Identifying the Development of a Tropical Depression into a Tropical Storm over the South China Sea

Tropical depressions formed over the South China Sea usually produce severe flooding and wind damage when they develop into a storm and make landfall. To provide an early warning, forecasters should know when, and if, a tropical depression will develop into a tropical storm. To better understand and predict such development, we examine the dynamic and thermodynamic variables of 74 tropical depressions over the South China Sea, 52 of which developed into storms, hereafter developing, with the remaining being classified as nondeveloping. Using the National Centers for Environmental Prediction Final (NCEP FNL) data, verified with ECMWF forecast data, we examine the dynamic and thermodynamic statistics that characterize these tropical cyclones. Based on these characteristics, we propose seven criteria to determine whether a tropical depression will develop. Five had been used before, but two new criteria are also found to be useful. These two are associated with the diabatic heating rate and help to determine whether a tropical cyclone diurnal cycle exists and whether the convection system remains intact in the center: 1) presence of a regular diurnal variation of the diabatic heating rate at the center and 2) occurrence of specific peaks in the radiative-heating profile. We test all seven criteria on all tropical depression cases in 2018/19 before the system developed or decayed, showing that these criteria can help to operationally identify whether or not a tropical depression develops into a tropical storm with an average lead time of 36.6 h. Significance Statement Tropical cyclones formed over the South China Sea usually produce severe flooding and wind damage. What determines if a given tropical depression will develop into a storm? We examine the dynamic and thermodynamic characteristics of tropical cyclones over the South China Sea, finding that established criteria plus characteristics of the diabatic heating rate can separate the developing from the nondeveloping cases. Applying these criteria to 11 tropical depression cases, we show that they can help forecasters predict whether or not a tropical depression will develop into a tropical storm.

Automated summary

Researchers examined the dynamic and thermodynamic variables of 74 tropical depressions over the South China Sea and proposed seven criteria to determine whether a tropical depression will develop into a tropical storm. These criteria, including the diabatic heating rate, can help forecasters identify the likelihood of tropical depression development with an average lead time of 36.6 hours.