Advection caused by the urban heat island circulation as a regulating factor on the nocturnal urban heat island

Abstract

Nocturnal atmospheric cooling rates were investigated in a medium sized town in Sweden. Rates were found to be very different, depending on the time of the evening/night, therefore the urban heat island (UHI) development was divided into three phases: a, differential cooling; b, transition; and c, stabilization. In phase a the UHI intensity increases by differential cooling between urban and rural areas. In phase b, after the urban heat island circulation (UHIC) starts, there is a drastic change in the rural cooling rate. The urban cooling rate is unchanged in the early evening, but about 2 h after the start of the UHIC there is a sudden increase of the cooling rate as the cool rural air reaches the city centre. The UHIC is therefore assumed to be an effective way of transporting sensible heat between the rural and urban areas. In phase c this coupling results in an equalization of the cooling rates at both the rural and the urban site from 1.5 K h−1 and 1.0 K h−1, respectively, to 0.5 K h−1. Once the UHIC is activated, the system is self-regulating since if one factor is changed some of the others have to change as well in order to preserve the balance. In phase c the advective flux is estimated to be −9±4 W m−2 and to give a central UHI potential cooling of −0.3 K h−1 of the. Copyright © 1999 Royal Meteorological Society