Tropical Cyclones and Resolution Sensitivity in HighResMIP GCMs

For the first time in the CMIP exercise, international modelling groups have come together under a coordinated protocol, HighResMIP, which is designed to investigate the role of model resolution in the simulation of climate processes. The protocol prescribes long simulations of 100 years each in both atmosphere-only mode (1950-2050), and coupled mode, in two sets: a) constant 1950s radiative forcing and b) historic forcing (1950-2014). The principal focus of the high-resolution simulations is at around 20km mesh size.

Past, opportunity-driven intercomparisons, such as carried out by the US CLIVAR Hurricane Working Group, have revealed that Tropical Cyclone track densities start to be credibly represented at resolutions ~50km, including their interannual variability, while simulations of TC intensities start to be more realistic for models at ~20km and beyond.

New results from the current HighResMIP exercise, using six GCMs so far, confirm past findings in terms of the increased realism of Tropical Cyclone simulations, and further stress the substantial impact of refining resolution from ~100km to ~20km. The unprecedented length of the simulations in these coordinated experiments also reveals significant responses to (forced and unforced) climate variability, impossible to address with typical 30-year simulations. The coordinated protocol additionally permits the investigation of the role model formulation (e.g. the use of stochastic physics), which, in individual cases, can be as significant as the impact of model resolution.