Numerical simulations of Hurricane Erin (2001) observed during the Fourth Convection and Moisture Experiment (CAMEX4) are conducted using the Penn State University/National Center for Atmospheric Research mesoscale model (MM5, Version 3.5). Variations in Erin's evolution, thermodynamic structure and hydrometeor fields are found when different horizontal or vertical resolutions are used. Simulations with higher spatial resolution, especially when extra layers are added near the surface, tend to give a more intense hurricane. Fine horizontal resolution simulations (2km) tend to produce stronger vertical motion and generate more precipitation. The physical reasons for variations between different resolution simulations are investigated by examining model prognostic equations. Impacts on the quantitative precipitation forecasts (QPFs) for tropical cyclones are discussed.