Characterizing the Urban Heat Island with a Dense Sensor Network

Abstract:

Urban heat islands (UHIs) occur when urban and suburban areas experience temperatures that are elevated relative to their rural surroundings because of differences in the fraction of gray and green infrastructure. Traditional methods of characterizing UHIs rely on the comparison of near-surface air temperature measurements between few sites in an urban area with those in a nearby rural area. This methodology assumes (1) that the UHI can be characterized by the difference in air temperature from a small number of points, and (2) that these few points represent the urban and rural signatures of the region. While this methodology makes it possible to compare the UHI of cities around the globe, it ignores the rich information that could be gained from measurements across the urban to rural transect. This transect could traverse elevations, water bodies, vegetation fraction, and other land surface properties.

We deployed a network of ~200 temperature sensors across the Twin Cities Metropolitan Area (TCMA) beginning in July 2011 and continuing to the present. Our network covers a 5000-km² area encompassing the cities of Minneapolis and Saint Paul as well as suburban and rural areas within the seven-county region. The TCMA includes ~3.4 million people, nearly 900 lakes, and two major interstate highways and a beltway system. We employed a cokriging method to interpolate sensor measurements onto a continuous grid using satellite-based impervious surface fraction data.

Our results show consistent warm anomalies, when compared with a background rural temperature, over the two downtown cores and along major highways and dense suburban areas. Anomalies switch between positive and negative values (i.e., warmer or cooler than the rural, respectively) depending on season over large lakes. The warm season (May – October) average TCMA UHI (all urban areas – all rural areas) peaks at night at 1.5 °C, while the cold season (November – April) UHI is ~1 °C and is less variable over the diurnal cycle. When the top 25 UHI events of 2011 – 2014 are averaged, there is a maximum UHI of 5 °C in the cold season and ~4.5 °C in the warm season centered on downtown Minneapolis with a value near 3 °C in suburban areas in both seasons. These results yield insights into UHI behavior that cannot be gleaned from sparse observational methods.