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At least 253 records · Page 14
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NASA NTRS1994

Transport into the troposphere in a tropopause fold/cut-off low system

A tropopause fold developed on the western flank of a trough in the 300 mb flow on 6 Oct. 1990. Radiosonde ascents over western Europe showed very dry stable layers beneath the jet stream in the potential temperature range 310 to 315 K. These were evident on profiles from 12h on 6 Oct. to 00h on 8 Oct. ECMWF model assimilations were examined for this period to determine how well the model represented the radiosonde observations. Humidity fields were found to give better agreement than potential vorticity, probably because the PV is affected by the limited vertical resolution of the model. Isentropic trajectories were calculated for the air in the fold as represented by the ECMWF assimilation at 00h on 7 Oct. Those on the western edge of the fold split from the main flow and transferred to the troposphere, while those on the eastern side ended up in the cut-off low. A lower bound of 1.1 x 10(exp 14) kg is estimated for the amount of stratospheric air transferred into the troposphere by this fold.

Vaughan, G.↗
NASA NTRS1994

Observed changes in the vertical profile of stratopheric nitrous oxide at Thule, Greenland, February - March 1992

Using a ground-based mm-wave spectrometer, we have observed stratospheric N2O over Thule, Greenland (76.3 N, 68.4 W) during late February and March, 1992. Vertical profiles of mixing ratio ranging from 16 to 50 km were recovered from molecular emission spectra. The profiles of early March show an abrupt increase in the lower-stratosphere N2O mixing ratio similar to the spring-to-summer change associated with the break up of the Antarctic polar vortex. This increase is correlated with changes in potential vorticity, air temperature, and ozone mixing ratio.

Emmons, Louisa K.↗
NASA NTRS1994

An observational study of the ozone dilution effect: Ozone transport in the austral spring stratosphere

In a previous observational analysis, Atkinson et al (1989) ascribed a sudden decrease in Southern Hemisphere midlatitude total ozone during December 1987 to an 'ozone dilution effect' brought about by the breakup of the polar stratospheric vortex at that time. A question alluded to but unanswered by that study was the degree to which the observed total ozone decrease might have been caused by the quasi-horizontal equatorward transport of 'ozone hold' air from within the vortex, and to what degree by the vertical advection from lower levels of air naturally low in ozone, a dynamical adjustment process which must accompany the equatorward outbreak of a discrete high-latitude airmass. In the present study, analyses of Ertel potential vorticity, TOMS total ozone, and SAGE and ozone sonde vertical profile data are employed using a novel technique to examine the 1987 event in greater detail, to answer this question. Recent progress is then reported in refining the technique and extending the investigation to examine the dynamical evolution of the austral spring stratosphere during other recent years, to shed more light on the precise nature, frequency, and severity of such 'ozone dilution' events, and the effect that this process may have on long term ozone behavior in the Southern Hemisphere.

Atkinson, Roger J.↗
NASA NTRS1994

Application of a planetary wave breaking parameterization to stratospheric circulation statistics

The planetary wave parameterization scheme developed recently by Garcia is applied to statospheric circulation statistics derived from 12 years of National Meteorological Center operational stratospheric analyses. From the data a planetary wave breaking criterion (based on the ratio of the eddy to zonal mean meridional potential vorticity (PV) gradients), a wave damping rate, and a meridional diffusion coefficient are calculated. The equatorward flank of the polar night jet during winter is identified as a wave breaking region from the observed PV gradients; the region moves poleward with season, covering all high latitudes in spring. Derived damping rates maximize in the subtropical upper stratosphere (the 'surf zone'), with damping time scales of 3-4 days. Maximum diffusion coefficients follow the spatial patterns of the wave breaking criterion, with magnitudes comparable to prior published estimates. Overall, the observed results agree well with the parameterized calculations of Garcia.

Randel, William J.↗
NASA NTRS1994

Observations of the 2-day wave in NMC stratospheric analyses

Observational characteristics of the 2-day wave, a westward-propagating zonal wave 3 oscillation in the summer subtropical upper stratosphere and mesosphere, are studied based on five years of National Meteorological Center (NMC) operational stratospheric analyses. These data show episodic occurrence of the 2-day wave in the upper stratosphere in January (centered near 20 deg S) and July - August (centered near 20 deg N). These episodes are strongly correlated with observed reversals of the zonal mean potential vorticity gradient near the core of the summer easterly jet, consistent with previous suggestions that the 2-day wave is generated by an in situ instability of this jet. On the other hand, the horizontal and vertical structure of the waves is very similar to that calculated by Salby for a global normal-mode Rossby wave. The combination of normal-mode structure and instability signature suggests that the 2-day wave is a near-resonant mode forced by dynamical instability.

Randel, William J.↗
NASA NTRS1994

The Eady problem for a basic state with zero PV gradient but beta does not equal 0

The classic Eady problem is modified to include beta does not equal 0, but with the basic distributions of temperature and zonal flow adjusted to preserve zero meridional gradients of basic-state potential vorticity in the fluid interior. Much of the mathematical simplicity of the classic problem is retained; however, the results differ in important ways. Specifically, the instability now has a long-wave cutoff in addition to the traditional short-wave cutoff. The former is associated with the fact that the phase speeds of the edge waves begin to differ so much as wavenumber is reduced that the two edge waves can no longer interact in order to form unstable modes. For the unstable modes, this manifests itself in that the steering level for unstable modes is always below the middle of the fluid and approaches the lower boundary near the long-wave cutoff. Relatedly, the amplitude of the unstable geopotential perturbations is larger at the upper boundary than at the lower boundary. Finally, below the long-wave cutoff, one of the neutral waves has a phase speed that becomes increasingly easterly as wavenumber decreases. This allows a resonant response to planetary-scale stationary forcing.

Lindzen, Richard S.↗
NASA NTRS1994

The permeability of the Antarctic vortex edge

Mixing and cross-vortex mass transport along isentropic surfaces in the lower stratosphere are investigated with a 'contour advection' technique and a semi-Lagrangian transport model for the Antarctic winter of 1993 using analyzed winds from the United Kingdom Meteorological Office data assimilation system. Results from the 'contour advection' technique show that at the vortex edge there exists a potential vorticity (PV) contour that has the smallest lengthening rate. This PV contour is referred to as the 'line of separation' because it essentially separates the inner and outer vortex. The average e-folding time for the lengthening of the 'line of separation' increases monotonically with altitude, ranging from about 7 days on the 350 K isentropic surface to about 105 days on the 500 K isentropic surface. The results also suggest the existence of a transition layer around the 400 K isentropic surface, above which the vortex is nearly completely isolated from the midlatitudes and below which the vortex is less isolated. Results from a semi-Lagrangian transport model with an idealized tracer initially inside the inner vortex show that at 425 K and above virtually no tracer is transported out of the vortex during a 40-day integration starting from July 21, 1993. At 400 K and below a small amount of the tracer is transported out of the vortex while the bulk of the tracer remains confined within the inner vortex.

Chen, Ping↗
NASA NTRS1994

Climatology of large-scale isentropic mixing in the Arctic winter stratosphere from analyzed winds

Dynamic isolation of the winter Arctic circumpolar vortex is studied using analyzed winds derived from geopotential height fields. Isentropic trajectories are calculated for assemblages of particles initialized on uniform latitude-longitude grids. Transport across isolines of Ertel potential vorticity (PV) is used to characterize the mixing processes of ejection of vortex air and entrainment of midlatitude air into the vortex. During January and February a barrier to mixing, where exchange of air is inhibited, typically forms near the vortex boundary. At 450 K, transport across the barrier is predominantly in the form of thin filaments of particles ejected from the vortex. These filaments tend to wrap around the vortex, creating a layered structure of vortex and midlatitude air at the vortex edge. Near or total splits of the vortex into two or more distinct vortex fragments are quite common based on these trajectory calculations. Significant entrainment deep into the vortex is rare and results from only a limited number of the splitting events. During December and March the mixing barrier is less evident due to nonconservative factors during the spin-up and breakdown of the vortex, respectively. In December both ejection and entrainment are only weakly inhibited by the mixing barrier. Exchange in March is dominated by ejection of air from the vortex.

Dahlberg, Steven P.↗
NASA NTRS1994

Stratospheric warmings during February and March 1993

Two stratospheric warmings during February and March 1993 are described using United Kingdom Meteorological Office (UKMO) analyses, calculated potential vorticity (PV) and diabetic heating, and N2O observed by the Cryogenic Limb Array Etalon Spectrometer (CLAES) instrument on the Upper Atmosphere Research Satellite (UARS). The first warming affected temperatures over a larger region, while the second produced a larger region of reversed zonal winds. Tilted baroclinic zones formed in the temperature field, and the polar vortex tilted westward with height. Narrow tongues of high PV and low N2O were drawn off the polar vortex, and irreversibly mixed. Tongues of material were drawn from low latitudes into the region between the polar vortex and the anticyclone; diabatic descent was also strongest in this region. Increased N2O over a broad region near the edge of the polar vortex indicates the importance of horizontal transport. N2O decreased in the vortex, consistent with enhanced diabatic descent during the warmings.

Manney, G. L.↗
NASA NTRS1994

Three-dimensional evolution of water vapor distributions in the Northern Hemisphere stratosphere as observed by the MLS

The three-dimensional evolution of stratospheric water vapor distributions observed by the Microwave Limb Sounder (MLS) during the period October 1991 - July 1992 is documented. The transport features inferred from the MLS water vapor distributions are corroborated using other dynamical fields, namely, nitrous oxide from the Cryogenic Limb Array Etalon Spectrometer instrument, analyzed winds from the U.K. Meteorological Office (UKMO), UKMO-derived potential vorticity, and the diabatic heating field. By taking a vortex-centered view and an along-track view, the authors observe in great detail the vertical and horizontal structure of the northern winter stratosphere. It is demonstrated that the water vapor distributions show clear signatures of the effects of diabatic descent through isentropic surfaces and quasi-horizontal transport along isentropic surfaces, and that the large-scale winter flow is organized by the interaction between the westerly polar vortex and the Aleutian high.

Lahoz, W. A.↗
NASA NTRS1994

On the motion of air through the stratospheric polar vortex

Trajectory calculations using horizontal winds from the U.K. Meteorological Office data assimilation system and vertical velocities from a radiation calculation are used to simulate the three-dimensional motion of air through the stratospheric polar vortex for Northern Hemisphere (NH) and Southern Hemisphere (SH) winters since the launch of the Upper Atmosphere Research Satellite (UARS). Throughout the winter, air from the upper stratosphere moves poleward and descends into the middle stratosphere. In the SH lower to middle stratosphere, strongest descent occurs near the edge of the polar vortex, with that edge defined by mixing characteristics. The NH shows a similar pattern in late winter, but in early winter strongest descent is near the center of the vortex, except when wave activity is particularly strong. Strong barriers to latitudinal mixing exist above about 420 K throughout the winter. Below this, the polar night jet is weak in early winter, so air descending below that level mixes between polar and middle latitudes. In late winter, parcels descend less and the polar night jet moves downward, so there is less latitudinal mixing. The degree of mixing in the lower stratosphere thus depends strongly on the position and evolution of the polar night jet and on the amount of descent experienced by the air parcels; these characteristics show considerable interannual variability in both hemispheres. The computed trajectories provide a three-dimensional picture of air motion during the final warming. Large tongues of air are drawn off the vortex and stretched into increasingly long and narrow tongues extending into low latitudes. This vortex erosion process proceeds more rapidly in the NH than in he SH. In the lower stratosphere, the majority of air parcels remain confined within a lingering region of strong potential vorticity gradients into December in the SH and April in the NH, well after the vortex breaks up in the midstratosphere.

Manney, G. L.↗
NASA NTRS1994

The anomalous Arctic lower stratospheric polar vortex of 1992-1993

Potential vorticity (PV) gradients defining the lower stratospheric vortex during the 1992-1993 winter were anomalously strong and persistent compared to those during the last 16 Arctic winters. For approximately equal to 3 months PV gradients were closer to typical Antarctic values than to most Arctic values. Air motion diagnostics computed for 3-dimensional air parcels confirm that the 1992-1993 Arctic lower stratospheric vortex was substantially more isolated than is typical. Such isolation will delay and reduce the export of the higher ozone typical of the winter lower stratospheric vortex to mid-latitudes. This may have contributed to the record-low total ozone amounts observed in northern mid-latitudes in 1993.

Manney, G. L.↗
NASA NTRS1994

Mixing by barotropic instability in a nonlinear model

A global, nonlinear, equivalent barotropic model is used to study the isentropic mixing of passive tracers by barotropic instability. Basic states are analytical zonal-mean jets representative of the zonal-mean flow in the upper stratosphere, where the observed 4-day wave is thought to be a result of barotropic, and possibly baroclinic, instability. As is known from previous studies, the phase speed and growth rate of the unstable waves is fairly sensitive to the shape of the zonal-mean jet; and the dominant wave mode at saturation is not necessarily the fastest growing mode; but the unstable modes share many features of the observed 4-day wave. Lagrangian trajectories computed from model winds are used to characterize the mixing by the flow. For profiles with both midlatitude and polar modes, mixing is stronger in midlatitude than inside the vortex; but there is little exchange of air across the vortex boundary. There is a minimum in the Lyapunov exponents of the flow and the particle dispersion at the jet maximum. For profiles with only polar unstable modes, there is weak mixing inside the vortex, no mixing outside the vortex, and no exchange of air across the vortex boundary. These results support the theoretical arguments that, whether wave disturbances are generated by local instability or propagate from other regions, the mixing properties of the total flow are determined by the locations of the wave critical lines and that strong gradients of potential vorticity are very resistant to mixing.

Bowman, Kenneth P.↗
NASA NTRS1994

Quasi-horizontal transport and mixing in the Antarctic stratosphre

The quasi-horizontal transport and mixing properties of the Antarctic stratosphere are investigated with a simi-Lagrangian transport model and a 'contour advection' technique for the winter and spring of 1992 using analyzed winds from the United Kingdom Meteorological Office data assimiliation system. Transport calculations show that passive tracers are well mixed inside the polar vortex as well as in the midlatitude 'surf zone.' A the vortex edge, strong radial gradients in the tracer fields are well preserved, and their evolutions follow that of the potential vorticity until some time after the breakdown of the polar vortex. In the middle stratosphere there is little tracer exchange across the vortex edge in August and September. Some vortex air is eroded into the surf zone in filamentary form in October, and very strong exchange of air occurs between high and middle latitudes in November. In the lower stratosphere the vortex is not so isolated from the midlatitudes as in the middle stratosphere, and there is more mass exchange across the vortex edge. Calculations of the lengthening of material contours using the contour advection technique show that in the middle stratosphere, strong stirring (i.e., stretching and folding of material elements) occurs in the inner vortex, while the strongest stirring occurs in the midlatitude surf zone and the weakest occurs at the vortex edge. In the lower strtosphere, strong stirring occurs in the inner vortex. Stirring is moderate at the vortex edge and in the midlatitudes.

Chen, Ping↗
NASA NTRS1993

Barotropic simulation of large-scale mixing in the Antarctic polar vortex

Theory and observations suggest that the Antarctic polar vortex is relatively isolated from midlatitudes, although others have interpreted the observations to indicate that there is substantial mixing from the interior of the vortex into middle latitudes. The equivalent barotropic model of Salby et al. is used to study quasi-horizontal mixing by the large-scale flow in the lower stratosphere during Southern Hemisphere spring, which is when the Antarctic ozone hole appears and disappears. The model is forced by relaxation to observed climatological monthly mean zonal-mean winds and by an idealized wave 1 or 2 forcing at the lower boundary. Mixing and transport are diagnosed primarily through Lagrangian tracer trajectories. For September, October, and November basic states, there is little or no mixing in the interior of the vortex. Mixing occurs near the critical lines for the waves: in the tropics and subtropics for a stationary wave 1, and in midlatitudes on the equatorial flank of the jet for an eastward-moving wave 2. For the December basic state, the wave 2 forcing rapidly mixes the interior of the vortex. Mixing of Lagrangian tracer particles can be significant even when the waves do not `break', as evidenced by the potential vorticity field. In the model there does not appear to be any significant transport of air out of the interior of the polar vortex prior to the vortex breakdown. The principal factor that leads to the vortex breakdown and mixing of the vortex interior is the deceleration of the jet to the point where winds in the interior of the vortex are close to the phase velocity of the wavenumber 2 forcing. The tracer transport is very similar to many aspects of the behavior of the total ozone field during the spring season.

Bowman, Kenneth P.↗
NASA NTRS1993

Intraseasonal variability in a barotropic model with seasonal forcing

It has recently been suggested that oscillatory topographic instability could contribute to low-frequency variability over the Northern Hemisphere midlatitudes. A barotropic potential vorticity model, with a hierarchy of forcing and topography configurations on the sphere, is used to investigate the nature of low-frequency oscillations induced by such instabilities. Steady-state solutions of the model include multiple unstable equilibria that sustain oscillatory instabilities with periods of 10 - 15 days, and 150 - 180 days, for a realistic forcing pattern, Time-dependent solutions exhibit chaotic behavior with episodic oscillations, featuring both the intraseasonal (35 - 50 day) and biweekly (10 - 15 day) modes. The former is dominated by standing spatial patterns, the latter by traveling wave patterns. The phases of the intraseasonal oscillation are robust for all cases, exhibiting a clear oscillatory exchange of atmospheric angular momentum with the solid earth via mountain troque. It is demonstrated, through linear stability analysis on the sphere, that the intraseasonal oscillations are induced by topographic instabilities. The role of the seasonal cycle is studied by prescribing an annual cycle in the forcing. In this case, the winter forcing is more favorable than the summer for the occurrence of episodic intraseasonal oscillations. Recent observations are consistent with this model result.

Strong, Christopher M.↗
NASA NTRS1994

Ozone heating and the destabilization of traveling waves during summer

The effects of ozone heating on the linear stability of lower stratospheric traveling waves of the summertime, extratropical circulation are examined. Based on coupled equations for the quasigeostrophic potential vorticity and ozone volume mixing ratio, it is shown that the diabatic heating arising from ozone advection can offset the damping due to Newtonian cooling, leading to wave amplification and significant changes in the structure and zonally rectified fluxes of the wave fields in both the lower stratosphere and troposphere. The vertical profile of the zonal mean wind plays a crucial role in determining whether the ozone heating destabilizes eastward and/or westward traveling disturbances.

Nathan, Terrence R.↗
NASA NTRS1995

An analysis of the Antarctic Halogen Occultation Experiment trace gas observations

Analysis of the version 16 Halogen Occultation Experiment (HALOE) CH4 data shows that this long-lived trace gas is well correlated with potential vorticity (PV) computed from National Meteorological Center balanced winds. Analyzing late September and October 1992 data, we show that very low CH4 values are confined to the interior of a vortex edge defined by the maximum gradient in PV. The CH4 and HF time tendency is used to estimate the descent rate in the Antarctic vortex. After removing a component of the trend correlated with the HALOE sampling pattern, we compute the lower stratosphere vertical descent rates and net heaing rates in the spring Antarctic vortex. Our computations of the spring Antarctic vortex heating rates give -0.5 to -0.1 K/day. Over the winter season, the overall lower stratospheric descent rate averages about 1.8-1.5 km/month. These computations are in line with radiative transfer estimates of the heating and descent rate. The HALOE data thus appear to be consistent with the picture of an isolated lower stratospheric Antarctic vortex.

Schoeberl, Mark R.↗