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Idealized integral studies of the dynamics of atmospheric inertia–gravity waves (IGWs) from their sources in the troposphere (e.g., by spontaneous emission from jets and fronts) to dissipation and mean flow effects at higher altitudes could contribute to a better treatment of these processes in IGW parameterizations in numerical weather prediction and climate simulation. It seems important that numerical codes applied for this purpose are efficient and focus on the essentials. Therefore, a previously published staggered-grid solver for f-plane soundproof pseudoincompressible dynamics is extended here by two main components. These are 1) a semi-implicit time stepping scheme for the integration of buoyancy and Coriolis effects, and 2) the incorporation of Newtonian heating consistent with pseudoincompressible dynamics. This heating function is used to enforce a temperature profile that is baroclinically unstable in the troposphere and it allows the background state to vary in time. Numerical experiments for several benchmarks are compared against a buoyancy/Coriolis-explicit third-order Runge–Kutta scheme, verifying the accuracy and efficiency of the scheme. Preliminary mesoscale simulations with baroclinic wave activity in the troposphere show intensive small-scale wave activity at high altitudes, and they also indicate there the expected reversal of the zonal-mean zonal winds. [ABSTRACT FROM AUTHOR]

Many operational numerical weather prediction (NWP) systems now extend into the stratosphere and are beginning to be used to generate forecasts beyond conventional 5-10-day periods out to seasonal time scales. Past observational and modeling studies have shown that the quasi-biennial oscillation (QBO) in equatorial stratospheric winds can play an important role in stratosphere-troposphere dynamical coupling over these longer time scales. Consequently, stratosphere-resolving NWP models used to generate seasonal forecasts should contain the necessary physics to generate and maintain the QBO. This study describes several key modifications that were necessary to produce a QBO in a high-altitude NWP model, which include an increase in model vertical resolution, implementation of a computationally efficient stochastic gravity wave drag parameterization, and reductions in the amount of horizontal and vertical diffusion in the stratosphere. Results from a 10-yr free-running model simulation with these modifications show that the westerly QBO phase produces lower temperatures and stronger westerly flow in the Northern Hemisphere (NH) winter polar stratosphere compared to the easterly QBO phase. Ensembles of 120-day simulations over the December-March period show that these modifications replace persistent easterly flow in the equatorial lower stratosphere with a more realistic transition from easterly to westerly flow. The resulting changes in planetary wave propagation produce a statistically significant response in the dynamics of the NH extratropical stratosphere consistent with the Holton-Tan relationship. The westerly shift in equatorial winds also produces a significant response in the NH extratropical troposphere, where the sea level pressure differences in winter resemble the positive phase of the northern annular mode. [ABSTRACT FROM AUTHOR]

The link between poleward-breaking Rossby waves and stratospheric inertia–gravity waves is examined. With a visual inspection of Ertel’s potential vorticity maps based on ECMWF analyses it was found that Rossby wave–breaking events occurred over northern Europe in about 40% of the winter days in 1999–2003. The majority of them were breaking poleward downstream. A total of 10 field campaigns were performed in the winters of 1999–2002 at Kühlungsborn, Germany (54°N, 12°E). They are related to such events and can be considered as representative for poleward-breaking Rossby waves. Inertia–gravity wave properties are diagnosed from radiosonde observations. They appeared to be shallower, slower, and stronger than the climatological mean for the north German lowlands. Hence, Rossby wave–breaking events are linked with strong stratospheric inertia–gravity wave activity. A novel parameterization of inertia–gravity wave generation and propagation is proposed. The stratospheric inertia–gravity wave action in the 16–20-km height range was parameterized with the synoptic-scale cross-stream ageostrophic wind, which accounts for imbalances in the upper-tropospheric jet streak. This empirical relationship is supported with quasigeostrophic theory. Effects of damping and critical level absorption are taken into account with Wentzel–Kramers–Brillouin theory. For verification of the parameterization with homogeneous meteorological fields in space and time, the 10 field campaigns were hindcasted with the nonhydrostatic fifth-generation Pennsylvania State University–National Center for Atmospheric Research Mesoscale Model. About 80% of the variance in inertia–gravity wave action was found to be explained. For the 10 campaigns a close link was found between the poleward-breaking Rossby waves and the strong stratospheric inertia–gravity waves. The role of the polar vortex was twofold: first, it forced the poleward-oriented Rossby waves to break downstream and to form strong tropospheric jet streaks generating inertia–gravity waves. Second, the strong winds in the stratosphere favored the upward propagation of the inertia–gravity waves. The proposed new parameterization of inertia–gravity wave generation and propagation was validated and can be used to deduce mesoscale wave intensity from synoptic flow characteristics during poleward Rossby wave–breaking events. [ABSTRACT FROM AUTHOR]

A high-altitude version of the Navy Operational Global Atmospheric Prediction System (NOGAPS) spectral forecast model is used to simulate the unusual September 2002 Southern Hemisphere stratospheric major warming. Designated as NOGAPS-Advanced Level Physics and High Altitude (NOGAPS-ALPHA), this model extends from the surface to 0.005 hPa (∼85 km altitude) and includes modifications to multiple components of the operational NOGAPS system, including a new radiative heating scheme, middle-atmosphere gravity wave drag parameterizations, hybrid vertical coordinate, upper-level meteorological initialization, and radiatively active prognostic ozone with parameterized photochemistry. NOGAPS-ALPHA forecasts (hindcasts) out to 6 days capture the main features of the major warming, such as the zonal mean wind reversal, planetary-scale wave amplification, large upward Eliassen–Palm (EP) fluxes, and splitting of the polar vortex in the middle stratosphere. Forecasts beyond 6 days have reduced upward EP flux in the lower stratosphere, reduced amplitude of zonal wavenumbers 2 and 3, and a middle stratospheric vortex that does not split. Three-dimensional EP-flux diagnostics in the troposphere reveal that the longer forecasts underestimate upward-propagating planetary wave energy emanating from a significant blocking pattern over the South Atlantic that played a large role in forcing the major warming. Forecasts of less than 6 days are initialized with the blocking in place, and therefore are not required to predict the blocking onset. For a more thorough skill assessment, NOGAPS-ALPHA forecasts over 3 weeks during September–October 2002 are compared with operational NOGAPS 5-day forecasts made at the time. NOGAPS-ALPHA forecasts initialized with 2002 operational NOGAPS analyses show a modest improvement in skill over the NOGAPS operational forecasts. An additional, larger improvement is obtained when NOGAPS-ALPHA is initialized with reanalyzed 2002 fields produced with the currently operational (as of October 2003) Naval Research Laboratory (NRL) Atmospheric Variational Data Assimilation System (NAVDAS). Thus the combination of higher model top, better physical parameterizations, and better initial conditions all yield improved forecasting skill over the NOGAPS forecasts issued operationally at the time. [ABSTRACT FROM AUTHOR]