Simulations of the Extratropical Transition of Tropical Cyclones

A conceptual model of the transformation stage of extratropical transition by Klein et al. (1999; Wea. and Forecasting) describes a three-step progression of the interaction between the tropical cyclone and the pre-existing midlatitude baroclinic zone based on satellite imagery and 1 degree lat./long. resolution Navy Operational Global Atmospheric Prediction System analyses. The three-step conceptual model provides the framework for idealized simulations that are intended to examine the primary effects of the interaction between a baroclinic zone and a tropical cyclone. Simulations are performed using the U.S. Navy's Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS).

Three major environmental interactions that are likely to be important during transformation are examined with these simulations: (i) response to the low-level temperature gradient associated with the baroclinic zone; (ii) response to the sea-surface temperature gradient; and (iii) interaction with the vertical wind shear associated with the midlatitude synoptic pattern. Although the vertical wind shear is related to baroclinic effects in both the low levels and aloft, one objective of these simulations is to separate the upper-level and low-level related processes during transformation. No attempt is made to represent all of the midlatitude circulation variability in these simulations. Instead, the intent is to examine the basic processes by specifying somewhat idealized circulations based on the Klein et al. study.

In the set of simulations, the interactions of a tropical cyclone with idealized environments that are based on composite wind fields of the three steps of transformation are investigated. This interaction is first explored with attention focussed on the structural changes of the tropical cyclone as it progresses through the three steps of transformation. The cloud evolution in the simulations is validated against observed cloud structures. In addition, the physical characteristics of the transformed system, particularly the above three processes that lead to the observed asymmetric structure of the tropical cyclone remnants at the end of transformation are examined.