The Potential Uses of Filamentation Time As a Diagnostic Tool in Tropical Cyclone Intensity Forecasting.

Rozoff et al. (2006) popularized the use of a concept borrowed from two-dimensional fluid dynamics known as filamentation time. The behaviour of the gradient of any scalar quantity such as relative vorticity or vertical velocity over time can be categorised into two distinct regimes. In the rotational regime coherent structures tend to stay well defined and only rotation of the gradient scalar occurs, while in the filamentary regime structures do not stay coherent indefinitely and are instead sheared and stretched into long, thin, filaments. The filamentation time is linked to how rapidly this process occurs for the case of the filamentary regime. Regions of low filamentation time (under the typical convective overturning time of 30 minutes) are notable for where high wavenumber convection is suppressed in favour of line-like filaments of convection e.g. Li and Wang (2012) and Wang (2008). The size, shape, and location of rapid filamentation zones in tropical cyclones offer crucial insight into the development and evolution of tropical cyclones. For example, the development of an initial filamentation zone outside the radius of maximum wind speed may herald intensification while concentric rings of rapid filamentation zones may indicate an in-progress eyewall replacement cycle.

Given the potential utility of filamentation time as a diagnostic metric for tropical cyclone dynamical development there is motivation to re-evaluate this metric as well as the theory and assumptions behind it, particularly since there has been relatively little research on the subject since the first few years after Rozoff’s seminal paper. The purpose of this poster is to provide a clarified understanding of the applications of filamentation time to tropical cyclone research and test assumptions from the theory in a real tropical cyclone using model output data (convection permitting high resolution MetUM simulations) with the aim of testing the validity of the assumptions in different contexts (for example close to the radius of maximum wind speed vs far from the radius of maximum wind speed). By testing some of the assumptions made in the theory ,using a state of the art high resolution numerical model, and showing how the results of how the filamentation time (and regime) vary when assumptions are relaxed, the validity of the paradigm and its future applications may be rigorously tested.