The relationship between precipitation and extra-tropical cyclone intensity in different idealised climates

Extra-tropical cyclones (ETCs) are the main cause of precipitation in the mid-latitudes. Some, but not all, ETCs can lead to heavy precipitation and flooding. Furthermore, not all extreme ETCs in terms of maximum vorticity – a commonly used measure of ETC intensity – lead to extreme precipitation. Therefore, this study quantifies the relationship between maximum vorticity and ETC related precipitation in the current and future climates, and determines how this relationship depends on the structure and characteristics of the ETC. These aims are addressed using three 10-year-long aqua-planet simulations performed with a state-of-the-art global model, OpenIFS. The simulations only differ in terms of the specified sea surface temperature (SST) distributions which are held fixed in time. The control simulation has the well-known “QObs” SST distribution, the uniform warming simulation has a 4K warming relative to the control simulation applied everywhere, and the polar amplification experiment has a 5K warming applied poleward of 45 degrees. The feature tracking software TRACK is applied to objectively identify and track all ETCs in each experiment and obtain the maximum 850-hPa vorticity of each ETC. k-means clustering is then applied to the precipitation field surrounding each ETC to group the ETCs into clusters with similar precipitation structures.

In all experiments, ETCs with stronger maximum vorticity were associated with more precipitation. This relationship was strongest in the uniform warming simulation and weakest in the control simulation. Considerable spread, particularly in the uniform warming experiment, was present in this correlation suggesting that not all ETCs have a strong correlation between maximum vorticity and precipitation. This is confirmed by the clustering results. The k-means clustering identifies 4 distinct types of ETCs which are present in all experiments. For two types of ETCs – those located at high latitudes with weak precipitation and those with the precipitation located mainly in the centre of the ETCs – there is only a very weak relationship between maximum vorticity and precipitation. The strongest relationship between ETC maximum vorticity and precipitation occurs for ETCs that have most precipitation associated with the warm front. These results will be presented along with dynamical interpretations.