Impact of the Horizontal Heat Flux in the Mixed Layer on an Extreme Heat Event in North China: A Case Study
Extreme heat over the North China Plain is typically induced by anomalous descending flows associated with anticyclonic circulation anomalies. However, an extreme heat event that happened in the North China Plain region on 12–13 July 2015, with maximum temperature higher than 40°C at some stations, was characterized by only a weak simultaneous appearance of an anomalous anticyclone and descending flow, suggesting that some other factor(s) may have induced this heat event. In this study, we used the forecast data produced by the Beijing Rapid Updated Cycling operational forecast system, which predicted the heat event well, to investigate the formation mechanism of this extreme heat event. We calculated the cumulative heat in the mixed-layer air column of North China to represent the change in surface air temperature. The cumulative heat was composed of sensible heat flux from the ground surface and the horizontal heat flux convergence. The results indicated that the horizontal heat flux in the mixed layer played a crucial role in the temporal and spatial distribution of high temperatures. The horizontal heat flux was found to be induced by distinct distributions of air temperatures and horizontal winds at low levels during the two days, implying a complexity of the low-level atmosphere in causing the extreme heat.
Key wordsextreme heat North China Plain horizontal heat flux sensible heat flux warm advection
2015年7月12日, 13日华北大范围地区出现极端高温天气, 部分站点观测地表气温达到40度以上. 华北平原的极端高温事件通常由反气旋环流异常导致的异常下沉气流引起, 而此次极端高温事件只伴随较弱的反气旋环流异常和下沉气流, 意味着其他因素导致此次极端高温天气. 北京市气象局业务预报系统准确预报了此次高温过程, 因此本文使用该系统的实际预报数据研究此次高温事件的形成机制. 本文利用华北地区混合层气柱中累积热量变化代表地表温度变化. 累积热量由地表感热通量和水平热量输送组成. 结果表明混合层中的热量水平输送对此次高温的时空分布起到决定性作用, 两天中都有暖平流向华北地区输送热量, 而温度场和风场在这两天具有不同的分布. 本文的研究结果说明低层大气在极端高温事件形成过程中有可能起到重要的作用.
关键词极端高温 华北平原 水平热量输送 混合层 暖平流
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This research was sponsored by the Ministry of Science and Technology of China (Grant No. 2015DFA 20870).
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