The forcing of large-scale waves in an explicit simulation of deep tropical convection

Abstract

The structure of emerging tropical waves forced by simulated deep convection is analysed with a view to understanding their energy source and wave propagation characteristics. The simulation is carried out using a cloud-resolving model run in a circum-equatorial domain and a strongly anisotropic horizontal grid providing 2.44 km resolution in longitude and 40 km in latitude. The analysis focusses on the vertical structure of wavenumber 10 disturbances with their eastward propagation and boomerang-shaped phase lines. Convective heat, moisture and momentum forcing functions are computed by coarse-graining their associated model fields and tendencies so that relative phase displacements between them can be examined. It is found that the waves are characterized by an available potential energy source located in the middle to upper troposphere due to the positive correlation of temperature and coarse-grain diabatic heating. The coarse-grained zonal momentum forcing is found to be far less coherent and the associated kinetic energy generation is small. The self-consistency of the proposed dynamical interpretation is tested in a simple linear model of equatorial beta-plane flow forced by a travelling heat source. Reasonable agreement between the linear model and the simulated wavenumber 10 disturbance is obtained if the heating function is decomposed into lower tropospheric Newtonian thermal damping and a prescribed sinusoidal heating function peaking in mid-troposphere.