The Height of the Tropopause

Principal Investigator: John Thuburn

Background: The tropopause is one of the most fundamental features of the temperature structure of the atmosphere. Despite this, it is still not clearly understood what determines its location, why it is so sharp in places, or even why it exists at all. The maintenance of the tropopause is intimately related to the exchange of air, moisture and trace chemicals between the troposphere and the stratosphere, which has implications for the chemistry of both regions.

Achievements: The simplest model that attempts to explain the height of the tropopause is a radiative-convective model. However, this predicts a tropospheric lapse rate that is much less stable than observed in the extratropics. Alternative theories, based on "baroclinic adjustment", have been proposed to explain the extratropical temperature structure. These radiative and dynamical mechanisms have been investigated using a version of the UGAMP GCM with increased vertical resolution in the region of the tropopause. A series of experiments with the GCM showed that the tropopause height is highly sensitive to changes in the surface temperature but relatively insensitive to changes in the ozone distribution or the planet's rotation rate.

A relationship between tropospheric lapse rate and tropopause height predicted by a simple radiative model was found to hold well in the extratropics of the GCM. The simple radiative model explains the sensitivity of tropopause height to surface temperature in terms of changes in the moisture distribution and its resulting radiative effects. However, baroclinic adjustment does not appear to occur in the GCM.

In the tropics the situation is again more complicated than suggested by the radiative-convective model. Between about 150 hPa and the tropopause the lapse rate is more stable than predicted by the radiative-convective model, and convection, radiation and mean upwelling all play a role in determining the temperature.

The four panels each show the zonal mean 30-day mean tropopause height as a function of latitude for a control GCM run (solid line) and two other runs. The top left panel shows that shifting the ozone profile downward about 7 km (dotted line) has little effect on the tropopause height. Removing the ozone completely (dashed line) drastically changes the stratospheric temperature profile but a well defined tropopause continues to exist only slightly higher than in the control run. The top right panel shows that increasing (dotted line) or decreasing (dashed line) the planet's rotation rate by 40% has a very small effect on tropopause height. The lower left panel shows that reducing the lower boundary temperature uniformly by 10 K (dotted line) or by 20 K (dashed line) causes a significant lowering of the tropopause in both the tropics and extratropics. This sensitivity occurs partly through changes in the moisture distribution and its radiative effects. The lower right panel shows that cooling the polar lower boundary by 20 K (dotted line) leads to a lowering of the extratropical tropopause while warming the polar lower boundary by 20 K (dashed line) leads to a raising of the extratropical tropopause. This is contrary to the predictions of baroclinic adjustment theory, but is consistent with the results shown in the lower left panel applied latitude by latitude.