The Impact Of Low-latitude Sub-seasonal Oscillations On The Summer Precipitation Over The Tibetan Plateau And Its Related Mechanisms

It is well known that the Tibetan Plateau(TP)acts as a source and a sink of lowfrequency oscillation in Asia,which plays a vital role in the Asian monsoon and related precipitation variablity.Previous studies have proposed that the atmospheric low-frequency oscillation in TP mainly originates from low latitudes.However,the influences of sub-seasonal oscillation from low latitudes on the variation of summer precipitation and mid-latitude oscillation over TP in summer are not fully understood.Investigating the effects of sub-seasonal oscillations of low latitudes on the summer precipitation over TP,therefore,boosts our understanding of the interaction between low-and mid-latitude fluctuations and the response of sub-seasonal precipitation variability in Asia.In the study,before using the regional climate model,the Weather Research and Forecasting(WRF),to explore the mechanism of low-latitude subseasonal oscillations on TP precipitation,we first evaluate the effects of various physical scheme combinations and applying interior nudging techniques in WRF model simulation of regional precipitation pattern.Secondly,we identify that the Boreal Summer Intraseasonal Oscillation(BSISO)can affects the summer mean rainfall over TP with observed datasets during 1999-2008.Then,with the chosen physical parameterizations combination and spectral nudging methodology,seven high-resolution WRF runs driven by filtered and raw ERA5 fields are established to investigate the influence of sub-seasonal oscillation from low latitude on precipitation over TP in 2018 summer as well as in the summers of 2005-2009.Finally,with the five-year simulations in the summers of 2005-2009,not only the dynamic process that the TP summer precipitation responds to key sub-seasonal oscillation is exhibited,but also the role configuration of the sub-seasonal oscillation in the energy conversion between fluctuation and mean flows over TP and surrounded regions has been reflected.The main results are as follows:(1)The precipitation simulation is most sensitive to convective parameterization and microphysical schemes in East Asia.It is clear that the performance of multiphysics simulations shows strong regional dependence.Specificly,the choice of land surface modle shows substantial influence in simulating precipitation in Tibetan Plateau.The combination of Noah the land surface process,G3 D cumulus parameterization,and CAM radiation scheme can produce the spatial and temporal features of extreme precipitation reasonably over China.(2)The application of interior nudging can improve the model's ability to reproduce precipitation and heavy rainfall events.Specifically,spectral nudging has an obvious advantage over grid nudging in simulating low-level circulation and temporal features of sub-seasonal rainfall.In addition,the option of nudging wavenumbers and variables shows fundamental effect on spectral nudged WRF runs.With cut-off wavelength less than 1000 km,corresponding four nudging wavenumber,the WRF model can capture more reliable sub-seasonal feature of precipitation.Meanwhile,in grid nudging experiments,shorter nudging time is efficient to reduce the simulated bias for rainfall and circulation pattern over most sub-regions.Also,the experiment spectrally nudged with kinematic variables,1-hour relaxation time and four nudging wavenumber can simulate more accurate precipitation over TP at multitimescales and associated large-scale circulation,particularly in summer and autumn.(3)BSISO shows a prominent influence on summer precipitation in TP,especially its quasi-biweekly oscillation.Under the influence of BSISO,the quasibiweekly oscillation of summer rainfall over TP migrates from southeast towards northwest over TP,which increases the precipitation amount over southwestern TP and central TP.Meanwhile,although the equatorial Rossby waves and Eastly waves can also trigger the atmospheric transient waves over the South China Sea,they have little effect on the mean flows and precipitation over TP.The different responses of atmospheric oscillation to the equatorial Rossby waves,eastly waves and BSISO indicate that the location near TP along north India-northern Bay of Bengalnorthwestern Indo-China could therefore be identified as the key area that bridges BSISO and summer rainfall over TP.(4)Behind the influence of BSISO on summer rainfall over TP are two major mechanisms.One is that the convection associated with BSISO upon northern Bay of Bengal can trigger Rossby wave train which features a northwestward migration arriving at about 85?E and affecting the southwestern fridge of TP.This process can transport water vapor from low latitude to southwestern TP by perturbation energy accumulation along the south side of southwestern TP.Another is that the effect of BSISO can lead to positive sensitive heat anomaly over the northwest of Southeast Asia and northern Bay of Bengal,resulting in increased instability energy on northern Bay of Bengal and enhanced convergence at lower level on land near 94?E.Similar to conditional instability of the second kind,the coupled convergence at lower level and divergence at higher level help near 94?E can help the moist-warm sea air elevate to southeastern TP and play an important role in sustaining the water vapor path at the southern side of TP.From the viewpoint of energy balance,the perturbation energy of the previous process is provided by the kinetic energy of the mean flows while the energy conversion of the latter one comes from the available potential energy of mean flows.Consequently,the energy conversion,in turn,forces mean flows and affects the mean pattern of summer rainfall and circulation over TP.