How Much Do Tropical Cyclones Grow in Size during Extratropical Transition?

Approximately half of landfalling tropical cyclones (TCs) along the eastern coast of the United States undergo extratropical transition (ET), necessitating the need to understand changes in the TC wind field during ET.   While previous studies have examined the evolution of inner core TC winds during ET, relatively few studies have examined the outer core TC winds and, more specifically, TC size.   These prior studies on TC size during ET suggest that TCs, on average, grow larger in size during ET, but the limited sample size of these studies raises questions about their broader applicability.  Building upon this prior research, the present study seeks to determine whether North Atlantic and western North Pacific TCs grow larger in size during ET, using a reanalysis-derived climatology of TC size data.

The results suggest that a majority of North Atlantic TCs (~60%) grow to be, on average, ~1.4 times larger than their initial size prior to ET onset.  North Atlantic TCs that undergo the largest size changes are those TCs that generally experience longer transition times, more strongly baroclinic environments, and larger changes in latitude during ET.  In the western North Pacific, a minority of TCs (~25%) grow to be, on average, ~1.3 times larger than their initial size prior to ET onset.  The relatively smaller fraction of western North Pacific TCs that grow larger in size during ET may result from their significantly larger initial size prior to ET onset, which is likely closer to extratropical cyclone size.  These results suggest that a substantial fraction of TCs grow larger in size during ET with the degree of growth likely sensitive to the initial TC size prior to ET onset and its background environment during ET.