Environmental Contributions to Tropical Cyclone Initial Size and Size Evolution

Tropical cyclone (TC) size determines the spatial extent of hazardous winds, heavy rainfall, and storm surge, yet the environmental processes governing TC size remain poorly understood. In this study, we examine the environmental controls on TC initial size and subsequent size evolution during intensification using global TC observations from 2001 to 2023. Large TCs at formation tend to occur in environments characterized by stronger vertical wind shear (VWS) and higher mid-level relative humidity, whereas sea surface temperature (SST) and relative SST are less important. The influence of environmental factors on size growth shows a clear latitudinal dependence. VWS is more strongly linked to TC size growth at lower latitudes, while SST and relative SST become increasingly important at higher latitudes. Environmental relative humidity consistently favors TC size expansion across both latitudes. Generalized additive models and random forest analyses reveal that initial size and intensity together explain more than half of the variance in TC size, highlighting a strong structural memory during intensification. After accounting for these storm characteristics, relative SST emerges as the most important environmental factor on TC size expansion. Positive relative SST, combined with abundant environmental moisture, might enhance outer core deep convection and promote radial wind field expansion, providing new insight into the environmental regulation of TC structural evolution.