Outer Spiral Rainbands Research Articles

An Explicit Simulation of Tropical Cyclones with a Triply Nested Movable Mesh Primitive Equation Model: TCM3. Part I: Model Description and Control Experiment*

Results from an explicit simulation of tropical cyclones are presented in this study. The numerical model used in the study is the triply nested movable mesh primitive equation model newly developed by the author. It uses the hydrostatic primitive equations with explicit treatment of cloud microphysics. The integration domain is triply nested by a two-way nesting strategy with the two interior meshes being movable following the model tropical cyclone. The model physics are chosen based on the up-to-date developments, including an E-e closure scheme for subgrid-scale vertical turbulent mixing [with E being the turbulent kinetic energy (TKE), and e the TKE dissipation rate]; a modified Monin‐Obukhov scheme for the surface flux calculation, with an option to include the effect of sea spray evaporation; an explicit treatment of mixed-ice phase cloud microphysics; and dissipative heating, which has been found to be important in tropical cyclones. New developments include a new iteration scheme to solve the nonlinear balance equation in s coordinates in the nested-mesh grids, which is used for model initialization; an initialization scheme for both TKE and its dissipation rate fields based on a level-2 turbulence closure scheme deduced from the TKE and its dissipation rate equations; and a modified formula for the timescale that determines the rate at which cloud ice converts to snow via the Bergeron process. The success of the multiply nested movable mesh approach and the conservative property of the numerical model is first tested with an experiment in which the model was initialized with an axisymmetric cyclonic vortex embedded in a uniform easterly flow of 5 ms 21 on an f plane, but with no model physics. Results from a control experiment with the full model physics are then discussed in detail to demonstrate the capability of the model in simulating many aspects of the tropical cyclone, especially the inner core structure and both the inner and outer spiral rainbands in the cyclone circulation. The vortex Rossby waves in the simulated tropical cyclone core region are also identified and analyzed. Sensitivity of the model results to various model physics and major physical parameters will be given in a companion paper.