The influence of TUTT cells on TC motion

Large-scale environmental influences on tropical cyclone (TC) motion have been studied extensively for decades. There are a few situations, however, where further research is still required. The Joint Typhoon Warning Center (JTWC) has repeatedly identified TC interactions with Tropical Upper Tropospheric Trough (TUTT) cells in the northwest Pacific Ocean as one such type of situation. Additionally, numerous publications have speculated as to whether a TUTT cell can actually affect TC motion. Examples include Hodanish and Gray (1993), Holland and Lander (1993), Chen and Chou (1994), Fitzpatrick et al (1995), Elsberry, (1995) and Wu and Kurihara (1996). To date, there are no known observational studies that prove this type of influence can and do occur nor is there any known operational guidance for such scenarios.

Our research is loosely based on similarities with observational binary TC interaction studies (Lander and Holland 1993). ECMWF ReAnalysis (ERA-40) fields are used in conjunction with the JTWC best track data to depict the historical state of the atmosphere from 1994 to 2001. TUTT cell centers are identified at 200 hPa via streamline analysis. With these data, we identify 10 cases in the northwest Pacific Ocean that had the TC follow a non-standard track when a TUTT cell was within 2000 km range. Other transient mesoscale or larger atmospheric features known to influence TC motion did not exist within 2000 km of the TC and its path. We use the 10 cases to explore the following questions:

1. How does a TUTT cell directly influence TC motion?

2. Does a TC appear to influence TUTT cell motion?

3. What environmental steering combination (layer, radial band) best fits an interaction?

4. What is the maximum separation distance for which an interaction can occur?

5. How does the intensity of either circulation affect an interaction?

6. How long do interactions last?

7. What TUTT cell characteristics are important for an interaction to occur?

8. Are there any key indicators an interaction is about to take place?

A conceptual model is being developed to provide operational guidance to the JTWC. The model was based on the deduced or inferred influence of a TUTT cell's wind field on TC motion. It quantifies decision-grade criteria for determining the likelihood of a TUTT cell's influence on TC track with five questions. The process can be completed in under a minute for a “situationally aware” forecaster by utilizing a yes/no checklist. If an interaction is likely, graphical scenarios provide the forecaster with TC track-bias guidance for application on future forecasts.

Our operational guidance will help to improve TC position forecasts for situations that occur at least once or twice per season in the northwest Pacific Ocean basin. We believe our results provide credence to the numerous researchers that speculated about the influence of a TUTT cell on TC motion.