Spatial variations of incoming longwave radiation in Goteborg, Sweden

A measurement programme was conducted in Goteborg Sweden, to examine the sp atial variations of incoming longwave irradiance on calm, cloudless nights. Both regional and local spatial variations were examined. Incoming longwav e irradiance data was obtained from mobile car transects, and at a fixed si te on a building roof at the city centre. Ancillary data included sky view factor at various transect locations, and balloon soundings of air temperat ure and humidity on one night. Measurements revealed that on average, incoming longwave irradiance at the fixed urban site was 11 W m(-2) higher than at the rural station, with vary ing differences for intervening sites. Bulk apparent sky emissivity was hig her at the most rural station compared to the fixed urban site, by about 0. 03 on average. Nighttime balloon measurements and a sensitivity analysis wi th a radiative transfer model argue that the bulk apparent sky emissivity d ifferences stem mainly from the temperature structure of the lower boundary layer which changes markedly from rural to urban areas. A good relationship was found between sky view factor and incoming longwave irradiance for a range of urban and park locations. The relationship appli es to both individual nights and average data. Using a simple obstruction m odel, canyon wall temperatures are derived, and the relationship between sk y view factor and wall temperature is examined.