Journal of Atmospheric and Terrestrial Physics最新文献

This tutorial review describes some possible future scenarios for changes in temperature and water vapor in the mesosphere-lower thermosphere (MLT) region (50–100 km). The structure and dynamics of this region are controlled by physical processes, some of which are very different than in the lower atmosphere, such as gravity-wave breaking, radiative transfer in non-local thermodynamic equilibrium and airglow cooling. The couplings between the various atmospheric properties are illustrated by the use of a 2D zonally-symmetric model ranging from 16 to 120 km. The importance of temperature and water vapor for the occurrence and scattered brightness of mesospheric clouds (at a height of about 83 km) is described in terms of their influence on nucleation, growth and sedimentation of ice particles. At the cold mesopause at high latitude, IR effects would warm the region without dynamical feedbacks, which in the 2D model to be described, cause a net cooling at all latitudes and seasons. The effects of a future doubling of carbon dioxide and methane (and a past halving) are examined by means of the same 2D model. All models predict a future lowering of temperature throughout much, if not all of the MLT region, as a result of enhanced IR cooling and dynamical feedbacks. The rise of methane will lead to an enhancement of water vapor concentrations throughout the upper atmosphere. The cloud existence region, defined in terms of water-ice saturation, is predicted to extend to lower-latitude, high population areas in the future. In a glacial-era scenario, the existence region is found to be confined to a small region near the summertime polar mesopause. Over the past century, with a doubling of methane and a 30% increase in carbon dioxide, the mesospheric cloud existence region may, have advanced from near the pole to its current location inside the 50°–90° latitude zone. The uncertainties in current models and need for further studies are discussed.

Space weather is a term which refers to the dynamic, highly variable conditions in the geospace environment. This includes conditions on the sun, in the interplanetary medium, and in the magnetosphere-ionosphere-thermosphere system. Adverse changes in the near-Earth space environment can diminish the performance and reliability of both spacecraft and ground-based systems. This, in turn, can spell major losses due to communication, navigation, power system, and reconnaissance satellite operational problems. This paper discusses some of the principal adverse space environmental effects presently known including trapped magnetospheric radiation, solar energetic particles, geomagnetic storms, and magnetospheric substorms. Methods for predicting changes in space weather are also considered. It is concluded that practical and effective methods of predicting and mitigating space weather effects are close at hand.

Numerous mechanisms have been proposed to account for the dissipation and saturation of gravity waves in the atmosphere. We review the leading wave dissipation paradigms and identify the experimental data required to test definitively the fundamental physics upon which these theories are based. We also examine the separability of the joint vertical wave number (m) and temporal frequency (ω) spectrum and the separability of the unambiguous two-dimensional horizontal wave number spectrum. Definitive tests of the Linear Instability (Dewan and Good, 1986), Saturated-Cascade (Dewan, 1994), and Diffusive Filtering Theories (Gardner, 1994) and separability of the (m, ω) spectrum are within the observational capabilities of modern remote sensing instruments. The Diffusive Damping (Weinstock, 1990) and Doppler Spreading Theories (Hines, 1991) are untestable in their present forms. Separability of the two-dimensional horizontal wave number spectrum is difficult to test using current technology, although analysis of airglow images may provide some insight.

Weather disturbances in the ionosphere-thermosphere system can have a detrimental effect on both ground-based and space-based systems. Because of this impact and because our field has matured, it is now appropriate to develop specification and forecast models, with the aim of eventually predicting the occurrence, duration, and intensity of weather effects. As part of the new National Space Weather Program, the CEDAR community will focus on science issues concerning space weather, and this tutorial/review is an expanded version of a tutorial presentation given at the recent CEDAR annual meeting. The tutorial/review provides a brief discussion of weather disturbances and features, the causes of weather, and the status of weather modeling. The features and disturbances discussed include plasma patches, boundary and auroral blobs, sun-aligned polar cap arcs, the effects of traveling convection vortices and SAID events, the lifetime of density structures, sporadic E and intermediate layers, spread F and equatorial plasma bubbles, geomagnetic storms and substorms, traveling ionospheric disturbances (TID's), and the effects of tides and gravity waves propagating from the lower atmosphere. The tutorial/review is only intended to provide an overview of some of the important scientific issues concerning ionospheric-thermospheric weather, with the emphasis on the ionosphere. Tutorials on thermospheric and magnetospheric weather issues are given in companion papers.

This paper reviews the current state of comprehensive, three-dimensional, time-dependent modelling of the circulation in the middle and upper atmosphere from a meteorologist's perspective. The paper begins with a consideration of the various components of a comprehensive model (or general circulation model, GCM), including treatments of processes that can be explicitly resolved and those that occur on scales too small to resolve (and that must be parameterized). The typical performance of GCMs in simulating the tropospheric climate is discussed. Then some important background on current ideas concerning the general circulation of the stratosphere and mesosphere is presented. In particular, the transformed-Eulerian mean flow formalism, the role of vertically-propagating internal gravity waves in driving the large-scale circulation, and the notion of a stratospheric surf zone are all briefly reviewed. Using this background as a guide, some middle atmospheric GCM results are discussed, with a focus on simulations made recently with the GFDL ‘SKYHI’ troposphere-stratosphere-mesosphere GCM. The presentation attempts to emphasize the interaction between theory and comprehensive modelling. Many theoretical notions cannot be confirmed in detail from observations of the real atmosphere due to the various limitations in the observational methods, but can be very completely examined in GCMs in which every atmospheric variable is known perfectly (within the limits of the numerical methods). It will be shown that our understanding of both the role of gravity waves in the general circulation and the nature of the stratospheric surf zone has benefited from analysis of GCM results.

From the point of view of the upper atmosphere, one of the most interesting aspects of GCMs is their ability to generate a self-consistent field of upward-propagating gravity waves. This paper concludes with a discussion of the gravity wave field in the middle atmosphere of GCMs. Comparisons of the explicitly-resolved gravity wave field in the SKYHI model with observations are quite encouraging, and it seems that the model is capable of producing a gravity wave field with many realistic features. However, the simulated horizontal spectrum of the eddy momentum fluxes associated with the waves is quite shallow, suggesting that much of the spectrum that is important for maintaining the mean circulation is not explicitly resolvable in current GCMs. A brief discussion of current efforts at parameterizing the mean flow effects of the unresolvable gravity waves is presented.

The complex demodulation as a spectral technique has been used for the quasi-continuous determination of the actual frequencies of Schumann resonances. Applying this method, the first three modes of the vertical electric component have been measured regularly in the Nagycenk Observatory (47.6°N, 16.7°E) since May 1993.

Stratospheric temperatures were measured by lidar at McMurdo station, Antarctica (78°S, 167°E) during two late spring months (September-October) in 1991 and 1992, and during the period March-October in 1993 and 1994. The stratosphere was found to be quite active, with one major and several minor warmings occurring in 1993 and 1994, and showing the expected behaviour of a distinct region of high temperatures, formed in the polar mesosphere, descending with time and warming the stratopause region. A relative maximum of the stratopause temperature was observed in July 1994, and differences between two years in terms of the time development of average temperature in the different stratospheric layers and in terms of the average temperature variability over single months are pointed out. Monthly mean temperature profiles determined from lidar observations are compared with a reference atmosphere (CIRA86). Fair agreement, with discrepancies less than ±4 K, in June, July and August in the middle stratosphere and just above the stratopause was found.

We attempt to find the northern hemisphere zonal wavenumber for a striking quasi-2-day wave “event” or “burst” observed near 90 km altitude in the summer of 1992. A unique set of data on the upper atmosphere from nine radar sites is analysed (spacings ∼400– ∼ 12,000 km), and compared with expectations from models. The 2-day wave phase comparison, which finds zonal wavenumber m = 4, is conclusive. Determination of n, which defines the meridional wave amplitude structure, is not attempted, as the sites here have only a small latitude spread (21°N to 55°N). Also the amplitude seems to be unstable showing some sort of modulation which is not simultaneous at all sites. Finally, the radars have not been “calibrated” against each other in terms of wind speed. This calibration would have to be done before small differences in wave amplitude could be believed. A similar event in 1991 for which fewer sites are available is also discussed. Here the choice between m = 3 and 4 is not as clear.

During the normal electrojet period, a solar flare produces a positive change in the horizontal (H) field, negative changes in the eastward (Y) field and a negative change in the vertical (Z) field at a northern electrojet station. On average, the ΔY is about 40% of ΔH. During a counter electrojet period, ΔH, due to a solar flare, is negative and ΔY and ΔZ are positive. During a partial counter electrojet period, ΔH may be smaller at equatorial stations compared with other low latitude stations, and ΔY may be positive, or sometimes of very small magnitude. The observed change of ΔY at an electrojet station is suggested to be the combined effect of the flare on the associated Sq current system and on electrojet related meridional currents. These data confirm the seat of the equatorial meridional current to be in the ionospheric E layer.

Ionospheric electron content was monitored from sites in or near Tromsø, Norway, for six months of 1993, using the transmissions from the satellites of the Global Positioning System (GPS). The data have been used for preliminary studies of two important phenomena of the high-latitude ionosphere: the main trough and the incidence of large irregularities. The latitude and motion of the trough were determined on several occasions during the spring period, and the results compared with previous data. Best agreement is with the formula of Collis and Haggstrom (1988). The incidence of large irregularities was surveyed during a four-month period, approximately from the summer solstice to the autumnal equinox, and the variation with time of day and magnetic activity has been determined. It was found that irregularities are considerably larger by night than by day, but that they are enhanced during both periods by increased magnetic activity. Statistical results are presented. It is suggested that these irregularities are the same as the “auroral blobs” previously studied by incoherent-scatter radar.