Roles of Soil Moisture–Temperature Coupling in Three Types of Heatwaves over the Great Bay Area of China

China’s Greater Bay Area (GBA) is one of the fastest urbanizing regions in the world, featured by its complex land surface and unique geography. In this study, heatwaves (HWs) in the GBA during the summers (June, July, and August) of 1961–2020 are analyzed by using observational and reanalysis datasets. The results indicate that 70% of daytime HWs occur in the northern forested areas of the GBA, 65% of nighttime HWs are observed in the cropland and forest areas around the GBA, and 75% of compound HWs occur in the urban and southern coastal areas of the GBA. Daytime HWs are featured by lower near-surface specific humidity and drier soil moisture, while nighttime HWs are often accompanied by relatively wetter conditions. For compound HWs, they are jointly affected by the conditions of the above two types. During daytime and compound HWs, soil moisture dries and recovers quickly, exacerbating the high temperatures of HWs through strong soil moisture–temperature coupling that far exceeds the climatology. Nighttime HWs lack this coupling and are primarily driven by atmospheric factors, with high temperatures maintained mainly by increased water vapor. 

中国的大湾区（GBA）是世界上城市化速度最快的地区之一，其地理位置独特、土地表面状况复杂。本文利用观测资料和再分析资料对1961–2020年夏季（6–8月）大湾区的热浪进行了分析。结果表明，大湾区北部森林地区的热浪占白天高热天气的70%，大湾区周边农田和森林地区热浪占夜间高热天气的65%，大湾区城市和南部沿海地区热浪占复合高热天气的75%。白天高温天气的特点是较低的近地表比湿和较干燥的土壤湿度，而夜间高温天气往往伴随着相对潮湿的条件。对于复合热浪，它们受上述两类条件的共同影响。在白天和复合高温天气，土壤水分干燥，恢复迅速，土壤湿度和气温强烈耦合，异常远远超过气候态，加剧了高温天气。夜间热浪缺乏这种耦合，主要由大气因素驱动，高温主要由增加的水蒸气维持。