Water vapour multi-vortex structure under the interactions of typhoons and mid-low latitude systems during extreme precipitation in North China

Abstract

With diverse atmospheric circulation and complex mechanisms, typhoon remote rainstorms (TRR) are closely related to a variety of mid-low latitude systems. The anomalous signatures in such processes can improve forecasting heavy rainfall associated with typhoons and fill the gaps in current physical conceptual models of TRR. Using the optimal typhoon path dataset, ground observation data, and the National Centres for Environmental Prediction (NCEP) reanalysis dataset, the impact of anomalous water vapour transport on excessive rainfall in North China is investigated. The vector empirical orthogonal function (VEOF) analysis is applied to typhoon precipitation events from 1970 to 2021, and three major modes are extracted to reflect the anomalous water vapour transport for typhoon precipitation. Mode 1 reflects a moisture circulation pattern in North China due to the coexistence of typhoons and remote and direct precipitation, with the highest probability of heavy precipitation. The differences in remote precipitation areas and intensities are attributed to the location of typhoons in the western North Pacific and the shifting of the subtropical high ridge. To characterise the water vapour transport circulation of TRR in North China, we proposed the ‘multi-vortex’ idea. The transmission of the multi-vortex ensures a continuous supply of TRR water vapour, and the enhancement of the multi-vortex is significantly linked to the enhancement of remote precipitation. Three extreme northern rainstorms, 75·8 Henan, 7·21 Beijing, and 7·20 Zhengzhou rainstorms, have anomalous multi-vortex water vapour convergence, similar to Mode 1. The mechanism of water vapour driven by multi-vortex in the three severe rainstorm events is more extreme than in usual TRR events in North China. The stronger Indian low vortex and relatively southerly subtropical highs can intensify the southwest branch of the double water vapour transport branch, whereas the strengthening of typhoons in the western North Pacific facilitates water vapour transport through the channel on the southwestern side of the subtropical highs and then transports it to key rainstorm areas, promoting TRR development. Therefore, the combination of subtropical highs, typhoons, and the anomalous multi-vortex structure may help in the establishment of key indicators of extreme precipitation in North China.