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Leonhard Pfister

Abstract

Satellite and rocket observations indicate that barotropically unstable waves may exist in the upper stratosphere. To gain some understanding of the effects of vertical mean flow variation on barotropic instability with a view toward stratospheric applications, a numerical method used by other investigators for tropospheric baroclinic instability was employed to establish the structures, phase speeds and growth rates of the normal modes of a number of idealized, barotropically unstable, mean zonal wind fields in a quasi-geostrophic, Boussinesq framework. Both horizontally symmetric and asymmetric flows were considered, but in all cases the flows were vertically symmetric, with depth scales large enough to preclude baroclinic instability. Results showed that the horizontal structure of the waves was affected only slightly by vertical mean flow variation. Growth rates, however, were strongly affected, with reductions of 40% (horizontally symmetric) and 30% (horizontally asymmetric) from the respective two-dimensional values for the largest vertical scales expected in the stratosphere. Vertical structure displayed phase variation in the direction of mean wind shear and amplitude decay with distance from the level of strongest horizontal shear. The scale of variation was on the order of the geometric mean of the vertical mean flow scale and the vertical penetration depth, in analogy with baroclinic waves in latitudinally slowly varying flows. Some deviations from this behavior was found for horizontally symmetric flows, however, where the vertical scale of variation of the amplitude significantly exceeded that of the phase.

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Leonhard Pfister

Abstract

anally and seasonally averaged zonal wind fields in the summer mesosphere are unstable in the Charney-Stern sense, with a primarily baroclinic component. Two-dimensional stability analyses show peaks in the unstable wave growth spectrum at zonal wavenumbers 2–4, with periods of 1.4–3 days, for a variety of basic state flows. Wavenumbers of peak growth are consistently lower for the two-dimensional analyses than for the one-dimensional analyses, with the lowest wavenumber peaks found for the basic state flows with the smallest latitudinal scale. For strongly baroclinic basic state flows, wave growth was inhibited by introducing stronger latitudinal variations in the basic state flow; however, for marginally unstable flows, stronger latitudinal variations slightly enhance the peak growth rates.

The strong thermal damping in the mesosphere reduces growth rates by 20–60% of the damping rate, but has little effect on the wave structure. The primary significance of the thermal damping is that basic state flows cannot be substantially less unstable than the weakly unstable flows considered here in order to allow growing waves.

The growth spectrum peak at zonal wavenumber 3 with a period close to two days is a good match for the observed two-day wave phenomenon, but the calculated temperature structures have their maximum amplitude at 40–60° latitude, rather than the observed 20°. Other periodicities ranging from 1.7 to 4 days have also been observed in the meteor region at middle and high latitudes, and these could be explained by baroclinic instability as well.

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Richard E. Young, Howard Houben, and Leonhard Pfister

Abstract

A three-dimensional, spherical, primitive equation eigenvalue model is used to investigate the baroclinic stability properties of the wind and temperature fields in the Venus atmosphere as measured by Pioneer Venus. It is found that baroclinic instability occurs in the region of the middle cloud deck. The most unstable modes have growth times less than eight days and are vertically confined to the region near the middle cloud layer. The most unstable baroclinic mode at zonal wavenumber 2 has characteristics similar to those observed for the high latitude rotating dipole thermal feature. Certain planetary scale baroclinic modes can penetrate to relatively high altitudes under the right circumstances, and may therefore explain some of the wave features observed between 60 and 90 km. For example, thermal oscillations with periods between four and seven days occurring at middle latitudes have characteristics which appear to be consistent with computed properties of planetary scale baroclinic modes.

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Leonhard Pfister, Walter Starr, Roger Craig, Max Loewenstein, and Marion Legg

Abstract

Measurments of temperature and ozone from instrumental aircraft in the tropical lower stratosphere show the presence of small-scale disturbances generated by 1) underlying cumulus convection and 2) Kelvin-Helmholtz instability. The disturbances associated with underlying convection have peak-to-peak vertical parcel excursions of ∼300 m. Flying conditions were smooth, suggesting an ensemble of gravity waves and little or no turbulent mixing. It is nevertheless possible that these waves break at other altitudes, leading to turbulent mixing and net fluxes of vertically stratified tracers.

Disturbances attributed to KH instability implied vertical parcel excursions of 300–400 m. The disturbances coincided with rough flying conditions, suggesting turbulent mixing. A linear stability analysis of the atmospheric basic state defined by high-resolution radiosondes shows fastest growing waves with horizontal wavelengths of 1.4–1.8 km, consistent with the aircraft observations. The strong shears responsible for the KH instability are due to large-scale waves propagating into a region of small intrinsic frequency. Radiosonde observations show that the zonal length scale of them waves is ∼1000 km.

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Leonhard Pfister, Stanley Scott, Max Loewenstein, Stuart Bowen, and Marion Legg

Abstract

The importance of the momentum flux of topographically generated mesoscale gravity waves to the extratropical middle atmosphere circulation has been well established for over a decade. Estimates of the zonal forcing due to tropical mesoscale gravity waves, however, are hampered by lack of data on their primarily convective sources. The advent of aircraft measurements over tropical convective systems now makes such estimates possible without the use of ad hoc assumptions about amplitudes and phase speeds.

Aircraft measurements from NASA's 1980 Panama and 1987 STEP/Australia Missions show that convectively generated disturbances observed just above the tropopause have horizontal scales comparable to those of the underlying anvils (about 50–100 km) with peak-to-peak isentropic surface variations of about 300–400 m. Satellite imagery of tropical anvil evolution indicates a typical lifetime of about five hours. Assuming that each convective system's impact on the stratosphere can be modeled as a time-dependent “mountain” with the preceding spatial and time scales, the excited spectrum of gravity waves can be calculated. A suitable quasilinear wave-mean flow interaction parameterization and satellite-derived cloud area statistics can then be used to evaluate the zonal acceleration as a function of altitude induced by gravity waves from mesoscale convective systems.

The results indicate maximum westerly accelerations due to breaking mesoscale gravity waves of almost 0.4 m s−1/day in the upper stratosphere (in the region of the semiannual oscillation) during September, comparable to but probably smaller than the accelerations induced by planetary-scale Kelvin waves. Calculated easterly accelerations due to breaking mesoscale gravity waves in the QBO region below 35 km are smaller, accounting for about 10% of the required zonal acceleration.

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Richard E. Young, Richard L. Walterscheid, Gerald Schubert, Leonhard Pfister, Howard Houben, and Duane L. Bindschadler

Abstract

This paper extends the study of stationary gravity waves generated near the surface of Venus reported previously by Young et al. to include finite amplitude effects associated with large amplitude waves. Waves are forced near the surface of Venus by periodic forcing. The height-dependent profiles of static stability and mean wind in the Venus atmosphere play a very important role in the evolution of the nonlinear behavior of the waves, just as they do in the linear wave solutions. Certain wave properties are qualitatively consistent with linear wave theory, such as wave trapping, resonance, and wave evanescence for short horizontal wavelengths. However, the finite amplitude solutions also exhibit many other interesting features. In particular, for forcing amplitudes representative of those that could be expected in mountainous regions such as Aphrodite Terra, waves generated near the surface can reach large amplitudes at and above cloud levels, with clear signatures in the circulation pattern. At still higher levels, the waves can reach large enough amplitude to break, unless damping rates above the clouds are sufficient to limit wave amplitude growth. Well below cloud levels the waves develop complex flow patterns as the result of finite amplitude wave–wave interactions, and waves are generated having considerably shorter horizontal wavelengths than that associated with the forcing near the surface. Nonlinear interactions can excite waves that are resonant with the background wind and static stability fields even when the primary surface forcing does not, and these waves can dominate the wave spectrum near cloud levels. A global map of Venus topographic slopes derived from Magellan altimetry data shows that slopes of magnitude comparable to or exceeding that used to force the model are ubiquitous over the surface.

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Thomas P. Ackerman, Kuo-Nan Liou, Francisco P. J. Valero, and Leonhard Pfister

Abstract

The interaction of infrared and solar radiation with tropical cirrus anvils is addressed. Optical properties of the anvils are inferred from satellite observations and from high-altitude aircraft measurements. An infrared multiple-scattering model is used to compute heating rates in tropical anvils. Layer-average heating rates in 2 km thick anvils were found to be on the order of 20 to 30°K day−1. The difference between heating rates at cloud bottom and cloud top ranges from 30 to 200°K day−1, leading to convective instability in the anvil. The calculations are most sensitive to the assumed ice water content, but also are affected by the vertical distribution of ice water content and by the anvil thickness. Solar heating in anvils is shown to be less important than infrared hearing but not negligible The dynamical implications of the computed heating rates are also explored and we conclude that the heating may have important consequences for upward mass transport in the tropics. The potential impact of tropical cirrus on the tropical energy balance and cloud forcing are discussed.

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Aurélien Podglajen, T. Paul Bui, Jonathan M. Dean-Day, Leonhard Pfister, Eric J. Jensen, M. Joan Alexander, Albert Hertzog, Bernd Kärcher, Riwal Plougonven, and William J. Randel

Abstract

The contribution of turbulent mixing to heat and tracer transport in the tropical tropopause layer (TTL) is poorly constrained, partly owing to a lack of direct observations. Here, the authors use high-resolution (20 Hz) airborne measurements to study the occurrence and properties of small-scale (<100 m) wind fluctuations in the TTL (14–19 km) over the tropical Pacific. The fluctuations are highly intermittent and appear localized within shallow (100 m) patches. Furthermore, active turbulent events are more frequent at low altitude, near deep convection, and within layers of low gradient Richardson number. A case study emphasizes the link between the turbulent events and the occurrence of inertio-gravity waves having small horizontal or vertical scale. To evaluate the impact of the observed fluctuations on tracer mixing, their characteristics are examined. During active events, they are in broad agreement with inertial-range turbulence theory: the motions are close to 3D isotropic and the spectra follow a −5/3 power-law scaling. The diffusivity induced by turbulent bursts is estimated to be on the order of 10−1 m2 s−1 and increases from the top to the bottom of the TTL (from ~2 × 10−2 to ~3 × 10−1 m2 s−1). Given the uncertainties involved in the estimate, this is in reasonable agreement (about a factor of 3–4 lower) with the parameterized turbulent diffusivity in ERA-Interim, but it disagrees with other observational estimates from radar and radiosondes. The magnitude of the consequent vertical transport depends on the altitude and the tracer; for the species considered, it is generally smaller than that induced by the mean tropical upwelling.

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Eric J. Jensen, Rei Ueyama, Leonhard Pfister, Thaopaul V. Bui, R. Paul Lawson, Sarah Woods, Troy Thornberry, Andrew W. Rollins, Glenn S. Diskin, Joshua P. DiGangi, and Melody A. Avery

Abstract

Numerical simulations of cirrus formation in the tropical tropopause layer (TTL) during boreal wintertime are used to evaluate the impact of heterogeneous ice nuclei (IN) abundance on cold cloud microphysical properties and occurrence frequencies. The cirrus model includes homogeneous and heterogeneous ice nucleation, deposition growth/sublimation, and sedimentation. Reanalysis temperature and wind fields with high-frequency waves superimposed are used to force the simulations. The model results are constrained by comparison with in situ and satellite observations of TTL cirrus and relative humidity. Temperature variability driven by high-frequency waves has a dominant influence on TTL cirrus microphysical properties and occurrence frequencies, and inclusion of these waves is required to produce agreement between the simulated and observed abundance of TTL cirrus. With homogeneous freezing only and small-scale gravity waves included in the temperature curtains, the model produces excessive ice concentrations compared with in situ observations. Inclusion of relatively numerous heterogeneous ice nuclei (N IN ≥ 100 L−1) in the simulations improves the agreement with observed ice concentrations. However, when IN contribute significantly to TTL cirrus ice nucleation, the occurrence frequency of large supersaturations with respect to ice is less than indicated by in situ measurements. The model results suggest that the sensitivity of TTL cirrus extinction and ice water content statistics to heterogeneous ice nuclei abundance is relatively weak. The simulated occurrence frequencies of TTL cirrus are quite insensitive to ice nuclei abundance, both in terms of cloud frequency height distribution and regional distribution throughout the tropics.

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Eric J. Jensen, Leonhard Pfister, David E. Jordan, Thaopaul V. Bui, Rei Ueyama, Hanwant B. Singh, Troy D. Thornberry, Andrew W. Rollins, Ru-Shan Gao, David W. Fahey, Karen H. Rosenlof, James W. Elkins, Glenn S. Diskin, Joshua P. DiGangi, R. Paul Lawson, Sarah Woods, Elliot L. Atlas, Maria A. Navarro Rodriguez, Steven C. Wofsy, Jasna Pittman, Charles G. Bardeen, Owen B. Toon, Bruce C. Kindel, Paul A. Newman, Matthew J. McGill, Dennis L. Hlavka, Leslie R. Lait, Mark R. Schoeberl, John W. Bergman, Henry B. Selkirk, M. Joan Alexander, Ji-Eun Kim, Boon H. Lim, Jochen Stutz, and Klaus Pfeilsticker

Abstract

The February–March 2014 deployment of the National Aeronautics and Space Administration (NASA) Airborne Tropical Tropopause Experiment (ATTREX) provided unique in situ measurements in the western Pacific tropical tropopause layer (TTL). Six flights were conducted from Guam with the long-range, high-altitude, unmanned Global Hawk aircraft. The ATTREX Global Hawk payload provided measurements of water vapor, meteorological conditions, cloud properties, tracer and chemical radical concentrations, and radiative fluxes. The campaign was partially coincident with the Convective Transport of Active Species in the Tropics (CONTRAST) and the Coordinated Airborne Studies in the Tropics (CAST) airborne campaigns based in Guam using lower-altitude aircraft (see companion articles in this issue). The ATTREX dataset is being used for investigations of TTL cloud, transport, dynamical, and chemical processes, as well as for evaluation and improvement of global-model representations of TTL processes. The ATTREX data are publicly available online (at https://espoarchive.nasa.gov/).

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