A recent study suggested that we have probably approached the limit of predictability for tropical cyclone (TC) track prediction. If that's true, this would be bad news for disaster prevention and mitigation.

"Because there's a diminishing trend in the reduction of positional error in National Hurricane Center (NHC) tropical cyclone forecasts, there seems to be little room for improvement," said Dr. ZHOU Feifan, a scientist from the Institute of Atmospheric Physics (IAP) of the Chinese Academy of Sciences, referring to a question asked by Landsea and Cangialosi in a paper published in 2018.

ZHOU used the same dataset as in LC18 (except excluding tropical depressions) and an approach called Statistical Analysis and Forecast Error estimation proposed by Zoltan Toth and his collaborators at the National Oceanic and Atmospheric Administration of the USA.

In a study published in the Bulletin of the American Meteorological Society, ZHOU and Zoltan Toth explored what the past trend was in the reduction of TC forecast track error, and how such errors may be further reduced in future decades.

Fengyun Satellite image of Typhoon Maysak which plowed into the Korean Peninsula in early September (Image by National Satellite Meteorological Center of China Meteorological Administration) 

In accordance with theoretical expectations, they found that the true forecast track error (i.e., forecast minus real TC position) increased exponentially with lead time. The 24-hour forecast error growth rate appeared to be rather stable over the years, with only relatively small year-to-year fluctuations, possibly influenced by seasonal circulations such as ENSO or MJO.

"The exponential growth of true forecast track error implies that the dynamics of TC motions could be viewed as linear, and that there is no model induced error in TC position forecasts. In other words, the transposition of TCs is dominated by the large-scale environmental circulation, which is well simulated in modern numerical weather prediction (NWP) models," said ZHOU.

Interestingly, ZHOU and Toth also found that the true analysis error also changed exponentially over long periods of time. "The near-exponential reduction of analysis error that we found over the years means that initial errors in the NHC official forecasts are reduced by approximately the same fraction each year. This suggests that the efficiency of international NWP research and development affecting the official forecasts, by and large, is constant over the years," said ZHOU.

Based on these features, the team set up an error model using just four parameters. Assuming that the level of investments, and the pace of improvements to the observing, modeling, and data assimilation systems continued unabated, their four-parameter error model indicated that the time limit of predictability at the 181 nm error level that was reached at day five in 2017, may be extended beyond six / eight days in 10 / 30 years' time.

"That is to say in 10 years' time, the forecast skill at day six would be the same as it was at day five in 2017. Considering also some results from Zhang et al., we can add that this one day per decade extrapolated error reduction, given the assumption above, may hold for at least 25 years into the future. That's a long time, with lots of potential for TC track error reductions along the way," said Toth.

The more optimistic assessment of TC track predictability in the current study is apparently due to their recognition that true forecast error behaves exponentially, i.e., it grows over forecast days, and is reduced by NWP development over the years.