Snowmelt sensitivity to radiation in the urban environment

Abstract

Despite having the same snowmelt processes, snowpacks in urban environments experience a range of conditions different from those of rural areas. Melt is intensified at some sites due to greater radiative energy. Shading, however, can reduce radiation and melt at other sites. Changes to the radiation balance and snowpack processes have been investigated. A physical snowpack model was developed and tested against data from an impervious study plot in Sweden. Estimated surface runoff compared favourably with that measured. An urban radiation scheme captured the observed net allwave radiation well. Series of sensitivity analyses were made by perturbing the scheme to represent three urban locations: open ground and the southern (sunny) and north (shaded) sides of a hypothetical building. Cloudiness, albedo, wall temperature and sky view were altered to reproduce common urban conditions. Even without perturbation, the shaded and sunny sides of the building had different radiation fluxes-the south side experienced a daily average net radiation enhancement of 15 W m-2 and the north a decrease of 35 W m-2. This pattern was reflected in melt. Perturbation exaggerated the disparity. Sensibilité de la fonte des neiges au rayonnement en milieu urbainRésumé Bien que les processus soient identiques, les conditions de la fonte des neiges en milieu urbain sont différentes de celles des zones rurales. Pour certains sites, la fonte est accélérée en raison d'une énergie radiative plus importante, alors que pour d'autres sites elle est ralentie à cause de l'ombre qui cause une diminution de l'exposition au rayonnement. Les modifications du bilan radiatif et des processus du manteau neigeux ont été étudiées. On a élaboré et évalué un modèle physique du manteau neigeux à partir de données obtenues sur une parcelle bitumée en Suède. Le ruissellement de surface simulé se corrèle bien au ruissellement mesuré. Le schéma du rayonnement en milieu urbain simule bien le bilan radiatif, courtes et grandes ondes, mesuré. Des séries d'analyses de sensibilité ont été réalisées en perturbant le schéma initial afin de représenter trois types de sites urbains; sol nu, exposition au sud (ensoleillée) et au nord (ombragée) d'un hypothétique bâtiment. La fraction nuageuse, l'albédo, la température des murs et la fraction de ciel dégagé sur le site ont été modifiés afin de reproduire les conditions urbaines courantes. Même sans l'intégration de ces perturbations, les flux radiatifs des façades ombragées et ensoleillées du bâtiment sont différents (allant de +15 à -35 W m" 2 ). La fonte de la neige est donc favorisée au sud et entravée au nord du bâtiment. Les perturbations ne font qu'augmenter cette disparité.

Development of the Surface Urban Energy and Water Balance Scheme (SUEWS) for cold climate cities

“…Valeo and Ho, 2004;Lemonsu et al, 2010;Leroyer et al, 2010) and Scandinavia (e.g. Semádeni-Davies et al, 1998). These emphasize the need for the correct description of snow cover in hydrological models.…”

“…These studies have found a strong seasonality in the energy exchanges and a need for the correct estimation of anthropogenic heat emissions from building sources, notably heating in winter. Similarly, the few hydrological studies have shown strong seasonality in stormwater runoff and differences in the amount of the snowmelt when compared to natural environments (Bengtsson and Westerström, 1992;Semádeni-Davies and Bengtsson, 1998;Valtanen et al, 2013).…”

“…The energy required for snowmelt can be of the same magnitude as the sensible and latent heat fluxes . Snow impacts water availability and its melt may cause springtime floods in urban areas (Semádeni-Davies and Bengtsson, 1998). To keep cities operational, snow is often redistributed within neighbourhoods and/or is transported away (Semádeni-Davies and Bengtsson, 1998Bengtsson, , 1999, which impacts both the energy and water cycles.…”

“…Besides re-freezing of melted water, the snowmelt runoff from the snowpack is delayed by the amount of water the snow can hold (Bengtsson, 1982;Semádeni-Davies and Bengtsson, 1998). In SUEWS, this liquid water retention capacity (C R ) is calculated as a function of snow density (ρ s , kg m −3 ) (Anderson, 1976;Jin et al, 1999):…”

Observations of urban snow properties in Calgary, Canada

“…In a case study of the town of Milton in southern Ontario, Farrell et al (2001) found that the annual peak flow is typically the result of spring flowrates. Semádeni-Davies and Bengtsson (1998) reported that northern Sweden experiences its most severe urban flooding during spring melt. Thorolfsson and Brandt (1996) reported that, in many urban areas in Norway, snowmelt and rain on frozen ground forms the most adverse runoff conditions, and not high-intensity rainfalls.…”