Numerical Simulation and Mechanism of the Influence of Poyang Lake on the Intensity of Severe Convection over the Lake
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摘要: 湖陆下垫面的非均匀性对强对流天气的发展演变有很大的影响。鄱阳湖是我国最大的淡水湖，湖面积具有明显的月变化和季节变化，而模式中的下边界一般默认湖面积不变，这与实际情况的差异较大，必然带来模式预报误差。利用WRF模式对夏季夜间发生在鄱阳湖地区的一次强对流天气过程进行数值模拟，并通过湖面积变化的敏感性试验，深入研究鄱阳湖对强对流天气发展演变的影响及其机理，结果表明：夏季夜间湖面上空2m温度明显高于陆面，向湖陆风在湖面上空辐合上升，岸边则存在下沉辐散气流。这导致降水在湖西岸减弱、湖上空增强。随后用去湖敏感性试验印证了鄱阳湖的暖湖效应，湖泊的存在能够通过激发陆风次级环流对湖西岸（湖面）上空降水起抑制（促进）作用。去湖试验的降水在湖西岸增强20%，在湖面上空减弱16%，体现出湖泊对降水强度的重要影响。此外，还发现湖面积扩大1.5、2.5、3.5、4.0倍的扩湖敏感性试验的降水在湖面上空分别增幅7%、16%、30%、43%，进一步证实了强对流强度对鄱阳湖面积变化较为敏感。这指示我们在预报夏季夜间穿湖而过的强对流天气时，应重点关注其可能存在的入湖前减弱、入湖后增强的变化趋势。同时，在利用数值模式模拟湖区强对流天气过程时，如果湖面积与模式中默认的湖面积相差较大，则应考虑将实际湖面积引入模式下边界，以期提升模式对于湖区对流的预报能力。Abstract: The inhomogeneity of the lake and land underlying surfaces has large influences on the development and evolution of severe convective weather. Poyang Lake is the largest freshwater lake in China, and its area exhibits distinct monthly and seasonal variations. However, the underlying surface of the lake in the Weather Research and Forecast (WRF) model is usually assumed to be constant, which is different from the real situation and leads to model forecast errors. In this paper, the WRFV3.9 model was used to simulate a severe convective weather event that occurred over the Poyang Lake region during a summer night. Lake area sensitivity experiments were conducted to analyze the influence of Poyang Lake on the development and evolution of severe convective weather and its mechanism. The results showed that the 2 m temperature over the lake surface was significantly higher than that over the land surface during summer night. The land breeze converged and rose over the lake region, while there was a sinking and divergent airflow over the shore, which caused precipitation to decrease over the west bank of the lake but increased over the lake. The subsequent lake-removing sensitivity experiment further confirmed the warming effect of Poyang Lake. Poyang Lake could inhibit precipitation occurred over the west bank of the lake and intensify precipitation over the lake by stimulating the secondary circulation of land-breeze. The precipitation of the lake-removing experiment increased by 20% over the west side of the lake, and decreased by 16% over the lake, suggesting the impact of the lake on precipitation intensity. In addition, it was found that the precipitation over the lake increased by 7%, 16%, 30% and 43% in the lake-expanding sensitivity experiments, where the lake area was expanded by 1.5, 2.5, 3.5 and 4 times, respectively, further confirming the sensitivity of severe convection intensity to the area of Poyang Lake. These results indicated that attention should be paid to the possible trend of convections weakening (enhancing) before (after) entering the Poyang Lake region during summer nights. Meanwhile, when a numerical model was used to simulate the severe convection weather, the real lake area could be introduced into the underlying surface of the model when the real lake area differed greatly from the default climatic value used in the numerical model, to improve the forecasting capability for severe convective weather occurring over lake regions.
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图 2 2020年6月29日12: 00基于ERA5再分析资料绘制的200 hPa位势高度（黑色实线，单位：dagpm）、温度（红色虚线，单位：℃，下同）、水平风场（风向标，单位：m·s-1，下同）、水平散度（阴影，单位：10-5 s-1）（a）；500 hPa位势高度（b，黑色实线，单位：dagpm）；700 hPa位势高度（黑色实线，单位：dagpm）、比湿（阴影，单位：g·kg-1），棕色实线表示切变线（c）
图中红色方框表示后文数值模拟中的d03区域。
图 3 模式三层嵌套模拟区域（a）；CTL中d03区域的海拔高度（单位：m，蓝色阴影表示水体区域），点L（116.35 °E，29.02 °N），W（116.00 °E，28.97 °N）（b）；CTL（c）、NOL（d）、EXL1.5（e）、EXL2.5（f）、EXL3.5（g）、EXL4.0（h）中d03区域的下垫面类型分布
图 5 2020年6月29日18: 20 CTL的（a）2 m温度（阴影，单位：℃）和10 m风场（箭头，单位：m·s-1）；（b）垂直速度（阴影，单位：m·s-1）、水平风与垂直风合成的矢量风（箭头，垂直速度扩大20倍，单位：m·s-1）、散度（等值线，单位：10-4 s-1，间隔为：10-4 s-1）沿AB的垂直剖面（横坐标上红色实线表示CTL的鄱阳湖区域，下同）
图 6 2020年6月29日CTL 21: 00（a）、21: 40（b）散度（阴影，单位：10-3 s-1）、垂直涡度（等值线，单位：10-3 s-1）、水平风与垂直风合成的矢量风（箭头，垂直速度扩大5倍，单位：m·s-1）；21: 00（c）、21: 40（d）垂直速度（阴影，单位：m·s-1）、相当位温（等值线，单位：K）沿GH的垂直剖面
图 7 2020年6月29日CTL22:00（a）、22:40（b）、23:00（c）散度（阴影，单位：10-3 s-1）、垂直涡度（等值线，单位：10-3 s-1，间隔为：0.5×10-3 s-1）、水平风与垂直风合成的矢量风（箭头，垂直速度扩大5倍，单位：m·s-1）；22:00（d）、22:40（e）、23:00（f）垂直速度（阴影，单位：m·s-1）、相当位温（等值线，单位: K，间隔为: 3 K）沿AB的垂直剖面
图 9 2020年6月29日18: 20 CTL-NOL的2 m温度差（阴影，单位: ℃）和10 m水平风矢量差（箭头，单位：m·s-1）（a）；水平风与垂直风合成的矢量风差（箭头，垂直速度扩大5倍，单位: m·s-1）及其散度差（等值线，单位: 10-4 s-1，间隔为：2 × 10-4 s-1）、水汽通量散度差（阴影，单位: 10-8 g·s-1·hPa-1·cm-2）（b）
图 10 2020年6月29日EXL1.5（a~d）、EXL2.5（e~h）、EXL3.5（i~l）、EXL4.0（m~p）模拟的22: 00—23: 00的逐20 min降水量
阴影，单位：mm，灰色加粗实线表示湖面积扩大后的湖泊边界线（下同）。
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