Evaluation Of Tropical Cyclone Characteristics Over The Northwest Pacific Simulated By FGOALS-f3-H Atmospheric And Coupled Model

Tropical cyclones(TCs)in the Northwest Pacific(WNP)are active and frequent,and they are one of the most serious meteorological hazards affecting the coastal areas of China and Southeast Asia.However,it is still challenging for global climate models to accurately model the characteristics of TC activity in this region,especially the distribution of generation frequency(TCGF)and its interannual variability.In this study,for the FGOALS-f3-H model in the Sixth International Coupled Model Comparison Program(CMIP6)High Resolution Coupled Model Comparison Program(High Res MIP),we optimize the relevant parameters of the TC track detection framework Tempest Extremes based on historical simulations(1950-2014)of atmospheric model and sea-air coupled experiments to systematically evaluate the simulations and differences of the basic activity characteristics(genesis frequency,track distribution and wind pressure relationship)of WNP TC in July-October by model atmospheric and sea-air coupled experiments.Focusing on the influence mechanism of the West Pacific Tropical Easterly Jet(WP＿TEJ)and ENSO,the model simulations of TCGF climate state and interannual variability and the sources of bias are explored in depth using the potential generation index(GPI)and its key environmental factors to provide an important basis for model improvement.Specific findings are as follows.(1)The FGOALS-f3-H atmospheric experiment can simulate the regions where the observed WNP TC generation frequency is concentrated(the South China Sea in the west,east of the central Philippines,and near Guam in the east),but the average overestimation is 9.1 TCs/year,and the overall generation location point is east-north,with some underestimation in the western sea and overestimation in the eastern sea;the atmospheric experiment can capture the high value area of TC path density in the northeastern Philippines.However,due to the high density of northward paths of typhoons and strong typhoons,the frequency of TCs in the central and eastern northern seas is high;the atmospheric test can simulate strong typhoons,and the wind-pressure relationship is stronger than that observed;the spatial distribution of GPI deviations simulated by the atmospheric test is more consistent with the distribution characteristics of TCGF deviations,and the analysis of the contribution of its key terms shows that the overestimation of TCGF in the eastern seas is mainly due to the model’s overestimation of the 600 hPa relative humidity and 500 hPa relative humidity in the region.The overestimation of TCGF in the eastern part of the ocean is mainly due to the model overestimation of 600 hPa relative humidity and 500 hPa vertical velocity,while the underestimation of TCGF in the western part of the ocean is due to the model underestimation of ambient vertical wind shear.(2)The coupled ocean model improves the overestimation of TC generation frequency by the atmospheric model,and the average deviation is reduced by 3.4 TCs/year,which is closer to the observation,especially the general overestimation of TCGF in the eastern part of WNP and TC path density in the central part of WNP by the atmospheric model is significantly reduced,and the simulation of wind-pressure relationship is more reasonable,but the frequency of super typhoon is still underestimated;however,the TC generation location in the coupled experiment is southeastward,which leads to the overestimation of TCGF in the eastern part of WNP.However,the TC generation location in the coupled experiment is to the southeast,resulting in the underestimation of TCGF in the central WNP and overestimation in the southeastern WNP,which corresponds to the overestimation of TC paths and frequency in the region.Using the contribution of the GPI deviation term,it is concluded that the overestimation of TCGF in the southeastern part of WNP is mainly due to the 600 hPa relative humidity and ambient vertical wind shear,but the underestimation of TCGF in the central part of WNP is due to the reduction of the maximum potential intensity MPI after the air-sea coupling,which is different from the deviation mechanism of TCGF in the atmospheric model.(3)The West Pacific tropical easterly jet(WP＿TEJ)can directly regulate the key atmospheric environmental factors in the GPI through the upper-level convergence and vertical circulation caused by the non-geostrophic winds north of its inlet area,and the changes in its position and intensity are important diagnostic factors affecting the TCGF spatial distribution climate state and interannual variability simulation.From the perspective of climate state,the WP＿TEJ simulated by the FGOALS-f3-H atmospheric model is strong and northerly,and the vertical circulation induced by high-altitude dispersion in the inlet area leads to the deviation of the key contribution of GPI index,and the monsoon trough is also strong and northerly accordingly,resulting in the positive deviation of TCGF in the eastern sea.However,the WP＿TEJ simulated by the sea-air coupling experiment is significantly eastward,and the vertical circulation and low-level monsoon trough triggered by it extend eastward accordingly,resulting in the positive deviation of TCGF in its southeastern waters.From the perspective of interannual variability,the atmospheric model better simulates the interannual variability of WNP TCGF with a correlation coefficient of 0.51 with the observation,but overestimates its variability,which is related to the excessive interannual variability of the simulated WP＿TEJ intensity;however,the simulated ability of interannual variability of TCGF in the air-sea coupled experiment decreases instead,and the interannual variability becomes weaker,which is related to the small interannual variability of the simulated WP＿TEJ intensity This corresponds to the smaller interannual variability of the simulated WP＿TEJ intensity.(4)The interannual variability of WP＿TEJ position and intensity has a significant response to the ENSO signal.The atmospheric model is driven by the fixed sea surface temperature(SST)and simulates the response of TCGF activity to strong El Ni(?)o and strong La Ni(?)a years,but the WP＿TEJ variability corresponding to strong and weak ENSO signals is large and the interannual variability of TCGF is larger.However,the SST simulated by the sea-air coupling experiment is biased,showing a warm bias at low equatorial latitudes and a cold bias in the middle and high latitudes of the WNP.The corresponding WP＿TEJ has a weak intensity and small variability,and its position is always significantly eastward,resulting in insufficient ability to simulate the interannual variability of mid-level upward motion and ambient vertical wind shear,and the simulated TCGF of the corresponding coupled experiment is reduced relative to the atmospheric experiment,and the generation position is too eastward and the interannual variability is small.Therefore,the bias of SST simulation affects the simulation of ENSO signal and the response of WP＿TEJ to it,which restricts the ability of climate model to simulate the interannual variability of TCGF.Paying attention to the response angle of air-sea coupling and key atmospheric circulation system WP＿TEJ is the key to improve the model’s ability to simulate and predict the characteristics of WNP TC activity.