsevier BV
Date: 08-2023
Publisher: Wiley
Date: 10-2003
Publisher: American Geophysical Union (AGU)
Date: 06-1996
DOI: 10.1029/96WR00287
Publisher: American Geophysical Union (AGU)
Date: 28-06-2013
DOI: 10.1002/GRL.50642
Publisher: Springer Science and Business Media LLC
Date: 05-2001
DOI: 10.1038/35078064
Publisher: American Geophysical Union (AGU)
Date: 1992
DOI: 10.1029/91WR02482
Publisher: American Geophysical Union (AGU)
Date: 10-2013
DOI: 10.1002/2013EO400012
Publisher: Springer Science and Business Media LLC
Date: 07-1995
DOI: 10.1007/BF00721045
Publisher: American Geophysical Union (AGU)
Date: 10-2015
DOI: 10.1002/2015JG003185
Publisher: Wiley
Date: 13-10-2010
Publisher: American Geophysical Union (AGU)
Date: 10-2018
DOI: 10.1029/2018JG004637
Publisher: Wiley
Date: 15-08-2002
No related grants have been discovered for Gabriel Katul.