On the Genesis and Intensification of Hurricanes
In general, observations indicate that only a small fraction in a fixed framework (from 140° W to 20° E) covering African to the eastern Pacific regions of the easterly waves that occur in a single hurricane season contribute to tropical cyclogenesis. However, this small fraction includes a large portion of named storms. In addition, a study by Dunkerton et al. (2009) has shown that named storms in the Atlantic and eastern Pacific basins are almost all associated with wave breaking of a tropical easterly waves. We will discuss fundamental dynamical and physical processes related to hurricane genesis. We will show that the coexistence of an African easterly wave that is breaking and a region of weak meridional potential vorticity (PV) gradient over several days might be a major factor determining whether or not tropical disturbances develop into hurricanes. This finding answered the long-standing question of why only a small fraction of African easterly waves contribute to hurricane genesis.
We will also review how well the prediction of hurricane and tropical cyclones has improved in the last two decades. The following image shows the simulated specific humidity (altitude 325m) at 06Z, September 1, 1998, for a 10000 x 10000 km numerical simulation over the Gulf of Mexico of hurricane Earl (September 1998). This image is approximately showing only 1% of the total surface of the computing domain. The pixels are at the model 1km horizontal resolution. The simulation shows realistic instabilities and sheared disturbances evolving around the spinning hurricane.