Using an advanced research version of the Weather Research and Forecasting (ARW WRF) model, a high-resolution numerical simulation is conducted to simulate part of Katrina's life cycle before and during its landfall in Louisiana. The model simulations are validated by observations made from the NASA TRMM satellite data and radar and dropsonde observations collected by the NOAA Hurricane Research Division (HRD) operational/research flights. Results show that the simulation captures the main features of Hurricane Katrina, based on comparisons with the observations. The simulation results are then used to diagnose the characteristics of Hurricane Katrina's evolution and changes in its intensity and structure, as well as the influences of environmental boundary layer conditions during its landfall. Specifically, the interaction of the hurricane's vortex and secondary circulations with environmental conditions in the atmospheric boundary layer will be examined. Details will be reported in the conference.