5.6
Interaction between urban climate and snow
Annette Semadeni-Davies, Lund University, Lund, Sweden
The presence of snow in an urban area has a dramatic effect on the urban radiation balance both due to the high albedo and emissivity of snow. But how does the climate of the urban canyon effect snowmelt? Finding the answer to this question is of practical value to urban water managers, particularly in these days of sustainable water management where drainage via a pipe network is being replaced by above ground structures for stormwater storage and treatment. Such structures are designed to both reduce flood risk and improve urban water quality. Knowledge of snowmelt processes and modelling techniques for urban drainage have not kept pace with the new stormwater management ethos. This paper will review work done in the last decade on the interaction between urban snowpacks and their surround. It is noted that the lack of knowledge extends to both urban hydrologists and climatologists. Arnfield’s (2003) comprehensive review of urban climate, for instance, mentions snow only in passing while stormwater systems in cold regions continue to be designed for summer storm-bursts rather than low intensity but long duration snowmelt (Semadeni-Davies, 2000).
Marsalek (1991) pointed out over a decade ago that stormwater systems are, ironically, prone to failure at the time of year when flows are likely to be greatest and with the worst water quality. Adherence to traditional design methods and poor modelling tools are partly to blame. To illustrate, despite proving valuable for large-scale snow modelling, the degree-day melt routine found in urban drainage models has neither a spatial or temporal scale fine enough for simulating combined system overflows (Matheussen and Thorolfsson, 1999). These are caused by high flows in sanitary sewers and result in untreated wastewater being released directly into receiving waters. Bartošová and Novotny (1999) too showed a need for improved time resolution for simulating spring water quality.
But to improve the situation requires both data collection and basic research. Up to date, most work has focused on radiation as solar (shortwave) radiation is recognised as the major driver of snowmelt. There is very little information on the turbulent heat fluxes at the street-level that can be used for melt modelling. During the 1980s and early 1990s, work in Sweden (Bengtsson, 1983, 1984, 1990; Westerström, 1984, 1990; Bengtsson and Westerström, 1992) and Canada (Xu and Buttle, 1987; Buttle and Xu, 1988; Buttle et al, 1990; Todhunter et al., 1992) identified fundamental differences between rural and urban snowmelt processes. They found that snow properties such as density and albedo varied both between town and country and within the town depending on landuse. Semadeni-Davies (2000) confirmed their findings and presented a snow survey for the city of Luleå in northern Sweden. But it is not only changed snow properties that affect melt. Takamura (1992) found that the skywards spectral reflectance in a city in general decreases as one moves from suburbs to the centre due to the low albedo of surface materials and increasingly dense canyon geometry. Winter-time aerial measurements are very pronounced, Takamura and Toritani (1994) found that reflectivity over a snow-covered suburb was 0.4 compared to 0.7 for nearby rural areas. Semadeni-Davies and Bengtsson (1998) and Semadeni-Davies (2002) developed and tested a simplified canyon model to simulate shading and enhancement. Differences in net radiation over snow between open sites and snow lying to the north or south of a building can range from –120 to +150 W m-2.
Thus melt and runoff generation occurs at different times and rates. Town centres can have melt rates almost double that of residential areas due to low snow albedo and radiation enhancement from buildings. Town centres, though, are most likely to have both snow removal policies and combined sewer systems.
This paper is both a summary and continuation of the state-of-the-art review found in a UNESCO special report on urban drainage in cold regions (Chapter 2, Semadeni-Davies and Bengtsson, 2000). Both snow distribution and energy balance are discussed. How to improve temporal and spatial resolution with limited budgets and limited data availability are ongoing problems, however, recent coupling between major urban drainage models such as SWMM and MOUSE and Geographic Information Systems offers a glimmer of hope. While full physically-based snow melt modelling is still out of the question, GIS could allow improved representations of snow. Melt due to snow handling (e.g., de-icing salts, street warming) and water quality issues are outside of the scope.
Session 5, the energy and water balance of cities (parallel with session 6)
Tuesday, 24 August 2004, 8:30 AM-10:00 AM
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