Diabatic cross-isentropic dispersion in the lower stratosphere

A significant contribution to vertical dispersion of tracers in the stratosphere arises from the variability in the diabatic heating of air parcels. Air parcels starting on a given isentropic surface experience different time histories of diabatic heating, which causes vertical dispersion across isentropic surfaces. We refer to this process as “diabatic cross-isentropic dispersion,” or “diabatic dispersion” for brevity. The present study investigates diabatic dispersion in the lower stratosphere by computing parcel trajectories initialized uniformly over the 500 K surface on January 1, 1993. Parcels are followed for 2 months using analyzed winds and diabatic heating rates computed from analyzed temperatures. Diabatic dispersion depends on the statistics of the large-scale horizontal eddy motion as well as on the spatial structure of the diabatic heating field. The trajectory statistics suggest that the polar vortex, surf zone, tropics, and extratropical summer hemisphere are, to varying extents, isolated from each other and that the diabatic dispersion within each of these regions is different. In both the surf zone and southern hemisphere extratropics the dispersion is initially advective, with potential temperature variance 〈δθ(t)2〉 increasing as t2 as time t increases. After about 1 month, the dispersion becomes diffusive in the sense that 〈δθ(t)2〉 ∼2Kθθt and with a diffusivity Kθθ in the range 2–6 K2 d−1, roughly equivalent to Kzz ∼0.1–0.2 m2 s−1. The emergence of a diffusive regime is discussed in terms of loss of memory of diabatic heating along parcel paths, as measured by the decay of the Lagrangian autocorrelation function. Diabatic dispersion within the tropics and polar vortex over the 2-month period is more than an order of magnitude smaller and is less clearly diffusive. The diabatic dispersion of parcels moving poleward out of the tropics into either hemisphere is faster than either differential advection or diffusion, and in this cas 〈δθ(t)2〉 increases as t3. For the total ensemble of all parcels, the potential temperature variance increases as t2, consistent with global-scale differential advection by the mean diabatic circulation. This is inconsistent, at least on the 2-month timescale considered here, with a one-dimensional diffusive model of vertical dispersion.

[1]  N. Kampen,et al.  Stochastic processes in physics and chemistry , 1981 .

[2]  Richard Swinbank,et al.  A Stratosphere-Troposphere Data Assimilation System , 1994 .

[3]  E. Dewan.  Turbulent Vertical Transport due to Thin Intermittent Mixing Layers in the Stratosphere and Other Stable Fluids. , 1981, Science.

[4]  Geoffrey Ingram Taylor,et al.  Diffusion by Continuous Movements , 1922 .

[5]  J. Lumley,et al.  A First Course in Turbulence , 1972 .

[6]  M. Yamamoto,et al.  Seasonal variability of vertical eddy diffusivity in the middle atmosphere: 1. Three‐year observations by the middle and upper atmosphere radar , 1994 .

[7]  J. Bacmeister,et al.  Stratospheric horizontal wavenumber spectra of winds, potential temperature, and atmospheric tracers observed by high-altitude aircraft , 1996 .

[8]  D. Lilly,et al.  Stratospheric Mixing Estimated from High-Altitude Turbulence Measurements , 1974 .

[9]  C. Trepte,et al.  A climatology of stratospheric aerosol , 1994 .

[10]  R. A. Plumb.  A “tropical pipe” model of stratospheric transport , 1996 .

[11]  H. Levy,et al.  Three‐dimensional simulations of stratospheric N2O: Predictions for other trace constituents , 1986 .

[12]  S. Corrsin.  Limitations of Gradient Transport Models in Random Walks and in Turbulence , 1975 .

[13]  P. Newman,et al.  Computations of diabatic descent in the stratospheric polar vortex , 1994 .

[14]  L. Polvani,et al.  On the Subtropical Edge of the Stratospheric Surf Zone , 1995 .

[15]  John Thuburn,et al.  Numerical advection schemes, cross‐isentropic random walks, and correlations between chemical species , 1997 .

[16]  M. McIntyre.  The stratospheric polar vortex and sub-vortex : fluid dynamics and midlatitude ozone loss , 1995, Philosophical Transactions of the Royal Society of London. Series A: Physical and Engineering Sciences.

[17]  William R. Young,et al.  Shear-Flow Dispersion, Internal Waves and Horizontal Mixing in the Ocean , 1982 .

[18]  M. Juckes,et al.  A high-resolution one-layer model of breaking planetary waves in the stratosphere , 1987, Nature.

[19]  H. Kida.  General Circulation of Air Parcels and Transport Characteristics Derived from a hemispheric GCM: Part 1. A Determination of Advective Mass Flow in the Lower Stratosphere@@@第一部下部成層圏の質量移流の決定 , 1983 .

[20]  P. Haynes,et al.  Quantification of lower stratospheric mixing processes using aircraft data , 1997 .

[21]  A. O'Neill,et al.  Isentropic Mass Exchange between the Tropics and Extratropics in the Stratosphere , 1994 .

[22]  K. Shine.  Sources and sinks of zonal momentum in the middle atmosphere diagnosed using the diabatic circulation , 1989 .

[23]  J. Holton.  A dynamically based transport parameterization for one-dimensional photochemical models , 1986 .

[24]  A. Douglass,et al.  Thermodynamic balance of three-dimensional stratospheric winds derived from a data assimilation procedure , 1993 .

[25]  J. Mahlman.  Mechanistic Interpretation of Stratospheric Tracer Transport , 1985 .

[26]  M. McIntyre.  Middle Atmospheric Dynamics and Transport: Some Current Challenges to our Understanding , 1990 .

[27]  P. Newman,et al.  An objective determination of the polar vortex using Ertel's potential vorticity , 1996 .

[28]  Richard B. Rood,et al.  An assimilated dataset for Earth science applications , 1993 .

[29]  D. Fahey,et al.  Quantifying Transport Between the Tropical and Mid-Latitude Lower Stratosphere , 1996, Science.

[30]  R. Swinbank,et al.  Characteristics of stratospheric winds and temperatures produced by data assimilation , 1997 .

[31]  T. Shibata,et al.  The El Chichon volcanic cloud in the stratosphere - Lidar observation at Fukuoka and numerical simulation , 1984 .

[32]  C. Long,et al.  Use of volcanic aerosols to study the tropical stratospheric reservoir , 1996 .

[33]  M. McCormick,et al.  Spatial changes in the stratospheric aerosol associated with the north polar vortex , 1983 .

[34]  Andrew J. Watson,et al.  Evidence for slow mixing across the pycnocline from an open-ocean tracer-release experiment , 1993, Nature.

[35]  P. Mote,et al.  An atmospheric tape recorder: The imprint of tropical tropopause temperatures on stratospheric water vapor , 1996 .

[36]  A. O'Neill,et al.  Rapid descent of mesospheric air into the stratospheric polar vortex , 1993 .

[37]  D. Waugh.  Seasonal variation of isentropic transport out of the tropical stratosphere , 1996 .

[38]  G. Taylor.  Dispersion of soluble matter in solvent flowing slowly through a tube , 1953, Proceedings of the Royal Society of London. Series A. Mathematical and Physical Sciences.

[39]  R. Woodman,et al.  Evaluation of effective eddy diffusive coefficients using radar observations of turbulence in the stratosphere , 1984 .

[40]  Malcolm K. W. Ko,et al.  Interrelationships between mixing ratios of long‐lived stratospheric constituents , 1992 .

[41]  Mark R. Schoeberl,et al.  The structure of the polar vortex , 1992 .

[42]  J. Eluszkiewicz,et al.  Dynamics of wintertime stratospheric transport in the Geophysical Fluid Dynamics Laboratory SKYHI general circulation model , 1995 .

[43]  Ka Kit Tung,et al.  On the Two-Dimensional Transport of Stratospheric Trace Gases in Isentropic Coordinates , 1982 .

[44]  R. A. Plumb,et al.  The Zonally Averaged Transport Characteristics of the GFDL General Circulation/Transport Model , 1987 .

[45]  J. Holton,et al.  Stratosphere‐troposphere exchange , 1995 .