The Inner-Core Size Increase of Typhoon Megi (2010) During its Rapid Intensification Phase

Abstract

ABSTRACT While Typhoon Megi (2010) shared many common features of tropical cyclones (TCs) that crossed Luzon Island in northern Philippines, it experienced a significant inner-core size increase with little eyewall contraction during its rapid intensification (RI) phase. This is unusual since the majority of TCs experience an eyewall contraction with little or a slow inner-core size increase during RI. The inner-core size increase during RI of Typhoon Megi was simulated reasonably well using the Advanced Research Weather Research and Forecasting (ARW-WRF) model with both dynamical initialization and large-scale spectral nudging. In this paper processes responsible for the inner-core size increase of Typhoon Megi during its RI phase were analyzed based on a control simulation. It is shown that the inner-core size increase was primarily related to the binary interaction of Megi with a large-scale low-level depression in which Megi was embedded. The shearing/merging of the large-scale depression with Megi and the subsequent axisymmetrization led to the strengthening of the outer circulation of Megi. Both the moist condition in the low-level depression and the binary interaction contributed to active spiral rainbands. Diabatic heating in spiral rainbands enhanced low-level inflow, which brought absolute angular momentum inward, increasing tangential wind speed outside the eyewall, thus leading to the outward expansion of tangential wind and the increase of the inner-core size of Megi.