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Murry Salby and Fabrizio Sassi

Abstract

Climate properties regulated by convection, such as water vapor, cloud cover, and related distributions, are undersampled in asynoptic data from an individual orbiting platform, which must therefore be restricted to time-mean distributions. A procedure is developed to identify small-scale undersampled variance in asynoptic data and reject it, leaving a more accurate representation of large-scale variance that describes the organization of climate properties. The procedure is validated against high-resolution distributions that have been constructed from six satellites simultaneously observing the earth. Observing the high-resolution distributions asynoptically is shown to result in sampling error at large scales that is as great as the large-scale signal present, limiting the usefulness of the raw asynoptic data to time-mean distributions. However, processing the asynoptic data to reject undersampled incoherent variability reduces the error variance to 10% or less, yielding a fairly accurate representation of large-scale coherent variability, which then can be mapped synoptically on periods as short as 2.0 days. Made possible then are studies of how cloud, water vapor, and related distributions are organized by unsteady elements of the general circulation, which cannot be studied in the raw asynoptic data.

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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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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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Murry Salby, Fabrizio Sassi, Patrick Callaghan, William Read, and Hugh Pumphrey

Abstract

Thermal and humidity structures near the tropical tropopause are studied using microwave satellite retrievals of water vapor, along with contemporaneous dynamical structure in ECMWF analyses and cold clouds in high-resolution global cloud imagery. Examined during November 1991–February 1992, these fields all vary coherently with the outflow from convective centers—in the upper troposphere as well as in the lowermost stratosphere. The outbreak of deep convection is accompanied by diabatic heating below a level between 250 and 150 mb but by diabatic cooling at higher levels. The reversal from heating to cooling is broadly consistent with cumulus detrainment. Through irreversible mixing, that process serves as a heat source for the environment below the level of neutral buoyancy (LNB) but as a heat sink at higher levels. Calculations, inclusive of entrainment, place the LNB very near the observed reversal from heating to cooling.

The outbreak of convection is also accompanied by humidification below 125 mb but by dehydration at higher levels. The reversal from humidification to dehydration coincides with levels where environmental conditions approach saturation. Those conditions suggest the efficient removal of total water from cumulus updrafts, leaving dessicated air to ventilate to higher levels. Cumulus detrainment then acts to humidify the environment beneath the zone of nearly saturated environmental conditions, while dehydrating it at higher levels. Dry air emerges from the region of the coldest cloud. It then extends into the winter hemisphere, along streamlines that characterize the Hadley circulation.

Coinciding with diabatic cooling are stratospheric convergence and downwelling. These features of stratospheric motion amplify simultaneously with divergence at tropospheric levels, which represents the major outflow from deep convection. The deepest convection, found over the equatorial Pacific, coincides with the highest moist static energy. The latter yields an LNB that is some 3 km higher over the equatorial Pacific than elsewhere, in agreement with the observed reversal from heating to cooling. Observed brightness temperatures place the level at which cumulus anvils are most extensive very near the cold point over the equatorial Pacific. This, in turn, lies near the tropical tropopause throughout the Tropics. Collectively, these features suggest that the coldest cloud, found over the equatorial Pacific, plays a key role in maintaining temperature and humidity near the tropical tropopause.

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Fabrizio Sassi, R. R. Garcia, D. Marsh, and K. W. Hoppel

Abstract

This paper compares present-day simulations made with two state-of-the-art climate models: a conventional model specifically designed to represent the tropospheric climate, which has a poorly resolved middle atmosphere, and a configuration that is built on the same physics and numerical algorithms but represents realistically the middle atmosphere and lower thermosphere. The atmospheric behavior is found to be different between the two model configurations, and it is shown that the differences in the two simulations can be attributed to differences in the behavior of the zonal mean state of the stratosphere, where reflection of quasi-stationary resolved planetary waves from the lid of the low-top model is prominent; the more realistic physics in the high-top model is not relevant. It is also shown that downward propagation of zonal wind anomalies during weak stratospheric vortex events is substantially different in the two model configurations. These findings extend earlier results that a poorly resolved stratosphere can influence simulations throughout the troposphere.

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Fabrizio Sassi, Rolando R. Garcia, and Byron A. Boville

Abstract

The middle atmospheric version of the NCAR Community Climate Model (CCM2) has been used to study the development of the equatorial semiannual oscillation (SAO) in the stratosphere. The model domain extends from the ground to about 80 km, with a vertical resolution of 1 km. Transport of nitrous oxide (N2O) with simplified photochemistry is included in the calculation to illustrate the influence of tropical circulations on the distribution of trace species. Diagnosis of model output reveals two distinct phases in the evolution of the zonal mean state on the equator. In early December, a strong and broad easterly jet appears near the stratopause in connection with a midlatitude wave event (sudden stratospheric warming) that reverses the winter westerlies of the Northern Hemisphere throughout the upper stratosphere. When the wave forcing dies out, the radiative drive allows the westerlies to recover at midlatitudes, while easterlies persist in the tropics. The resulting strong meridional gradient of the zonal mean wind provides favorable conditions for the development of inertial instability at lower latitudes. The meridional circulation associated with the instability shapes the “nose” of the easterly jet, reducing the extension of the unstable region.

In equinoctial conditions, a jet of westerlies appears in the lower equatorial mesosphere and descends to lower attitudes; positive accelerations associated with the descending westerlies are due primarily to Kelvin waves. The descent of the westerly jet does not reproduce well the observed behavior of the SAO westerly phase, either in amplitude or in the extent of downward propagation. As a consequence, the model does not simulate the “double peak” observed in the tropical distribution of N2O. Comparison of wave amplitudes in the model with those derived from satellite observations shows that the calculated amplitudes are larger than observed in the upper stratosphere. It follows that inadequate shows wave forcing is not the cause of the weak westerly phase in the model, and that some other mechanism must be responsible for the generation of the strong westerly phase observed.

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Chaim I. Garfinkel, Dennis L. Hartmann, and Fabrizio Sassi

Abstract

Regional extratropical tropospheric variability in the North Pacific and eastern Europe is well correlated with variability in the Northern Hemisphere wintertime stratospheric polar vortex in both the ECMWF reanalysis record and in the Whole Atmosphere Community Climate Model. To explain this correlation, the link between stratospheric vertical Eliassen–Palm flux variability and tropospheric variability is analyzed. Simple reasoning shows that variability in the North Pacific and eastern Europe can deepen or flatten the wintertime tropospheric stationary waves, and in particular its wavenumber-1 and -2 components, thus providing a physical explanation for the correlation between these regions and vortex weakening. These two pathways begin to weaken the upper stratospheric vortex nearly immediately, with a peak influence apparent after a lag of some 20 days. The influence then appears to propagate downward in time, as expected from wave–mean flow interaction theory. These patterns are influenced by ENSO and October Eurasian snow cover. Perturbations in the vortex induced by the two regions add linearly. These two patterns and the quasi-biennial oscillation (QBO) are linearly related to 40% of polar vortex variability during winter in the reanalysis record.

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Jadwiga H. Richter, Fabrizio Sassi, and Rolando R. Garcia

Abstract

Middle atmospheric general circulation models (GCMs) must employ a parameterization for small-scale gravity waves (GWs). Such parameterizations typically make very simple assumptions about gravity wave sources, such as uniform distribution in space and time or an arbitrarily specified GW source function. The authors present a configuration of the Whole Atmosphere Community Climate Model (WACCM) that replaces the arbitrarily specified GW source spectrum with GW source parameterizations. For the nonorographic wave sources, a frontal system and convective GW source parameterization are used. These parameterizations link GW generation to tropospheric quantities calculated by the GCM and provide a model-consistent GW representation. With the new GW source parameterization, a reasonable middle atmospheric circulation can be obtained and the middle atmospheric circulation is better in several respects than that generated by a typical GW source specification. In particular, the interannual NH stratospheric variability is significantly improved as a result of the source-oriented GW parameterization. It is also shown that the addition of a parameterization to estimate mountain stress due to unresolved orography has a large effect on the frequency of stratospheric sudden warmings in the NH stratosphere by changing the propagation of stationary planetary waves into the polar vortex.

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Andrew J. Charlton, Lorenzo M. Polvani, Judith Perlwitz, Fabrizio Sassi, Elisa Manzini, Kiyotaka Shibata, Steven Pawson, J. Eric Nielsen, and David Rind

Abstract

The simulation of major midwinter stratospheric sudden warmings (SSWs) in six stratosphere-resolving general circulation models (GCMs) is examined. The GCMs are compared to a new climatology of SSWs, based on the dynamical characteristics of the events. First, the number, type, and temporal distribution of SSW events are evaluated. Most of the models show a lower frequency of SSW events than the climatology, which has a mean frequency of 6.0 SSWs per decade. Statistical tests show that three of the six models produce significantly fewer SSWs than the climatology, between 1.0 and 2.6 SSWs per decade. Second, four process-based diagnostics are calculated for all of the SSW events in each model. It is found that SSWs in the GCMs compare favorably with dynamical benchmarks for SSW established in the first part of the study.

These results indicate that GCMs are capable of quite accurately simulating the dynamics required to produce SSWs, but with lower frequency than the climatology. Further dynamical diagnostics hint that, in at least one case, this is due to a lack of meridional heat flux in the lower stratosphere. Even though the SSWs simulated by most GCMs are dynamically realistic when compared to the NCEP–NCAR reanalysis, the reasons for the relative paucity of SSWs in GCMs remains an important and open question.

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Edwin P. Gerber, Amy Butler, Natalia Calvo, Andrew Charlton-Perez, Marco Giorgetta, Elisa Manzini, Judith Perlwitz, Lorenzo M. Polvani, Fabrizio Sassi, Adam A. Scaife, Tiffany A. Shaw, Seok-Woo Son, and Shingo Watanabe

Advances in weather and climate research have demonstrated the role of the stratosphere in the Earth system across a wide range of temporal and spatial scales. Stratospheric ozone loss has been identified as a key driver of Southern Hemisphere tropospheric circulation trends, affecting ocean currents and carbon uptake, sea ice, and possibly even the Antarctic ice sheets. Stratospheric variability has also been shown to affect short-term and seasonal forecasts, connecting the tropics and midlatitudes and guiding storm-track dynamics. The two-way interactions between the stratosphere and the Earth system have motivated the World Climate Research Programme's (WCRP) Stratospheric Processes and their Role in Climate's (SPARC) activity on Modelling the Dynamics and Variability of the Stratosphere-Troposphere System (DynVar) to investigate the impact of stratospheric dynamics and variability on climate. This assessment will be made possible by two new multimodel datasets. First, roughly 10 models with a well-resolved stratosphere are participating in the Coupled Model Intercomparison Project phase 5 (CMIP5), providing the first multimodel ensemble of climate simulations coupled from the stratopause to the sea floor. Second, the Stratosphere Resolving Historical Forecast Project (Strat-HFP) of WCRP's Climate Variability and Predictability (CLIVAR) program is forming a multimodel set of seasonal hind-casts with stratosphere-resolving models, revealing the impact of both stratospheric initial conditions and dynamics on intraseasonal prediction. The CMIP5 and Strat-HFP model datasets will offer an unprecedented opportunity to understand the role of the stratosphere in the natural and forced variability of the Earth system and to determine whether incorporating knowledge of the middle atmosphere improves seasonal forecasts and climate projections.

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