Quasi-balanced dynamics in tropical cyclones

Abstract

In chapter 24, we have examined balanced dynamics from the potential vorticity (PV) perspective with different approximations. Strictly speaking, the balanced dynamics in mesoscale convective system (MCSs) or tropical cyclones (TCs) concerns mostly with horizontal flows, commonly known as the primary circulations. As a result, the balanced flows contain no vertical circulation (i.e., vertical motion and across-isobaric flows) and no time-dependent component. For TCs with high degree of axisymmetry, such a vertical circulation, consisting of convergent inflow in the planetary boundary layer (PBL), slantwise upward motion in the eyewall, and divergent outflow in the upper troposphere (referred to as secondary or transverse circulation, SC), plays an important role during the development of TCs. This is mainly because it is the SC that is responsible for the continuous adjustment of balanced flows in time.