Numerical Simulation of the Development of AMEX Tropical Cyclone Irma

Abstract

High resolution observational data from the Australian Monsoon Experiment have been used to verify simulations of the development of tropical cyclone Irma. From a small amplitude, prehurricane cloud cluster, the FSU high resolution regional prediction model quite skillfully simulates the temporal and spatial structure changes during development. The results are, however, sensitive to the initial windfield and somewhat sensitive to both the initial moisture field and the imposed boundary conditions. Temporal changes in the symmetric and asymmetric structure of the observed and simulated disturbances are compared. Apart from other well-documented necessary conditions, changes in the storm's vertical structure, and the development of horizontal asymmetries appear to be crucial features of the simulated development. The associated physical processes are discussed. Deficiencies in the simulation are lack of diurnal modulation of the vertical motion field, larger than diagnosed values of vertical motion over the genesis area, and an upper level flow that is too divergent and anticyclonic. We speculate that more accurate representation of diurnal effects, and a parameterization of momentum transports by cumulus clouds will reduce these deficiencies.