Trends in the Extratropical Tropopause

[contributed by Lorenzo Polvani, Seok-Woo Son and Thomas Birner]

1. Introduction

The purpose of this page is to describe how one goes about computing trends in the height and temperature of the tropopause in the CCMVal simulations. The emphasis here is on the global tropopause, with special attention to the extra-tropical tropopause. TTL matters are described in detail here.

2. Data and Calculations

Needless to say, one may use the tropopause pressure and temperature that ought to be archived by the individual modeling teams onto the CCMVal data archive. If these are not available, and they are not as of this writing, one can compute the tropopause height using the methodology in Reichler et al. (2003), briefly sketched here.

One starts from the temperature T on pressure levels p, and computes the lapse rate

where k=R/cp, R is the gas constant for dry air, cp the specific heat capacity of air at constant pressure, and g is the gravitational acceleration.

Using linear interpolation between any two pressure levels, one first finds the lowest level at which the lapse rate falls below 2K/km, and then verifies that the average of the lapse rate between that level and all higher levels within 2 km remains below 2K/km. If the latter does not hold, one proceeds to the next higher level, until the second criterionis satisfied.

This procedure is the *precise* definition of tropopause pressure as specified by the WMO (WMO 1957). In practice, for for most CCMVal data, the tropopause is likely to be well defined, and correct numbers can be obtained regardless of the detailed procedure for calculating tropopause pressure. This will be especially true when the procedure is applied to monthly mean temperature fields. However, the strict definition may be important when considering instantaneous three-dimentional temperature field, especially at high vertical resolution.

Once the tropopause pressure is known, linear interpolation is again used to compute the temperature at that pressure.

3. Example

Here we show the time evolution of the tropopause pressure from a subset of the CCMVal simulations, plus the NCEP/NCAR reanalysis for the period 1980-2100.

Specifically: (a) whole globe, (b) tropics, (c) SH extratropics, and (d) NH extratropics (averages are done with the cosine latitude weight factor). All time series are slightly smoothed, using an 11-year running mean filter. Thick gray and black lines denote the average of all four runs and three long-term runs, respectively. The red line shows the NCAR/NCEP reanalysis.

4. References

S.W. Son, L.M. Polvani, D.W. Waugh, et. al.: The tropopause in the 21st as simulated using stratosphere-resolving chemistry-climate models, J. Geophys. Res, in preparation (2007)

T. Reichler, M. Dameris, and R. Sausen: Determining the tropopause height from gridded data, Geophys. Res. Lett.,Vol. 30, No. 20, 2042, doi:10.1029/2003GL018240 (2003)

World Meteorological Organization: Meteorology - a three-dimensional science, 230 WMO Bull., 6, 134-138 (1957)