摘要:

This paper is a review of the observational, experimental, theoretical, and numerical studies of mesoscale shallow convection (MSC) in the atmosphere. Typically, MSC is 1 to 2 km deep, has a horizontal length scale of a few to a few tens of kilometers, and takes distinctive planforms: linear and hexagonal. The former is called a cloud street, roll, or band, while the latter is called mesoscale cellular convection (MCC), comprising three‐dimensional cells. MSC is characterized by its shape, horizontal extent, convective depth, and aspect ratio. The latter is the ratio of the horizontal extent to that in the vertical. For cells the horizontal extent is their diameter, whereas for rolls it is their spacing. Rolls usually align along or at angles of up to 10° from the mean horizontal wind of the convective layer, with lengths from 20 to 200 km, widths from 2 to 10 km, and convective depths from 2 to 3 km. The typical value of aspect ratio ranges from 2 to 20. Rolls may occur over both water surface and land surfaces. Mesoscale convective cells may be divided into two types: open and closed. Open‐cell circulation has downward motion and clear sky in the cell center, surrounded by cloud associated with upward motion. Closed cells have the opposite circulation. Both types of cell have diameters ranging from 10 to 40 km and aspect ratios of 5 to 50, and both occur in a convective layer with a depth of about 1 to 3 km. Both the magnitude and direction of horizontal wind in the convective layer change little with height. MSC results from a complex and incompletely understood mix of processes. These processes are outlined, and their interplay is examined through a review of theoretical and laboratory analyses and numerical modeling of

ISSN:8755-1209    

DOI:10.1029/96RG02623

年代:1996

数据来源: WILEY

摘要:

The scaling properties of earthquake populations show remarkable similarities to those observed at or near the critical point of other composite systems in statistical physics. This has led to the development of a variety of different physical models of seismogenesis as a critical phenomenon, involving locally nonlinear dynamics, with simplified rheologies exhibiting instability or avalanche‐type behavior, in a material composed of a large number of discrete elements. In particular, it has been suggested that earthquakes are an example of a “self‐organized critical phenomenon” analogous to a sandpile that spontaneously evolves to a critical angle of repose in response to the steady supply of new grains at the summit. In this stationary state of marginal stability the distribution of avalanche energies is a power law, equivalent to the Gutenberg‐Richter frequency‐magnitude law, and the behavior is relatively insensitive to the details of the dynamics. Here we review the results of some of the composite physical models that have been developed to simulate seismogenesis on different scales during (1) dynamic slip on a preexisting fault, (2) fault growth, and (3) fault nucleation. The individual physical models share some generic features, such as a dynamic energy flux applied by tectonic loading at a constant strain rate, strong local interactions, and fluctuations generated either dynamically or by fixed material heterogeneity, but they differ significantly in the details of the assumed dynamics and in the methods of numerical solution. However, all exhibit critical or near‐critical behavior, with behavior quantitatively consistent with many of the observed fractal or multifractal scaling laws of brittle faulting and earthquakes, including the Gutenberg‐Richter law. Some of the results are sensitive to the details of the dynamics and hence are not strict examples of self‐organized criticality. Nevertheless, the results of these different physical models share some generic statistical properties similar to the “universal” behavior seen in a wide variety of critical phenomena, with significant implications for practical problems in probabilistic seismic hazard evaluation. In particular, the notion of self‐organized criticality (or near‐criticality) gives a scientific rationale for the a priori assumption of “stationarity” used as a first step in the prediction of the future level of hazard. The Gutenberg‐Richter law (a power law in energy or seismic moment) is found to apply only within a finite scale range, both in model and natural seismicity. Accordingly, the frequency‐magnitude distribution can be generalized to a gamma distribution in energy or seismic moment (a power law, with an exponential tail). This allows extrapolations of the frequency‐magnitude distribution and the maximum credible magnitude to be constrained by observed seismic or tectonic moment release rates. The answers to other questions raised are less clear, for example, the effect of the a priori assumption of a Poisson process in a system with strong local interactions, and the impact of zoning a potentially multifractal distribution of epicentres with smooth polygons. The results of some models show premonitory patterns of seismicity which could in principle be used as mainshock precursors. However, there remains no consensus, on both theoretical and practical grounds, on the possibility or otherwise of reliable inte

ISSN:8755-1209    

DOI:10.1029/96RG02808

年代:1996

数据来源: WILEY

摘要:

The North Atlantic Current is a well‐defined western boundary current that flows north along the east side of the Grand Banks from 40° to 51°N, where it turns sharply to the east and begins its journey across the ocean. The current is unique in transporting warm tropical waters to much higher latitudes than any other western boundary current and thus plays a crucial role in ameliorating the climate of the European subcontinent. The North Atlantic Current originates in the Gulf Stream when the latter curves north around the Southeast Newfoundland Rise, a major submarine ridge that stretches SE from the Grand Banks. A well‐defined front delineates the path of the current as long as it flows north as a western boundary current. After the current turns east in the north, it broadens into a widening band of eastward drift without a sharp or permanent front in the sense of the eastward flowing Gulf Stream after it separates from Cape Hatteras. The North Atlantic Current transports more than 40 Sv (1 Sv = 106m³ s−1) in the south and about 20 Sv by the time it flows east across the Mid‐Atlantic Ridge. The currents along the northward flowing front are quite swift, with typical maximum average speeds in the upper 300 m near 1 m s−1(= 2 knots). The current meanders almost as wildly as a “snaking” river, but unlike steep meanders in the Gulf Stream these meanders appear to be stable, and with one exception have not been observed to break off to form pools of warm and/or cold waters as frequently occurs in the Gulf Stream. The meanders appear to be induced by major topographic features along the path of the current, namely, the Southeast Newfoundland Rise, the Newfoundland Seamounts, and Flemish Cap. Strong recirculations develop on the concave side of the meanders. One of these, the “Mann eddy” at the first meander crest of the North Atlantic Current, should be regarded as a permanent feature of the North Atlantic circulation. Other meanders also contain recirculations that can persist for months. Under certain conditions these can merge together to form an extended SW flow (recirculation) just east of the No

ISSN:8755-1209    

DOI:10.1029/96RG02214

年代:1996

数据来源: WILEY

摘要:

Atmospheric loss from planetary atmospheres is an important geophysical problem with implications for planetary evolution. This is a multidisciplinary research field that requires an expertise in a wide range of subjects including statistical mechanics, fluid mechanics, plasma physics, collision theory, and surface science. This paper is a review of the current state of our understanding of atmospheric loss from the terrestrial planets. A detailed discussion is provided of the basic concepts required to understand the processes occurring in the high‐altitude portion of a planetary atmosphere referred to as the exosphere. Light atomic species with sufficient translational energy can escape from an atmosphere. The translational energy required for escape could be thermal energy and proportional to the ambient temperature or the result of some collisional processes energizing the species above thermal energies. These collisional processes, which include charge exchange and dissociative recombination between energetic ions, neutrals, and electrons, are referred to as nonthermal escape processes. We highlight the similarities and differences in the important escape mechanisms on the terrestrial planets and comment on application of these mechanisms to evolutionary theories of the terrestrial atmospheres. The emphasis in this paper is directed toward the need to consider the exosphere as collisiona

ISSN:8755-1209    

DOI:10.1029/96RG02213

年代:1996

数据来源: WILEY