The tropical cyclone—jet interaction
Eric D. Rappin, Univ. of Wisconsin, Madison, WI; and G. J. Tripoli and M. C. Morgan
Current theories on the maximum potential intensity (MPI) of tropical cyclones (TCs) are largely based on the energetics of a steady state system. The MPI is obtained by defining an equilibrium between the air sea interaction (primary source) and frictional dissipation (primary sink). Frictional dissipation occurs during inflow in the form of surface friction, and in the outflow, where energy is dissipated overcoming the ambient torque in developing the anticyclonic outflow. At steady state the dissipation in the outflow is negligable as the anticyclone is ejecting flow into a preconditioned environment built from earlier outflow. Prior to steady state, the work needed to expand the outflow can result in a temporary steady state, termed the intermediate potential intensity (IPI), at a reduced level from the MPI. Once the outflow is established, growth to the MPI will resume. The goal of this work is to show that an outflow environment characterized by low vorticity significantly decreases the expenditure of energy in building a mature anticyclone, therefore minimizing the IPI duration and allowing the TC to obtain it's MPI more rapidly.
Preliminary results using a three-dimensional primitive equation model, show the existence of an IPI in the mean sea level pressure evolution, consistent with the aforementioned postulate. At the IPI, both the total kinetic energy and the kinetic energy of the anticyclonic motion in the outflow layer (OFL) increase while the kinetic energy of the cyclonic motion in the OFL decreases. During the period that the TC is at its IPI, the growth of the kinetic energy in the OFL is manisfest as the development of an OFL anticyclone. The anticylone , in fact, grows at the expense of the primary TC circulation. The IPI phase ends once the anticyclone is fully developed and the kinetic energy of the anticyclonic motion has leveled off. Rapid deepening begins anew and the kinetic energy of the cyclonic motion in the OFL increases.
A second simulation was run in which a TC was placed on the anticyclonic shear side of a jet. The TC in this second simulation maintained its IPI for a much shorter period and was therefore able to reach the steady state MPI more rapidly than the first simulation. At the IPI in the TC-jet run, the increases in kinetic energy of the anticyclonic motion in the OFL were more modest as the TC's initial OFL was characterized by low vorticity. In addition, instead of a decrease in kinetic energy of the cyclonic motion in the OFL, there was a slow but steady increase once the IPI was reached.
Extended Abstract (224K)
Session 13A, Tropical Cyclone Intensity Change II: Environmental Factors
Thursday, 6 May 2004, 10:15 AM-11:45 AM, Le Jardin Room
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