华南沿海暖海雾过程中的湍流热量交换特征

Turbulent Heat Exchange in a Warm Sea Fog Event on the Coast of South China

湍流交换是海雾中的关键物理过程,在海雾的热量和水汽平衡过程中起重要作用。本文根据2007年3月24～25日一次海雾的外场观测数据,分析了海雾过程中近海面湍流热量交换特征;并在区分风切变机械湍流与雾顶长波辐射冷却热力湍流的基础上,着重分析了两种不同性质的湍流对海雾发展和维持的作用。结果表明:(1)本次海雾是在西南低压和变性冷高压的控制下,来自南海东部暖水区的空气平流在近岸冷海面上形成的暖海雾;暖海雾中的湍流热量交换过程比冷海雾更为复杂;(2)在暖海雾的形成和消散阶段,风切变机械湍流的热量输送起主要作用;而在发展和维持阶段,既有风切变机械湍流的热量输送作用,也有雾顶长波辐射冷却热力湍流的热量输送作用;(3)风切变机械湍流向冷海面输送热量,对近海面空气起到降温和增湿作用;热力湍流同样向冷海面输送热量,但对雾层起到增温和降湿作用;(4)暖海雾中的湍流热量交换机制与雾层的非充分混合结构有密切关系。

Turbulent exchange is one of the key physical processes in sea fog formation,development and dissipation,and plays an important role in the heat and water vapor balance in the fog layer.Based on in situ observation of a sea fog event during 24-25 March 2007,the characteristics of turbulent heat flux and its effect on fog evolution were examined by distinguishing the turbulent heat transport induced by wind shear and by fog-top longwave radiation cooling.Results show that：（1）the observed sea fog occurs over the cold water near the coast of the South China Sea（SCS）and results from advection of warm and moist air from the Philippine Sea,which is under the control of the low pressure in Southwest China and the transformed cold high pressure over the ocean.The turbulent heat exchange associated with the warm sea fog is more complicated than that during the cold sea fog.（2）During the formation and dispersion phase of the warm sea fog,the heat transport is primarily induced by wind shear,whereas during the development/persistence phase,the turbulent heat transport is induced by both the wind shear and the longwave radiation cooling at the fog top.（3）The wind shear-induced turbulence transports heat to the sea surface,favoring cooling and moistening of the air near the sea surface.The heat transport resulting from the fog-top longwave radiation cooling also transports heat downward but warms and dries the air in the sea surface layer.（4）The mechanism of heat exchange during the warm sea fog has a close relationship with the immature mixing structure in the fog layer.