Decadal variations in the nocturnal heat island of London

Abstract

Urban landscapes typically experience higher air temperatures than the surrounding countryside due to absorption of solar heat by the fabric of buildings and trapping of outgoing longwave radiation within enclosed spaces. There is also less evapotranspiration from paved areas compared with vegetated surfaces, leaving a greater fraction of solar energy for surface heating. Artificial heat sources from transport, industry, air conditioning and space heating can further contribute to local warming. Although such phenomena are well-understood, urban heat islands (UHIs) can be contentious because of their potential to corrupt estimates of global near-surface temperature trends (Parker, 2010). There are also concerns that UHIs may intensify under future climate conditions, thereby amplifying heat stress on urban residents and habitats (Wilby, 2008;McCarthy et al., 2010).Thanks to the pioneering work of Luke Howard (1833), the UHI of central London has probably attracted more attention than that of any other city. The earliest digitized temperature series for the London area originate from Kew Observatory (1881) (1931). These data show that the UHI varies over a range of timescales: on average peaking at night, towards the end of the week, and in summer (Wilby, 2003a;2008). The locus of the maximum temperature anomaly is known to be highly mobile as it drifts across central London in response to prevailing wind speed and direction (Graves et al., 2001; McGregor et al., 2006).London's UHI has also varied over recent decades. Using temperature differences between SJP (urban) and WIS (rural) during day and night, Lee (1992) found that the nocturnal UHI intensified during the period 1962-1989. The same index was used by Wilby (2003b) to show that the intensity of London's nocturnal UHI in summer increased by 0.12 degC per decade over the period . However, the most recent assessment suggests that temperatures at SJP have not increased any more than those at ROTH or WIS since 1907 (Jones and Lister, 2009). This is a finding of some consequence for urban planners and building designers; it further opens the debate about which meteorological stations can be included in global temperature datasets.The purpose of this article is to reconcile the apparent discrepancy between these studies by uncovering the causes of multidecadal variations in London's UHI. Accordingly, we investigate two groups of meteorological factors: first, potential influences of record length, homogeneity and quality on long-term trends, and secondly, possible changes in the frequency and properties of weather patterns favouring intense heat-island episodes. We pay particular attention to the summer nocturnal heat island because the climatic signature is strongest and has the greatest health implications at these times. Data and methodsWe employ both daily and monthly mean maximum (Tx) and minimum (Tn) temperatures measured at SJP and WIS. As the name suggests, SJP is not located in a highly developed area of central London and is known to be cooler than ...

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