A simple, vertically resolved model of tropical disturbances with a humidity closure

A vertically resolved linear model for large-scale motions in an equatorial non-rotating atmosphere is presented. Precipitation is forced by tropospheric precipitable water, cloud–radiation interactions (CRI), and wind-induced surface heat exchange (WISHE). The dynamics of the model are based on the assumption that the vertical heating profile has the shape of the first baroclinic mode. The vertical profiles of vertical velocity, temperature perturbation, etc. are calculated using an upward radiation boundary condition. The modelled modes are of three types: fast gravity waves that resemble adiabatic modes with the vertical wavelength twice the depth of the troposphere; convectively coupled gravity modes that are damped and move with a phase speed of ≈17 m s−1; and the unstable moisture mode. The value of the model is that under a single dynamical assumption it yields the observed phase speed for the convectively coupled gravity waves that map to Kelvin waves in the equatorial beta plane case while still producing the unstable moisture mode. However, the model lacks the precipitation physics needed to produce the observed destabilization of the gravity mode.