The Tropical Cyclone as a Divergent Source in a Background Flow

Abstract

The interactions between the outflow of a tropical cyclone (TC) and its background flow are explored using a hierarchy of models of varying complexity. Previous studies have established that for a select class of TCs that undergo rapid intensification in moderate values of vertical wind shear, the upper-level outflow of the TC can block and re-route the environmental winds, thus reducing the shear and permitting the TC to align and subsequently to intensify. We identify in satellite imagery and reanalysis datasets the presence of tilt nutations and evidence of upwind blocking by the divergent wind field, which are critical components of Atypical Rapid Intensification. We then demonstrate how an analytical expression and a shallow water model can be used to explain some of the structure of upper-level outflow. The analytical expression shows that the dynamic high inside the outflow front is a superposition of two pressure anomalies caused by the outflow’s deceleration by the environment and by the environment’s deceleration by the outflow. The shallow water model illustrates that the blocking is almost entirely dependent upon the divergent component of the wind. Finally, using a divergent kinetic energy budget analysis, we demonstrate that in a full-physics TC, upper-level divergent flow generation occurs in two phases: pressure-driven and then momentum-driven. The change happens when the tilt precession reaches left-of-shear. When this change occurs, the outflow blocking extends upshear. We discuss these results with regards to prior severe weather studies.