CMCC Webinar
17 October 2024, 12:00 CEST
To join the webinar, register here

Speaker:
Pier Luigi Vidale – NCAS / University of Reading

Moderator:
Enrico Scoccimarro – CMCC

Abstract
For decades we have known that Tropical Cyclones (TCs) are an emergent phenomenon in GCMs, with TC-like vortices appearing even in coarse (mesh size of~100km) GCMs, and credible inter annual variability represented in most basins with mesh sizes of ~50km). HighResMIP, promoting the use of 20km GMCs, consolidated these advances with heterogenous ensembles of atmosphere-only and coupled GCMs, exercised under a well-defined protocol.

New GCM simulations at ~10km and sub-10km, from EU projects EERIE and NextGEMS continue to show this convergent behaviour, with nearly the entire spectrum of TC intensities reproduced at 10km, and realistic rapid intensification (RI) reproduced at 4-2km. Further, GCMs in the 10-1km resolution range produce Tropical Cyclones that spend most of their lifetime, including the phase at which they reach maximum intensity, within the tropics, unlike TCs in the 100-20km range of resolution, which reach maximum intensity outside the tropics.

These new results are important for two reasons: first, in coarse (100-20km) GCMs, TCs reach their maximum intensity in the extra tropics, failing to release latent heat in the tropics, thus contributing to erroneous tropical circulation; they grow instead by exploiting baroclinicity in the extra-tropics, while undergoing extra-tropical transition. At the same time, our estimates of the risk of TC landfall in the tropics versus extra-tropics are systematically wrong, unless we exercise GCMs in the 10-1km range. Downscaling coarse GCMs is obviously not a solution to these important errors in the simulation of TCs and their interaction with the large scale environment.

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