Numerical Investigation Of A Torrential Rain Event In Xinxiang 2016 With Convection-resolving Simulations And Sensitivity Experiments

During 8-9 July 2016,the once-in-a-century extreme rainfall event struck Xinxiang(which is located in Henan Province,China).Operational models forecasted the rainfall but failed to predict the extreme precipitation amount.To reproduce this high-impact weather event,Nanjing University's version of the Weather Research and Forecasting(WRF-NJU)model was used to hindcast the event at 1 km horizontal resolution.The simulation successfully reproduced the Xinxiang 2016 event not only in terms of the rainfall pattern and rainfall time series,but also for the maximum precipitation and reflectivity.Based on the simulations,this extreme rainfall event was divided into three stages which are the initiation and developing stage,the maintenance and mature stage,and the dissipation stage.During the initiation and developing stage,the mountain blocked easterly wind converged with a typhoon's outer ring southeasterly wind to trigger the first convective cell.This cell was controlled by the 700hPa steering wind.It went upslope of mountains and developed.Evaporation of the subsequent rainfall led to the accumulation of cold air at the foot of mountain,and formed cold pool.New cells were continuously generated at the edge of cold pool while old cell dissipated.This train effect was responsible for the moderated rainfall in this stage.During the maintenance and mature stage,developed new cell moved northeastward along the Taihang mountain and merged with the existing MCS.The merged MCS was the main rainfall producing system in this event.Low-level inflows forced this MCS into a stagnation before impassable blockings,which are the mountsins,resulting in a long rainfall duration.The horizontal equivalent potential temperature((?)e)gradient due to the different characteristics of the two converging air mass,which are from the typhoon outflow and the anti-cyclonic system over Shandong province,provided blocking on the east.These two synoptic systems were well matched at this stage.Location of the horizontal(?)e gradient line barely moved.This provided the large scale maintenance condition.Meanwhile,the mesoscale terrain blocked the MCS from moving west.The inertial oscillation-induced nocturnal low-level jet enhanced the 850 hPa southerly inflows.The combined southerly winds had an equivalent magnitude as the convective outflow,which were essential to the sustained location of the precipitation MCS.At the dissipation stage,due to the strengthening of the synoptic anticyclonic system and the weakening of typhoon,the anticyclonic flow moved southward and so did the horizontal 9e gradient line and the MCS.Meanwhile,the nocturnal low-level jet dissipated due to the clockwise veering of the wind.The balance between inflow and convective outflo wno longer existed.MCS weakened and moved away from Xinxiang,and this event came to an end.In the analysis,large-scale synoptic condition,mesoscale mountain and nocturnal low-level jet are vital for sustaining this long duration rainfall event.To further investigate the role of the low-level jet and the mountain in this event,two sensitivity experiments were conducted.The first experiment(NoHFX)prevented the low-level jet from occurring by turning off the surface heat flux.The NoHFX experiment confirmed the important role of the nocturnal low-level jet in terms of transporting water vapor and promoting vertical motion.The nocturnal low-level jet affected rainfall rate through those two factors instead of precipitation efficiency,and its effects on rainfall duration has also been confirmed.In another sensitivity experiment(NoTER)where the mountain terrain is removed,the long duration rainfall event no longer occurred at Xinxiang.The NoTER experiment confirmed the mountain's effect in terms of upslope lifting and water vapor accumulation.It also showed that terrain may strengthen water vapor transport under certain conditions.