Transport and mixing in the extratropical tropopause region in a high vertical resolution GCM

Summary:
Detailed thermal, dynamic, and chemical structures of the extratropical upper troposphere and lower stratosphere (UTLS) have been studied using aircraft, sonde, satellite, and GPS measurements. These observational studies have revealed large vertical gradients in temperature and chemical constituent concentrations near the extratropical tropopause (i.e., the tropopause inversion layer (TIL) and the extratropical transition layer (ExTL)).

Numerical models such as general circulation models (GCMs) or chemical transport models (CTMs) are powerful tools for comprehending dynamic and thermodynamic processes. However, current models do not fully succeed in simulating the location and strength of static stability in the TIL. Iinsufficient model resolution may lead to unrealistic air transport around the ExTL. Moreover, small-scale transport processes by convection, turbulence, and gravity waves are generally parameterized in current models because of insufficient resolutions, although such small-scale motions play an important role in transporting air around the extratropical tropopause.

In this study, we adopted a vertically highly resolved GCM to clarify atmospheric structures and transport and mixing processes in the extratropical tropopause region. The high resolution GCM, which has a vertical resolution of about 300 m, can resolve fine atmospheric structures around the tropopause. This study has provided new insights into dynamic and thermodynamic processes in the extratropical tropopause region, especially related to formation of the ExTL and TIL.

	Strong temperature gradient forms a TIL, and the ExTL also lies at this level. The similar locations of the TIL and ExTL imply that the chemical and thermal structures interact with each other in the extratropical tropopause region. Thermal and dynamic structures simulated by the high resolution (T213L256) GCM were consistent well with observations, which showed a rapid increase in static stability and MPV just above the extratropical tropopause with a thickness of about 2 km, indicating the TIL. In contrast, the TIL simulated by the low resolution (T42L32) GCM was located at higher altitudes than indicated by observations and the high resolution GCM.
	The high resolution GCM simulates fine structures in the mean-meridional circulation in the UTLS region. The extratropical poleward flow nearly coincides with the dynamical tropopause, between approximately 1 to 5 PVU in the winter hemisphere. The mean downward velocity strongly converges around 25-40 K above the extratropical tropopause at high latitudes of the NH during the winter.
	Dominant transport processes in the extratropical tropopause region were clarified by using a PV budget analysis. The diabatic source/sink effect on the PV gradient is less significant than transport effects in the extratropical UTLS, and therefore, the PV analysis results can be used for explaining transport characteristics in these regions. Although the results of the gradient genesis analysis can differ among PV and chemical tracers because of a difference in their vertical profiles, the PV analysis results imply that the formation of large chemical tracer concentration gradients around the ExTL primarily arises from the mean downward advection (MEZ1&MEZ2)in the lower levels and isentropic (EDY, during winter) and vertical (EDZ, during summer) mixing in the upper levels.
	Analysis of a thermodynamic equation elucidated the relationship between the formation mechanisms of the TIL and the ExTL. Stratification by radiation (QRAD) and downward advection of the static stability profile (MEZ2) play important roles in determining seasonal variation in the static stability of the TIL. The summertime maximum in static stability in the TIL is primarily a result of radiation (QRAD). In the lower part TIL, downward advection of high static stability air (MEZ2) dominantly increases the static stability. To summarize, we have clarified that the locations of the TIL and ExTL are similar, as a result of common dynamic processes and interaction between constituent distributions and thermal structures in the extratropical tropopause region; i.e., the downward advections of constituent gradients and heat in the lower levels, and the mixing of constituents, including water vapor, and the radiative stratification effect caused by large water vapor variations in the upper levels.
	Transport of tropospheric wet air induced by eddy transports creates large gradients in water vapor concentrations in the ExTL. Large variations in water vapor concentrations contribute to the formation of the TIL through the diabatic effect of radiation, particularly in the upper TIL.
The relative contributions of atmospheric waves with different scales, including resolved gravity waves, to the driving of the mean meridional circulation have been examined using the downward control calculation. The E-P flux associated with gravity waves diverges and induces a mean equatorward flow in the extratropical tropopause region. It partly cancels a mean poleward flow induced by the planetary and synoptic-scale wave E-F P flux convergence near the extratropical tropopause.
Strong isentropic mixing is observed between 20 K below and 10 K above the tropopause from autumn to spring. Vertical mixing are substantial in the tropopause region during summer, but are strongly suppressed just above the well-mixed region throughout the year.
Isentropic mixing Vertical mixing In order to investigate the relative contributions of atmospheric motions with different scales to isentropic mixing, the eddy PV flux can be decomposed into individual zonal wavenumber components. Planetary waves largely contribute to isentropic mixing in the stratosphere, while both planetary-scale and synoptic-scale waves cause strong isentropic mixing below the lowermost stratosphere. The small horizontal-scale motions (s > 21) provide an important contribution to the total isentropic mixing just above the tropopause. The enhanced contribution of the small horizontal-scale mixing is a unique and important characteristic of the extratropical tropopause region. The vertical dispersion by the small-scale eddy motions with horizontal wavelengths is clearly strengthened just above the tropopause. The analysis results confirm the occurrence of obvious three-dimensional mixing by small-scale motions around the TIL.
	Small horizontal-scale disturbances are large over regions with possible sources of gravity waves (high mountains, cyclones, fronts, and convection) at the TIL level.
	To summarize, the small-scale dynamics associated with the propagation and breaking of gravity waves play important roles in driving tracer transports by both the mean-meridional circulation and three-dimensional mixing in the extratropical tropopause region. It may be important to include these small-scale dynamic effects in GCMs or CTMs to obtain better simulations of the TIL and ExTL.
References: Miyazaki K., S. Watanabe, Y. Kawatani, Y. Tomikawa, K. Sato, and M. Takahashi, Transport and mixing in the extratropical tropopause region in a high vertical resolution GCM. Part I: Potential vorticity and heat budget analysis, Journal of the Atmospheric Sciences, Vol. 67, No. 5, 1293-1314. Miyazaki K., K. Sato, S. Watanabe, Y. Tomikawa, Y. Kawatani, and M. Takahashi, Transport and mixing in the extratropical tropopause region in a high vertical resolution GCM. Part II: Relative importance of large-scale and small-scale dynamics, Journal of the Atmospheric Sciences, Vol. 67, No. 5, 1315-1336.