Wintertime radiation and energy budget along an urbanization gradient in Montreal, Canada.

This study reports on the radiation and energy balance of three sites (rural, suburban, and urban) located along an urbanization gradient in the Montreal, QC, region for two winters (December-March) with contrasting snow regimes. The urban and suburban sites had similar albedo which was about half that at the rural site during the snow-cover period. Temporal variability in albedo was attributable to the presence of snow on rooftops at the urban site and to a site-specific response to cloudiness at the suburban site. As compared to the suburban site, the urban site showed higher albedo inducing lower net radiation ( Q*) which was compensated for by greater anthropogenic heat flux ( QF), so that the urban site had highest total available energy ( Q* + QF). Hourly QF estimates were a significant term in the winter energy budget analysis. QF was dominated by building heating at both urbanized sites, while vehicular traffic contributed to rush hour peaks. Daytime total available energy was mostly dissipated as sensible heat flux ( QH) at the beginning of the winter season and mostly stored (Δ QS) towards the end of the winter at both urbanized sites. Daytime energy partitioning into QH and Δ QS was correlated with air temperature with no significant differences between urbanized sites. On a daily time scale, available energy was mostly stored before noon and dissipated as QH in the afternoon at both urbanized sites. Urbanized sites showed differences in diurnal variability of QH and Δ QS occurring in the afternoon and evening. Latent heat flux ( QE) was low throughout winter and accounted for 10% of the total available energy during daytime at the urbanized sites. Water vapour emissions showed intra-urban differences in their response to wintertime climatic conditions. Copyright © 2010 Royal Meteorological Society [ABSTRACT FROM AUTHOR]