The Research On The Contribution Of Initial Conditions And Air-sea Interaction To Extended-range Predictability

The memory of the initial conditions and the air-sea interaction are the main predictability source on extended range forecast,their relative role and importance is the important basis for development and improvement of extended range forecast.Numerical model is the efficient way to extended range forecast,however,because of there are not fully aware of the initial conditions and the air-sea interaction to contribute to extended range forecast,which is better to extended range forecast between atmospheric general circulation model and air-sea coupled model remains to be determined.Based on the new operational dynamical extended-range forecast model(BCC_AGCM)and climate system model(BCC_CSM)of National Climate Center of China Meteorological Administration(NCC/CMA),the advantages and disadvantage of two sets model system will be comprehensively evaluated.This study will investigate the predictability source for AsianAustralian monsoon region(AAM),which provided certain indicating significance and some reference value.In this study,SST and atmospheric circulation in coupled model has been evaluated,the atmospheric component and ocean component's performance of the coupled system has been evaluated;on the one hand,atmospheric circulation model has been evaluated by numerical experiments,on the other hand the memory of initial conditions and air-sea interaction for extended range forecast has been investigated;the relative role of initial conditions and the air-sea interaction for atmospheric circulation in the northern hemisphere in the extended time range has been investigated,the contribution and mechanism of air-sea interaction has been studied;the predictable patterns of AAM has been studied,the effect of air-sea interaction on the predictability of the AAM region has been discussed,the main predictability source on extended range forecast has been revealed,which laid a foundation for the development of extended range forecast.The major results and conclusions of this study are summarized as follows:(1)Summer prediction skill of global mid-latitudes and low latitudes sea surface temperature(SST)in BCC_CSM retrospective forecasts is assessed.The results indicate that the SST forecasts exhibit certain skills in middle and low latitude areas and the skills over much of low latitude areas are better.Further analysis reveals that model forecasts in different sea areas have different skills.Recent investigations have revealed that prediction skill is closely related to the ENSO phases of previous winter.In most cases,with the lead time becoming longer,the model will have a drop in forecast skill for summer SST and these indexes.(2)The summer atmospheric moisture content and the characteristics of moisture transport in the middle and lower reaches of the Yangtze River are analyzed,and the relationship between rainfall and the summer monsoon are discussed.The pattern of moisture transport simulation capabilities are evaluated,and possible reasons for the model error are analyzed.Further studies show that the simulation of the deviation of the Yangtze River moisture transport mainly caused by an analog deviation the summer monsoon,and the simulation of the weak monsoon is better than the simulation of strong monsoon.(3)Four experiments with different combinations of atmospheric initial condition and sea surface temperature(SST)boundary forcing are designed in this study to investigate the roles of atmospheric initial condition and SST boundary forcing in extended-range forecast over different regions around globe.Results show that the skills in extended prediction are strongly dependent on initial condition at lead times less than 3 weeks,and different initial conditions with the same boundary conditions lead to significant differences in the prediction skills at sub-monthly time scales.However,the initial condition can still provide some useful information for the prediction exceeding monthly time scales.From a global perspective,SST boundary forcing affects the prediction skill at lead times of more than one week.As for the extended forecast,impacts of the lower boundary forcing are concentrated in low-latitude regions,and are more stably shown in the forecast of geopotential height at 500 hPa.Significant improvements caused by the boundary forcing are shown in the forecast after the 5th pentad.(4)The contribution of air-sea interaction on the extended-range prediction of geopotential height at 500 hPa in the northern extratropical region has been analyzed with a coupled model from Beijing Climate Center and its atmospheric components.Under the assumption of the perfect model,the extended-range prediction skill was evaluated.The coupled model has more prediction skill than its atmospheric model,the air-sea interaction in July made a greater contribution than the other months.The prediction skill of extratropical in the coupled model reaches 16–18 days in all months,while the atmospheric model reaches 10–11 days in January,April,and July and only 7–8 days in October,indicating that the air-sea interaction can extend the prediction skill of the atmospheric model by about 1 week.The errors of both the coupled model and the atmospheric model reach saturation in about 20 days,suggesting that predictable range are less than 3 weeks.(5)The contribution of air-sea interaction to predictable mode and predictability of the Asian-Australian monsoon region has been analyzed.The results show that the air-sea interaction can significantly reduce the prediction error of the AAM region.The sea surface temperature in the warm pool area is an important predictable source of the region,and the model error is reduced when the sea surface temperature anomaly is significant.Using the MSN EOF method to extract the most predictable modes of the two models,the first two major modalities have significant anomalies in the Northwest Pacific region.The first mode is strongly controlled by SSTAs of the central Pacific Ocean,which is inconsistent with the observation,resulting in the two models with the corresponding explanatory variance is larger than the observation;the second mode has significant correlation with the air-sea interaction of the warm pool area,simulation capability is significantly higher than the atmospheric model.The simulation mode of the second mode of the coupled mode is also significantly better than the first mode.To sum up,for the atmospheric circulation model,the skills in extended prediction are strongly dependent on atmospheric initial condition at lead times less than three weeks,SST forcing affects the prediction skill at lead times of more than one week.Compared with atmospheric circulation model,the coupled model has more predictability than the former,the air-sea interaction can extend the prediction skill of the atmospheric model by about 1 week.This illustrates the air-sea interaction can provides more predictability.