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Equatorial waves are a major driver of widespread convection in Southeast Asia and the tropics more widely, a region in which accurate heavy rainfall forecasts are still a challenge. Conditioning rainfall over land on local equatorial wave phases finds that heavy rainfall can be between 2 and 4 times more likely to occur in Indonesia, Malaysia, Vietnam, and the Philippines. Equatorial waves are identified in a global numerical weather prediction ensemble forecast [Met Office Global and Regional Ensemble Prediction System (MOGREPS-G)]. Skill in the ensemble forecast of wave activity is highly dependent on region and time of year, although generally forecasts of equatorial Rossby waves and westward-moving mixed Rossby–gravity waves are substantially more skillful than for the eastward-moving Kelvin wave. The observed statistical relationship between wave phases and rainfall is combined with ensemble forecasts of dynamical wave fields to construct hybrid dynamical–statistical forecasts of rainfall probability using a Bayesian approach. The Brier skill score is used to assess the skill of forecasts of rainfall probability. Skill in the hybrid forecasts can exceed that of probabilistic rainfall forecasts taken directly from MOGREPS-G and can be linked to both the skill in forecasts of wave activity and the relationship between equatorial waves and heavy rainfall in the relevant region. The results show that there is potential for improvements of forecasts of high-impact weather using this method as forecasts of large-scale waves improve. [ABSTRACT FROM AUTHOR]

Forecasting rainfall in the tropics is a major challenge for numerical weather prediction. Convection-permitting (CP) models are intended to enable forecasts of high-impact weather events. Development and operation of these models in the tropics has only just been realized. This study describes and evaluates a suite of recently developed Met Office Unified Model CP ensemble forecasts over three domains in Southeast Asia, covering Malaysia, Indonesia, and the Philippines. The fractions skill score is used to assess the spatial scale dependence of skill in forecasts of precipitation during October 2018–March 2019. CP forecasts are skillful for 3-h precipitation accumulations at spatial scales greater than 200 km in all domains during the first day of forecasts. Skill decreases with lead time but varies depending on time of day over Malaysia and Indonesia, due to the importance of the diurnal cycle in driving rainfall in those regions. Skill is largest during daytime when precipitation is over land and is constrained by orography. Comparison of CP ensembles using 2.2-, 4.5-, and 8.8-km grid spacing and an 8.8-km ensemble with parameterized convection reveals that varying resolution has much less effect on ensemble skill and spread than the representation of convection. The parameterized ensemble is less skillful than CP ensembles over Malaysia and Indonesia and more skillful over the Philippines; however, the parameterized ensemble has large drops in skill and spread related to deficiencies in its diurnal cycle representation. All ensembles are underspread indicating that future model development should focus on this issue. [ABSTRACT FROM AUTHOR]

Equatorially trapped waves, such as Kelvin Waves, Equatorial Rossby Waves and Westward‐moving Mixed Rossby–Gravity (WMRG) Waves, play a major role in organising tropical convection on synoptic to sub‐seasonal time‐scales. These waves have the potential to provide an important source of predictability for high‐impact weather in Southeast (SE) Asia and the Tropics more widely. To aid understanding of the role played in high‐impact weather by such waves, the observed statistical relationship between identified equatorial waves and heavy rainfall in SE Asia is examined for the period 1998–2016. Increases in the amount of precipitation and the likelihood of extreme precipitation in SE Asia are linked to all three types of waves that are included in analysis; Kelvin, equatorial Rossby and WMRG waves. There is both increased mean rainfall and increased probability of occurrence of heavy rainfall on days when high‐amplitude waves propagate over SE Asia. In particular, heavy precipitation can be up to three times more likely in regions of SE Asia during equatorial waves, including Malaysia, Indonesia and the Philippines. Kelvin waves have a large influence on heavy rainfall over Indonesia, WMRG and Kelvin waves impact Malaysia rainfall, and equatorial Rossby and WMRG waves are linked to increased rainfall over the Philippines. Based on this study it can be concluded that the probability of extreme precipitation in this region is dependent on equatorial wave activity. Therefore, the skill in probabilistic prediction of extreme precipitation in SE Asia would be expected to be conditional on the skill in equatorial wave prediction, and the modelled relationship between equatorial waves and convection. [ABSTRACT FROM AUTHOR]