A Simple Statistical Model for the Lifetime Evolution of Outer Tropical Cyclone Size

Knowledge of outer tropical cyclone (TC) size is crucial for properly simulating the hazards and risks associated with landfalling storms. Prior work has shown that outer TC size can vary substantially over the lifetime of the TC. However, TC hazard models have typically used a constant, basin-averaged outer TC size due to the absence of an outer TC size model and deficiencies in our understanding of which factors control outer TC size. The present study utilizes a reanalysis-derived outer TC size dataset to construct a simple statistical outer TC size model for North Atlantic TCs. Based on prior work, the outer TC size metric used in this study is the radius in which the azimuthal-mean surface azimuthal wind equals 8 m/s.

Our statistical model focuses on simulating three stages of outer TC size evolution: 1) genesis, 2) from genesis to lifetime maximum outer TC size, and 3) from lifetime maximum outer TC size to end of TC lifetime. The genesis model is constructed using a stepwise multilinear regression, which has chosen both the sea surface temperature and Julian day of genesis as model variables. The magnitude of growth following genesis is largely linearly dependent on the duration of TC growth, which is itself dependent on TC lifetime (i.e., long-lived TCs exhibit greater outer TC size changes). Similarly, the decrease in outer TC size following its lifetime maximum is also generally linearly dependent on the duration of outer TC size reduction. Together these three model components provide one of the first skillful outer TC size models. Both verification and application of the outer TC size model will be presented, with specific emphasis placed on analyzing results for Hurricane Harvey, Irma, and Maria (2017).