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Paul Spaete, Donald R. Johnson, and Todd K. Schaack

Abstract

Using data generated from a model simulation, the exchange of mass between the stratosphere and the troposphere is estimated for the Presidents' Day storm during a 24-h period beginning at 1500 UTC 18 February 1979. This 24-h interval coincided with a strongly developed tropopause depression and the onset of explosive surface cyclogenesis. The initial part of the study consists of identifying a surface of isentropic potential vorticity (IPV) to represent the tropopause. The 3.0-IPV-unit surface is chosen since the pressure distribution on this surface closely matches the tropopause pressures reported by radiosonde stations. The IPV surface portrays the depression of the tropopause associated with the polar-front jet and trough system accompanying the baroclinic amplification of the Presidents' Day storm.

Using a quasi-Lagrangian transport model, stratospheric–tropospheric mass exchange is estimated for the region including and immediately adjacent to the tropopause depression. The estimated mass transport from the stratosphere to the troposphere for the 24-h period is 5 × 1014 kg. The transport from the troposphere to the stratosphere is 2 × 1014 kg yielding a net transport across the tropopause of 3 × 1014 kg from the stratosphere to the troposphere. These results are confirmed by a second, independent model simulation.

The mass transport from stratosphere to troposphere across the 3.0-IPV surface coincides with descending air, often referred to as the “dry airstream,” arcing counterclockwise around the polar-front jet and trough system from northwest to east. Reverse transport from the troposphere to the stratosphere occurs northeast of the depression and agrees with trajectories of air parcels within the end region of rising “conveyer belts."

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Martin P. Hoerling, Todd K. Schaack, and Allen J. Lenzen

Abstract

The European Center for Medium Range Weather Forecasts (ECMWF) level IIIb dataset is used to construct global pressure analyses of the tropopause surface during January 1979. Two methods are employed: a dynamical method based on isentropic potential vorticity (IPV) and a thermal method based on lapse rate criteria. Regional tropopause pressure analyses are extracted from the global analyses and compared against distributions derived from rawinsonde data. The coarse vertical resolution of the ECMWF data compromises the ability to resolve abrupt stability changes between the troposphere and stratosphere and impacts tropopause analyses using both methods. Sensitivity of the derived tropopause pressures to a range of IPV and lapse rate thresholds is examined. For the assimilated dataset employed herein, 3.5 IPV units represent an optimal value for tropopause analysis outside the tropics. Modification of the WMO lapse rate criteria does not significantly improve tropopause analysis globally.

Both methods capture the large-scale features of the radiosonde-reported tropopause surface in the regional analyses, although each approach has limitations. The spatial structure and temporal evolution of the dynamically determined tropopause surface within a developing extratropical cyclone is found to be superior to that based on lapse rate criteria, while only the lapse rate method is a viable approach in the tropics.

We conclude that the pressure of the tropopause surface can be determined globally using ECMWF assimilated data. The preliminary results are encouraging and suggest that it is feasible to proceed beyond sounding analyses and case studies for determining the tropopause position. We view this to be an important first step toward implementing global studies of stratospheric–tropospheric exchange.

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Martin P. Hoerling, Todd K. Schaack, and Allen J. Lenzen

Abstract

Using a mathematical formulation of stratospheric-tropospheric (ST) exchange, the cross-tropopause mass flux is diagnosed globally for January 1979. Contributions by physical mechanisms including the diabatic transport and the quasi-horizontal adiabatic transport along isentropes that intersect the tropopause surface are evaluated. Both thermal and dynamical definitions of the tropopause are used.

Two regions of zonally integrated mass flux into the stratosphere are found, one over tropical latitudes associated with diabatic transports, and a second over subpolar latitudes associated with adiabatic transports. The ingress to the stratosphere in each of the latitude bands 50°–70°N and 40°–70°S is as intense as that occurring over the tropics, a feature of the global budget not previously documented. Compensating mass outflow from the stratosphere occurs mainly over midlatitudes near axes of strong upper-level westerlies.

Large zonal asymmetries are found in the regional patterns of ST exchange. Consistent with the concept of a stratosphere fountain, the tropical inflow to the stratosphere is maximized over the Australasian monsoon. The midlatitude mass outflow tends to be concentrated along stationary wave troughs, roughly in the vicinity of cyclogenetic areas. A mass transport into the stratosphere occurs downstream and poleward of the troughs. The extratropical pattern of time-averaged cross-tropopause mass flux thus appears to be interpretable within the framework of simple physical models on three-dimensional airmass trajectories in baroclinic disturbances.

While uncertainties concerning quantitative aspects of the global ST exchange remain, qualitative confirmation of the mass-transport diagnostics is found in independent studies of trace atmospheric constituents. In particular, the finding of mass inflow to the stratosphere at subpolar latitudes is consistent with satellite and aircraft measurements of high water vapor mixing ratios in the low stratosphere over these regions.

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Tom H. Zapotocny, Allen J. Lenzen, Donald R. Johnson, Todd K. Schaack, and Fred M. Reames

Abstract

Five- and 10-day inert trace constituent distributions prognostically simulated with the University of Wisconsin (UW) hybrid isentropic–sigma (θσ) model, the nominally identical UW sigma (σ) model, and the National Center for Atmospheric Research Community Climate Model 2 (CCM2) are analyzed and compared in this study. The UW θσ and σ gridpoint models utilize the flux form of the primitive equations, while CCM2 is based on the spectral representation and uses semi-Lagrangian transport (SLT) for trace constituents. Results are also compared against a version of the CCM that uses spectral transport for the trace constituent. These comparisons 1) contrast the spatial and temporal evolution of the filamentary transport of inert trace constituents simulated with the UW θσ and σ models against a “state of the art” GCM under both isentropic and nonisentropic conditions and 2) examine the ability of the models to conserve the initial trace constituent maximum value during 10-day integrations.

Results show that the spatial distributions of trace constituent evolve in a similar manner, regardless of the transport scheme or model type. However, when compared to the UW θσ model’s ability to simulate filamentary structure and conserve the initial trace constituent maximum value, results from the other models in this study indicate substantial spurious dispersion. The more accurate conservation demonstrated with the UW θσ model is especially noticeable within extratropical amplifying baroclinic waves, and it stems from the dominance of two-dimensional, quasi-horizontal isentropic exchange processes in a stratified baroclinic atmosphere. This condition, which largely precludes spurious numerical dispersion associated with vertical advection, is unique to isentropic coordinates. Conservation of trace constituent maxima in sigma coordinates suffers from the complexity of, and inherent need for, resolving three-dimensional transport in the presence of vertical wind shear during baroclinic amplification, a condition leading to spurious vertical dispersion. The experiments of this study also indicate that the shape-preserving SLT scheme used in CCM2 further reduces conservation of the initial maximum value when compared to the spectral transport of trace constituents, although the patterns are more coherent and the Gibbs phenomenon is eliminated.

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