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[AAS04-07] Statistical analysis of near-landfall Intensified Typhoons and their Dynamic and Thermal conditions
Keywords:near-lanfall tropical cyclones, offshore rapid intensification, dynamical conditions, thermal conditions
Introduction Unlike most tropical cyclones (TCs) that begin to weaken when approaching the land, there are some TCs that intensified near-landfall on the contrary. Those TCs bring a great challenge to typhoon forecasts and disaster prevention. In this paper, the basic characteristics of those TCs is firstly analyzed based on three best track datasets in the Northwestern Pacific. Then the fundamental spatial and temporal distribution of such TCs are further provided. Based on selected typical cases, the environmental dynamic conditions as well as local thermal conditions during the intensification periods before landfall are closely clarified.
Data and methods Based on three best track datasets JMA, CMA and JTWC from 1980 to 2021, the intensity variation characteristics of tropical cyclones in the Western Pacific within 48 hours before landfall were studied, and 11 common typical near-landfall intensified typhoon cases in the three datasets were selected for composite analysis. The environmental wind vertical shear, relative eddy flux convergence and local radiation flux during landings were calculated using ERA5 reanalysis.
Distribution of near-landfall intensity variation The intensity changes of 24hr, 12hr and 6hr before landfall were documented. According to statistical analysis, tropical cyclones whose intensity changes of 24hr before landfall exceeded 20kt and did not weaken within 48hr before landfall were defined as near-landfall intensification cases. Statistics show that those TCs have northwestward prevailing tracks. The high activated period is from July to September, and the probability during nighttime is significantly higher than that of daytime.
Environmental dynamic conditions Composite analysis of selected typical near-landfall intensified TCs shows that the vertical wind shear, which has no significant decreasing before landfall. Further analysis find out that the decreasing of local vertical wind shear and the increase of relative eddy flux convergence can favor the development of inner-core convection.
Local thermal conditions The asymmetric distribution of latent heat, sensible heat flux and long-wave radiation on the underlying surface around the typhoon before landfall is further analyzed. It is found that the asymmetric distribution of local non-adiabatic typhoon circulation is conducive to the development of offshore intensity.
Conclusions and discussion Under favorable dynamic and thermal processes. Statistical analysis of such typhoons reveals common characteristics and possible influential factors. Further simulation study of typical near-landfall intensified typhoons is expected to clarify the possible mechanism of topography during the offshore intensification process for those TCs.
Data and methods Based on three best track datasets JMA, CMA and JTWC from 1980 to 2021, the intensity variation characteristics of tropical cyclones in the Western Pacific within 48 hours before landfall were studied, and 11 common typical near-landfall intensified typhoon cases in the three datasets were selected for composite analysis. The environmental wind vertical shear, relative eddy flux convergence and local radiation flux during landings were calculated using ERA5 reanalysis.
Distribution of near-landfall intensity variation The intensity changes of 24hr, 12hr and 6hr before landfall were documented. According to statistical analysis, tropical cyclones whose intensity changes of 24hr before landfall exceeded 20kt and did not weaken within 48hr before landfall were defined as near-landfall intensification cases. Statistics show that those TCs have northwestward prevailing tracks. The high activated period is from July to September, and the probability during nighttime is significantly higher than that of daytime.
Environmental dynamic conditions Composite analysis of selected typical near-landfall intensified TCs shows that the vertical wind shear, which has no significant decreasing before landfall. Further analysis find out that the decreasing of local vertical wind shear and the increase of relative eddy flux convergence can favor the development of inner-core convection.
Local thermal conditions The asymmetric distribution of latent heat, sensible heat flux and long-wave radiation on the underlying surface around the typhoon before landfall is further analyzed. It is found that the asymmetric distribution of local non-adiabatic typhoon circulation is conducive to the development of offshore intensity.
Conclusions and discussion Under favorable dynamic and thermal processes. Statistical analysis of such typhoons reveals common characteristics and possible influential factors. Further simulation study of typical near-landfall intensified typhoons is expected to clarify the possible mechanism of topography during the offshore intensification process for those TCs.