Observational Signal of the Interaction Between Mountain–Plain Wind and Urban Breeze Under Weak Synoptic Systems

Abstract Valley winds and urban breezes induced by thermal circulation have been the subject of numerous studies; however, their interactions deserve further observational study under different geographical conditions globally. We propose quantitative criteria for analysis of observations, including identification of valley and mountain–plain wind episodes, determination of the direction and duration of valley winds, and exclusion of suspect cases induced by strong synoptic systems. We selected a subset of 345 valley wind days that occurred during 2013–2015 in Hangzhou, China. Valley winds in the study area occur mainly in autumn and early winter (annual frequency: 32%). To the west (east) of the urban center of Hangzhou are small mountains (is a plain). At night, the directions of mountain–plain circulation and urban thermal circulation are consistent in the region from the mountains to western parts of the city, resulting in a strengthened mountain wind at the surface. Their interaction also delays the reversal of the mountain wind to the plain wind in the morning, and slightly advances the reversal of the plain wind to the mountain wind in the evening. Conversely, in the plain area to the east of Hangzhou, the interaction delays the time of reversal of the plain wind to the mountain wind in the evening. Our results indicate that interactions between the urban breeze and local thermal circulations could have distinct impact on the environment of the city and surrounding areas, and thus they should be taken into consideration in relation to urban planning and management.