Trapped-fetch waves in a transitioning tropical cyclone (part II-analytical and predictive model)

The Canadian Hurricane Centre’s (CHC) analytical wind model (MacAfee and Peters, 1999) was further evaluated and refined using tropical cyclone (TC) data from the past tracks database. The model’s performance on these additional cases, in varying situations, confirmed its ability to depict the surface wind field around a TC. Using forecast values of model parameters (e.g. at a time node: maximum wind speed, minimum pressure, motion vector, etc.), the model was used, in real-time, by CHC meteorologists to analyze and predict the surface wind fields during Floyd (1999), Gert (1999), Irene (1999), and Jose (1999), all of which impacted on Atlantic Canada. CHC meteorologists agreed that the modeled wind fields increased their confidence in analyzing and predicting these TCs. Although statistical analysis of the model’s performance is ongoing, the utility of the generated wind field is no longer questioned. Hence, the output gridded wind field can be used, with confidence, as input to models where a detailed TC wind field is required.

As presented in Trapped-Fetch Waves in a Transitioning Tropical Cyclone (Part I—The Need and The Theory), the predictors for the wave height calculations are: wind speed, system speed, and fetch length. The system speed is a wind model input parameter; gridded wind speeds are output. Hence, by coupling the wind model with fetch determination and wave height calculation algorithms, a trapped fetch wave height model was developed.

In analytical mode, the trapped fetch wave height model can be applied to any TC in the past tracks database to seek out extreme events warranting further analysis. In predictive mode, wave height calculations provide guidance on areas of dangerously high waves such as those experienced during Luis (1995), but can also provide indirect evidence that the TC’s intensity and/or motion require adjustment. Further, the wave height evolution with time along the TC track may provide evidence that the TC is undergoing post-tropical transition (PTT). For example, observed and modeled wave heights, no longer in agreement, may indicate that the TC wind field is no longer representative, thereby, providing a “PTT signature.”

Several case studies will be shown in the poster session to illustrate, in detail, the concepts, theories, and modeling. Particular attention will focus on the real-time use of the model in identifying “huge” waves. The indirect evidence it may provide on TC structure changes will be outlined to provide a forum for informal discussion on this work, especially on the relevance, utility, and speculation of “PTT signatures.”