Extended Impact of Cold Air Invasions in East Asia in Response to a Warm South China Sea and Philippine Sea

During boreal winter,the invasion of cold air can lead to remarkable temperature drops in East Asia which can result in serious socioeconomic impacts.Here,we find that the intensity of strong synoptic cold days in the East China Sea and Indochina Peninsula are increasing.The enhanced synoptic cold days in these two regions are attributed to surface warming over the South China Sea and Philippine Sea(SCSPS).The oceanic forcing of the SCSPS on the synoptic cold days in the two regions is verified by numerical simulation.The warming of the SCSPS enhances the baroclinicity,which intensifies meridional wind and cold advection on synoptic timescales.This leads to a more extended region that is subject to the influence of cold invasion.

Tropical Stratospheric Forcings Weaken the Response of the East Asian Winter Temperature to ENSO

The El Niño-Southern Oscillation(ENSO)plays a critical role in predicting the winter surface temperature over East Asia.Numerous studies have attempted to improve the seasonal forecasting skill in view of the combined effects of ENSO and oceanic-tropospheric factors.However,high uncertainty and notable challenges still exist in using the ENSO to predict the surface temperature.Here,we showed that tropical stratospheric forcings(Quasi-Biennial Oscillation,QBO)could disrupt the response of the surface temperature to ENSO.The response of the East Asian surface temperature to El Niño/La Niña events evidently weakened in winter during the westerly/easterly phase of the QBO.This disruption has shown an increasing trend in recent decades,limiting the usefulness of ENSO alone as a seasonal predictor of the surface temperature.The modulation of the QBO on the East Asian surface temperature is achieved mainly by affecting subtropical zonal winds and North Pacific wave activity.Our analyses suggest that the QBO is a nonnegligible predictor in improving seasonal forecasts of the East Asian surface temperature,even comparable to the ENSO.

Improving the Nowcasting of Strong Convection by Assimilating Both Wind and Reflectivity Observations of Phased Array Radar:A Case Study

With the advent of the phased array radar(PAR)technology,it is possible to capture the development and evolution of convective systems in a much shorter time interval and with higher spatial resolution than via traditional Doppler radar.Research on the assimilation of PAR observations in numerical weather prediction models is still in its infancy in China.In this paper,the impact of assimilating PAR data on model forecasts was investigated by a case study of a local heavy rainfall event that occurred over Foshan city of Guangdong Province on 26 August 2020,via a series of sensitivity experiments.Both the retrieved three-dimensional wind and hydrometeor fields were assimilated through the nudging method with the Tropical Regional Assimilation Model for South China Sea_Rapid Update Cycle_1km(TRAMS_RUC_1km).The temperature and moisture fields were also adjusted accordingly.The results show that significant improvements are made in the experiments with latent heat nudging and adjustment of the water vapor field,which implies the importance of thermodynamic balance in the initialization of the convective system and highlights the need to assimilate PAR radar observations in a continuous manner to maximize the impact of the data.Sensitivity tests also indicate that the relaxation time should be less than 5 min.In general,for this case,the assimilation of PAR data can significantly improve the nowcasting skill of the regional heavy precipitation.This study is the first step towards operational PAR data assimilation in numerical weather prediction in southern China.