Simulation of the formation of Hurricane Isabel (2003)

Using the retrieved temperature and humidity profiles from the AIRS suite on the NASA Aqua satellite, the Saharan Air Layer (SAL) activity and its influence on the formation of Hurricane Isabel (2003) are simulated numerically with MM5. The warmth and dryness of the SAL (the thermodynamic effect) is considered by the nudging technique, which enables the model thermodynamic state to be relaxed to the profiles of the AIRS retrieved data for the regions without deep convection. Compared with the SAL effect that is only partially included in the initial and lateral boundary conditions, the simulated large-scale atmospheric circulation is fairly comparable to the NCEP reanalysis and the simulated SAL with nudging the AIRS data delays the formation of Hurricane Isabel and suppresses the development of the other tropical disturbance, suggesting that the AIRS retrieved temperature and humidity profiles provide an opportunity to investigate the influence of the SAL on Atlantic tropical cyclones. This case study confirms the argument by Dunion and Velden (2004) that the SAL can suppress Atlantic tropical cyclone activity by increasing the vertical wind shear, reducing the mean relative humidity, and stabilizing the environment at lower and middle levels.