Angular Momentum Transport in an Idealized Tropical-cyclone

Angular momentum transport in a developing tropical cyclone with the idealized axisymmetric structure was analyzed through numerical experiment with WRF model. The idealized axi-symmetric vortices to be used as the initial condition of the tropical cyclone simulation, was constructed using empirical functions. The initial vortices of empirical formula were designed to have important features of developing tropical cyclones. Time integration was carried out for 96 hours over the model domain of 3000 km X 3000 km. which is centered at 15 degrees in northern hemisphere. The tropical cyclones showed a steady development with a steep strengthening for the time interval from day 1 to day 3, giving the central pressure of 910 hPa by day 5. The total relative angular momentum (TRAM) in developing vortices was reduced with simulation time, which means that the developing vortex lost its cyclonic angular momentum itself. Also, the budgets of relative angular momentum integrated over the entire cylindrical volume showed that the angular momentum transport in idealized vortices was primarily maintained by vertical horizontal circulation. Surface frictional dissipation at larger radii was comparable to TRAM tendency. This result suggests that the main torque of the idealized vortex, associated with the angular momentum loss, is surface frictional toque. The budgets within each 100km-width radial band were analyzed to investigate transport of angular momentum. At radii smaller than 600 km, the angular momentum was transported inwardly and contributed to the vortex intensification. However, outward transported angular momentum was found to be dominant at larger radii due to the anti-cyclonic rotation in upper-level of vortex.