摘要:

Terpenoids (terpenes and their oxygenated derivatives) are common tropospheric species. Information concerning the terpenoids and assessing their importance in atmospheric processes is sparse and widely distributed, however. This review presents chemical structure, physical property, and source information for 45 terpenoids known to be emitted into the atmosphere. Rate constants for the reactions of the terpenoids with atmospheric scavengers are presented, and typical terpenoid lifetimes are derived. Chemical product studies are used to derive plausible reaction mechanisms for selected acyclic, monocyclic, and bicyclic terpenes. The atmospheric cycle of the terpenoids is discussed, and it is concluded that inadequate information currently precludes quantitative assessments of the relative impact of terpenoids on local, regional, and global atmospheric processes.

ISSN:8755-1209    

DOI:10.1029/RG017i005p00937

年代:1979

数据来源: WILEY

摘要:

The problem of constructing optimal linear prediction models by multivariance regression methods is reviewed. It is well known that as the number of predictors in a model is increased, the skill of the prediction grows, but the statistical significance generally decreases. For predictions using a large number of candidate predictors, strategies are therefore needed to determine optimal prediction models which properly balance the competing requirements of skill and significance. The popular methods of coefficient screening or stepwise regression represent a posteriori predictor selection methods and therefore cannot be used to recover statistically significant models by truncation if the complete model, including all predictors, is statistically insignificant. Higher significance can be achieved only by a priori reduction of the predictor set. To determine the maximum number of predictors which may be meaningfully incorporated in a model, a model hierarchy can be used in which a series of best fit prediction models is constructed for a (prior defined) nested sequence of predictor sets, the sequence being terminated when the significance level either falls below a prescribed limit or reaches a maximum value. The method requires a reliable assessment of model significance. This is characterized by a quadratic statistic which is defined independently of the model skill or artificial skill. As an example, the method is applied to the prediction of sea surface temperature anomalies at Christmas Island (representative of sea surface temperatures in the central equatorial Pacific) and variations of the central and east Pacific Hadley circulation (characterized by the second empirical orthogonal function (EOF) of the meridional component of the trade wind anomaly field) using a general multiple‐time‐lag prediction matrix. The ordering of the predictors is based on an EOF sequence, defined formally as orthogonal variables in the composite space of all (normalized) predictors, irrespective of their different physical dimensions, time lag, and geographic position. The choice of a large set of 20 predictors at 12 time lags yields significant predictability only for forecast periods of 3 to 5 months. However, a prior reduction of the predictor set to 4 predictors at 10 time lags leads to 95% significant predictions with skill values of the order of 0.4 to 0.7 up to 6 or 8 months. For infinitely long time series the construction of optimal prediction models reduces essentially to the problem of linear system identification. However, the model hierarchies normally considered for the simulation of general linear systems differ in structure from the model hierarchies which appear to be most suitable for constructing pure prediction models. Thus the truncation imposed by statistical significance requirements can result in rather different models for the two cases. The relation between optimal prediction models and linear dynamical models is illustrated by the prediction of east‐west sea level changes in the equatorial Pacific from wind field anomalies. It is shown that the optimal empirical prediction is statistically consistent in this case with both the first‐order relaxation and damped oscillator models recently proposed by McWilliams and Gent (but with somewhat different model parameters than suggested by the authors). Thus the data do not allow a distinction between the two physical models; the simplest acceptable model is the first‐order damped response. Finally, the problem of estimating forecast skill is discussed. It is usually stated that the forecast skill is smaller than the true skill, which in turn is smaller than the hindcast skill, by an amount which in both cases is approximately equal to the artificial skill. However, this result applies to the mean skills averaged over the ensemble of all possible hindcast data sets, given the true model. Under the more appropriate side condition of a given hindcast data set and an unknown true model, the estimation of the forecast skill represents a problem of statistical inference and is dependent on the assumed prior probability distribution of true models. The Bayesian hypothesis of a uniform prior distribution yields an average forecast skill equal to the hindcast skill, but other (equally acceptable) assumptions yield lower forecast skills more compatible with the usual hindcast‐averaged

ISSN:8755-1209    

DOI:10.1029/RG017i005p00949

年代:1979

数据来源: WILEY

摘要:

Energy budgets for the long‐time‐averaged circulation of mesoscale resolution numerical ocean circulation model (EGCM) experiments are the only widely used analysis methods that permit investigation into the eddy‐mean flow interaction of these model oceanic systems. All published energy results from the closed basin EGCM experiments are presented here in nondimensional form to facilitate comparison of the various types of behavior. The ubiquitous basin‐averaged energy analysis method is necessarily sensitive to changes in model forcing and dissipative processes, and it is shown that several natural energy measures of eddy‐mean flow interaction are strongly correlated with forcing and dissipation choices. In particular, it is found that by these energy measures, eddies are relatively unimportant in the mean flow when the horizontal momentum subgrid scale process corresponds to constant coefficient viscosity, but they are much more important when this process is modeled by a constant coefficient biharmonic operator. Energy budgets over subrogions of the full domain should offer increased insight into model mechanisms, but the use of quasi‐geostrophic dynamics can severely limit the number of independent energy pathways in the flow. Owing to these constraints the published regional two‐layer quasi‐geostrophic energy budgets offer information beyond that of the basin budgets. It is suggested that the assumptions of these quasi‐geostrophic models also limit the generality of inferences about the role of vertical processes based on these calculations. In none of the energy budgets reported is there significant direct driving of a mean flow by Reynolds stress work, and in only one case does mean buoyancy work act to try to increase mean flow kinetic energy. From these budgets it appears that if eddies are energetically important in the model flows, their only unambiguous role is as a pathway to dissipation. Further more detailed regional analysis is necessary to see if this result continues to hold when smaller horizontal subregions of the domain are analyzed. Such analysis activity, as well as more attention to alternative means of investigating eddy‐mean flow interaction, a

ISSN:8755-1209    

DOI:10.1029/RG017i005p00969

年代:1979

数据来源: WILEY

摘要:

The state of stress in the lithosphere provides strong constraints on the forces acting on the plates. The directions of principal stresses in the plates as indicated by midplate earthquake mechanisms, in situ stress measurements, and stress‐sensitive geological features are used to test plate tectonic driving force models, under the premises that enough data exist in selected areas to define a regionally consistent stress field and that for most such areas the dominant forces producing such stresses are plate tectonic in origin. Force models include buoyancy forces at ridges, subduction zones, and continental convergence zones and variously parameterized viscous shear between the lithosphere and the asthenosphere. A linear finite element method, based on the wave front solution technique, is used to predict the intraplate stress for each force model. Several long‐wavelength patterns for the orientation of horizontal principal deviatoric stresses are observable in the stress data. Maximum compressive stresses trend E‐W to NE‐SW for much of stable North America and E‐W to NW‐SE for continental South America. In western Europe the maximum compressive stresses trend NW‐SE, while in Asia the trend is more nearly N‐S, especially near the Himalayan front. In the Indian plate the trend varies from nearly N‐S in continental India to more nearly E‐W in Australia. Horizontal stresses are variable in Africa but tend to indicate a NW‐SE trend for the maximum compressive stress in west Africa and an E‐W trend for the minimum compressive stress in east Africa. Oceanic lithosphere away from plate boundaries is generally in a state of deviatoric compression, although few focal mechanisms can be constrained to define the orientation of the principal stresses.Comparison of stress orientations predicted for a wide range of driving force models to these regional stress observations provides a powerful test of the models. Ridge pushing forces are required in all models that match the stress orientation field. The net pulling force of subducted lithosphere, if such a force acts approximately symmetrically about the plate boundary, is at most a few times larger than other forces acting on the plates. Resistive forces associated with trench thrust faults and motion of the slab with respect to the mantle must therefore nearly balance the large gravitational potential of the slab. The upper limit on the ratio of net slab to ridge forces may be increased by less than a factor of 2 if net slab forces are reduced for the fastest moving plates by assuming that the resistance to subduction increases with convergence rate. Forces acting to resist further continental convergence along the Alpine‐Himalayan belt are important for models of the intraplate stress field in Europe, Asia, and the Indian plate. Inclusion of continental convergence zone forces, however, does not affect the upper bound on the ratio of net slab to ridge forces. A variety of possible lithosphere‐asthenosphere interactions have been tested. Resistive viscous drag forces acting on the base of the lithosphere improve the fit between calculated and observed stresses in the Nazca and South American plates as long as the drag coefficient is nonzero beneath oceanic lithosphere. The calculated intraplate stress field is not very sensitive to an increased drag coefficient beneath old oceanic lithosphere compared to young oceanic or continental lithosphere. Increasing the drag coefficient beneath continents compared to oceanic lithosphere by a factor of 5 or 10 has little effect on the overall fit of calculated stresses to observed stresses. Models in which viscous drag forces drive, rather than resist, plate motions are in poor agreement with intraplate stress data, although this lack of fit may depend on the oversimplified model of mantle flow patterns that has been assumed. A model of the driving mechanism in which slab forces act only on the plate with subducted lithosphere and where viscous drag forces are assumed to balance the torque on each plate produced by ridge and trench forces predicts stresses in good agreement with the data for most continental regions. The fit between calculated and observed stresses for this model is relatively poor for most oceanic regions. This model suggests, however, that it is probably an oversimplification to assume that the net force exerted by the slab acts symmetrically about the plate boundary, though some pulling force on the overthrust plate appears necessary. While the role of drag forces in the driving mechanism remains poorly constrained, the finite element technique that has been developed may be applied in the future to any specific and realizable flow pattern that

ISSN:8755-1209    

DOI:10.1029/RG017i005p00981

年代:1979

数据来源: WILEY

摘要:

This paper reviews the present state of knowledge concerning the relativistic particles of solar origin. The analytic techniques for obtaining information regarding the distribution of particles in the interplanetary space from ground‐based observations are described. The general features of pitch angle anisotropy of high‐energy particles are outlined. The concepts of relativistic particle propagation in the corona and interplanetary space are discussed with special reference to the relevant new ideas that have emerged from studies of energetic particle events. The experimental and theoretical arguments that indicate that particles can be accelerated to high energies by shock waves in the heliomagnetosphere are reviewed. Finally, a brief summary of the well‐established ideas and unsolved problems is pres

ISSN:8755-1209    

DOI:10.1029/RG017i005p01021

年代:1979

数据来源: WILEY

摘要:

Carbonaceous chondrites, generally considered to be the most primitive surviving materials from the early solar system, form a distinctive group in terms of bulk Mg/Si, Ca/Si, and Al/Si ratios. The carbonaceous chondrites can be subdivided into five groups (CI, CM, CR, CO, and CV) based on a number of petrologic and chemical criteria. Petrographic observations indicate that most carbonaceous chondrites have been processed, either by thermal metamorphism in the case of CO and CV chondrites or by low‐temperature aqueous alteration in the case of CI, CM, and CR chondrites. Thermal metamorphism resulted in Fe/Mg exchange between chondrules, olivine and pyroxene grains, and matrix, changes in the compositions of metal grains, and textural integration. Aqueous alteration probably produced hydrated phyllosilicate matrix phases and resulted in alteration of chondrules and replacement and vein filling by secondary carbonates and sulfates. The changes incurred during these processes appear to have been largely isochemical. However, if certain constituents behaved as open‐system components, volatile elements or compounds may have been depleted during metamorphism, and isotopic patterns may have been changed during aqueous alteration. The recognition of two different types of postaccretional processes resulting in petrological modifications necessitates a reinterpretation of the classification system for carbonaceous chondri

ISSN:8755-1209    

DOI:10.1029/RG017i005p01059

年代:1979

数据来源: WILEY