How Well Does the Met Office Numerical Weather Prediction System Forecast West Pacific Tropical Cyclone Precipitation?

Tropical cyclones (TCs) contribute a significant proportion of the precipitation which falls over south-east Asia and the west Pacific Ocean. TC trajectories are tracked in the JRA55 reanalysis and matched against those in the International Best Tracks (IBTrACS) archive. Up to 50% of annual precipitation over the South China Sea and Philippine Sea is associated with TCs, with a strong annual cycle, rising to 60-80% in July-October. The contribution of TCs is also strongly modulated by ENSO and MJO phase. For example, when active MJO convection is in the region of the Maritime Continent (MJO phases 4 to 6) the TC track density is at its highest, and over 90% of precipitation over the sea adjacent to the northern Philippines is attributable to TCs. TCs are also tracked in operational UK Met Office global forecasts from 2006 to 2016, and the contribution of TCs to precipitation is computed as a function of forecast lead time up to 6 days. TCs weaken with forecast lead time; hence, their fractional contribution to precipitation also reduces. The resolution of the operational model increased over the period analysed. In 2006 a grid spacing of approximately 60km at the equator was used; by 2016 this was approximately 25km. Forecasts run at a finer resolution show less weakening of TC intensity with forecast time. Track statistics verified against IBTrACS and ECMWF analyses show that the error in TC intensity is lower when the active MJO is passing through the east Maritime Continent and west Pacific Ocean (phases 6 and 7) than in other MJO phases.