摘要:

Nearly 50 coincident seismic reflection/refraction studies to depths of at least the Moho provide an improved understanding of the continental lithosphere. Some conclusions include the following: (1) A transparent upper crust, a common observation on vertical reflection profiles, cannot generally be correlated with velocity gradients or low‐velocity zones. Rather, a commonly transparent upper crust may be explained by short‐wavelength, steeply dipping features in the brittle upper crust and to a lesser degree by signal contamination from source‐generated noise. (2) The reflective lower crust in extensional terranes appears to be characterized by a high average seismic velocity (6.6–7.3 km/s) and to consist of laminated high‐ and low‐velocity layers with typical thicknesses of 100–200 m. (3) Landward dipping reflectors observed in the middle to lower crusts of convergent zones have been identified as paired high‐ and low‐velocity slabs which represent oceanic crust and mantle accreted via underplating to the continental margin. (4) The crust‐mantle boundary may differ sufficiently when imaged with vertical incidence and wide‐angle data to justify the retention, for the present, of the concept of separate reflection and refraction Mohos. While there is good evidence that these features are coincident within measurement uncertainties in most regions, recently recorded data from the Basin and Range admit the possibility for noncoincidence in that area. (5) Upper mantle reflections which cannot be migrated into the lower crust remain rare, despite isolated unequivocal examples. Thus the upper mantle appears to be relatively homogeneous at seismic reflection wavelengths and to lack the laminations inferred for the lower crust. The wide‐angle method will likely provide the most reliable information on the velocity structure and physical state of this portion of the lithosphere for some years to come. (6) There appear to be clear and consistent basic differences between convergent and extensional terranes which have been identified from coincident experiments; these differences may be sufficiently universal to infer the tectonic history of poorly exposed terranes. (7) No truly three‐dimensional coincident experiment (i.e., including three‐dimensional migration) has been conducted, but some three‐dimensional data have been collected using both methods. Measurements of attenuation, Poisson's ratio, and anisotropy within the crust using coincide

ISSN:8755-1209    

DOI:10.1029/RG025i004p00723

年代:1987

数据来源: WILEY

摘要:

In terms of effects on the stratosphere, the 1982 eruption of the Mexican volcano El Chichón (17°N) is believed to have been one of the two or three greatest in the past century. In terms of providing opportunities for testing physical and chemical models in the stratosphere, it is unprecedented. Earlier recent eruptions (Fuego in 1974, St. Helens in 1980, and Alaid in 1981) were not of the magnitude required to investigate the more subtle chemical and concomitant radiative and dynamical consequences, and while Agung in 1963 may have been, the necessary measurements are absent. Thus for the first time, trace chemical species such as HCℓ and OH were observed to increase, and other species, possibly related to these, were observed to decrease (O3, NO, and NO2) following the eruption. Volcanic SO2was observed from space for the first time, and its conversion into H2SO4, vapor, condensation nuclei, and finally, sulfuric acid aerosol were monitored closely by both in situ and remote measurements. In this manner the very uncertain reaction sequence, which converts SO2to H2SO4could be ascertained. The optical depth of about 0.3, produced by the sulfuric acid aerosol which formed after the eruption, ranks it with the largest in recent history (for example, Krakatau in 1883). At least three factors contributed to the severity of the eruption aftermath. These include the apparently large fraction of sulfur involved for an eruption of only moderate proportions, the latitude of the volcano, and the time of year of the eruption. As in the case of the eruption of Agung (8°S) in 1963, the stratosphere was observed to warm, especially in equatorial regions. However, the expected surface cooling did not apparently occur. A highly enhanced El Niño, which is followed by tropospheric warming, may have acted to counterbalance the surface cooling in the case of El Chichón. Evidence that enhanced El Niño events usually accompany major low‐latitude eruptions suggests a volcanic effect on global atmospheric transport, thus providing a climatic self‐hea

ISSN:8755-1209    

DOI:10.1029/RG025i004p00743

年代:1987

数据来源: WILEY

摘要:

The first assessments of the potential climatic effects of increased CO2were performed using simplified climate models, namely, energy balance models (EBMs) and radiative‐convective models (RCMs). A wide range of surface temperature warming has been obtained by surface EBMs as a result of the inherent difficulty of these models in specifying the behavior of the climate system away from the energy balance level. RCMs have given estimates of ΔTsfor a CO2doubling that range from 0.48° to 4.2°C. This response can be characterized by ΔTs= ΔRTG0/(1 ‐f), where ΔRTis the radiative forcing at the tropopause due to the CO2doubling (∼4 W m−2),G0is the gain of the climate system without feedbacks (∼0.3°C/(W m−2)), andfis the feedback. The feedback processes in RCMs include water vapor feedback (fis 0.3 to 0.4), moist adiabatic lapse rate feedback (fis −0.25 to −0.4), cloud altitude feedback (fis 0.15 to 0.30), cloud cover feedback (fis unknown), cloud optical depth feedback (fis 0 to −1.32), and surface albedo feedback (fis 0.14 to 0.19). However, these feedbacks can be predicted credibly only by physically based models that include the essential dynamics and thermodynamics of the feedback processes. Such physically based models are the general circulation models (GCMs). The earliest GCM simulations of CO2‐induced climate change were performed without the annual insolation cycle. These “annual mean” simulations gave for a CO2doubling a global mean surface air temperature warming of 1.3° to 3.9°C, an increase in the global mean precipitation rate of 2.7 to 7.8%, and an indication of a soil moisture drying in the middle latitudes. The first GCM simulation of the seasonal variation of CO2‐induced climate change was performed for a CO2quadrupling and obtained annual global mean surface temperature and precipitation changes of 4.1°C and 6.7%, respectively. Substantial seasonal differences in the CO2‐induced climate changes were found, especially in polar latitudes where the warming was maximum in winter and in the middle latitudes of the northern hemisphere where a soil moisture desiccation was found in summer. Recently, three CO2‐doubling experiments have been performed with GCMs that include the annual insolation cycle. These seasonal simulations give an annual global mean warming of 3.5° to 4.2°C and precipitation increases of 7.1 to 11%. These changes are approximately twice as large as those implied for a CO2doubling by the earliest seasonal simulation, apparently as a result of a positive cloud feedback. The geographical distributions of the CO2‐induced warming obtained by the recent simulations agree qualitatively but not quantitatively. Furthermore, the precipitation and soil moisture changes do not agree quantitatively and even show qualitative differences. In particular, the summertime soil moisture drying in middle‐latitudes is simulated by only one of the GCMs. In order to improve the state of the art in simulating the equilibrium climatic change induced by increased CO2concentrations, it is recommended first that the contemporary GCM simulations be analyzed to determine the feedback processes responsible for their differences and second that the parameterization of these processes in the GCMs be validated ag

ISSN:8755-1209    

DOI:10.1029/RG025i004p00760

年代:1987

数据来源: WILEY

摘要:

This is an analysis of diffusion of a scalar field by molecular transport and isotropic turbulence. Existing results are surveyed, and some new results are advanced. The discussion is supported with oceanographic and atmospheric observations of dispersion and diffusion. The existing results were originally obtained using a variety of mathematical techniques. However, all results are derived here using an approximate solution of the Lagrangian form of the advection‐diffusion equation. The approximation is equivalent to neglecting the spatial dependence of the transformation factors in the Lagrangian representation of the molecular flux divergence. Examinations of the diffusive subranges show the approximation to justified: infinitesimal line stretching is either controlled by relatively large scale shears (viscous‐diffusive subrange at large Prandtl number) or else is negligible during the diffusion process (inertia‐diffusive subrange at small Prandtl number). Estimation of scalar mean fields, total variances, and wave number spectra requires, in general, joint statistics of infinitesimal line stretching and either single particle displacement or particle pair separation. Normality is assumed for displacement statistics; separation statistics are determined from the Richardson‐Kraichnan equation. A simple derivation of that equation is presented here. Joint stretching‐separation statistics are modeled by a uniform shear flow, with time‐dependent amplitudes described by the Wiener process (white noise). With the possible exception of this random process, the only mathematics required here is elementary calculus, so details have been kept to a minimum. In the diffusion problems considered here, the turbulence is isotropic. However, both the approximate solution of the advection‐diffusion equation and the equations for joint displacements are equally valid for inhomogeneo

ISSN:8755-1209    

DOI:10.1029/RG025i004p00799

年代:1987

数据来源: WILEY