Lifetime Evolution of Outer Tropical Cyclone Size and Structure as Diagnosed from Reanalysis and Climate Model Data

Abstract

The present study examines the lifetime evolution of outer tropical cyclone (TC) size and structure in the North Atlantic (NA) and western North Pacific (WNP). The metric for outer TC size is the radius at which the azimuthal-mean 10-m azimuthal wind equals 8 m s −1 ( r 8 ) derived from the NCEP Climate Forecast System Reanalysis (CFSR) and GFDL High-Resolution Forecast-Oriented Low Ocean Resolution model (HiFLOR). Radial profiles of the azimuthal-mean 10-m azimuthal wind are also analyzed to demonstrate that the results are robust across a broad range of wind radii. The analysis shows that most TCs in both basins are characterized by 1) minimum lifetime r 8 at genesis, 2) subsequent substantial increases in r 8 as the TC wind field expands, 3) peak r 8 values occurring near or after the midpoint of the TC lifetime, and 4) nontrivial decreases in r 8 and outer winds during the latter part of the TC lifetime. Compared to the NA, WNP TCs are systematically larger up until the end of their lifetime, exhibit r 8 growth and decay rates that are larger in magnitude, and are characterized by an earlier onset of lifetime maximum r 8 near their lifetime midpoint. In both basins, the TCs exhibiting the largest r 8 increases are the longest lived, especially those that traverse the longest distances (i.e., recurving TCs). Finally, analysis of TCs undergoing extratropical transition (ET) shows that NA TCs exhibit negligible changes in r 8 during ET, while WNP ET cases either show r 8 decreases (CFSR) or negligible changes in r 8 (HiFLOR).