The Tricellular model is simply a chain showing the connections between 3 distinct cells namely the Hadley Cell, the Ferrel Cell and the Polar Cell. The origin of the Tricellular model is the equator extending outwards to the poles.
Air is heated at the equator and rises creating cumulonimbus clouds with heavy rain. The air continues outwards and descends at the 30o South and North of the equator creating a subtropical HP (Ferrel cell). This is another factor causing the creation of deserts at the subtropical highs e.gSahara Desert