1.2.3.4. Equilibrium level (‚anvil level‘)

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  • Author: SW101

THE ROLE OF THE TROPOPAUSE

The tropopause is the boundary in Earth’s atmosphere between the troposphere and stratosphere. It is a strong temperature inversion: the temperature drops with height as you go upwards in through the troposphere, but begins rising again as you cross the tropopause into the stratosphere.

The World Meteorological Organization defines the tropopause as: “The boundary between the troposphere and the stratosphere, where an abrupt change in lapse rate usually occurs. It is defined as the lowest level at which the lapse rate decreases to 2 °C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 °C/km.„ In other words, it is a boundary, a layer with a very small lapse rate, i.e. the temperature of the air changes very little with increasing altitude.

The height of the tropopause is not the same everywhere in the world. The tropopause height depends on the latitude: at latitudes above 60° (North or South) it is 9-10 km high, but it reaches up to 17-18 km close to the equator.

Strong updrafts, that form strong thunderstorms rise all the way up to the tropopause. The tropopause is a very strong temperature inversion that limits the growth of even the strongest updrafts. As the vertical temperature lapse rate in the stratosphere (above the tropopause) is positive, the updraft is not buoyant anymore and the tropopause is the ultimate limit of updraft upward development. Very little upward development is possible above this level. This is also why the strongest thunderstorms have large anvils that develop at the tropopause level.

Diagram of a Cumulonimbus capillatus incus with equilibrium level in the tropopause. The updraft remains buoyant throughout the troposphere: it cools as it rises (at the moist adiabatic lapse rate), but does not cool to the temperature of the surrounding air. As it reaches the tropopause, the surrounding air does not cool with increasing height anymore and the rising air in the updraft soon cools to the temperature of the surrounding air, not rising anymore. In effect it reaches the ceiling and cannot rise anymore. The upward momentum is transformed into lateral (sideways) momentum and the updraft spreads into anvil shape.