Marie Haeger-Eugensson

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

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 99 4W m 2 and to give a central UHI potential cooling of 0.3 K h 1 . Copyright

WINTER LAND BREEZE IN A HIGH LATITUDE COMPLEX COASTAL AREA

The winter land breeze (WLB) was studied in an area with complex topography on the Swedish West Coast (58°N). Methods used were mapping directions of smoke plumes, temperature soundings by tethered balloon and helicopter, three years of monitoring temperature and wind at three levels on a suspension bridge and on a mast, and meteorological observations from three stations located at different distances from the coast. To initiate the development of the WLB, the necessary temperature difference between sea and land was 5°C, but the magnitude of the difference appeared to have little influence. The direction of the typical WLB was at an oblique angle to the coast. The WLB started with a sudden wind shift as a result of frictional decoupling from the prevailing synoptic wind caused by increased stability. The flow, with a depth of about 100 m, developed within the nocturnal inversion at the level of the plateau-shaped mountains and overrode the cold air in the valleys and followed the uplifted inversion over...