  The Hadley circulation (or Hadley cell) is traditionally described as a
large-scale atmospheric circulation phenomenon driven by differential heating
of the Earth surface: warm, moist air rises near the equator, diverges poleward
in the upper troposphere, and subsides in the subtropics. In this article, the
mechanism of the Hadley circulation is revisited and a new model is provided to
explain its mechanism. The new model is based on a form of the atmospheric
dynamic equation which substitutes pressure with temperature and density;
thereby categorizing weather systems into thermal and dynamic systems. Such
classification is useful for explaining large-scale weather systems such as the
Hadley cell. The proposed explanation for the mechanism of the Hadley
circulation argues that subtropical highs are the driving force of the Hadley
cell, rather than the conventionally-believed ITCZ (Intertropical Convergence
Zone). To support our theory, we analyze the atmospheric air density flux
divergence with the results from the Community Earth System Model (CESM) and
derive a new continuity equation by adding source/sink terms, in which
evaporation serves as the air-mass source, and precipitation (condensation) as
the air-mass sink. Results found that the equatorial easterlies could be linked
to the solar diurnal cycle, demonstrating that the trade wind can be generated
by the solar diurnal cycle, especially in the spring and fall seasons, as well
as from the equatorial branch of the subtropical high.
