data that has a range spanning zero such as differences of a quantity from its mean.
These schemes are colour-blind friendly. In addition black or grey label contours are superimposed and labelled with the contour value. Coloured contours are plotted at half the interval of the black/grey contours.
Extratropical tropopause inversion layer
3.1 Introduction
A key feature of the extratropical upper troposphere and lower stratosphere is a region of stable air located just above the tropopause. This region is commonly referred to as the tropopause inversion layer (TIL) since it is directly related to an inversion in the vertical temperature gradient i.e the vertical temperature gradient changes from a roughly steady decrease in the troposphere to a strong increase just above the tropopause. The TIL is a few kilometers thick and is located about 0–1 km above the extratropical tropopause. Figure 3.1 is an illustration of the lower part of the atmosphere together with the extratropical TIL.
Figure 3.1: Schematic diagram of the lower part of the atmosphere showing the rough location of the TIL. The stratosphere is followed upwards by the mesosphere, thermosphere and exosphere. The diagram is not to scale.
Birner et al. (2002) noted the presence of this feature by looking at observational data. Figure 3.2, modified from Figure 1 in Birner et al. (2002), shows profiles of temperature and buoyancy frequency squared. The buoyancy frequency is a measure of how stable a parcel of air is with respect to its surroundings, i.e. its static stability.
The troposphere is characterized by low values of static stability and the stratosphere by higher values. The fields shown come from radiosonde data and from the ECMWF reanalysis data set (ERA-40) [Uppala et al. (2005)]. Reanalysis data products are generated by assimilating observational data from a wide range of sources such as radiosonde, aircraft and satellite measurements into a model. The vertical coordinate used by Birner et al. (2002) is a tropopause based height where the profiles are shifted so that the vertical variation in tropopause height over time is accounted for. This process is also referred to as compositing. The tropopause height can be defined in various ways and Birner et al. (2002) use the one based on the vertical temperature gradient. Further details on defining the tropopause will be given in the next section.
The horizontal lines in Figure 3.2 represent the tropopause and the sharp peak above the tropopause can be identified as the extratropical TIL. A simple vertical coordinate average does not capture the sharp transition in static stability seen in a tropopause based average.
Figure 3.2: [Modified from Figure 1 in Birner et al. (2002)] Mean profiles and corresponding relative standard deviations of (a) temperature, (b) buoyancy frequency squared for Munich.
Black full lines represent averages for radiosonde data. Dotted lines denote mean ERA-profiles.
Profiles are time-averaged (denoted by the overbar) relative to the respective tropopause height (horizontal lines). The grey shading indicates the tropopause region.
The TIL is in a region characterized by a strong connectivity between radiative, dynamical, chemical and microphysical processes and as a result is highly sensitive to climate change. Its presence can play a crucial role in affecting chemical transport
across the extratropical tropopause. Processes that contribute to the formation of the TIL are still not well understood and are a matter of active research. Understanding the structure of the static stability in the atmosphere is important because static stability is central to dispersion relations for atmospheric waves such as gravity and Rossby waves which all depend to some extent on the restoring force associated with buoyancy.
The midlatitude troposphere is dominated by eddies which affect the stratification.
Wirth (2003) looks more closely at the effect of those eddies and suggests that the asymmetry between upper level cyclones and anticyclones affects the local stratification around the tropopause in such a way that for anticyclonic flow, composited profiles of static stability reveal a TIL in the climatological mean. Randel et al. (2007b) points out that although the strength of the TIL is modulated by the local circulation, as expected from balanced dynamics [Wirth (2003)], the fact that the inversion layer is also present for cyclonic flow suggest additional forcing mechanisms. Randel et al. (2007b) conducted a set of radiative transfer calculations and showed that the lower stratospheric structure of ozone and water vapour produce the right pattern of temperature changes to force and maintain a TIL. However, a radiative mechanism also inherently includes transport effects by assuming the dynamics lead to the observed ozone and water vapour distributions. In the stratosphere, air rising from the tropical pipe is transported polewards and then descends over the high latitudes.
This circulation is called the Brewer–Dobson circulation [Haynes (2005)]. Birner et al.
(2006) examined the properties of the downwelling branch of the Brewer–Dobson circulation. Results from the Canadian Middle Atmosphere Model (CMAM) and ERA-40 reanalysis showed that the strong vertical gradients in the downwelling just above the tropopause help create a TIL. The time–dependent equation for the static stability derived in Birner et al. (2006) contains the gradient of the vertical velocity as a crucial forcing term.
The TIL has been shown to be present in reanalysis data [Birner (2006)] and quasi realistic global circulation models [Birner (2006), Miyazaki et al. (2010)] and in satellite temperature profiles [Randel et al. (2007b), Grise et al. (2010)]. Models produce a TIL which is broader and smoother than observational data, and the peak is not significantly enhanced by tropopause based (TB) averaging; in particular, the sharp transition in static stability is not seen. This has been attributed to their coarse vertical resolution and Hegglin et al. (2010) show that they behave similarly to observations where the high vertical resolution is degraded. Son and Polvani (2007) ran a simple general circulation model which did not include a realistic radiative forcings from
chemical species and found a broad TIL to be present in a TB vertical coordinate but did not compare to the simple averaged case. These results show that the TIL emerges as a result of the dynamics but Son and Polvani (2007) do not offer a dynamical mechanism.
In this chapter, we first explain the process of tropopause based averaging in Section 3.2.
Section 3.3 then shows static stability plots from satellite data and the effect of different methods of averaging on the data. Next, using a similar approach to Son and Polvani (2007), we attempt to reproduce a TIL in a simple general circulation model. The model and results are described in Section 4.5 and the sensitivity to model resolution is discussed. We also present a case where the model does not produce a TIL. Building on the model results from Section 3.4, Section 3.5 focuses on a possible dynamical mechanism for the formation of the TIL by focussing on the meridional circulation. We look in detail at the vertical velocity distribution above the tropopause but in contrast to Birner et al. (2006), we consider the steady state case and how wave forcings in the stratosphere can give rise to an upwelling that is consistent with increased static stability. Finally, Section 3.6 examines the effects of tropopause based averaging on idealised profiles of static stability. A brief summary and some concluding remarks are given in Section 3.7.