A Diagnostic part of the SPARC Chemistry-Climate Model Validation Activity

Back to Process Tables

Diagnosing the Tropical Tropopause Layer in Global Models

Andrew Gettelman, National Center for Atmospheric Research

(email: andrew at ucar.edu, phone: +1 303 497 1887)

Version 1.0, 1 November 2005.  Comments welcome: please email me.

PROCESS

The Tropical Tropopause Layer (TTL) is a transition region in the tropics between the radiative balance of the stratosphere and radiative-convective equilibrium in the troposphere. The TTL is essentially the region between these boundaries. There are multiple definitions possible, but the key for models is the following:

1. There is a separation between the cold point tropopause and the level at which the ozone starts to increase rapidly with height. The level at which ozone starts to increase is basically the level at which convection stops efficiently mixing air (Folkins 1999). In models this could be determined by ozone distributions, or the maximum height of convection
   
2. It is also critical that the level of zero radiative heating be about 15km, especially for clear sky (Gettelman et al 2004, Corti et al 2005).
   
3. There is a definable change in the dry static stability, especially in convective regions, near 10-12km, which is where the maximum convective outflow should be. This is a minimum in stability (buoyancy frequency), or minimum (max negative value) in temperature lapse rate, or a minimum in the potential temperature lapse rate (Gettelman and Forster 2002).
   

The basic definition of the TTL is from the level of minimum stability to the cold point. Probably the most convenient way then to define the TTL in models is to use the definition of the TTL from Gettelman and Forster (2002). This can be done using temperature and ozone fields, and some knowledge of heating. Ideally this should be done on model levels at has high temporal resolution as possible, but for inter model comparisons and comparisons with climatologies, even zonal monthly means on pressure are useful.

Diagnostics

Analysis could be done on 2D fields. Ideally use 3D fields (instantaneous) and average. Keys to look for:

1. CPT should be 'correct' temperature relative to GPS temperatures (mean at least).
2. CPT should be 16+/-1km, and above Q=0
3. Q=0 should be 15+/-1km and below CPT (delz between CPT and Q=0 also useful metric)
4. dO3/dz 'change' should occure BELOW CPT, ideally around 12-14km.
5. Min lapse rate should be 10-12km, higher in regions of active convection

Variables

Temperature, Ozone, LW Heating rates, SW heating rates

1. T to define altitude (pressure) of: (a) CPT and (b) minimum potential temperature lapse rate (dq/dz), and value of CPT.
2. Use heating rates to determine level of zero radiative heating  (all sky)
3. Use ozone profiles to define (a) minimum UT O3 and (b) O3 lapse rate change

Correlative data

(I will try to get more links here)

• GPS temperatures of CPT (seasonal mean)
  • GPS temperatures are best for absolute accuracy of tropical cold point temperatures
  • Good for both the mean (climatology) and total variability (std deviation, waves)
• Radiosonde seasonal mean CPT, min lapse rate
• Ozone profiles (SHADOZ archive is a good place to start)
• For comparison with models on standard pressure levels:
  • ECMWF (re)analyses or NCEP (re)analyses on pressure levels
  • Note this is for the bottom of TTL and CP altitude/pressure, not cold point temperature
  • Analyses have cold point temperatures which are biased high (Pawson & Fiorino, 1999)

Acknowledgements

Thanks to Thomas Birner for Comments

References

(click for link to pdf if available)

I. Folkins, M. Loewenstein, J. Podolske, S. J. Oltmans, and M. Proffitt. A barrier to vertical mixing at 14 km in the tropics: Evidence from ozonesondes and aircraft measurements. J. Geophys. Res., 104(D18):22,095-22,102, 1999

T. Corti, B. P. Luo, T. Peter, H. Voemel and Q. Fu. Mean radiative energy balance and vertical mass fluxes in the equatorial upper troposphere and lower stratosphere. Geophys. Res. Lett, 32, L06802, doi:10.1029/2004GL021889, 2005

A. Gettelman, P. M. F. Forster, M. Fujuwara, Q. Fu, H. Vomel, L. K. Gohar, C. Johanson, and M. Ammeraman. The radiation balance of the tropical tropopause layer. J. Geophys. Res., 109, doi: 10.1029/2003JD004190, 2004

A. Gettelman and P. M. F. Forster. A climatology of the tropical tropopause layer. J. Met. Soc. Japan, 80(4B):911-924, 2002

S. Pawson and M. Fiorino. A comparison of reanalyses in the tropical stratosphere. Part 1: thermal structure and the annual cycle. Climate Dynamics, 14:631-644, 1998