Abstract

This study investigates the roles of atmospheric moisture transport under the influence of topography for summer extreme precipitation over North China (NC) during 1979–2016. Based on rain gauge precipitation data and a reanalysis, 38 extreme precipitation days in NC during the 38 years were selected and associated moisture fluxes estimated. The results show that there is a dominant moisture influx of 311.8 kg m−1 s−1 into NC along its southern boundary from tropical oceans, and a secondary influx of 107.9 kg m−1 s−1 across its western boundary carried by mid-latitude westerlies. The outflux across the eastern boundary is 206.9 kg m−1 s−1 and across the northern boundary is 76.0 kg m−1 s−1, giving a net moisture gain over NC of 136.8 kg m−1 s−1. During extreme precipitation days, the moisture flux convergence (MFC) was much larger, exceeding 4 × 10−5 kg m−1 s−1. The MFC maximum core, the pronounced moisture transport, and the striking extreme precipitation zone over NC are all anchored to the east of the steep slopes of the surrounding topography. Moreover, a remarkably high humidity and strong upward motion also occur near steep slopes, indicating the critical role of the adjacent topography on the extreme precipitations. Simulations with and without the topography in NC using the Weather and Research Forecasting model for six selected out of the 38 extreme precipitation days demonstrate that the surrounding topography reinforces the MFC over NC by 16% relative to the case without terrain, primarily through enhanced wind convergence and higher moisture content, as well as stronger vertical motion induced by diabatic heating. The interactions between moisture convergence and topographic settings strengthen the extreme precipitation over NC.

Highlights

  • Extreme precipitation events lead to huge economic losses and fatalities globally every year (Mirza 2002; Jonkman 2005; Gemmer et al 2008)

  • An extreme precipitation day over North China (NC) is defined when the regional mean of daily precipitation over NC is greater than 22.7 mm

  • Totally 38 extreme precipitation days over NC were selected from the summers during 1979 and 2016

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Summary

Introduction

Extreme precipitation events lead to huge economic losses and fatalities globally every year (Mirza 2002; Jonkman 2005; Gemmer et al 2008). The socioeconomic conditions in NC are especially vulnerable to extreme precipitation, which cost thousands of millions of dollars in damage every year (Gao and Wang 2017). Extreme precipitation events over NC have occurred frequently. NC witnessed five extreme precipitation events in August 2010 (Orsolini et al 2015). On 21–22 July 2012, a disastrous rainstorm hit Beijing with a maximum daily precipitation reaching 460 mm (Liu and Wang 2013; Zhou et al 2013). Researching on extreme precipitation over NC is a crucial issue in terms of regional disaster prevention and mitigation,

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