Nudging地面加密观测改进数值模式对城市效应及其触发对流的模拟:一次夜间个例试验

城市(热力、动力、人类活动等)效应对城市及周边地区强降水有着重要的影响。由于涉及到多方面因素，在数值模式中合理描述城市效应仍然存在许多困难。采用Nudging地面加密观测间接引入城市效应(Nudg)和使用城市冠层模式直接引入城市效应(Urba)两种方式模拟了2017年5月7日发生在广东广州的一次极端降水过程。结果表明Nudging地面加密观测能够有效地在数值模式中引入城市效应，从而合理地再现出城市热岛及其触发的对流和强降水的时空演变，而使用城市冠层模式未能够模拟出城市热岛及其触发的对流和降水。具体而言，Nudg能够维持城市的热岛，在城市热岛的下风方最先触发了对流，对流快速发展并稳定维持，从而在广州城市下风(西北)方形成了强降水。然而，Urba试验中城市热岛迅速减弱消亡，从而未能在城市下风方触发出对流，导致降水模拟的失败；尽管后期在城市下风方边缘及北部山区有零星的雷达回波，但均未能够发展形成组织化的对流单体或系统。对本次强降水事件对流的触发而言，城市热岛的加热作用较城市的摩擦辐合更重要。提出的Nudging地面加密观测间接地引入城市效应的方法可用于与城市下垫面相关的各类天气事件的模拟研究。 

NUDGING SURFACE INTENSIVE OBSERVATIONS TO IMPROVE THE MODELING OF URBAN EFFECT AND ITS ASSOCIATED CONVECTION: A NOCTURNAL CASE STUDY

Urban (thermal, dynamical, and anthropogenic) effects have important influence on the heavy rainfall in cities and their surrounding areas. Urban effects involve many factors, and thus it is difficult to parameterize them in numerical models. In this work, urban effects are indirectly introduced by nudging the ground intensive observations (Nudg) or directly introduced by using an urban canopy module (Urba) to simulate an extreme rainfall event in Guangzhou on May 7, 2017. The results show that Nudg can effectively introduce urban effects into the WRF model, and thereby the temporal and spatial evolutions of urban heat island, convection, and rainfall are reproduced successfully. By contrast, the urban canopy module fails to replicate the urban heat island and its associated convection and rainfall. Specifically, Nudg maintains the urban heat island, and convection first occurs on the downwind side of Guangzhou. With the development and maintenance of the convection, strong rainfall forms. However, urban heat island quickly weakens, thus no convection is produced in the leeward side of Guangzhou. Although there are sporadic radar echoes in the leeward edge of the city and the northern mountainous areas in the later stage, none of them develop into an organized convective cell or system. For the triggering of the convection, the heating of urban heat island is more important than the convergence due to urban friction. Nudging the ground intensive observations used in this study to introduce urban effects indirectly can be used to simulate various weather events related to urban effects.