Question – The Tricellular meridional model of atmospheric circulation is a useful model for explaining Earth’s atmospheric phenomena. Explain how different factors form the Tri-cellular model. – 24 March 2022
Answer – The flow pattern of atmospheric winds is known as atmospheric general circulation. The flow pattern of winds is formed by the pressure gradient created due to uneven heating of the earth and atmosphere by solar radiation.
As opposed to Zonal overturning circulation, that conventionally represents the atmospheric horizontal motion of air depending on temperature gradients; the tri-cellular model envisages a three-cell model of meridional circulation of the atmosphere wherein the cellular circulation of air at each meridian or longitude is taken into account.
As per tri cellular meridional circulation of the atmosphere, there are three cells of atmospheric circulation depending on thermal and dynamic factors—tropical or hadley cell, ferrel cell and polar cell.
Fig: The Tri-cellular meridional model of atmospheric circulation
- Hedley Cell or Trade Wind Cell :
The cell located in the trade wind belt, or Hadley cell, is formed in the region of the belt of trade winds located between the subtropical high pressure belt and the equatorial low pressure belt. In the equatorial belt, low pressure is created as a result of insolation. Air rises and diverges to poles at the top of the atmosphere. At subtropics region, the wind cools and sinks, and creates dynamic subtropical highs.
The surface wind starts blowing horizontally from the subtropical air pressure area towards the equatorial low pressure, it is called permanent trade wind. Due to the high temperature in the equatorial region, it gets heated again and rises up. In this way, the contact of surface trade wind with tropospheric wind is maintained continuously.
- POLAR FRONT CELL/ MID-LATITUDE CELL/ FERREL CELL:
Feral cell located in the westerly wind belt is formed in the region of westerly wind located between the subtropical high pressure and sub-polar low air pressure belt. Surface winds blow from the subtropical high pressure area to the subpolar low pressure area, which rises due to the effect of the Coriolis force. The rising air moves horizontally due to the tropospheric barrier, and settles on the surface near latitude 25 to 35 degrees. From here it starts flowing again in the form of westerly wind towards the subpolar region. Due to this the westerly wind remains in constant contact with the tropospheric winds, and from this a cycle of winds develops. The polar front develops when westerly winds meet the polar cold winds in the subpolar regions. A temperate cyclone originates along the polar front, it follows the westerly winds from west to east, affecting a wide area of mid-latitude
- Polar Cell:
The smallest and weakest cells are the Polar cells, which extend from between 60 and 70 degrees north and south to the poles. Air in these cells sinks over the highest latitudes and flows out towards the lower latitudes at the surface. Polar easterlies blow from polar high-pressure areas to sub- polar or mid- latitude low pressure belts. Polar cold winds converge with warm westerlies near 60 degrees and 65 degrees latitude, forming the polar front. Poleward upper air descends as cool air at the poles & reinforces the polar high pressure.
Tri-cellular meridional model and Earth’s Atmospheric Phenomenon
- This circulation is fundamental in maintaining latitudinal heat balance of earth.
- Convective currents of Hadley cells are responsible for heavy precipitation along equator regions.
- Formation of jet streams is the result of these circulation, and upper atmosphere dynamics of rotating earth.
- Temperate cyclones are formed at the polar front formed by ferrel cell and polar cell.
- Subtropical highs are responsible for creating aridity, and sustaining off shore winds and trade wind deserts.
- The interaction of easterly and westerly creates frontogenesis which influences the climate of the subpolar area.
- The tropical deserts of the world are a manifestation of the air circulation of the Hadley cell.
- The Phenomenon of monsoon is heavily influenced by the Upper air circulation and the trade wind movement.
The tricellular meridional mecha-nism of general circulation of the atmosphere, thus, includes both, the horizontal and vertical components of atmospheric circulation. The Hadley, Ferrel, and polar cells operate at the largest scale of thousands of kilometers (synoptic scale) creating a variety of atmospheric phenomena affecting the entire life on the planet.