A modeling study of Hurricane Danny (1997) at landfall

Hurricane Danny (1997) was a small category 1 tropical cyclone that originated near the coast of Louisiana on the 14th of July 1997. The system moved very slowly in an east-northeast direction and became a hurricane 4 days later on the 18th. It first made landfall early on the 18th just northwest of the Mississippi River delta but moved back out to sea later that day. Before dawn on the 19th of July Danny reached the mouth of Mobile Bay in Southern Alabama and made landfall on the southeast side of the bay around midday on the 19th. The storm almost stalled for 24 hours in the Mobile Bay area while torrential rains devastated the region. Before entering the bay, Danny was a symmetric system. A strong asymmetric structure evolved, however, once the storm entered the bay. Intense convection occurred in the western eyewall with rainfall rates increasing dramatically at the same time. Precipitation nearly vanished in the eastern half of the storm. The change in rainfall distribution coincided with a change in the distribution of the low-level winds with maximum winds occurring in the western eyewall. Danny reached its maximum intensity during this asymmetric phase in Mobile Bay.

Using the PSU/NCAR mesoscale model, MM5, a fine resolution (around 1 km) simulation of hurricane Danny is carried out to investigate Danny's unusual evolution of rainfall and wind distribution while the system was located in the Mobile Bay area. The use of 1 km horizontal resolution allows explicit modeling of convection, a process crucial to the accurate simulation of hurricane precipitation distribution and amount. MM5 is initialized with GFDL initial fields while the addition of extra, WSR-88D (from KMOB), flight level and dropsonde, data will provide the best possible representation of the initial vortex. The simulated rainfall distribution is carefully validated against the excellent observations available for this storm due to its close proximity to Gulf coast radar sites. Furthermore, the influences of the warm waters of Mobile Bay and the surrounding topography will be explored.