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Rolando R. Garcia

Abstract

A zonally averaged, quasi-geostrophic residual Eulerian model is used to illustrate how the adjustment of the middle atmophere to externally imposed forcing depends on internal dissipative properties (parameterized as Newtonian cooling a and Rayleigh friction KR) and on the periodicity of the forcing. It is shown that when the problem is formulated in this manner, many well-known properties of the stratosphere/mesosphere system (e.g., the relative efficiency of wave versus diabatic driving of the meridional circulation, and the near radiative equilibrium of much of the stratosphere) are succinctly expressed in terms of the governing elliptic differential equation and its solutions. Despite its simplicity, the model is a useful heuristic tool for studying the response of the middle atmosphere to external forcing.

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Rolando R. Garcia

Abstract

A parameterization of planetary wave breaking in the middle atmosphere has been developed and tested in a numerical model which includes governing equations for a single wave and the zonal-mean state. The parameterization is based on the assumption that wave breaking represents a steady-state equilibrium between the flux of wave activity and its dissipation by nonlinear processes, and that the latter can be represented as linear damping of the primary wave. With this and the additional assumption that the effect of breaking is to prevent further amplitude growth, the required dissipation rate is readily obtained from the steady-state equation for wave activity; diffusivity coefficients then follow from the dissipation rate. The assumptions made in the derivation are equivalent to those commonly used in parameterizations for gravity wave breaking, but the formulation in terms of wave activity helps highlight the central role of the wave group velocity in determining the dissipation rate. Comparison of model results with nonlinear calculations of wave breaking and with diagnostic determinations of stratospheric diffusion coefficients reveals remarkably good agreement, and suggests that the parameterization could be useful for simulating inexpensively, but realistically, the effects of planetary wave transport.

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Rolando R. Garcia

The Eighth Conference on the Middle Atmosphere was held in Atlanta, Georgia, on 7–10 January 1992. Over 100 papers were presented, emphasizing areas of current interest such as advances in numerical modeling, the dynamics of the tropical middle atmosphere, polar photochemistry and dynamics, and new observational techniques. Two sessions highlighted preliminary results from the Upper Atmosphere Research Satellite.

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Fabrizio Sassi and Rolando R. Garcia

Abstract

Recent satellite observations suggest that convection over the tropical continents is capable of exciting wave motions over a wide range of spatial and temporal scales. An equatorial beta-plane model was used to investigate the forcing by convective heating of equatorial waves with zonal wavenumbers from 1 to 15 and a wide range of periods, including diurnal oscillations. Also studied are the propagation of these waves in the equatorial middle atmosphere and their role in driving the tropical semiannual oscillation (SAO). Specification of the heating distribution used to force the model is guided by observations and analyses of tropical convection. It was found that intermediate-scale Kelvin and inertia–gravity waves provide between 25% and 50% of the forcing necessary to drive the westerly phase of the SAO near the stratopause, while the remainder is supplied by planetary-scale Kelvin waves. In the mesosphere, intermediate-scale waves account for an even larger fraction of the force required to drive the westerly phase and they are solely responsible for driving the easterly phase. The resulting SAO agrees well with ground-based and satellite observations in both the stratosphere and mesosphere. The dependence of the simulated SAO on various model parameters has also been explored. A simulation wherein only planetary-scale waves (k = 1–3) are included yields a weaker than observed stratopause oscillation and fails to produce a mesospheric oscillation. If the full range of zonal wavenumbers (k = 1–15) is included but the diurnal component of the forcing is omitted, the stratopause oscillation is again weaker than observed, while the amplitude of the mesospheric oscillation is greatly diminished. These results suggest that strong excitation of intermediate-scale equatorial waves depends on the diurnal cycle of convection and that the waves thus excited play an important role in the forcing of the tropical semiannual oscillation.

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Natalia Calvo and Rolando R. Garcia

Abstract

Two simulations from the Whole Atmosphere Community Climate Model, covering the periods 1950–2003 and 1980–2050, are used to investigate the nature of the waves that force the increase of the tropical upwelling in the lower stratosphere as the concentration of greenhouse gases increases. Decomposition of the wave field resolved by the model into stationary and transient wavenumber spectra allows attribution of trends in the Eliassen–Palm (EP) flux and its divergence to specific wave components. This analysis reveals that enhanced dissipation of stationary planetary waves is the main driver of trends in the tropical upwelling in the lower stratosphere. The contribution of transient waves is smaller and is responsible mainly for trends in wave forcing in the subtropics and middle latitudes, which, however, provide only minor contributions to the mean tropical upwelling. Examination of individual wave structures shows that the stationary waves are tropical Rossby waves trapped in the upper troposphere and lower stratosphere, whereas the transient components are synoptic waves present in the subtropics and middle latitudes. The authors also present evidence that trends in resolved wave forcing in the lower stratosphere are due to both changes in wave transmissivity and changes in wave excitation, with the first mechanism dominating the behavior of the simulation during the last half of the twentieth century, while the second is clearly more important in the simulation during the first half of the twenty-first century.

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Fabrizio Sassi and Rolando R. Garcia

Abstract

A one-dimensional model that solves the time-dependent equations for the zonal mean wind and a wave of specified zonal wavenumber has been used to illustrate the ability of gravity waves forced by time-dependent tropospheric heating to produce a semiannual oscillation (SAO) in the middle atmosphere. When the heating has a strong diurnal cycle, as observed over tropical landmasses, gravity waves with zonal wavelengths of a few thousand kilometers and phase velocities in the range ±40–50 m s−1 are excited efficiently by the maximum vertical projection criterion (vertical wavelength ≈2 × forcing depth). Calculations show that these waves can account for large zonal mean wind accelerations in the middle atmosphere, resulting in realistic stratopause and mesopause oscillations. Calculations of the temporal evolution of a quasi-conserved tracer indicate strong down-welling in the upper stratosphere near the equinoxes, which is associated with the descent of the SAO westerlies. In the upper mesosphere, there is a semiannual oscillation in tracer mixing ratio driven by seasonal variability in eddy mixing, which increases at the solstices and decreases at the equinoxes.

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Lucrezia Ricciardulli and Rolando R. Garcia

Abstract

The forcing of equatorial waves by convective heating in the National Center for Atmospheric Research Community Climate Model (CCM3) is investigated and compared with the forcing deduced from observations of convective clouds. The analysis is performed on two different simulations, wherein convection is represented by the Zhang–McFarlane and the Hack parameterization schemes, respectively. Spectra of equatorial waves excited by convective heating (Rossby, Kelvin, and gravity waves) are obtained by projecting the heating field onto Hough modes; the dynamical response to the heating is then calculated in terms of the vertical component of the Eliassen–Palm flux, F z, focusing on waves that are able to propagate into the middle atmosphere. The same analysis is repeated using observations of outgoing longwave radiation as a proxy for tropical convection. Comparison of CCM3 results with those derived from observations indicates that high-frequency heating variability is underestimated in both CCM3 simulations, despite the fact that time-mean values of convective heating are well represented. Moreover, the two convective parameterization schemes differ substantially from each other: Compared to observations, F z is severely underestimated at most frequencies when CCM3 is run with the Zhang–McFarlane scheme. When the Hack scheme is used, F z at frequencies |ω| < 0.5 cycles per day is comparable to the observations, but it is underestimated at higher frequencies. Misrepresentation of the variability of convective heating is likely to have important consequences for the dynamical simulation of the middle atmosphere and even the troposphere.

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Dennis L. Hartmann and Rolando R. Garcia

Abstract

The role of planetary-scale waves in the transport of ozone in middle and high latitudes is studied with a linearized model which takes into account the coupling between radiation, chemistry and dynamics. The governing equations are a quasi-geostrophic potential vorticity equation and a continuity equation for perturbations of the O3 mixing ratio. These equations include the effects of absorption of solar radiation by O3, temperature-dependent O3 photochemistry and advection of O3 by wave motions. Model runs show that, with a basic state representative of winter conditions, ultralong waves can produce O3 perturbation which exhibit a phase shift of nearly 180° between the lower stratosphere (where the O3 perturbations are controlled by advection) and the upper stratosphere (where they are controlled by photochemical processes). This phase shift brings the O3 perturbation into phase with the eddy meridional velocity (and 180° out of phase with the eddy vertical velocity) at ∼45 km, giving rise to large poleward and downward zonally averaged transports. The effect on these transports of varying a number of basic-state parameters, such as mean meridional gradient of O3 and temperature dependence of the O3 reaction rates, is also discussed.

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Rolando R. Garcia and Jadwiga H. Richter

Abstract

This study documents the contribution of equatorial waves and mesoscale gravity waves to the momentum budget of the quasi-biennial oscillation (QBO) in a 110-level version of the Whole Atmosphere Community Climate Model. The model has high vertical resolution, 500 m, above the boundary layer and through the lower and middle stratosphere, decreasing gradually to about 1.5 km near the stratopause. Parameterized mesoscale gravity waves and resolved equatorial waves contribute comparable easterly and westerly accelerations near the equator. Westerly acceleration by resolved waves is due mainly to Kelvin waves of zonal wavenumber in the range k = 1–15 and is broadly distributed about the equator. Easterly acceleration near the equator is due mainly to Rossby–gravity (RG) waves with zonal wavenumbers in the range k = 4–12. These RG waves appear to be generated in situ during both the easterly and westerly phases of the QBO, wherever the meridional curvature of the equatorial westerly jet is large enough to produce reversals of the zonal-mean barotropic vorticity gradient, suggesting that they are excited by the instability of the jet. The RG waves produce a characteristic pattern of Eliassen–Palm flux divergence that includes strong easterly acceleration close to the equator and westerly acceleration farther from the equator, suggesting that the role of the RG waves is to redistribute zonal-mean vorticity such as to neutralize the instability of the westerly jet. Insofar as unstable RG waves might be present in the real atmosphere, mixing due to these waves could have important implications for transport in the tropical stratosphere.

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William J. Randel, Rolando Garcia, and Fei Wu

Abstract

The dynamical balances associated with upwelling in the tropical lower stratosphere are investigated based on climatological 40-yr ECMWF Re-Analysis (ERA-40) and NCEP–NCAR reanalysis data. Zonal mean upwelling is calculated from momentum balance and continuity (“downward control”), and these estimates in the deep tropics are found to be in reasonable agreement with stratospheric upwelling calculated from thermodynamic balance (and also with vertical velocity obtained from ERA-40). The detailed momentum balances associated with the dynamical upwelling are investigated, particularly the contributions to climatological Eliassen–Palm (EP) flux divergence in the subtropics. Results show that the equatorward extension of extratropical waves (baroclinic eddies and, in the NH, quasi-stationary planetary waves) contribute a large component of the subtropical wave driving at 100 hPa. Additionally, there is a significant contribution to subtropical forcing from equatorial planetary waves, which exhibit a strong seasonal cycle (a reversal in phase) in response to latitudinal migration of tropical convection. The observed balances suggest that the strong annual cycle in upwelling across the tropical tropopause is forced by subtropical horizontal eddy momentum flux convergence associated with waves originating in both the tropics and extratropics.

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