6. The Multicloud Model with Congestus Preconditioning

As we saw from the previous chapter, among all the theories mentioned so far, for convectively coupled waves, the stratiform instability seems to be the most promising one. However, it suffers from a major shortcoming that the linear instability is not self-sustained without the use of WISHE, which itself is deemed unphysical as demonstrated by the nonlinear simulation presented there. There is no observational evidence of sustained easterlies in the tropical atmosphere to force eastward moving (Kelvin and MJO) waves that are ubiquitous in the tropical atmosphere, through WISHE [248, 126]. While the stratiform instability is very appealing and appears to be physically sound [184, 175], something fundamental seems to be missing. The answer to this hard question is rooted from the observations led by Lin and Johnson [156, 157] Johnson et al. [99], using data from the Tropical Ocean Global Atmosphere Coupled Ocean Atmosphere Response Experiment (TOGA COARE). These authors discovered that unlike the common belief at that time, the dynamics of organized tropical convection involves three cloud types besides shallow cumulus clouds that are omnipresent. In addition to deep and stratiform clouds, congestus clouds that do not penetrate above the freezing level (4–5 km) play a major dynamical roles.