Urban and rural dewfall, surface moisture, and associated canopy-level air temperature and humidity measurements for vancouver, canada

Abstract

Canopy-level humidity is often less at night during fine weather in a mid-latitude city, compared to its rural surroundings. This feature has been attributed, in part, to reduced urban dew, but links are largely unproven, because urban dew data are rare. In this study, surface moisture (i.e., dew + guttation by blotting) and dewfall (by mini-lysimeter) were measured at rural and urban residential sites in Vancouver, Canada, during the summer of 1996. Air temperature and humidity were measured at both sites, and on rural-to-urban vehicle traverses. Weather and location effects were evident. Humidity data suggested the small (<1 g m )3 ) urban moisture excess observed on fine nights was linked to reduced urban dew. For grass, the frequency of moisture events, and surface moisture amounts, were similar for both sites. However, on grass, rural dewfall (mean=0.10 mm per night) was more than urban dewfall (mean=0.07 mm per night). On the other hand, data for a 'roof ' lysimeter (mean dewfall=0.12 mm per night) showed that an urban roof could rival rural grass as a favoured location for dewfall in Vancouver.

Water vapour variability induced by urban/rural surface heterogeneities during convective conditions

“…The urban area is also drier near the surface during daytime with a small urban surface moisture excess at night, as seen in previous studies (Lee, 1991;Deosthali, 2000;Richards, 2005). This typical feature is also illustrated in Figure 11 which shows the diurnal cycle of the simulated latent and sensible heat surface fluxes.…”

“…During daytime, the urban areas are significantly drier than the rural areas because of the lack of urban moisture sources available for evaporation. During nighttime, the situation is less clear, even though the rural areas seem slightly drier in the second half of the night (after 0200 UTC) probably due to larger surface dew in these areas (Richards, 2005) or a slightly convective situation in the urban area which maintains weak evaporation. The surface relative humidity is much lower in the urban areas due to the UHI (Figure 8(b)).…”

“…The urban effect on the temperature (the urban heat island, UHI) has been described for different climates and seasons by several authors (reviews by Oke, 1987;Arnfield, 2003). However the urban effect on water vapour has been documented for a few cases but only based on surface measurements (Lee, 1991;Jáuregui and Tejeda, 1998;Unger, 1999;Deosthali, 2000;Unkaševic et al, 2001;Richards, 2005). Indeed, water vapour monitoring is complicated due to the specific constraints of urban areas: soundings are seldom released in city centres, the natural high variability of water vapour is enhanced by the heterogeneities of the urban surface properties which affect the representativeness of surface measurements of urban water vapour and impact on numerical modelling which requires specific subgrid-scale parametrization (Masson, 2000(Masson, , 2006Dupont et al, 2004Dupont et al, , 2006.…”

A novel method for monitoring urban dew condensation and its application

“…This section discusses the dew days and the dew intensity of different landscapes with seasonal changes in urban ecosystems based on the dew intensity calculation formulas (1 and 2) and dew observation method mentioned in Section 2.2. The dew days were 13,18,15,24,23,16, and 20 from April to October with 129 dew days during the experimental period. The number of dew days accounted for 61.4% of the frost-free season.…”

“…In Vancouver, BC, the dew frequency in city was close to the suburbs [23]. Dew intensity was 0.11-0.13 mm in a grassy landscape, whereas it was 0.07-0.09 mm in the town [24]. Dew was collected using standard passive dew condensers on a rooftop in southwest Morocco, which has an annual accumulated dewfall of 18.85 mm [17].…”