Turbulence and convective inversion

Turbulence and convective inversion:

This type of inversion is produced at altitudes above the surface by mechanical processes. Turbulence and convection are the contributory factors in causing this type of inversion.

Because of the frictional forces eddies form in the lower layers of atmosphere which transport lower air to higher levels and bring back the upper air to the lower levels. Convectional currents set up in the air near the ground are mainly responsible for the exchange of air between upper and lower levels of the atmosphere.

The phenomena of turbulence and convection cause a thorough mixing of the atmosphere in turbulent layers. However, the turbulent or convective mixing is limited to a certain height beyond which it does not and cannot penetrate.

It is at this height that the convective inversion is formed. In the process of vertical mixing the air carried upward is cooled adiabatically. Similarly the air brought downwards heated at the same adiabatic rate.

After a prolonged mixing in the atmosphere, the air at the maximum height of turbulent penetration becomes colder than what it was before, and that at the bottom of the turbulence layer will be warmer than what it originally was.

The transition from this cold upper part of the turbulence zone to the air above with its temperature unaffected by adiabatic cooling comprises a temperature inversion.

Clouds, if they ever form in this inversion layer, are of stratus or stratocumulus type. In certain situations, turbulence in association with heat from the ground leads to the formation of cumulus or cumulonimbus clouds.

Turbulence inversion may occur at a low level or it may form at very high altitudes. In case the inversion has formed at lower levels, smoke, dust particles and other pollutants are carried up to the inversion where they spread beneath the inversion layer and form distinct smoke or haze lines in clear weather.

On the other hand, the anvil-shaped upper portion of cumulonimbus clouds is the result of inversion at considerable heights.

It may be interesting to note that stratiform clouds appearing in the sky are indicative of the presence of an inversion layer above them. Sometimes the upper air inversion, by imposing a restriction on their vertical growth, makes the cumulus clouds stunted in appearance.

Frontal inversion:

The inverted lapse rate at the front is called frontal inversion, when differing air masses are brought together by converging movements; the warmer air being relatively higher tends to overlie the colder and denser air in a horizontal layer.

However, because of the Coriolis force the boundary zone between the air masses with contrasting physical properties are never horizontal; they are rather sloping. In fact, the frontal zone itself is converted into inversion layer in which the lapse rate is inverted.

In other words, at the frontal zone as one move up from the lower to upper layers of the atmosphere, there is an increase in temperature with increasing altitudes.

The following characteristics distinguish frontal inversion from other types of inversion: - (a) The inversion layer associated with fronts is sloping, while in the other types of inversion it is horizontal, (b) In frontal inversion the moisture content shows a marked increase with elevation, while in other types the temperature increases and humidity decreases with the increasing elevation.

That is why above the inversion layer clouds are generally seen. In other words, the frontal inversions show an increasing specific humidity in the inversion layer.

It is generally seen that along the fronts the inversion, in the strictest sense of the term, is never found. What happens is that the actual lapse rate becomes very low in the inversion layer. The reason is quite simple.

The warmer air masses ascend the retreating wedge of cold air and they cool by adiabatic expansion. Because of expansional cooling of the upward moving air at adiabatic rate, the frontal inversions are rarely observed beyond the height of 2 kilometers.

At greater heights there is a marked decrease in the lapse rate in frontal zone, whereas just above the top of the clouds formed by the rising warm air currents the inversion is always present. Recent investigations attach special significance to frontal inversions in the origin of extra tropical cyclones.