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Roland A. Madden and Paul R. Julian

Abstract

Observational aspects of the 40–50-day oscillation are reviewed. The oscillation is the result of large-scale circulation cells oriented in the equatorial plane that move eastward from at least the Indian Ocean to the central Pacific. Anomalies in zonal winds and the velocity potential in the upper troposphere often propagate the full circumference of the globe. Related, complex convective regions also show an eastward movement. There is a zonally symmetric component to the oscillation. It is manifest in changes in surface pressure and in the relative atmospheric angular momentum. The oscillation is an important factor in the timing of active and break phases of the Indian and Australian monsoons. It affects ocean waves, currents, and air-sea interaction. The oscillation was particularly active during the First GARP (Global Atmospheric Research Program) Global Experiment year, and some features that were evident during the Monsoon Experiment are described.

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John Molinari and Michael Dudek

Abstract

Current approaches for incorporating cumulus convection into mesoscale numerical models are divided into three groups. The traditional approach utilizes cumulus parameterization at convectively unstable points and explicit (nonparameterized) condensation at convectively stable points, The fully explicit approach uses explicit methods regardless of stability. The hybrid approach parameterizes convective scale updrafts and downdrafts, but “detrains” a fraction of parameterized cloud and precipitation particles to the grid scale. This allows the path and phase changes of such particles to be explicitly predicted over subsequent time steps.

The traditional approach provides the only alternative for numerical models with grid spacing too large to resolve mesoscale structure. As grid spacing falls below 50 km, the traditional approach becomes increasingly likely to violate fundamental scale-separation requirements of parameterization, particularly if mesoscale organization of convection is parameterized as well. The fully explicit approach has no such limits, but it has repeatedly failed in mesoscale models in the presence of large convective instability. Although it is preferable under certain specialized circumstances, the fully explicit approach cannot provide a general solution for models with grid spacing above 5–10 km.

The hybrid approach most cleanly separates convective-scale motions from the slow growth, fallout, and phase changes of detrained hydrometeors that produces mesoscale organization of convection. It is argued that this characteristic removes the need to parameterize the mesoscale and thus reduces the scale-separation problems that may arise when the traditional approach is used. The hybrid approach provides in principle the preferred solution for mesoscale models, though such promise has yet to be fully realized.

In the absence of large rotation, the fundamental assumptions of cumulus parameterization begin to break down once grid spacing falls below 20–25 km. For models with such resolution, the time scale of the convection being parameterized approaches the characteristic time scale of the grid, and parameterized and unparameterized convective clouds often exist simultaneously in a grid column. Under such ambiguous circumstances, successful simulations have been produced only because parameterized convection rapidly gives way in the, model to its grid-scale counterpart. It is essential to understand the interactions between implicit and explicit clouds that produce this transition, and whether they represent physical processes in nature, before cumulus parameterization can be widely used in such high-resolution models. In a broader sense, more detailed analysis of why convective parameterizations succeed and fall is needed.

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Andrew Staniforth and Jean Côté

Abstract

The semi-Lagrangian methodology is described for a hierarchy of applications (passive advection, forced advection, and coupled sets of equations) of increasing complexity, in one, two, and three dimensions. Attention is focused on its accuracy, stability, and efficiency properties. Recent developments in applying semi-Lagrangian methods to 2D and 3D atmospheric flows in both Cartesian and spherical geometries are then reviewed. Finally, the current status of development is summarized, followed by a short discussion of future perspectives.

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William H. Raymond and Arthur Garder

Abstract

Low- and high-pass traditional recursive and implicit filters are reviewed. Some similarities and differences between these two forms are illustrated. The use of recursive filters in signal processing is contrasted with the needs in meteorology. The standard techniques used in building a recursive filter with specified characteristics are described. The desirability of high-order calculations is demonstrated. Some numerical results are presented to illustrate the differences in filter selectivity in the presence of topography. To make the implicit filters competitive with the traditional recursive formalism, efficient numerical matrix inversion procedures are employed in the application of both limited area and cyclic boundary conditions.

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Kuo-Nan Liou

Abstract

Current understanding and knowledge of the composition and structure of cirrus clouds are reviewed and documented in this paper. In addition, the radiative properties of cirrus clouds as they relate to weather and climate processes are described in detail. To place the relevance and importance of cirrus composition, structure and radiative properties into a global perspective, we present pertinent results derived from simulation experiments utilizing models with varying degrees of complexity, which have been carried out for the investigation of the influence of cirrus clouds on the thermodynamics and dynamics of the atmosphere. In light of these reviews, suggestions are outlined for cirrus-radiation activities aimed toward the development and improvement of weather and climate models for a physical understanding of cause and effect relationships and for prediction purposes.

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Scott R. Fulton, Paul E. Ciesielski, and Wayne H. Schubert

Abstract

Multigrid methods solve a large class of problems very efficiently. They work by approximating a problem on multiple overlapping grids with widely varying mesh sizes and cycling between thew approximations, using relaxation to reduce the error on the scale of each grid. Problems solved by multigrid methods include general elliptic partial differential equations, nonlinear and eigenvalue problems, and systems of equations from fluid dynamics. The efficiency is optimal: the computational work is proportional to the number of unknowns.

This paper reviews the basic concepts and techniques of multigrid methods, concentrating on their role as fast solvers for elliptic boundary-value problems. Analysis of simple relaxation schemes for the Poisson problem shows that their slow convergence is due to smooth error components; approximating these components on a coarser grid leads to a simple multigrid Poisson solver. We review the principal elements of multigrid methods for more general problems, including relaxation schemes, grids, grid transfers, and control algorithms, plus techniques for nonlinear problems and boundary conditions. Multigrid applications, current research, and available software are also discussed.

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Daniel Keyser and M. A. Shapiro

Abstract

This article presents a review of upper-level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves. The discussion begins by tracing present-day concepts concerning the structure of upper-level frontal systems, which are based on composite analyses of radiosonde and aircraft data, from their origins in the pioneering analyses of upper-air data in the 1930s. Perspectives from scales both smaller and larger than upper-level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper-level frontogenesis.

The dynamics of upper-level fronts are shown to comprise the interactions between the primary (geostrophic) and secondary (ageostrophic) circulations. To elucidate the mechanisms and feedbacks contributing to the evolution of upper-level fronts in relation to their setting within baroclinic waves, the two-dimensional theory of forced secondary circulations in the cross-front plane developed by Sawyer and Eliassen is presented and interpreted, and theoretical and numerical examples of the formation of upper-level fronts in idealized two-dimensional flows are reviewed. In the three-dimensional case, the presence of along-front ageostrophic circulations superimposed upon the cross-front ageostrophic circulations treated by the two-dimensional theory is discussed in terms of the gradient wind. The relative contribution of the along-front ageostrophic circulation to upper-level frontogenesis is considered in the context of the results from three-dimensional β-plane channel models of baroclinic wave growth.

Directions for future observational, diagnostic and theoretical investigation are identified, including the scale interactions between upper-level fronts, their environmental baroclinic waves and related low-level cyclones, and between upper-level fronts and mesoscale convective systems. The review concludes with a discussion of the potential role of recent innovations in remote-sensing technology and trends in numerical weather prediction using mesoscale models in motivating continuing interest and future advances in frontal research.

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T. N. Krishnamurti

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This paper presents a short summary of the Summer Monsoon Experiment (MONEX). The review is largely based on those papers that have made use of the summer MONEX observations during 1979. 0bservational aspects of this study emphasize the annual march of the monsoon rainfall belt from Indonesia to the foothills of the Himalayas, from the northern winter to the northern summer season and a reverse motion thereafter. The excellent FGGE/MONEX data sets have provided a detailed definition of the divergent wind; these are summarized with reference to the Hadley and the Walker circulations.

The manner in which monsoonal circulations respond to the evolving differential heating fields are presented via the mutual interactions among the rotational and divergent wind components. Specific examples of heat sources from the studies of Luo and Yanai highlight their contrast over different regions of the monsoon including the Tibetan Plateau. A problem of considerable interest in this context is the cooling of the Arabian Sea. A summary of results pertaining to this problem—especially the distribution of the wind stress curl—is highlighted.

The planetary boundary layer is another area of investigation which has drawn much interest, especially over the western Arabian Sea where the Somali jet exhibits interesting properties during summer monsoon. These studies cover modeling, theoretical and observational areas.

The onset and active monsoons were monitored by a large array of ship and research aircraft during MONEX. Studies in this area place an emphasis on observational, theoretical stability analysis and numerical weather prediction. The major results with respect to medium range prediction of the onset of monsoon and the formation and motion of a monsoon depression are summarized in the review.

A component of the MONEX observational program that is examined is the structure and maintenance of desert heat lows. A summary of these results includes the structure of the mixed layer, the day-night differences in the vertical motion profiles and the thermodynamic heat budget.

The final section of this review includes studies on low frequency modes—especially on the time scale of 30 to 50 days. It is becoming apparent that modulations of active and inactive spells of the monsoon are related to wave motions on this time scale. These MONEX data sets provide a strong signal for monitoring these waves. These wave motions on the planetary scale move eastward; on a more regional scale they move northward over the monsoon region. Their behavior is illustrated with respect to the onset, active and break monsoons.

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Graeme L. Stephens

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This paper presents a review of the various methods used to compute both the fluxes and the rate of heating and/or cooling due to atmospheric radiation for use in numerical models of atmospheric circulation. The paper does not follow, step by step, the solution to the relevant radiative transfer problem but rather concentrates on providing the reader with the physical basis underlying the various methods. The paper discusses, separately, the various parameterizations for the absorptions by water vapor, carbon dioxide and ozone and for the scattering and absorption associated with cloud (and hazes) and also provides some indication of their accuracy.

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William M. Frank

Abstract

In the past two decades there has been extensive research into the nature of atmospheric convection and scale interactions in cumulus regimes. A major goal of these efforts has been to advance the state of the art in cumulus parameterization. This paper reviews the cumulus parameterization problem in terms of fundamental principles, goals and dynamics constraints as they apply to parameterization in mesoscale and large scale numerical models. Several popular current schemes are discussed in terms of their relationships to these overall aspects of the problem.

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