An Intensity and Size Phase Space for Tropical Cyclone Structure and Evolution

Intensity and size are important to characterize a tropical cyclone (TC), but there are a wide variety of ways that both metrics are defined. TC intensity can refer to either a maximum sustained wind speed at some height level or central surface pressure minimum, and TC size may refer to the radius of maximum wind, the radius of gale force wind, or be based on other criteria. While different definitions of TC intensity and size have useful applications, there are varying amounts of redundant information and covariations between some size and intensity variables that make investigating physical relationships more challenging. In this study, we use aircraft observations and Best Track information to calculate an empirical orthogonal function analysis that yields new, orthogonal metrics of TC intensity and size. The new, linearly independent metrics reduce a seven-dimensional space of co-varying parameters into a simplified, two-dimensional phase space in which key TC structural changes can be visualized and historically contextualized. Additionally, our analysis introduces a new parameter that is a simplified measure of the wind decay outside the radius of maximum tangential velocity. We show that this decay parameter is nearly orthogonal to the new intensity and size metrics and is useful for identifying TC maturity. We demonstrate the utility of the new phase space by first comparing the structural evolution of the large Hurricane Rita (2005) and small Hurricane Charley (2004) using observations, as well as comparing two modeling simulations of Hurricane Rita with different initial conditions in the phase space.