Impact of urbanization on changes in temperature extremes in Beijing during 1978-2008

To quantify the impact of urbanization on changes in observed surface air temperature and extremes, a homogenized dataset of daily mean, maximum and minimum temperature (T (m), T (max) and T (min)) observations at 20 stations in Greater Beijing (GBJ) from 1978 to 2008 is analyzed. In contrast with previous studies, a cluster analysis is done to objectively classify observing stations into four categories (urbanized, suburban, rural and mountain), which is checked with remote-sensing night-light images since the 1990s. At urbanized sites, there is an added warming trend in annual mean T (m) representing an average of 10.9% (up to 18.4% or 0.12A degrees C/decade at the most strongly influenced site) of overall warming. Corresponding contributions for T (min) are 12.7% (up to 20.8% or 0.19A degrees C/decade) and 24% for diurnal temperature range DTR (up to 37.4% or 0.149A degrees C/decade) over the last three decades. Although it has not had a significant impact on daytime records (T (max)), urbanization has enhanced the increasing (decreasing) trend of extremely warm (cold) nights by an average of 12.7% or 2.07 d/decade (29.0% or 5.06 d/decade) at the urbanized sites since the 1970s.