Reflectivity structure of hurricanes undergoing an eyewall replacement cycle

Understanding the hurricane life cycle is critical to successful, life-saving predictions. Major hurricanes often undergo a process called an eyewall replacement cycle (ERC) in which the original eyewall contracts while a new eyewall forms at a radial distance outward from the eye with a nearly precipitation free "moat" separating the original eyewall from the secondary eyewall. Hurricanes undergoing an ERC exhibit substantial and sometimes quick shifts in maximum wind speeds, as well as significant broadening of the surface wind field. These shifts in the structure of the hurricane are of vital importance to forecasters, who must predict both hurricane intensification and de-intensification, yet the process of eyewall replacement is not well understood. This study seeks to improve understanding of the ERC empirically, by examining a set of ERC cases from the Atlantic Basin and West Pacific Basin in order to document the changes that occur during eyewall replacement. The data are three-dimensional reflectivity measurements obtained by the Tropical Rainfall Measuring Mission (TRMM) Precipitation Radar for category 4 and 5 hurricanes occurring in the years 1998 - 2007. The cases are divided into two groups: those exhibiting characteristics of an ERC and those not. For the ERC cases, the data are divided into annular regions such that regions 1, 2 and 3 contain the primary eyewall, the moat and the secondary eyewall, respectively. For the non-ERC cases, the data are divided into similar annular regions defined by proximity to the eye. Statistical frequency analysis is then conducted to determine the type and amount of precipitation in each region and comparisons are made between the ERC cases and the non-ERC cases, as well as between the ERC cases in the two ocean basins. Results indicate differences in reflectivity structure between the ERC and non-ERC cases and that storms undergoing eyewall replacement have similar structures in the NW Pacific and Atlantic ocean basins.