Dynamics of Disturbances on the Intertropical Convergence Zone

The dynamics of disturbances on a theoretically derived Intertropical Convergence Zone are studied. Radiative cooling over a hemisphere is parameterised by a Newtonian cooling law, relative to a radiative equilibrium temperature which decreases from equator to pole. Release of latent heat of condensation in the tropics is treated as a function of convergence in the planetary boundary layer. The zonally symmetric field of motion which evolves in response to these sources of energy shows a concentrated region of rising motion near, but not at, the equator. The associated lowlevel wind field possesses a strong cyclonic shear. Asymmetric perturbations periodic in longitude are introduced by means of truncated Fourier series. The wavelengths are chosen to correspond to maximum instability in the ITCZ, and as such are too small to permit baroclinic instability in middle latitudes. In this way, the stability of the low latitude flow is examined while excluding middle latitude perturbations. The low-level wind field in the vicinity of the ITCZ is found to be barotropically unstable, the wavelength of maximum growth rate being about 2000 km. The corresponding e-folding time is found to be of the order of two days, depending on the frictional and heating coefficients. The perturbations are allowed to grow to finite amplitude. In the initial stages of growth, the Reynolds stresses supply most of the perturbation energy. At the mature stage, the energy is provided mainly by direct conversion of condensationally produced eddy available potential energy. Further growth is then limited by frictional dissipation of kinetic energy. The mature disturbance shows a highly asymmetric cell of concentrated rising motion, bounded by regions of weak sinking motion, propagating towards the west at about 13 kts. The disturbance is 'warm-core', having a much larger amplitude at the lower than at the upper level. The mean flow is in turn influenced by the disturbance through the mechanisms of Reynolds stresses, eddy conduction and the modification of the mean flow condensational heating through boundarylayer pumping. The influence is seen on the mean temperature and zonal wind fields, and may extend to latitudes poleward of where the perturbation amplitude in situ has decreased to zero. A framework is thus provided for viewing tropical disturbances as an integral component of the general circulation of the atmosphere. Thesis Supervisor: Jule G. Charney Title: Sloan Professor of Meteorology