Simulated precipitation structure of the extratropical transition of tropical cyclones

The extratropical transition (ET) of a tropical cyclone has been described as a continuous process that can be divided into two stages by Klein et al. (2000). The first stage, which they call transformation, includes the initial structural changes that a recurving tropical cyclone undergoes as it begins to interact with a pre-existing baroclinic zone and the associated wind shear. If the transformed storm begins to re-deepen as a baroclinic cyclone, the second stage, re-intensification, commences. In this presentation we will investigate the spatial patterns of precipitation that develop during the transformation and re-intensification stages of ET. The model setup is similar to that used in Ritchie and Elsberry (2003). The tool used is the U.S. Navy Coupled Ocean-Atmosphere Model Prediction System (COAMPS) described by Hodur (1997). Just the atmospheric portion of the system is used for this study. The model domain is configured with a coarse and fine mesh with grid spacing of 81 km and 27 km, respectively.

A series of sixteen idealized simulations have been run where the initial relative positions of the tropical cyclone and upper-level trough are varied to simulate different phasings between the two systems. The resulting simulations fall into the categories of strongly re-intensifying, moderately re-intensifying, and dissipating cases of ET. All cases have been examined for their differences in the structural development of the extratropical cyclone. It is found that distinct spatial patterns of precipitation develop when the storm is going to re-intensify or dissipate. It seems likely that there will be some advantage to using these spatial patterns of rainfall as a predictor for detecting a reintensifying or dissipating system. In the presentation, two simulations will be examined to highlight the differences in the precipitation patterns that develop for re-intensifying and dissipating cases of ET.