Rapid Contraction of the Radius of Maximum Tangential Wind and Rapid Intensification of a Tropical Cyclone

Abstract

Abstract During the intensification of a tropical cyclone (TC), the radius of maximum tangential wind (RMW) often contracts due to the spin‐up of tangential wind being much faster inside the RMW than at the RMW itself. However, rapid RMW contractions prior to rapid intensification (RI) of TCs are observed. Statistical analysis of best‐track data for the North Atlantic between 2000 and 2017 indicates that rapid RMW contraction (≥20 nmi within 24 h) does not necessarily cause RI (≥30 kt within 24 h) of a TC. An efficient TC RI is generally seen if the RMW contracts to ≤30 nmi. The change (contraction or expansion) of the RMW is prevalent for weak storms, and RMW contraction is found to be at its most rapid for TC initial intensities ≤60 kt. Rapid RMW contraction and simultaneous RI generally occurs when the initial RMW is >30 nmi. Convective heating associated with very deep convective clouds with infrared brightness temperatures <208 K appearing toward the storm center is important for RMW contraction and sufficient amounts of the convective heating are important for intensification. Less convective heating is required to have the same intensification rate for a storm with an initially smaller RMW. A storm is more likely to experience RI with relatively less convective heating after the RMW contracts to a smaller size. This study demonstrates the role of convective heating on RMW contraction and storm spin‐up through different processes in the early stages of a TC.