Prediction Ability And Physical Processes Of The ECMWF Model For The Tropical Cyclogenesis Over The Western North Pacific Ocean

Tropical cyclone(TC)is one of the most destructive weather systems with huge social lost.Once a tropical disturbance is formed,it can develop into a completely destructive typhoon in 2-3 days under certain favorable environmental conditions.If the TC genesis time and location over the ocean can be accurately predicted,the governments and agencies can gain additional time to prepare for impending threats and reduce the damages caused by TC.This paper presents a comprehensive statistical assessment of the performance of the high-resolution European Centre for Medium-Range Weather Forecasts(ECMWF)deterministic forecast in predicting TC genesis over the western North Pacific between2007 and 2018 using The International Grand Global Ensemble(TIGGE)data.Each genesis forecast at each lead time is classified into one of these categories: well-predicted(WP),early formation(EF),late formation(LF)or failed prediction(FP).Based on the synoptic patterns at the actual genesis time,the genesis of a particular TC is grouped into one of five flow patterns: monsoon shear line(SL),monsoon confluence region(CR),monsoon gyre(GY),easterly wave(EW),and preexisting tropical cyclone(PTC).Any case that does not fall in any of these patterns is labelled as unclassified flow pattern(UCF).(1)Overall,the prediction skill of SL cases is the highest,followed by CR,GY,PTC,and the EW and UCF patterns have the lowest skill.No significant improvement in the model performance is found during the 12 years,probably partly due to the low percentage of cases with the SL pattern in the later years.The prediction skills of TC genesis in May,June,September and October are the highest,while those in July,August and November are the lowest.TC that form further to the south are easier to predict in the SL pattern.The predicted skill at a 5-day forecast lead time of SL pattern is the highest but quite low(16.2%),which has the highest frequency of occurrence.It is very important to study the evolution of large-scale dynamic and thermodynamic variables of SL flow pattern in 5-day forecast lead time for improving the prediction skills of ECMWF model for TC genesis.(2)The physical processes of TC genesis from the initial forecast time(Day-5)to genesis time(Day 0)under the SL flow pattern are mainly influenced by three aspects: the environmental forcing,internal thermodynamic and dynamic processes.The environmental forcing in the WP cases provides favorable external conditions for genesis.The northeasterly to the north and southwesterly flow to the south of the pre-genesis vortex begin to strengthen at Day-4 and Day-3 respectively,and then the maximum wind centres move close to the genesis position(GP)from Day-3 to Day 0.At 200 h Pa,the divergence of anticyclone in the northeast of the WP type is stronger from Day-5 to Day 0.This monsoon circulation help to form a more obvious large-scale convergence in the lower troposphere,which provides favorable conditions for the convergence of water vapor and the enhancement of radial wind.(3)Evolution of the WP cases TC can be explained by Bottom-up theory.At Day-5,the convection near the GP absorbs the large-scale convergence of water vapor and further enhances the low-level radial wind.One day later(Day-4),combining with the increasing of radial wind convergence,more absolute angular momentum is imported into the inner core and the initial vortex are spinned up.At the same time,the increasing of moist convection near the core enhances the surface latent heat flux(SLHF)and reduces the mean sea level pressure(MSLP),supporting the import of low-level absolute angular momentum and the development of low-level vortex.As the low-level vorticity continues to increase and extend upward,the outflow of absolute angular momentum in the upper layer increases sharply around Day-2.5,maintaining strong convection and gradually increasing the low-level vorticity.At Day-2,the secondary circulation system is established and the vertical wind shear becomes favorable for TC formation.(4)Moist-convection appears to be the first important factor affecting TC genesis,and the positive feedback processes are also the important.At Day-5,the enhancement of the moist-convection leads to the decreasing of MSLP and the increasing of surface latent heat flux(Day-3),which in turns promoting low-level water vapor convergence.It is a positive feedback process in the internal thermal process.In addition,the strong convection inside the TC core absorbs the large-scale water vapor around the GP,which further enhancing the low-level radial wind.The contraction of radial wind convergence and the increasing of meridional gradient would in turns cause the convergence of water vapor,which is a positive feedback process between environmental forcing and internal dynamics.At Day-2,with the establishment of the secondary circulation system,the upper-level angular momentum outflow and the low-level angular momentum inflow are maintained,and the convective movement is developed.Under these conditions,the tropospheric humidity and SLHF increase continuously,further increasing the vorticity in the convective region.It is a positive feedback process among environmental forcing,internal thermal forcing and dynamic forcing.