Effects of background state on tropical cyclone size over the Western North Pacific and Northern Atlantic

Observational studies have shown that tropical cyclone (TC) size, commonly measured by the radius of the 17ms(-1) surface wind speed, in the western North Pacific (WNP) is statistically larger than its counterpart in the North Atlantic (NA). In this study we conduct idealized simulations for TC developments using high-resolution WRF model to understand the reason behind the size difference and relative contributions from among temperature, moisture and wind fields to modulate the TC size. Climatological mean states of the WNP and NA were computed using long-term analysis fields and used as background fields for the simulations. With identical initial vortices, TCs in the WNP environment evolve to larger sizes than those in the NA environment, consistent with previous observational studies and our own analysis using best track data. Experiments were designed to separate impacts of the specific humidity, wind fields, and the temperature profiles. Our simulations indicate that the temperature profile is the dominant factor in controlling the TC size with its influence about twice larger than from the specific humidity or the wind fields. The background climatological state in the WNP has a higher SST and a lower tropopause temperature than in the NA. This condition is favorable for more intense TCs. As the size is proportional to the intensity of the storm, this more unstable atmospheric condition is the reason behind the larger size observed in the WNP. The size of the inner core, represented by the radius of the maximum wind, settled down quickly in the early stage and is less correlated to the evolution of the outer size which continues to increase with the intensity increasing. The more favorable condition for the TC development and larger size in the WNP also correspond to the larger surface entropy flux.