CONVECTIVE INTERACTION WITH BOUNDARY-LAYER DYNAMICS IN THE DEVELOPMENT OF A TROPICAL INTRASEASONAL SYSTEM

Tropical boundary-layer flows interact with the free tropospheric circ ulation and underlying sea surface temperature, playing a critical rol e in coupling collective effects of cumulus heating with equatorial dy namics. In this paper a unified theoretical framework is developed in which convective interaction with large-scale circulation includes thr ee mechanisms: convection-wave convergence (CWC) feedback, evaporation -wind (EW) feedback, and convection-frictional convergence (CFC) feedb ack. We examine the dynamic instability resulting from the convective interaction with circulation, in particular the role of CFC feedback m echanism. CFC feedback results in an unstable mode that has distinctiv e characteristics from those occurring from CWC feedback or EW feedbac k in the absence of mean flow. The instability generated by CFC feedba ck is of low frequency with a typical growth rate on an order of 10(-6 ) s-1. The unstable mode is a multiscale wave packet; a global-scale c irculation couples with a large-scale (several thousand kilometers) co nvective complex. The complex is organized by boundary-layer convergen ce and may consist of a few synoptic-scale precipitation cells. The he ating released in the complex in turn couples the moist Kelvin wave an d the Rossby wave with the gravest meridional structure, forming a dis persive system. The energy propagates slower than the individual cells within the wave packet. A transient boundary layer is shown to favor planetary-scale instability due to the frictionally created phase shif t between the maximum vertical motion and the heating associated with boundary-layer convergence. The implications of the theory to the basi c dynamics of tropical intraseasonal oscillation are discussed.