Radiation balance of urban snow: a water management perspective

The radiation balance of urbanised catchments differs from their rural coun terparts, with snowpacks experiencing either enhanced or decreased irradian ce depending on snowpack location and condition. As snowmelt is largely dri ven by radiation inputs, changes to localised irradiance (and melt rates) h ave implications for urban runoff generation. Storm- and wastewater drainag e systems in cold regions are currently dimensioned for rain according to p ractices developed for temperate climates. They are not designed to cope wi th wintry conditions, which can lead to surface flooding, hydraulic overloa ds and poor water quality at receiving waters. Net allwave radiation measurements over snow made at the Swedish city of Lu lea during April 1997 and 1998 are presented. The 1997 measurements were ma de in the vicinity of a matt-black-painted metal building at Lulea Universi ty of Technology, whereas the 1998 measurements are from a specially constr ucted 3 X 6-m black plastic-clad wall built on an open field just outside t he town. Black minimises multiple reflections between the snow and walls, w hile maximising absorption of shortwave radiation by walls. The data were c ompared to the outputs of an urban radiation model, The results show that u rban structures significantly alter radiation over snow. The temperature of the south-facing walls translates to longwave enhancements in the order of 150 W m(-2) for several metres from the walls on sunny days. Shaded snow n ear the north-facing wall showed a net allwave radiation loss of the same o rder of magnitude. Radiation inputs to snow are similar both to the north a nd south of walls when the sky is overcast. The need to include snowmelt en ergetics within design and management techniques is discussed in light of t he results. (C) 2001 Elsevier Science B.V. All rights reserved.