A Historical Analysis of the Mature Stage of Tropical Cyclones

The characteristics of tropical cyclone intensity and size during the mature stage are presented. Rooted in the classic description by Herbert Riehl, the mature stage is identified as the period from the time of lifetime maximum intensity to the time of lifetime maximum size. This study is the first to analyse the global climatology of the mature stage of tropical cyclones in detail. Three basic features at the mature stage are observed: the reduction of intensity, the outward expansion of the eyewall, and the increase of tangential wind in the outer primary circulation.

Globally, about a quarter of tropical cyclones undergo the mature stage. High intensity at the end of the immature stage favors the likelihood of the occurrence of the mature stage. The intensity reduction during the mature stage is considerable with nearly three-quarters of cyclones decreasing by more than 10%, which makes the conventional “steady-state” presumption questionable. The increase in the radius of damaging-force wind is typically about 50 km, while the decrease in maximum wind speed is typically 20% at the mature stage. However, the average integrated kinetic energy and hence destructive potential increases substantially by about 70%. This is consistent with our finding that most of the highly damaging landfalling hurricanes undergo a mature stage.

Based on our observational analyses, the previously idealized simulations, and the classic definition by Riehl, the TC mature stage can be better described. The mature stage is the period when the TC intensity decreases progressively with the circulation expanding. Our modification to the classic definition by Riehl is that we specify the progressive decay of intensity during the mature stage. The destructive potential of a storm usually continues increasing during this stage if the decrease of intensity is not too large.

The TCs undergo the mature stage more frequently over the North Atlantic and Western North Pacific than other ocean basins. If a TC has a large lifetime maximum size, there is a high probability that the TC has undergone the mature stage. It is also found that high intensity at the end of the immature stage favors the mature stage.

Climate change may further favor the formation of the mature stage. The understanding of the mature stage is of great importance to improve TC destructive potential forecasting. The TC destructive potential can increase significantly at the mature stage due to the increase in size. Intensity downgrades during the mature stage may be misinterpreted as they are mostly not accompanied by an overall danger reduction. Some attention should be given to this when issuing public intensity downgrades during the mature stage.