中国西北地区复合极端高温-降水事件可能成因

Mechanisms of compound extreme heat-precipitation events in northwestern China

摘要:

基于1961—2018年欧洲中期天气预报中心的再分析资料、中国高分辨率格点资料,采用合成分析方法,揭示了中国西北地区复合极端高温-降水事件(compound extreme heat-precipitation events,CEHPE)的成因,并探讨了与无强降水热浪事件的异同。结果表明,CEHPE是由上游向东南方向传播的准正压反气旋环流异常引起:当异常环流中心接近关键区(95°～105°E,35°～40°N)时,与之相关的异常下沉运动一方面使得大气下沉增温,增加了低层大气温度;另一方面下沉运动减少云量,增强了向下短波辐射并加热地表,被加热的地表又向上释放了更多的长波辐射,进一步加热低层大气,最终导致了热浪的形成。在热浪发生时,由于地表热强迫,对流层低层发展出低压异常,其伴随的异常辐合,引导东南方向水汽至关键区,提高关键区大气水汽含量。增加的大气水汽含量和增高的气温使得热浪期间关键区大气不稳定能量异常增强。当准正压反气旋异常东移出关键区时,热浪结束,同时上游方向有异常气旋靠近,关键区位于对流层中高层的槽前脊后,有助于高层辐散低层辐合,因此强对流迅速发展,关键区出现极端降水,完成一次CEHPE的过程。无强降水热浪事件与CEHPE的热浪在强度和形成机制上几乎相同,但在热浪结束时,准正压反气旋异常水平范围较大,虽强度减弱,但中心仍位于关键区附近,因此对流层低层低压及其伴随的辐合上升异常的发展受到压制,导致对流层低层气旋、水平水汽辐合、垂直水汽输送以及不稳定能量异常小于同时期的CEHPE,无法引发强降水事件。

Abstract:

In the context of global warming,the frequency of compound extreme weather and climate events has increased,posing significant challenges to environmental stability and societal resilience.Understanding the mechanisms driving these events is crucial for improving their prediction and mitigation.This study examines Compound Extreme Heat-Precipitation Events (CEHPE) in Northwest China,a region frequently impacted by such phenomena.The study aims to elucidate the weather-scale dynamics underpinning CEHPE and distinguish them from isolated heatwave events to enhance our understanding of these processes.
CEHPEs are defined as the occurrence of heavy precipitation within 3 days after the end of a heatwave,excluding instances of heavy precipitation during the heatwave itself.Heatwaves are identified when the DMT exceeds the 90th percentile for at least three consecutive days during summer (June-August).This percentile is calculated for each grid cell and each date using a 31-day sliding window based on 30-year baseline period (1961—1990).Heavy precipitation events are defined as daily precipitation exceeding the 95th percentile of precipitation on rainy days (≥0.1 mm/d).
The results reveal that CEHPE are triggered by a quasi-barotropic wave train propagating southeastward from upstream regions.When an anticyclonic circulation center approaches the key region (95°-105°E,35°-40°N),anomalous descent leads to adiabatic warming,raising lower tropospheric air temperatures.Simultaneously,reduced cloud cover enhances downward shortwave radiation,heating the surface.The heated surface emits more longwave radiation,further warming the lower troposphere and initiating a heatwave.During the heatwave,surface thermal forcing generates a low-level cyclonic circulation anomaly,which directs moisture from the southeast into the key region.The resulting increase in moisture and air temperature enhances atmospheric instability.
At the end of the heatwave,when the quasi-barotropic anticyclonic anomaly moves eastward and is succeeded by a cyclonic circulation anomaly,the key region lies west of the upper-level ridge and east of the upper-level trough.This configuration promotes upper-level divergence,lower-level convergence,and rapid convective development,culminating in extreme precipitation and completing the CEHPE cycle.
Mere heatwave events share similar intensity and formation mechanisms with CEHPE.However,in these cases,the quasi-barotropic anticyclonic anomaly expands and weakens but remains centered over the key region,suppressing the development of low-level cyclonic circulation anomalies.Consequently,weaker low-level convergence,moisture transport,and atmospheric instability prevent heavy precipitation.Furthermore,the local weakening of the anomalous anticyclone inhibits the approach of upstream cyclonic circulation anomalies,further reducing the likelihood of precipitation.
This study elucidates the mechanisms underlying CEHPE in Northwest China and highlights their distinct differences from isolated heatwaves.However,the external forcings contributing to these differences remain unclear and merit further investigation.