Advancing tropical cyclone simulations for improved climate predictions and resilience

by Alexander J. Baker from the National Centre for Atmospheric Science and Department of Meteorology at the University of Reading

Tropical cyclones, including hurricanes and typhoons, rank among the most costly natural hazards. Recent research has highlighted an increasing trend in the destructiveness of tropical cyclones over the last few decades, as well as the disproportionately high exposure of socioeconomically underprivileged populations to their impacts globally. Simulating realistic tropical cyclones is therefore a vital ambition of weather and climate modelling—and of the nextGEMS project. Mainly, because while the physical processes governing a tropical cyclone’s lifecycle are relatively well understood, many of these processes occur at scales below those resolved by global climate models. Consequently, it is anticipated that increasing the resolution of models will help simulate more realistic tropical cyclones.

What is a tropical cyclone?

Tropical cyclones are born from ‘seeds’—tropical waves or rotating clusters of individual thunderstorms—in a process lasting from hours to weeks. This occurs at low latitudes over warm tropical oceans and, usually, at least a thousand kilometres from the equator because it is where planetary rotation is sufficient to aggregate convective activity into a coherent whirling phenomenon called vortex. Once formed, tropical cyclones mature, increasing in intensity. Typically, they move westward and poleward before reaching the midlatitudes, where they weaken over a cooler ocean surface or transform into frontal weather systems.

Understanding simulated tropical cyclones

In research carried out the University of Reading, we used a tracking algorithm to identify tropical cyclones in nextGEMS data. We analysed various simulated tropical cyclone characteristics, and I’ll describe two key ones here: the frequency of tropical cyclones and how quickly they intensify. Both characteristics are related to cyclone predictability and societal impacts.

Simulated tropical cyclone frequency is affected by both model resolution and model physics (see bar chart). For ECMWF’s Integrated Forecasting System (IFS) model, the annual number of tropical cyclones is closer to observations at higher resolutions in both cycle 2 and 3 simulations. Cyclone frequency also changes with ocean model / resolution. IFS simulations in cycle 3 at 9 km resolution in the atmosphere reveal an increase in cyclone frequency when using the Finite-Element/volume Sea ice-Ocean Model (FESOM) ocean model at 5km, compared with the Nucleus for European Modelling of the Ocean (NEMO) model at 0.25 º. The MPI-M’s Icosahedral Non-hydrostatic Weather and Climate (ICON) model shows little sensitivity to changes in atmospheric resolution, but shows significant sensitivity to model physics. Simulated cyclone frequency is too high in cycle 2 but is closer to observations in cycle 3.

On the other hand, how quickly tropical cyclones intensify increases with finer atmospheric resolution (see graph). At low resolution (e.g., the IFS at 28 km), even the most intense cyclones mature too slowly and reach a peak wind speed that is much lower than observed in the real world. When resolution is increased, however, the rate of intensification becomes much more realistic, and mimics observations closely at resolutions of 4.4 km (for the IFS) and 5 km (for ICON). nextGEMS simulations also contain examples of rapid intensification of tropical cyclones, forecasts of which are still frequently error prone. Finer resolution may therefore be a key component of improving forecasts of such events.

Data produced by model-development cycles in nextGEMS have demonstrated that tropical cyclone realism improves significantly with an increase in atmospheric resolution from that of previous-generation, HighResMIP-type models to a few kilometres. The forthcoming production of 30-year simulations in nextGEMS will help better understand the overall role of intense tropical cyclones in the climate system.

Reference

Baker, A. J., Vannière, B., and Vidale, P. L. On the realism of tropical cyclones simulated in global storm-resolving climate models. To be submitted to Geophysical Research Letters.