Tropopause

THE TROPOPAUSE IS the boundary region dividing the troposphere, the lowest layer of the atmosphere, from the overlying stratosphere. Since the tropospheric and stratospheric air masses have rather distinct features, in correspondence to each surface location, the tropo-pause height is the level in the vertical where abrupt changes in the physical and chemical properties of the atmosphere are observed.

Three different definitions are typically adopted. The thermal tropopause is related to the change of the sign of the vertical derivative of the temperature (lapse rate), which is negative in the troposphere and positive in the stratosphere.

The World Meteorological Organization defines the tropopause as the lowest level where the absolute value of the temperature lapse rate decreases to 2K/km. or less, with the average lapse rate between this level and all higher levels within 1.2 mi. (2 km.) not exceeding 2K/km. The dynamical tropopause is defined in terms of sharp changes in the potential vor-ticity (much higher in the stratosphere), which measures stratification and rotation of the air masses. An abrupt increase (decrease) with height of the ozone (water vapor) mixing ratio indicates the presence of the chemical tropopause. In spite of the necessity of choosing phenomenological thresholds, the three definitions of the tropopause are quite consistent.

TROPICAL TROPOPAUSE

Typically, the tropopause height decreases with latitude, being around 3.7 mi. (6 km.) near the poles and 11 mi. (18 km.) near the equator. Whereas radiative and convective processes with time scale of the order of one week to one month basically determine the properties of the tropical tropopause, in the midlati-tudes a relevant role is played also by baroclinic-fuelled extra-tropical cyclones, having a typical time scale of a few days, in such a way that the tropopause readjusts its height in such a way as to act effectively as a stabilizing mechanism limiting the growth of the weather perturbations. The tropopause is not a hard boundary: exchanges of tropospheric and stratospheric air occur through various mechanisms, including vigorous thunderstorms and midlatitude perturbations.

The globally averaged tropopause height tends to increase if the troposphere warms up and/or the stratosphere cools down, and the height change is approximately proportional to the difference between the tropospheric and stratospheric temperature changes. Therefore, the mean tropopause height can act as a robust indicator of climate change. Recent climate simulations have shown that the estimated increase after 1979 of about 492 ft. (150 m.) may be primarily explained by anthropogenic causes, namely the stratospheric cooling driven by ozone depletion and the tropospheric warming driven by increases in the greenhouse gases concentration. Considering natural processes, episodic, short-lived strong reductions of the globally averaged mean tropopause height are caused by large explosive volcanic eruptions, which warm the troposphere and cool the stratosphere.

SEE ALso: Atmospheric General Circulation Models; Atmospheric Vertical Structure; Cyclones.

BIBLIoGRAPHY. World Meteorological Organization, "Meteorology: A Three Dimensional Science', WMO Bulletin (v.6, 1957); B.D. Santer "Contributions of Anthropogenic and Natural Forcing to Recent Tropopause Height Changes," Science (v.301/5632, 2003).

VALERIO LUCARINI University of Bologna

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