Assessment of connections between atmospheric planetary waves and extreme rainfall events

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NERC:Anna Whitford:NE/S007512/1Extreme rainfall events are recognised as causing significant socioeconomic damage and loss of life as the intense rainfall can generate dangerous floods, particularly in small steep catchments and urban areas. These extreme rainfall events are driven by a combination of thermodynamic (linked to temperature) and dynamic (linked to atmospheric circulation) forces on both local and regional scales. While research in recent years has revealed much about the thermodynamic drivers of extreme rainfall, there is still much we do not know about the dynamical drivers (Westra et al., 2014). These dynamic drivers range from very large, planetary scale atmospheric waves (Rossby waves) down to very localised small-scale convective currents. In this project we are focussing on the large-scale atmospheric drivers, in particular the relationships between Rossby waves and extreme rainfall events. These large-scale atmospheric waves have been shown in previous studies to be drivers of extreme heatwave events and also of 5 to 7-day extreme rainfall events. These connections occur for both anonymously high amplitude waves and also for very slow-moving (quasi-stationary) waves (Petoukhov et al., 2013; Wolf et al., 2018; Kornhuber et al., 2019). We therefore aim to confirm whether these connections also occur for shorter multi-day (3 days and less) extreme rainfall events and then investigate whether there are also connections to daily and sub-daily extreme events. Midlatitude waveguides are narrow bands of high altitude wind (jets) which occur in the atmosphere and can effectively trap the Rossby waves within the latitude band of the waveguide. Midlatitude waveguides have been found to occur in conjunction with anomalously high quasi-stationary wave activity and extreme heat events (Petoukhov et al., 2013; White, 2019), suggesting they have an influence on the development of conditions conducive to anomalous Rossby wave development. This project aims to identify whether the same waveguide - Rossby wave associations are present for extreme rainfall events. Once any connections between the Rossby waves and extreme rainfall have been identified, we will use output from seasonal to subseasonal climate forecast models and Rossby wave indices to investigate whether it is possible to produce forecasts of extreme rainfall event occurrence on seasonal timescales based on the Rossby wave activity.It has been shown that, due to climate change, the conditions which support the development of anomalous RWs have increased in frequency (Petoukhov et al., 2013; Kornhuber et al., 2019). At the same time, the evidence for increases in the intensity and frequency of extreme rainfall due to global warming (Trenberth et al., 2003) indicates we urgently need to understand the drivers behind these damaging events. Therefore, improving our understanding of how anomalous RWs effect extreme rainfall is vital to enhance our ability to forecast and prepare for these events and any associated flooding. This project would ultimately provide a key piece of research towards this understanding and the results will help improve our ability to project future extreme event frequency. The outcomes of this project could have large positive impacts for the insurance industry and for local and national government agencies e.g. the UK Environment Agency. Being able to forecast on a seasonal to sub-seasonal basis, when these damaging extreme events may occur would allow insurers and local governments to implement plans and strategies to help prepare before these events occurred, thereby increasing resilience and mitigating the negative socioeconomic impacts of any resulting flooding