Daytime Local Circulations and Their Interactions in the Seoul Metropolitan Area

Daytime local circulations and their interactions in the Seoul, South Korea, metropolitan area are investigated using a high-resolution mesoscale model. It is found that the urban-breeze circulation interacts strongly with other local circulations, such as sea-breeze, cross-valley, and river-breeze circulations. Inland penetration of the sea breeze is retarded in the morning when moving over the coastal urban area because of the increased surface roughness but is accelerated in the afternoon after passing through the urban area as a result of the landward-convergent flow induced by urban heat islands in the inland urban areas. In the valley region, the cross-valley circulation prevails in the morning and the urban-breeze circulation prevails in the afternoon. The mountainside urban-breeze circulation weakens because of the cross-valley circulation in the opposite direction. On the other hand, the plainside urban-breeze circulation away from the mountain increases in strength as a result of the combined effect of heating from the urban surface and subsidence heating associated with the cross-valley circulation. This strengthened urban breeze acts to inhibit penetration of the sea breeze farther inland, causing the sea breeze to stagnate in Seoul. In the vicinity of the Han River, convergence zones with strong updrafts are formed as a result of the interaction between the urban breeze and the river breeze. When the sea-breeze front encounters the strong updrafts, the vertical velocity of the front at the intersection points increases.