Changes in the Deep Convective Structures of Tropical Cyclones Associated with Diurnal Pulse

Forecasting tropical cyclone (TC) intensity change, especially rapid intensification, still faces great challenges, as intensity change is impacted by complex processes across multiple scales involving both environmental forcing and internal structures. The diurnal cycle is likely one such process. One of the most prominent diurnal cycle features in TCs is the radially outward propagation of a cooling signal in the upper-level clouds, the so-called diurnal pulse (DP). Studies suggested that some of the DPs occur in the deeper convective layer and therefore may impact the TC structure and intensity. This study investigates changes in the cloud and precipitation structures of TCs experiencing DPs, using over 15 years of satellite passive microwave and spaceborne precipitation radar observations. Our results suggest significant changes in the TC’s inner region associated with DPs: 1) The cold clouds containing both ice and liquid particles in the inner core become more frequent and extended when DP occurs, especially in the downshear-left quadrant of weak TCs and upshear region of moderate-to-strong TCs. The occurrence of DP also helps to maintain a relatively symmetric distribution of cold clouds even under high-shear environments. 2) The inner-core warm cloud dominated by liquid hydrometeors markedly increases in the right-of-shear during DP events, indicating new development of convection in these quadrants. 3) Spaceborne radar observations show a significant convective deepening and increase of moderate-to-deep convection in the TC’s inner region on DP days, particularly the upshear-right quadrant. In short, the inner region convective structures of TCs become deeper and more symmetric with the occurrence of DPs, which is beneficial for TC intensification.