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Defining Binary Tropical Cyclones

Research team from the Institute of Atmospheric Physics, Chinese Academy of Sciences address issue with characterising binary tropical cyclones through two key parameters; movement distance between each other and presence of a mutual anti-clockwise spin.

In the Northwest Pacific, most tropical cyclones occur alone during their lifetime. However, sometimes two or several tropical cyclones exist simultaneously. Generally, two tropical cyclones occurring simultaneously are referred to as binary tropical cyclones, and they concurrently perform a mutual counter clockwise spin and move closer to each other when at a relatively close range. This phenomenon was first noted by Fujihara in 1921.

There are three existing binary tropical cyclones definitions with different criteria based on the separation distance, tropical cyclone intensity and the coexistence time, which seem to be objective factors. However, as there is so explanation or reason in existence regarding the origins of these definitions, in truth they are mainly subjective.

To address this problem, Professor Fumin Ren and his research team, looked at of two important parameters of binary tropical cyclones. These two parameters were, the two tropical cyclones moving closer to each other and performing a mutual counter clockwise spin. The team carried out a study that analysed two best-track datasets, provided by the China Meteorological Administration (CMA) and the Joint Typhoon Warning Center (JTWC). They established an objective standard, which includes a main standard for defining binary tropical cyclones and a secondary standard for identifying typical and atypical binary tropical cyclones, based on the high level of consistency between the two datasets.

“The main standard has two requirements: two coexisting TCs are a pair of BTCs if the separation distance is less than 1800 km, and if this separation maintains for at least 12 hours,” said Professor Ren, with Chinese Academy of Meteorological Sciences.

The above research has been accepted and published by Advances in Atmospheric Sciences.

Binary tropical cyclones often bring extreme precipitation when they make landfall, and cause serious disasters. For example, the strong typhoon “Morak” in 2009 caused about 700 deaths or disappearances over the southern Taiwan Island.

“We still do not have a clear picture of the characteristics of binary tropical cyclone interactions in China’s offshore area, and this is what we are going to investigate next,” added Professor Ren. [APBN]