25 research outputs found
In-Situ Measurements and Remotely Sensed Estimations of Surface Fluxes over the Southern Great Plains of the United States
Evapotranspiration (ET) is an important variable in the hydrologic cycle. As such, improved understanding of the spatial and temporal variability of ET is critical to weather and climate analysis and prediction, water management resources, agriculture, land-use and land-change projects, and ecological research. Eddy covariance flux towers were deployed over grasslands and winter wheat at the United States Department of Agriculture site near El Reno, Oklahoma and at the Marena Oklahoma In-Situ Sensor Testbed (MOISST). Ten total years of surface energy, water, and carbon fluxes were paired with fifty total years of data from twelve additional flux towers across the Southern Great Plains (SGP) for a regional land-surface analysis using the Breathing Earth System Simulator (BESS). BESS is a land-surface model that couples land-atmosphere processes to estimate ET and gross primary production (GPP). The study results show that BESS and the observations yield good agreement with R2 values of 0.74. Further, BESS outperformed other ET estimates including a two-source surface energy balance model (Gowda et al. 2013), an empirical model (Wagle et al. 2016), and the MODIS ET product. ET decreases from southeast to northwest across the SGP, ranging from 300-1000 mm/year. ET varies more in the southwest portion of the SGP (50-100 mm/year) and less in the northeast (10-40 mm/year). Using BESS to analyze long-term ET, it was determined that, on average, 74% of the precipitation received in the SGP is re-distributed by ET. However, the results also noted that BESS lacks the ability to accurately depict ET patterns during flash drought conditions, as seen during 2012, but can depict drought and pluvial conditions when soil moisture and near-surface atmospheric conditions are in equilibrium
Assessing agricultural drought in summer over Oklahoma Mesonet sites using the water-related vegetation index from MODIS.
Agricultural drought, a common phenomenon in most parts of the world, is one of the most challenging natural hazards to monitor effectively. Land surface water index (LSWI), calculated as a normalized ratio between near infrared (NIR) and short-wave infrared (SWIR), is sensitive to vegetation and soil water content. This study examined the potential of a LSWI-based, drought-monitoring algorithm to assess summer drought over 113 Oklahoma Mesonet stations comprising various land cover and soil types in Oklahoma. Drought duration in a year was determined by the number of days with LSWI <0 (DNLSWI) during summer months (June-August). Summer rainfall anomalies and LSWI anomalies followed a similar seasonal dynamics and showed strong correlations (r 2 = 0.62-0.73) during drought years (2001, 2006, 2011, and 2012). The DNLSWI tracked the east-west gradient of summer rainfall in Oklahoma. Drought intensity increased with increasing duration of DNLSWI, and the intensity increased rapidly when DNLSWI was more than 48 days. The comparison between LSWI and the US Drought Monitor (USDM) showed a strong linear negative relationship; i.e., higher drought intensity tends to have lower LSWI values and vice versa. However, the agreement between LSWI-based algorithm and USDM indicators varied substantially from 32 % (D 2 class, moderate drought) to 77 % (0 and D 0 class, no drought) for different drought intensity classes and varied from âŒ30 % (western Oklahoma) to >80 % (eastern Oklahoma) across regions. Our results illustrated that drought intensity thresholds can be established by counting DNLSWI (in days) and used as a simple complementary tool in several drought applications for semi-arid and semi-humid regions of Oklahoma. However, larger discrepancies between USDM and the LSWI-based algorithm in arid regions of western Oklahoma suggest the requirement of further adjustment in the algorithm for its application in arid regions
Based on Atmospheric Physics and Ecological Principle to Assess the Accuracies of Field CO2 /H2O Measurements From Infrared Gas Analyzers in Closed-Path Eddy-Covariance Systems
Field CO2 /H2O measurements from infrared gas analyzers in closed-path eddy-covariance systems have wide applications in earth sciences. Knowledge about exactness of these measurements is required to assess measurement applicability. Although the analyzers are specified with uncertainty components (zero drift, gain drift, cross-sensitivities, and precision), exactness for individual measurements is unavailable due to an absence of methodology to comprehend the components as an overall uncertainty. Adopting an advanced definition of accuracy as a range of all measurement uncertainty sources, the specified components are composited into a model formulated for studying analyzersâ CO2 /H2O accuracy equations. Based on atmospheric physics and environmental parameters, the analyzers are evaluated using the equations for CO2 accuracy (±0.78 ”molCO2 molâ1, relatively ±0.18%) and H2O accuracy (±0.15 mmolH2 O molâ1). Evaluation shows that precision and cross-sensitivity are minor uncertainties while zero and gain drifts are major uncertainties. Both drifts need adjusting through zero/span procedures during field maintenance. The equations provide rationales to guide and assess the procedures. H2O span needs more attentions under humid conditions. Under freezing conditions while H2O span is impractical, this span is fortunately unnecessary. Under the same conditions, H2O zero drift dominates H2O measurement uncertainty. Therefore, automatic zero becomes a more applicable and necessary tactic. In general cases of atmospheric CO2 background, automatic CO2 zero/ span procedures can narrow CO2 accuracy by 36% (±0.74 to ± 0.47 ”molCO2 molâ1). Automatic/manual H2 O zero/span procedures can narrow H2O accuracy by 27% (±0.15 to ±0.11 mmolH2O molâ1). While ensuring system specifications, the procedures guided by equations improve measurement accuracies
Doping and energy dependent microwave conductivity of kinetic energy driven superconductors with extended impurities
Within the framework of the kinetic energy driven superconducting mechanism,
the effect of the extended impurity scatterers on the quasiparticle transport
of cuprate superconductors in the superconducting state is studied based on the
nodal approximation of the quasiparticle excitations and scattering processes.
It is shown that there is a cusplike shape of the energy dependent microwave
conductivity spectrum. At low temperatures, the microwave conductivity
increases linearly with increasing temperatures, and reaches a maximum at
intermediate temperature, then decreases with increasing temperatures at high
temperatures. In contrast with the dome shape of the doping dependent
superconducting gap parameter, the minimum microwave conductivity occurs around
the optimal doping, and then increases in both underdoped and overdoped
regimes.Comment: 9 pages, 3 figure
Singular Fermi Liquids
An introductory survey of the theoretical ideas and calculations and the
experimental results which depart from Landau Fermi-liquids is presented.
Common themes and possible routes to the singularities leading to the breakdown
of Landau Fermi liquids are categorized following an elementary discussion of
the theory. Soluble examples of Singular Fermi liquids (often called Non-Fermi
liquids) include models of impurities in metals with special symmetries and
one-dimensional interacting fermions. A review of these is followed by a
discussion of Singular Fermi liquids in a wide variety of experimental
situations and theoretical models. These include the effects of low-energy
collective fluctuations, gauge fields due either to symmetries in the
hamiltonian or possible dynamically generated symmetries, fluctuations around
quantum critical points, the normal state of high temperature superconductors
and the two-dimensional metallic state. For the last three systems, the
principal experimental results are summarized and the outstanding theoretical
issues highlighted.Comment: 170 pages; submitted to Physics Reports; a single pdf file with high
quality figures is available from http://www.lorentz.leidenuniv.nl/~saarloo
Non Fermi-Liquid States and Pairing of a general Model of Copper-Oxide Metals
A model of copper-oxygen bonding and anti-bonding bands with the most general
two-body interactions allowable by symmetry is considered. The model has a
continuous transition as a function of hole-density x and temperature T to a
phase in which a current circulates in each unit cell. This phase preserves the
translational symmetry of the lattice while breaking time-reversal invariance
and the four-fold rotational symmetry. The product of time-reversal and
four-fold rotation is preserved. The circulating current phase terminates at a
critical point at . In the quantum-critical region about this point
the logarithm of the frequency of the current fluctuations scales with their
momentum. The microscopic basis for the marginal Fermi-liquid phenemenology and
the observed long wavelength transport anomalies near are derived from
such fluctuations. The symmetry of the current fluctuations is such that the
associated magnetic field fluctuations are absent at oxygen sites and have the
correct form to explain the anomalous copper nuclear relaxation rate.
Cross-overs to the Fermi-liquid phase on either side of and the role of
disorder are briefly considered. The current fluctuations promote
superconductive instability with a propensity towards ``D-wave" symmetry or
``extended S-wave"symmetry depending on details of the band-structure.Comment: 85 pages RevTex,15 figures available from the autho