Global Average Model Research Articles

A Mantle Plume Beneath South China Revealed by Electrical Conductivity Obtained from Three-Dimensional Inversion of Geomagnetic Data.

A three-dimensional electrical conductivity model of the mantle beneath South China is presented using the geomagnetic depth sounding method in this paper. The data misfit term in the inversion function is measured by the L1-norm to suppress the instability caused by large noises contained in the observed data. To properly correct the ocean effect in responses at coastal observatories, a high-resolution (1° × 1°) heterogeneous and fixed shell is included in inversion. The most striking feature of the obtained model is a continuous high-conductivity anomaly that is centered on ~(112° E, 27° N) in the mantle. The average conductivity of the anomaly appears to be two to four times higher than that of the global average models at the most sensitive depths (410-900 km) of geomagnetic depth sounding. Further analysis combining laboratory-measured conductivity models with the observed conductivity model shows that the anomaly implies excess temperature in the mantle. This suggests the existence of a mantle plume, corresponding to the Hainan plume, that originates in the lower mantle, passes through the mantle transition zone, and enters the upper mantle. Our electrical conductivity model provides convincing evidence for the mantle plume beneath South China.

Quantifying Location-Specific Impacts of Pavement Albedo on Radiative Forcing Using an Analytical Approach

High-albedo materials reflect more solar radiation and, thereby, alter the earth's radiative balance. Increasing pavement albedo, therefore, has been considered as a technological strategy to mitigate global warming. Previous studies have evaluated this strategy using global average models. To factor this effect into life cycle assessments, location-specific models of the albedo effect for pavements are required. A parametric analytical model is developed to estimate the radiative forcing (RF) using a novel model form and an iterative solution approach. The new model is extended to estimate the corresponding global warming potential (GWP) over an analysis period of 50 years for an albedo change in a pavement surface. This was applied to quantify the GWP impacts of increasing pavement albedo in 14 cities across various climate zones in the US. For the United States, the GWP in kg CO2 equivalent per square meter of altered surface ranges from 0.8 to 1.6 per 0.01 change in albedo, a range of more than 40%. Analysis of a hypothetical albedo change to all darker pavements in the US would produce a negative RF of a magnitude equivalent to that associated with a reduction in CO2 emissions of more than 17 Mton per year.

Robust latent-variable interpretation of in vivo regression models by nested resampling

Simple multilinear methods, such as partial least squares regression (PLSR), are effective at interrelating dynamic, multivariate datasets of cell–molecular biology through high-dimensional arrays. However, data collected in vivo are more difficult, because animal-to-animal variability is often high, and each time-point measured is usually a terminal endpoint for that animal. Observations are further complicated by the nesting of cells within tissues or tissue sections, which themselves are nested within animals. Here, we introduce principled resampling strategies that preserve the tissue-animal hierarchy of individual replicates and compute the uncertainty of multidimensional decompositions applied to global averages. Using molecular–phenotypic data from the mouse aorta and colon, we find that interpretation of decomposed latent variables (LVs) changes when PLSR models are resampled. Lagging LVs, which statistically improve global-average models, are unstable in resampled iterations that preserve nesting relationships, arguing that these LVs should not be mined for biological insight. Interestingly, resampling is less discriminatory for multidimensional regressions of in vitro data, where replicate-to-replicate variance is sufficiently low. Our work illustrates the challenges and opportunities in translating systems-biology approaches from cultured cells to living organisms. Nested resampling adds a straightforward quality-control step for interpreting the robustness of in vivo regression models.

Rayleigh wave constraints on the structure and tectonic history of the Gamburtsev Subglacial Mountains, East Antarctica

The Gamburtsev Subglacial Mountains (GSM), located near the center of East Antarctica, remain one of the most enigmatic mountain ranges on Earth. A lack of direct geologic samples renders their tectonic history almost totally unconstrained. We utilize teleseismic Rayleigh wave data from a 2 year deployment of broadband seismic stations across the region to image shear velocity structure and analyze the lithospheric age of the GSM and surrounding regions. We solve for 2‐D phase velocities and invert these results for 3‐D shear velocity structure. We perform a Monte Carlo simulation to improve constraints of crustal thickness and shear velocity structure. Beneath the core of the GSM, we find crustal thickness in excess of 55 km. Mantle shear velocities remain faster than global average models to a depth of approximately 250 km, indicating a thick lithospheric root. Thinner crust and slower upper mantle velocities are observed beneath the Lambert Rift System and the Polar Subglacial Basin. When compared with phase velocity curves corresponding to specific tectonothermal ages elsewhere in the world, average phase velocity results for the GSM are consistent with regions of Archean‐Paleoproterozoic origin. Combined with radiometric ages of detrital zircons found offshore, these results indicate a region of old crust that has undergone repeated periods of uplift and erosion, most recently during the Mesozoic breakup of Gondwana. Lower crustal seismic velocities imply a moderately dense lower crust beneath the core of the GSM, but with lower density than suggested by recent gravity models.

An Instance-Based Method for Remaining Useful Life Estimation for Aircraft Engines

Under customer service agreements (CSA), engine operational data are collected and stored for monitoring and analysis. Other data sources provide damage assessments that are either provided post-maintenance or analytically assessed. This paper takes advantage of these data and investigates local fuzzy models to determine the remaining useful life (RUL) of an engine or engine component. Local fuzzy models are related to both kernel regressions and locally weighted learning. The particular local models described in this paper are not based on individual models that consider the track history of a specific engine nor are they based on a global average model that would consider the collective track history of all the engines. Instead, for a given engine or component, this local fuzzy model defines a cluster of peers in which each of these peers is a similar instance to this given engine with comparable operational characteristics; the RUL prediction for this given engine is obtained by a fuzzy aggregation of its peers’ RUL. We combine the fuzzy instance-based approach with an evolutionary framework for model tuning and maintenance. This evolutionary tuning process is repeated periodically to automatically update and improve the fuzzy models such that they can be updated to date with the latest collection of data. This fuzzy instance-based approach is applied to predicting the RUL of a commercial engine validated with post-maintenance assessment.

A new global average model of the coupled thermosphere and ionosphere

A model representing the global average ionosphere and thermosphere (GAIT) is developed as a tool to explore the response of the coupled system to changes in the input solar irradiance between 3 and 360 nm. The GAIT model self‐consistently solves the coupled continuity, momentum, and energy equations for the three major neutral species, N2, O2, and O, as well as minor neutral constituents important to the global energy budget. In the ionosphere the model includes five different ion species and two excited states of O+. The GAIT model also includes an approximate treatment of photoelectrons, in order to calculate secondary ionization and thermal electron volume heating rates. After examining the sensitivity of the GAIT model to uncertainties in key reaction rates and inputs, we consider its response to four different extreme ultraviolet irradiance models. With three of the irradiance models we reproduce the expected variation in exospheric temperature to within 2%; however, the dynamic range of the fourth is deemed to be too small. We conclude with an analysis of the model's wavelength‐dependent sensitivity to input solar photons.

Variation of transition zone high‐velocity anomalies and depression of 660 km discontinuity associated with subduction zones from the southern Kuriles to Izu‐Bonin and Ryukyu

The seismic structure of the transition zone beneath the northwestern Pacific ocean is studied by comparing regional distance seismic waveforms with synthetic seismograms. By modeling waveforms we incorporate the travel times of secondary waves that are not included in traveltime data used in most P wave tomography experiments. These secondary waves are highly sensitive to the velocity above the 660 km discontinuity and thus add information on the seismic structure of the transition zone independent of that used in many tomography experiments. We used just two one‐dimensional models in the waveform comparison. The iasp91(slightly modified in the shallower depths as aiasp) is used as a global average structure, and model M3.11 is used as representative of transition zone structure near a deep subducting slab. Model M3.11 is faster than iasp91 from 525 to 660 km depth but also has a 30 km deeper “660” km discontinuity. Our results show strong variations in the transition zone structure beneath the northwestern Pacific. Data that sample beneath the northwestern Japanese subduction zone and the southern Kuriles are well fit by model M3.11. The deep structure beneath the Bohai Sea, to the west of Korea, also appears to show slab‐related velocity anomalies in the transition zone. In contrast, the seismic data sampling beneath the Yellow Sea to the East China Sea are better modeled using the global average model iasp91. Data that sampled beneath the Philippine Sea showed evidence for anomalously high velocity in the transition zone, but there is little apparent change in the depth of the 660 km discontinuity there. The overall results are in agreement with past work indicating fairly broad regions where subducting plate is lying flat or piling up within the transition zone, although we find that the region where this occurs, and thus the total volume of slab within the upper mantle, is considerably less than that seen in past studies. The unusual structure beneath the Philippine plate may be due to the relatively recent descent of slab beneath this region. The slab itself is cold, and thus anomalously fast, but the deeper mantle may not have cooled enough for the 660 km discontinuity to have deepened.

Comparison of plate and asthenospheric flow models for the thermal evolution of oceanic lithosphere

Although seafloor depth and heat flow for young oceanic lithosphere can be described by modeling the lithosphere as the boundary layer of a cooling halfspace, a long standing question has been why data at older ages deviate from those expected for a halfspace. Two classes of models have been proposed for these deviations. In one, heat added from below “flattens” depth and heat flow. In the other, asthenospheric flow beneath the lithosphere perturbs the depths. We compare recent versions of the model classes: the GDH1 thin‐lithosphere plate model [Stein and Stein, 1992] and an asthenospheric flow model [Phipps Morgan and Smith, 1992]. The plate model fits heat flow data better than the flow model for all cases considered, and topographic data in all but one case. The flow model significantly overpredicts depths for the North Atlantic, because the assumed asthenospheric flow in the plate motion direction would yield deepening for old ages rather than the observed flattening. Overall, the GDH1 global average model does better than this flow model, whose parameters were fit to specific plates. Moreover, plate models fit to specific plates do better than the flow model. Plate models thus appear more useful than this flow model, suggesting that deviations from a cooling halfspace are largely thermal in origin.

Comparative terrestrial planet thermospheres: 1. Solar cycle variation of global mean temperatures

Global average models for the thermospheres of Venus, Earth, and Mars are used to calculate the solar cycle variations of the global mean temperatures using the compositional profiles for each planet. All models use the same absolute solar EUV and UV fluxes but scaled for planetary distances. Photochemistry appropriate to each planet is used (or assumed) with similar rate coefficients for common physical and chemical processes. In particular, the CO2 15‐μm cooling rates for all three planets are determined using a common rate coefficient for collisional excitation of the CO2 bending mode by atomic oxygen (relaxation rate 10−12 cm³ s−1). Solar EUV and UV heating efficiencies are calculated self‐consistently for Earth, but prescribed for Venus (15/22%) and Mars (18/22%) according to independent calculations. Eddy diffusion profiles are prescribed for each planet in accord with previous studies that compared model predictions with available satellite observations. The global mean models are run to steady state for both solar minimum (F10.7 = 70) and solar maximum (F10.7 = 240) conditions. The results show that the solar cycle global mean exospheric temperature variation is about 76 K for Venus (172 to 248 K), 518 K for Earth (737 to 1255 K), and 110 K for Mars (180 to 290 K). A thermal balance analysis shows that the small exospheric temperature variation on Venus occurs because of the strong radiative damping by CO2 15‐μm cooling. The peak CO2 cooling occurs at the altitude of maximum solar heating, and efficiently radiates it to space. On Earth, the increased solar heating occurs at a higher altitude than the peak in the infrared cooling. Consequently, it must be thermally conducted down to the altitude of the peak infrared cooling before it is radiated to space. An increase in the thermally conducted heat flux requires an increase in the vertical temperature gradient which results in a larger exospheric temperature variation. On Mars, the increased solar heating also occurs at a higher altitude than the peak cooling and likewise must be conducted downward before radiating to space. Furthermore, CO2 cooling is not as effective on Mars as it is on Venus because of lower O/CO2 ratios. These two factors yield a Mars solar cycle variation of global mean temperatures that is larger than for Venus.

How will changes in carbon dioxide and methane modify the mean structure of the mesosphere and thermosphere?

A global average model of the coupled mesosphere, thermosphere and ionosphere is used to examine the effect of trace gas variations on the overall structure of these regions. In particular, the variations caused by CO2 and CH4 doublings and halvings from present day mixing ratios are presented. The results indicate that the mesosphre and thermosphere temperatures will cool by about 10K and 50K respectively as the CO2 and CH4 mixing ratios are doubled. These regions are heated by similar amounts when the trace gas mixing ratios are halved. Compositional redistributions also occur in association with changes in the temperature profile. The results show that global change will occur in the upper atmosphere and ionosphere as well as in the lower atmosphere during the 21st century.

On the global mean structure of the thermosphere

A self‐consistent global average model of the coupled thermosphere and ionosphere has been developed and used to examine the global mean structure of these regions for solar minimum and maximum conditions. The model uses a parameterized version of the solar EUV and UV flux measured from the Atmospheric Explorer satellite to calculate photoionization rates of the major ion species and photo‐dissociation rates for O2 and N2. The ionospheric calculation solves for the vertical distributions of O+, including ambipolar diffusion, and of NO+, O2+, N2+, and N+, assuming photochemical equilibrium with respect to O+. It also solves the electron and ion energy equations as well as the coupled minor neutral constituent diffusion equations for N(²D), N(4S), and NO. The neutral gas energy equation is solved considering heating from photoelectrons, O2 absorption in the Schumann‐Runge continuum and bands, excess energy from exothermic ion‐neutral and neutral‐neutral chemical reactions, thermal electron, ion, and neutral collisions, O3 absorption in the Hartley bands, and atomic oxygen recombination and cooling from molecular and eddy thermal conduction and NO and CO2 IR radiation. The O2 photodissociation rates are used in a model of major species diffusion to solve for the O, O2, and N2 number densities. From specified lower‐boundary conditions and arbitrary initial conditions, both the thermospheric and the ionospheric structure evolve self consistently with time for specified solar flux values. The calculated globally averaged thermospheric and ionospheric structure compares well with the globally averaged structure predicted by the thermospheric and ionospheric empirical models based on data for both solar minimum and maximum conditions by considering solar EUV and UV radiation and aurora particle precipitation and joule heat sources to represent high‐latitude processes. Total joule heating is 7 × 1010 W for geomagnetic quiet conditions for both solar minimum and maximum. The model results show that the dominant heat sources for the thermosphere are collisions between ambient electrons, ions, and neturals above about 250 km, exothermic ion‐neutral chemical reactions between 200 and 250 km, exothermic neutral‐neutral chemical reactions between 200 and 150 km, and heating in the Schumann‐Runge continuum and bands below 150 km. Dominant cooling mechanisms are downward molecular thermal conduction above 150 km and NO and CO2 radiation below 150 km. The results also show that NO cooling must be considered in the thermal energy balance if the effect of minor neutral constituent chemical heating is considered in a definition of the neutral gas solar heating efficiency. Otherwise, the heat input into the thermosphere is overestimated.

Frequency dependence of Q in the mantle underlying the shield areas of Eurasia, Part III: The Q model

Summary. A large set of teleseismic body wave data covering a broad range in frequency was analysed to determine the frequency arid depth dependence of Q for P- and S-waves under the northern shield areas of Eurasia. Based on numerous f* estimates for P- and S-waves covering the seismic band between 0.02 and 8 Hz, a Q model of the Eurasian shield was constructed. The data require a model in which Q increases with frequency and which is characterized by Q values in the upper mantle that are generally higher than those of global average models. The model with the best fit includes a minimum in Q between about 100 and 200 km depth and high Q values on the order of thousands throughout most of the deeper mantle. These results are generally consistent with proposed models of attenuation as a thermally activated process which is influenced by the temperature and pressure in the earth. Preliminary results suggest that t* under tectonic regions is higher than t* under shield regions for all frequencies over the 0.02 to 8 Hz frequency range suggesting that Q varies regionally as well as with frequency and depth.

Great-circle Rayleigh wave attenuation and group velocity, Part III: Inversion of global average group velocities and attenuation coefficients

Average shear-velocity models for the upper mantle have been derived by controlled Monte Carlo inversion of global average Rayleigh wave group velocity (GAGV) data for periods between 50 and 300 seconds. GAGV data have been corrected for attenuative dispersion using a method based on the theory of Liu, Anderson and Kanamori. Two types of model bounds have been used with one- or two-layer low-velocity zones beginning at depths of 70 and 100 km. All models fitting GAGV data within one standard deviation have low-velocity zones in the 100–200 km depth range. Models with low-velocity zones beginning at 70 km, as well as 100 km, fit GAGV data within one standard deviation, so the average thickness of the lithosphere (taken as the depth to the top of the low-velocity zone) cannot be determined with precision. Global average models for shear-wave attenuation ( Q −1 β ) have been derived from global average Rayleigh wave attenuation coefficients for periods between 50 and 300 s and average shear-velocity models. Zones of high Q −1 β coincide with the low-velocity zones of all shear-velocity models, however, models with low-velocity zones beginning at a depth of 70 km have the highest-attenuation layer in the lower half of the low-velocity zone. Resolution kernels for these attenuation models show that parameters for layers shallower than the lower part of the low-velocity-high-attenuation zone are strongly coupled but are distinct from the lower part of this zone. This suggests that the deeper part of the low-velocity-high-attenuation zone is the most mobile part of the zone or that on the average, the top of the zone is deeper than 70 km. The average Q β of the lithosphere, low-velocity zone, and sub-low-velocity layer (asthenosphere) are approximately 200, 85–110 and 170–200, respectively.

A Global Average Model of Atmospheric Aerosols for Radiative Transfer Calculations

A global average model is proposed for the size distribution, chemical composition, and optical thickness of stratospheric and tropospheric aerosols. This aerosol model is designed to specify the input parameters to global average radiative transfer calculations which assume the atmosphere is horizontally homogeneous. The model subdivides the atmosphere at multiples of 3 km, where the surface layer extends from the ground to 3 km, the upper troposphere from 3 to 12 km, and the stratosphere from 12 to 45 km. A list of assumptions made in construction of the model is presented and discussed along with major model uncertainties. The stratospheric aerosol is modeled as a liquid mixture of 75% H2SO4 and 25% H2O, while the tropospheric aerosol consists of 60% sulfate and 40% soil particles above 3 km and of 50% sulfate, 35% soil particles, and 15% sea salt below 3 km. Implications and consistency of the model are discussed.