Using the Penn State-NCAR mesoscale model MM5, high resolution simulations (3 km) of Hurricane Erin are conducted covering the time period between September 7 and September 11 2001. Because Erin never achieved landfall, this tropical cyclone represents an ideal environment for better quantifying sensitivities of the storms to the representation of microphysical processes, such as sedimentation and conversions between various liquid and ice species. In this study, we examine how the track, minimum surface pressure, and precipitation amounts vary depending on the scheme used to represent microphysical processes. Because extensive observations of this storm were obtained during the Convection and Moisture Experiment Phase 4 (CAMEX4) by instruments installed on the DC-8 and ER-2, a detailed comparison with observations is also possible.