Effect of environmental moisture on rainbands in Hurricanes Katrina and Rita (2005)

The interaction of the environmental water vapor distribution around a tropical cyclone (TC), rainbands, and inner-core dynamics can affect hurricane structure and intensity change, which is not well understood. Although previous studies have addressed various aspects of this problem, a full three way interaction and its implications for hurricane intensity change has not been documented. Using data collected during the Hurricane Rainband and Intensity Experiment (RAINEX) in Hurricanes Katrina and Rita, the three way interaction of the environment moisture, rainbands, and inner-core dynamics can be evaluated. The TRMM TMI total precipitable water (PW) data with 1/4 degree horizontal resolution, TRMM TMI rainrate data with a 5 km horizontal resolution and the GPS dropsondes with a ½ second temporal resolution are used to characterize the environmental moisture. The high resolution model output from the real-time MM5 forecasts of Hurricanes Katrina and Rita are used to investigate the complex interactions in both storms. The model forecasts were made using a vortex-following nested grid with horizontal resolutions of 15, 5, and 1.67km, respectively. There were 28 vertical sigma levels. Observational data and high resolution model output show strong moisture gradients in the outer rainband region in Rita with a dry, stable outer environment. Additionally, Rita's rainbands had high circularity and evolved into a secondary eyewall. Model output demonstrated that the potential vorticity (PV) is enhanced in the rainband region in Rita and led to the development of a secondary wind maximum. The environmental water vapor distribution may play a role in enhancing the rainbands that developed into a secondary eyewall, leading to a temporary weakening of the hurricane. Analyses of RAINEX observation and the model output suggest that it was the environmental water vapor distribution and associated atmospheric instability that is responsible for the rainband structures observed in Rita. In contrast, Katrina, which had no secondary eyewall while being investigated by RAINEX, had a relatively weak moisture gradient surrounding the storm. There were not persistent outer rainbands with high circularity. This may explain the different evolution in Katrina compared with Rita. These differences between the two storms, and the implications on TC intensity change, will be presented at the conference.