The diurnal cycle and the meridional extent of the tropics

An explanation is proposed for the meridional extent of the Hadley cells: the diurnal waves forced by solar heating trapped between −30°and 30°. These baroclinic waves, we propose, give rise to the daily nature of tropical meteorology, and ultimately to the intensified mixing and circulation associated with the Hadley cells and the shape of the tropopause. This conceptual explanation is validated with basic scaling arguments and with two diurnally forced one-layer models: One linear and the other nonlinear, the latter with a simple closure for mixing and entrainment by breaking waves.