Introducing synthetic convectively coupled Kelvin waves into the Met Office Unified Model

The Maritime Continent (MC) is the rainiest region on Earth, where extreme precipitation constitutes a major hazard. Convectively coupled Kelvin waves (CCKWs) are weather systems that travel eastwards across the equatorial waveguide and can trigger convection in their convergent phase. CCKWs are linked to up to a fourfold increase in precipitation rates across the equatorial Maritime Continent (Ferrett et al., 2020). However, not all CCKWs produce precipitation extremes. Recent studies reveal that CCKWs arriving in phase with the local diurnal cycle of convection may be more likely to cause high impact weather events or when part of a multiscale interaction with the MJO, organising the large-scale precipitation on a more localised scale (Baranowski et al., 2016; Baranowski et al., 2020; Latos et al., 2021; Senior et al., 2023).

Current methods for studying these mechanisms have some limitations. For example, composite studies of CCKWs are useful for revealing statistical links but smooth out key interactions. Case studies are useful for identifying mechanisms in particular high-impact weather events but are difficult to generalise. Modelling such high-impact weather events provides additional insights; however, lacks the capability for fine-tuning.

Hence, we have developed a methodology for introducing synthetic CCKWs into convection-permitting Met Office Unified Model (MetUM) forecasts. This involves generating 3D CCKW structures on key dynamical fields using ERA5 data and adding these to the model’s initial conditions. The methodology will be presented, and a comparison of diagnostics from control and perturbation experiments will be provided. We will then discuss how the methodology will be applied to studying the mechanisms through which CCKWs cause precipitation extremes across various locations in the MC. Since CCKWs are an important dynamical predictor of extreme precipitation, understanding these mechanisms is crucial for providing accurate forecasts of hazardous weather in the MC.

Baranowski, D.B. et al., (2016) Phase locking between atmospheric convectively coupled equatorial Kelvin waves and the diurnal cycle of precipitation over the Maritime Continent. Geophysical Research Letters, 43(15), 8269–8276. https://doi.org/10.1002/2016GL069602.

Baranowski, D.B. et al., (2020) Social-media and newspaper reports reveal large-scale meteorological drivers of floods on Sumatra. Nature Communications, 11, 2503. https://doi.org/10.1038/s41467-020-16171-2.

Ferrett, S. et al., (2020) Linking extreme precipitation in Southeast Asia to equatorial waves. Quarterly Journal of the Royal Meteorological Society, 146(727), 665–684. https://doi.org/10.1002/qj.3699.

Latos, B. et al., (2021) Equatorial waves triggering extreme rainfall and floods in Southwest Sulawesi, Indonesia. Monthly Weather Review, 149(5), 1381–1401. https://doi.org/10.1175/MWR-D-20-0262.1.

Senior, N.V. et al., (2023) Extreme precipitation at Padang, Sumatra triggered by convectively coupled Kelvin waves. Quarterly Journal of the Royal Meteorological Society, 149(755), 2281–2300.  https://doi.org/10.1002/qj.4506