PHASE-LAG BETWEEN DEEP CUMULUS CONVECTION AND LOW-LEVEL CONVERGENCE IN TROPICAL SYNOPTIC-SCALE SYSTEMS

This study deals with the origin of the phase lag between deep cumulus convection and low-level convergence in tropical synoptic-scale syste ms, known since 1974. Several possible causes, including 1) propagatio n of the heating field, 2) beta, 3) vertical shear of the basic flow, and 4) vertical tilt of the heat source, are examined. The last one is found to be the reason for the phase lag. The vertical tilt of the he at source occurs as a result of evolution and propagation of mesoscale convective systems within the synoptic system. During this evolution the change of vertical heating profile results in the tilt of heating field. Previous efforts of incorporating such phase lag in wave-CISK s tudies are commented on.