Using satellite observations and high-resolution ensemble simulation output, researchers uncover the triggers of tropical cyclone onset of rapid intensification.
Dr Duan Yihong and his team of researchers from the Hainan Key Laboratory of South China Sea Meteorological Disaster Prevention and Mitigation, the State Key Laboratory of Severe Weather, China Meteorological Administration, and the Australian Bureau of Meteorology published their findings in Advances in Atmospheric Sciences.
Tropical cyclone cloud structures are observed using satellites and the evolution of surrounding weather systems, to assist weather forecasting. Changes in rapid intensity often occurs together with rapid re-organisation of the tropical cyclone’s mesoscale cloud and precipitation structures.
Super Typhoon Rammasun in 2014 was the strongest-recorded tropical cyclone at landfall over mainland China since 1949. Using satellite observations and high-resolution ensemble simulation output, the researchers were able to gather findings about precursors of Super Typhoon Rammasun onset of rapid intensification (RI).
As illustrated above of the two ensemble simulated tropical cyclone sets undergoing different timing for RI onset. For the early RI onset member, the minimal ventilation, undiluted clouds and moisture from the down-shear quadrant are then wrapped inwards to the up-shear left quadrant to form the deep convections (left-hand panel). For the late RI onset member, these processes are inhibited by stronger vertical wind shear, initially resulting in poor vertical coherence of the circulation, lesser moisture and larger ventilation (right-hand panel).
For the early RI onset member, strong ascent develops over the down-shear flank at radii beyond the radius of maximum wind, merging inward-penetrating inner cloud bands that become deep convections while the vortex core becomes upright. Thus, the Synchronization Index, a simple measure of the amount of coherence in the vertical structure of the circulation, is proposed by Dr Noel Davidson, one of the authors of the current study.
“The Synchronization Index presented here is a useful way of monitoring vortex structure, though it is only based upon our analysis of an ensemble of RI forecasts for one case study,” explains Dr Duan. “We plan to extend the study by making further analyses and budget diagnoses from the members, to better understand the processes that influence RI.” [APBN]