There is a new paper that examines the role of urbanization on extreme temperatures (h/t to Dallas Staley). The role of landscape change on surface air temperatures is significant and illustrates one reason why the use of these temperatures as part of the construction of a multi-decadal estimate of global warming produces biases.
The new article is
Grossman-Clarke et al, 2010: Contribution of Land Use Changes to Near-Surface Air Temperatures during Recent Summer Extreme Heat Events in the Phoenix Metropolitan Area. J of Applied Meterology and Climatology. DOI: 10.1175/2010JAMC2362.1
The impact of 1973–2005 land use–land cover (LULC) changes on near-surface air temperatures during four recent summer extreme heat events (EHEs) are investigated for the arid Phoenix, Arizona, metropolitan area using the Weather Research and Forecasting Model (WRF) in conjunction with the Noah Urban Canopy Model. WRFsimulations were carried out for each EHEusing LULC for the years 1973, 1985, 1998, and 2005. Comparison of measured near-surface air temperatures and wind speeds for 18 surface stations in the region show a good agreement between observed and simulated data for all simulation periods. The results indicate consistent significant contributions of urban development and accompanying LULC changes to extreme temperatures for the four EHEs. Simulations suggest new urban developments caused an intensification and expansion of the area experiencing extreme temperatures but mainly influenced nighttime temperatures with an increase of up to 10 K. Nighttime temperatures in the existing urban core showed changes of up to ~2 K with the ongoing LULC changes. Daytime temperatures were not significantly affected where urban development replaced desert land (increase by ~1 K); however, maximum temperatures increased by ~2–4 K when irrigated agricultural land was converted to suburban development. According to the model simulations, urban landscaping irrigation contributed to cooling by 0.5–1 K in maximum daytime as well as minimum nighttime 2-m air temperatures in most parts of the urban region. Furthermore, urban development led to a reduction of the already relatively weak nighttime winds and therefore a reduction in advection of cooler air into the city.
One excerpt from the paper reads
“Results show that urban land use characteristics that have evolved over the past ~35 years in the Phoenix metropolitan region have had a significant impact on extreme near-surface air temperatures occurring during EHEs in the area. Simulated maximum daytime and minimum nighttime temperatures were notably higher because of the conversion of agricultural to urban land use [by ~(2–4 and 8–10 K), respectively]. The conversion of desert to urban land use led to a significant increase in nighttime air temperatures (6–7 K) and slight increases in daytime temperatures of about 1 K. Current landscaping irrigation contributes to cooling of those parts of the region that have extensive areas of mesic vegetation (~2 K), but little benefit (~0.5 K) is obtained from irrigation for themore extensive areas characterized with predominantly xeric residential and commercial– industrial land use.
These results agree well with Brazel et al. (2000) who found no significant increase in average maximum June daytime air temperatures after ;1940 in the center of the urban area and on average slightly higher maximum daytime temperatures at a rural site southeast of the Phoenix metropolitan region. They also found that the average June minimum nighttime temperatures were on average 5 K (maximum up to ~10 K) higher at the Phoenix Sky Harbor Airport in comparison to the rural site with a slight upward trend with the expansion of the urban area after 1950.”