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D. G. Andrews and B. J. Hoskins

Abstract

During the final stages of the formation of fronts, semi-geostrophic theory predicts kinetic and available potential energy spectra that closely approximate a −8/3 power law as a function of horizontal wavenumber.

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D. G. Andrews and M. E. McIntyre

Abstract

The theorems, which exhibit the role of wave dissipation, excitation and transience in the forcing of mean flow changes of second order in wave amplitude by arbitrary, small-amplitude disturbances, are obtained 1) for the primitive equations in pressure coordinates on a sphere, and 2) in a more general form (applicable for instance to nonhydrostatic disturbances in tornadoes or hurricanes) establishing that no approximations beyond axisymmetry of the mean flow are necessary. It is shown how the results reduce to those found by Boyd (1976) for the case of sinusoidal, hydrostatic waves with exponentially growing or decaying amplitude, and it is explained why the approximation used by Boyd in the thermodynamic equation is not needed. The reduction to Boyd's results entails the use of a virial theorem. This theorem amounts to a generalization of the “equipartition” law derived in an earlier paper (Andrews and Mclntyre, 1976). That derivation appeared to rely on an assumption about relative phases of disturbance Fourier components; the present derivation shows that no such assumption is in fact necessary.

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D. G. Andrews and M. F. McIntyre

Abstract

A simple multiple-scale expansion procedure is given for calculating corrections to the structure of equatorial planetary waves in the presence of weak shear and dissipation. For upward-propapting Rossby-gravity (Yanai-Maruyama) and Kelvin (Wallace-Kousky) waves, explicit results are obtained for the case of Newtonian cooling and Rayleigh friction, correct to the first two orders in the ratio μ of wave to mean-flow height scales. The results are used in a direct calculation of the horizontal Reynolds stress uv′¯ and demonstrate the strong dependence of u&primev′¯ on the ratio of friction to cooling coefficients.

In certain parameter regimes of interest in the tropical stratosphere, a slight north-south asymmetry in the y profile of ū can cause changes in the wave structure such that the mean zonal acceleration ∂ū/∂t tends to have the same asymmetry. That is, there may be a tendency for asymmetries in ū(y) to amplify in the presence of dissipating waves.

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D. G. Andrews and M. E. McIntyre

Abstract

Using a new generalization of the Eliassen-Palm relations, we discuss the zonal-mean-flow tendency ∂ū/∂t due to waves in a stratified, rotating atmosphere, with particular attention to equatorially trapped modes. Wave transience, forcing and dissipation are taken into account in a very general way. The theory makes it possible to discuss the latitudinal (y) and vertical (z) dependence of ∂ū/∂t qualitatively and calculate it directly from an approximate knowledge of the wave structure. For equatorial modes it reveals that the y profile of ∂ū/∂t is strongly dependent on the nature of the forcing or dissipation mechanism. A by-product of the theory is a far-reaching generalization of the theorems of Charney-Drazin, Dickinson and Holton on the forcing of ∂ū/∂t by conservative linear waves.

Implications for the quasi-biennial oscillation in the equatorial stratosphere are discussed. Graphs of y profiles of ∂ū/∂t are given for the equatorial waves considered in the recent analysis of observational data by Lindzen and Tsay (1975). The y profile of ∂ū/∂t for Rossby-gravity and inertio-gravity modes, in Lindzen and Tsay's parameter ranges, prove extremely sensitive to whether or not small amounts of mechanical dissipation are present alongside the radiative-photochemical dissipation of the waves.

The probable importance of low-frequency Rossby waves for the momentum budget of the descending easterlies is suggested.

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D. G. Andrews, J. D. Mahlman, and R. W. Sinclair

Abstract

The Eliassen-Palm flux is important in analytical studies of small-amplitude waves where it provides a powerful and elegant tool for the description of wave propagation in mean zonal shear flows, as well as for analysis of the effective mean zonal force induced by the waves. Furthermore, it has recently been used as a diagnostic in a number of studies of atmospheric data and numerical models of specific dynamical phenomena. In this paper, we apply it to the GFDL "SKYHI" global general circulation model of the troposphere-stratosphere-mesosphere, and describe computations of the primitive equations, isobaric coordinate form of the Eliassen-Palm flux and its divergence under conditions of annual-mean insolation.

The Eliassen-Palm flux diagnostics show a clear picture of planetary wave propagation from the midlatitude troposphere into the stratosphere and mesosphere. In the tropics, the presence of Kelvin waves confuses the picture somewhat (because their phase speeds are eastward with respect to the mean flow) and necessitates additional analysis which is given elsewhere.

We find the Eliassen-Palm diagnostics lead to new insights on the forcing of mean flows by the eddies. The implications of the fact that the model waves are not close to "non-acceleration conditions" and the importance of mean diabatic effects in our 30-day average statistics are considered in Appendix B. An important finding is that zonal westerly flows are strongly decelerated by eddies in the midlatitude upper troposphere and the mesosphere, in sharp contrast to the apparent implication of traditional zonal mean balances.

On the other hand, the forcing of mean accelerations by waves in the tropics is essentially in agreement with that found in earlier studies. The above inferences from the Eliassen-Palm diagnostics concerning the effect of eddies on zonal flows have been tested in a companion model experiment in which eddies propagating out of the troposphere are strongly damped. This experiment shows the resultant zonal flow accelerations to be fully consistent with the inferences from the Eliassen-Palm diagnostics.

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L. D. Williams, R. G. Barry, and J. T. Andrews

Abstract

The variation over uneven terrain of the daily total of incident shortwave (global) radiation under cloudless conditions may be estimated by existing methods for calculating direct and diffuse solar radiation on a slope. A computer program for performing these calculations, incorporating a technique to determine when the direct rays of the sun are screened by the horizon at each point, is described. The adequacy of the approximation for diffuse radiation is considered by comparison with published data. Computations for an area of east Baffin Island, Northwest Territories, Canada, demonstrate that the occurrence of glaciers there is influenced both by elevation and by solar radiation. The potential of such computations as an aid in selecting station sites for climatological studies is also discussed.

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S. M. Rosier, B. N. Lawrence, D. G. Andrews, and F. W. Taylor

Abstract

Dynamical fields based on temperature measurements from the Improved Stratospheric and Mesospheric Sounder on the Upper Atmosphere Research Satellite are presented for the Northern Hemisphere stratosphere for the period 28 October 1991 through 18 January 1992. Interpretation of these fields gives a picture of the dynamical evolution of this period in terms of the zonal-mean fields and the synoptic structures. Among the features of interest are the movements of the zonal-mean jets and several periods of stratospheric warming, culminating in a near-major warming in January.

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T. B. Richardson, P. M. Forster, T. Andrews, O. Boucher, G. Faluvegi, D. Fläschner, Ø. Hodnebrog, M. Kasoar, A. Kirkevåg, J.-F. Lamarque, G. Myhre, D. Olivié, B. H. Samset, D. Shawki, D. Shindell, T. Takemura, and A. Voulgarakis

Abstract

The response of the hydrological cycle to climate forcings can be understood within the atmospheric energy budget framework. In this study precipitation and energy budget responses to five forcing agents are analyzed using 10 climate models from the Precipitation Driver Response Model Intercomparison Project (PDRMIP). Precipitation changes are split into a forcing-dependent fast response and a temperature-driven hydrological sensitivity. Globally, when normalized by top-of-atmosphere (TOA) forcing, fast precipitation changes are most sensitive to strongly absorbing drivers (CO2, black carbon). However, over land fast precipitation changes are most sensitive to weakly absorbing drivers (sulfate, solar) and are linked to rapid circulation changes. Despite this, land-mean fast responses to CO2 and black carbon exhibit more intermodel spread. Globally, the hydrological sensitivity is consistent across forcings, mainly associated with increased longwave cooling, which is highly correlated with intermodel spread. The land-mean hydrological sensitivity is weaker, consistent with limited moisture availability. The PDRMIP results are used to construct a simple model for land-mean and sea-mean precipitation change based on sea surface temperature change and TOA forcing. The model matches well with CMIP5 ensemble mean historical and future projections, and is used to understand the contributions of different drivers. During the twentieth century, temperature-driven intensification of land-mean precipitation has been masked by fast precipitation responses to anthropogenic sulfate and volcanic forcing, consistent with the small observed trend. However, as projected sulfate forcing decreases, and warming continues, land-mean precipitation is expected to increase more rapidly, and may become clearly observable by the mid-twenty-first century.

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F. Martin Ralph, Paul J. Neiman, David E. Kingsmill, P. Ola G. Persson, Allen B. White, Eric T. Strem, Edmund D. Andrews, and Ronald C. Antweiler

Abstract

Data from the California Land-Falling Jets Experiment (CALJET) are used to explore the causes of variations in flood severity in adjacent coastal watersheds within the Santa Cruz Mountains on 2–3 February 1998. While Pescadero Creek (rural) experienced its flood of record, the adjacent San Lorenzo Creek (heavily populated), attained only its fourth-highest flow. This difference resulted from conditions present while the warm sector of the storm, with its associated low-level jet, high moisture content, and weak static stability, was overhead. Rainfall in the warm sector was dominated by orographic forcing. While the wind speed strongly modulated rain rates on windward slopes, the wind direction positioned the edge of a rain shadow cast by the Santa Lucia Mountains partially over the San Lorenzo basin, thus protecting the city of Santa Cruz from a more severe flood. Roughly 26% ± 9% of the streamflow at flood peak on Pescadero Creek resulted from the warm-sector rainfall. Without this rainfall, the peak flow on Pescadero Creek would likely not have attained record status.

These results are complemented by a climatological analysis based on ∼50-yr-duration streamflow records for these and two other nearby windward watersheds situated ∼20 to 40 km farther to the east, and a comparison of this climatological analysis with composites of NCEP–NCAR reanalysis fields. The westernmost watersheds were found to have their greatest floods during El Niño winters, while the easternmost watersheds peaked during non–El Niño episodes. These results are consistent with the case study, that showed that the composite 925-mb, meridionally oriented wind direction during El Niños favors a rain shadow over the eastern watersheds. During non–El Niño periods, the composite, zonally oriented wind direction indicates that the sheltering effect of the rain shadow on the eastern watersheds is reduced, while weaker winds, less water vapor, and stronger stratification reduce the peak runoff in the western watersheds relative to El Niño periods.

These case study and climatological results illustrate the importance of conditions in the moisture-rich warm sector of landfalling Pacific winter storms. Although many other variables can influence flooding, this study shows that variations of ±10° in wind direction can modulate the location of orographically enhanced floods. While terrain can increase predictability (e.g., rainfall typically increases with altitude), the predictability is reduced when conditions are near a threshold separating different regimes (e.g., in or out of a rain shadow).

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T. Wood, A. C. Maycock, P. M. Forster, T. B. Richardson, T. Andrews, O. Boucher, G. Myhre, B. H. Samset, A. Kirkevåg, J.-F. Lamarque, J. Mülmenstädt, D. Olivié, T. Takemura, and D. Watson-Parris

Abstract

Rapid adjustments—the response of meteorology to external forcing while sea surface temperatures (SST) and sea ice are held fixed—can affect the midlatitude circulation and contribute to long-term forced circulation responses in climate simulations. This study examines rapid adjustments in the Southern Hemisphere (SH) circulation using nine models from the Precipitation Driver and Response Model Intercomparison Project (PDRMIP), which perform fixed SST and coupled ocean experiments for five perturbations: a doubling of carbon dioxide (2xCO2), a tripling of methane (3xCH4), a fivefold increase in sulfate aerosol (5xSO4), a tenfold increase in black carbon aerosol (10xBC), and a 2% increase in solar constant (2%Sol). In the coupled experiments, the SH eddy-driven jet shifts poleward and strengthens for forcings that produce global warming (and vice versa for 5xSO4), with the strongest response found in austral summer. In austral winter, the responses project more strongly onto a change in jet strength. For 10xBC, which induces strong shortwave absorption, the multimodel mean (MMM) rapid adjustment in DJF jet latitude is ~75% of the change in the coupled simulations. For the other forcings, which induce larger SST changes, the effect of SST-mediated feedbacks on the SH circulation is larger than the rapid adjustment. Nevertheless, for these perturbations the magnitude of the MMM jet shift due to the rapid adjustment is still around 20%–30% of that in the coupled experiments. The results demonstrate the need to understand the mechanisms for rapid adjustments in the midlatitude circulation, in addition to the effect of changing SSTs.

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