The effects of sea-surface temperature and static stability on WRF simulated intensity and track of selected 2005 Atlantic Hurricanes

Numerical experiments were designed and performed to investigate the effects of sea-surface temperature (SST) on some of the 2005 Atlantic hurricanes using the Weather Research Forecasting model (WRF). The selected hurricanes were Hurricanes Katrina, Ophelia, Rita, and Wilma. For each of these hurricanes, we modified the SST in the NCEP 1°x1° gridded observation file and used it to initialize the WRF model. The SST was successively lowered by 2°C for the entire simulation domain, and the maximum simulated hurricane intensities were compared with the intensity obtained from using the unaltered SST. Preliminary results expectedly showed that the maximum intensity of all of the simulated hurricanes decreased as the SST decreased. However, Hurricane Wilma required the SST to be lowered more than other selected hurricanes in order for the initial disturbance to fail to develop.

The results of the SST experiments suggested that lowering the SST stabilized the atmosphere and, therefore, limited the maximum hurricane intensities. However, the initial disturbance of Wilma was able to slightly develop despite very low SST. This led us to investigate the static stability in the initial hurricane environment as another important controlling factor on hurricane intensity. Our first set of experiments involved successively lowering the initial SST and the atmospheric temperature by 2°C while preserving the humidity everywhere in and around the initial disturbance of Hurricane Wilma. The simulation results also showed a decrease in hurricane intensity with decreasing SST, but not as rapidly as lowering the SST alone. Our next set of experiments involved successively lowering SST and the atmospheric temperature profile moist adiabatically by 2°C. The results of these experiments showed that the maximum intensities of the simulated hurricanes were not significantly reduced even though the SST was lowered to 20°C with the atmospheric temperature profile lowered moist adiabatically accordingly! This would indicate that the conditional instability or CAPE in and around the initial disturbance is a very important determining factor on the maximum hurricane intensity. This would also challenge the notion of a threshold SST (i.e. 26°C) necessary for hurricane formation.

We also performed SST experiments on Hurricane Ophelia. The SST immediately around Ophelia was progressively lowered to simulate the effect of upwelling as Ophelia stalled off the southeast coast of the U.S. The results showed that the intensity and track of the simulated Ophelia agreed better with observations.

Details of the above-mentioned simulations are planned to be presented in the conference.