Improvements in tropical cyclone forecasting using CCAM compared with the operational Unified Model: A case study of tropical cyclone Idai

Abstract

Abstract Tropical cyclones are among the most destructive weather systems, yet prediction skill in the South-west Indian Ocean (SWIO) lags due to limited observations and modelling constraints. This study evaluates the Conformal-Cubic Atmospheric Model (CCAM) in simulating track and intensity of Tropical Cyclone Idai, relative to the operational Unified Model (UM) at the South African Weather Service (SAWS). Simulations were performed at multiple resolutions (CCAM: 25 km, 6 km; UMGA: 10 km; UM: 4.4 km) and lead times (72 h, 48 h, 24 h), validated against independent observational and reanalysis datasets. Both models reproduced Idai’s track more accurately at finer resolution and shorter lead times. CCAM exhibited a southward track bias, while UM deviated northward. Landfall position and timing were generally captured, with position errors reduced to < 30 km at 24 h, although CCAM 6 km showed a late landfall bias of ~3 h at 72 h. Intensity was underestimated by all simulations, however, CCAM 6 km better matched best-track winds and central pressures, while UM 4.4 km aligned more closely with ERA5. Rainfall patterns differed, with CCAM overestimating rainfall extent and intensity, whereas UM 4.4 km more realistically captured IMERG and CHIRPS patterns, though with weaker magnitudes. Results show that high horizontal resolution is crucial for representing TC characteristics, with microphysical processes becoming increasingly influential at convection-permitting scales. Differences in model initialisation also contributed to track and intensity biases. These findings underscore the value of high-resolution modelling, improved initialisation and observations to advance TC forecasting and early warning in the SWIO.