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Murry L. Salby

Abstract

A one to one correspondence between alias-free asynoptic data and twice-daily synoptic maps is established in the Synoptic Retrieval Theorem. The uniqueness follows from an extension of the Sampling Theorem.

A Fast Fourier Transform Scheme is defined for retrieving the unique sequence of synoptic maps from the asynoptic observations. The procedure involves the construction of space-time spectra from “irregular,” combined asynoptic data. This is accomplished by application of the asynoptic form of the space-time transform. The “regular” sequence of synoptic maps is then recovered by application of the synoptic form of the inverse space-time transform. Twice-daily, synoptic sequences retrieved in this fashion, contain exactly the correct spectral contribution resolvable in both types of data. The technique conserves information and fully utilizes the information content of combined, asynoptic observations. Furthermore, it is directly amenable to parallel processing of data over large ensembles of latitudes and pressure levels. Temporal evolution is retrieved equally well for both statistically stationary and nonstationary processes.

Intermediate products of the synoptic inversion are global spectra. Their availability during the procedure allows the removal of latitudinally incoherent noise by low-pass filtering. It also makes possible the expansion of observed fields, or equivalently their wavenumber-frequency components, in arbitrary sets of spherical functions. In particular, the projection of remotely derived geopotential components onto Hough harmonics, facilitates the construction of “global” wind fields, thereby circumventing the equatorial problem characteristic of geostrophic treatments.

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Murry L. Salby

Abstract

The existence of planetary normal modes in the presence of realistic mean fields is examined. For sufficiently large wavenumber m, or meridional index n, the response of the Rossby modes is diffused beyond identification. This is primarily a result of the Doppler shifting of mean winds and supersedes the increasing role of dissipation.

Several initial modes for the first few wavenumbers should be both realizable and identifiable in typical conditions. “At least” the first three modes of wavenumber 1, the first two of wavenumber 2, and the first of wavenumber 3 should occur with periods isolated to within 12.5% of median values. The mode structures for the first four modes of wavenumbers 1, 2 and 3 are insensitive to the mean fields in the lowest two scale heights. In addition, the response of each of these is readily discernable in both equinox and solstice conditions.

The modes' horizontal character is notably robust. Although the solutions typically exhibit regions where they are affected by the mean fields, the domain of influence is local. Vertical growth rates tend to be magnified in regions where the winds are weak westerly relative to the wave or the temperature gradient is equatorward, while amplitudes evanesce in regions of strong westerlies or poleward temperature gradient. The former give rise to enhanced amplitudes in the equinox stratosphere and the summer mesosphere.

Results calculated here for the first symmetric wavenumber 1 mode are in close agreement with those found by Geisler and Dickinson (1976). Moreover, the estimate for the possible spread of variance compares favorably with the 4–6 day range existing in the observational evidence. Calculations for the second symmetric wavenumber 1 mode support Madden's (1978) identification of the 16-day wavenumber 1 disturbance with the (m, nm)=(1, 3) mode. In the presence of uniform surface forcing, the peak response is very near 16 days. More importantly, the estimate of possible spread in variance is compatible with the observed 1–3 week range for the disturbance. Although its structure is largely unaffected in the first few scale heights, the mode attains large amplitudes in the winter stratosphere of the solstice configuration. Finally, a number of observed features of the 2-day wave in the upper atmosphere suggest its identification with the third Rossby-gravity mode, which corresponds well in both temporal and spatial character.

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Murry L. Salby

Abstract

The influence of mean field variations on the realization of planetary normal modes is investigated by examining the mode response and structure in the presence of simple background nonuniformities. Gradients in the mean wind and temperature fields have the collective effect of depressing, shifting and broadening the characteristic response. While nonuniformities in both the wind and temperature fields contribute to the reduction in resonant response, spectral shifting and broadening are induced principally by variations in the mean wind field. The eigenperiods, for the most part, are influenced by the mean winds in the lowest three scale heights.

The characteristic structure is modified through a change in evanescence according to the local index of refraction. In regions of reduced refractive index, for example, strong westerlies relative to the wave or a poleward temperature gradient, the mode's vertical growth rate is retarded. Conversely, in regions of increased refractive index, for example, weak westerlies relative to the wave or an equatorward temperature gradient, the vertical growth of amplitude is enhanced. Amplitudes decay sharply into regions where the winds are easterly relative to the wave (negative refractive index). While the vertical character is sensitive to the mean fields throughout, the horizontal structure is influenced only locally. This robust nature of the eigenstructure prevails in even the most drastic simple configurations, e.g., when absorbing critical surfaces are present.

A general decline in the characteristic response results with increasing mean field variation. The same tendency is observed as the region of nonuniformity is moved to lower levels and lower latitudes. No appreciable change in the global response occurs with the introduction of a critical surface aloft.

The various influences which degrade the resonant response increase with wavenumber m and meridional index n. Hence in realistic conditions, the response of the Rossby modes will necessarily be suppressed and diffused beyond identification for sufficiently large m or n.

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Murry L. Salby and Patrick Callaghan

Abstract

The diurnal cycle present in many climate properties is undersampled in asynoptic data, which, through aliasing, introduces a bias into time-mean behavior derived from satellite measurements. This source of systematic error is investigated in high-resolution Global Cloud Imagery (GCI), which provides a proxy, with realistic space–time variability, for several climate properties to be observed from space. The GCI, which resolves mesoscale and diurnal variability on a global basis, is sampled asynoptically according to orbital and viewing characteristics from one and multiple platforms. Sampling error is then evaluated by comparing the resulting time-mean behavior against the true time-mean behavior in the GCI.

The bias from undersampled diurnal variability is most serious in polar-orbiting measurements from an individual platform. However, it emerges even in precessing measurements, which drift through local time, because diurnal variability is still sampled too slowly to be truly resolved in such observations. A “mean diurnal cycle” can be constructed by averaging precessing measurements, provided that the ensemble of observations at individual local times is large enough (e.g., that observations are averaged over a long enough duration). The pattern of time-mean error closely resembles the pattern of error in the mean diurnal cycle. Time-mean behavior can therefore be determined only about as accurately as can the mean diurnal cycle. Determining accurate time-mean properties often requires averaging measurements from an individual platform over several months, which cannot be performed without contaminating mean behavior with seasonal variations. The sampling limitations from an individual orbiting platform are alleviated by sampling from multiple platforms, which provide observations frequently enough in space and time to determine accurate monthly mean properties.

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Gene L. Francis and Murry L. Salby

Abstract

Temperatures over the Antarctic plateau are sharply colder than those over its maritime surroundings. The sharp temperature contrast due to Antarctica is conveyed upward through 9.6-μm absorption by ozone, which shapes the thermal structure in the stratosphere. The radiative impact of Antarctica on the polar stratosphere is investigated in three-dimensional integrations of the nonlinear primitive equations, coupled to a full radiative-transfer calculation that is performed with and without clouds. Cooling associated with Antarctica depresses radiative-equilibrium temperatures by as much as 10 K. This direct radiative influence emerges clearly at high latitudes of the lowermost stratosphere. It is accompanied elsewhere by temperature changes of opposite sign, which result indirectly through adiabatic warming by the induced residual meridional circulation. Collectively, these influences reinforce the polar-night vortex, shift the jet axis poleward, and intensify downward transport over the polar cap by the residual circulation. In this way, radiative forcing from below contributes significantly to the features that distinguish the Antarctic vortex from the Arctic vortex.

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John W. Bergman and Murry L. Salby

Abstract

The spectrum of equatorial wave activity propagating vertically into the stratosphere is calculated from high-resolution imagery of the global convective pattern. Synoptic Global Cloud Imagery (GCI), constructed from six satellites simultaneously observing the earth, is used to diabatically force the linearized primitive equations. Having resolution of 0.5 deg and 3 h, that imagery captures the dominant scales of organized convection, including several harmonics of the diurnal cycle. Its global coverage with high space–time resolution allows the GCI to represent heating variability and dynamical behavior excited by it over a wide range of scales.

The dynamical response above the heating is evaluated globally in terms of a space–time spectrum of Hough modes, one which includes planetary-scale Kelvin waves, Rossby waves, and gravity waves down to the resolution of the GCI. The geopotential response, which is indicative of temperature fluctuations observed by satellite, is very red in frequency. Therefore, planetary-scale waves with periods longer than two days dominate the spectrum of geopotential, while high-frequency gravity waves make a comparatively small contribution. Some 80% of the geopotential variance is accounted for by the Kelvin and gravest-symmetric Rossby modes, while the Rossby–gravity mode is comparatively weak. In horizontal eddy motion, the excited wave spectrum is still dominated by planetary-scale components. However, meridional wind fluctuations associated with the Rossby–gravity mode have variance comparable to that of zonal wind fluctuations associated with the Kelvin mode, even though the Rossby–gravity mode is nearly invisible in the geopotential response. Estimates of tropospheric heating lead to amplitudes and propagation characteristics that are broadly consistent with satellite and radiosonde observations of wave activity in the lower stratosphere.

The space–time spectrum of EP flux is significantly whiter than the response in either geopotential or motion. Gravity waves of small scale and high frequency carry a large fraction of the upward flux. Although it dominates eastward variance of geopotential and motion, the Kelvin mode carries only about 50% of the eastward EP flux at phase speeds of 20–40 m s−1 and only 35% of the total eastward flux transmitted to the stratosphere. The remainder is carried by the gravity wave spectrum, which carries nearly all of the westward flux at phase speeds greater than 20 m s−1. The gravity wave spectrum also contributes significantly at phase speeds of 10–20 m s−1, where only 25% of the flux is accounted for by zonal wavenumbers less than 20. The broad nature of the gravity wave spectrum suggests its absorption at critical levels will be distributed over a deep layer of the middle atmosphere.

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Murry L. Salby and Patrick F. Callaghan

Abstract

Recent evidence points to a decadal modulation of the quasi-biennial oscillation (QBO), one that varies with the 11-yr cycle of UV irradiance and ozone heating in the upper stratosphere. Interaction between the QBO and the Hadley circulation is considered here through an analysis that accounts for cyclic variations in their relationship, which may cancel and, hence, be invisible in the long-term average.

The analysis reveals coherent changes in the tropical stratosphere and troposphere. Involving periods shorter than 5 yr, their relationship manifests itself in major properties associated with the QBO and the Hadley circulation. Like the QBO’s relationship to the polar stratosphere, its relationship to the Hadley circulation reverses on the time scale of a decade. The systematic swing in their relationship leads to two important implications: 1) Interannual changes of one circulation operate coherently with changes of the other, reflecting their interaction. 2) At least one is influenced by a decadal variation. The latter is interpreted in light of the cyclic variation of ozone heating in the upper stratosphere, where the phase of the QBO is set.

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John W. Bergman and Murry L. Salby

Abstract

The ISCCP-C2 cloud climatology is used to describe the three-dimensional structure of cloud diurnal variations and to investigate their relationship to local climatological conditions. The latter follows from the regression of diurnal components onto climatological state variables.

Four important diurnal cloud categories are identified. The diurnal variation of maritime high-cloud fraction Chi maximizes at 1700 local solar time (LST) and is strongest over maritime convective locations where the mean high-cloud fraction is C hi > 0.1. The diurnal variation of maritime low-cloud fraction maximizes at 0400 LST and is strong over maritime nonconvective locations where C hi < 0.1. Diurnal variations of high-cloud fraction (persistent during the night, minimum at 1100 LST) and low-cloud fraction (1300 LST maximum) are strong over all continental locations in the latitude band 40°S–40°N.

In each cloud category, most of the diurnal amplitude and phase at individual locations is explained by the regression of diurnal amplitude onto only three climatological state variables. For most categories, the diurnal amplitude has its strongest relationship with mean cloud fraction. The relationship between relative diurnal amplitude (amplitude divided by the mean) and other climatological properties is then particularly meaningful. The relative amplitude of maritime high-cloud fraction is related to the mean total-cloud fraction and the noon-time solar zenith angle, which measures the solar diurnal amplitude. The diurnal amplitude of maritime low-cloud fraction does not have its strongest relationship with the mean low-cloud fraction, but has strong relationships to the upper-level cloud fraction, cloud-top height, and the solar diurnal amplitude. The relative amplitude of continental high-cloud fraction is related most strongly to the time-mean surface temperature, the diurnal amplitude of surface temperature, and the solar diurnal amplitude. The relative amplitude of continental low-cloud fraction has strong relationships with atmospheric moisture content and the diurnal amplitude of surface temperature.

In contrast to amplitude, diurnal phase does not exhibit a strong relationship with any climatological variable. Instead, it is uniform within individual categories, which makes cloud diurnal variations independent of geographical location and, therefore, highly spatially coherent. The spatial coherence of cloud diurnal variations makes them an important ingredient of climate, one that affords some predictability in terms of local climatological conditions.

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Murry L. Salby and R. G. Roper

Abstract

Meteor wind data taken over Atlanta (34°N, 84°W) during the three-year interval 1974–77 are analyzed for periodic fluctuations of a recurrent nature. Power spectra and cross spectra of the zonal and meridional velocities between 80 and 100 km are constructed for several samples.

Ensemble statistics indicate the regular appearance of periods near 17, 8, 5, 2, 1.6 and 1.2 days. Most of these display smaller spectral values near 90 km. The general seasonal behavior has maximum values in winter and minimum values in summer, paralleling the observed variation of traveling wave energy flux in the stratosphere.

The regularity with which these oscillations appear, tempts an association with the periods of atmospheric normal modes.

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Murry L. Salby and Patrick F. Callaghan

Abstract

Interannual changes of the stratospheric circulation are studied in relation to coherent changes of the tropospheric circulation. Emerging over the winter pole is a clear signature of adiabatic warming and anomalous downwelling. Reflecting an intensification of the Brewer–Dobson circulation, the signature of anomalous downwelling extends from stratospheric levels into the troposphere. Compensating for it at subpolar latitudes is a signature of adiabatic cooling and anomalous upwelling. Equally coherent, the signature of anomalous upwelling occupies the same levels as the signature of anomalous downwelling. Inside the tropical troposphere, anomalous cooling is replaced by anomalous warming. It reflects an intensification of organized convection and the Hadley circulation, one that accompanies the intensification of the Brewer–Dobson circulation.

These signatures of anomalous vertical motion represent changes that operate coherently in the stratosphere and troposphere. They share major features with the Arctic Oscillation. Extending across the tropopause, they couple the stratosphere and troposphere through a transfer of mass. By modifying vertical motion inside the Tropics, anomalous upwelling influences organized convection. Support for this interpretation comes from anomalous divergence in the tropical upper troposphere; it is shown to vary coherently with anomalous downwelling in the Arctic stratosphere. Exhibiting analogous behavior are changes of the tropical tropopause. Coupled to stratospheric changes, these variations of the tropical circulation act to organize convection about the equator, favoring a split ITCZ. They reflect as much as 40% of the interannual variance of tropical divergence, representing an important complement to ENSO.

Much of the covariance between the polar stratosphere and the tropical troposphere is concentrated at periods shorter than 5 yr. Included is variability that is associated with the quasi-biennial oscillation (QBO) in the tropical stratosphere. Also included is biennial variability, which accompanies the QBO in the polar stratosphere. These stratospheric variations involve the same time scales as biennial variability in the tropical troposphere, which likewise influences convection.

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