Impacts of Small-Scale Urban Encroachment on Air Temperature Observations

A field experiment was performed in Oak Ridge, Tennessee, with four instrumented towers placed over grass at increasing distances (4, 30, 50, 124, and 300 m) from a built-up area. Stations were aligned in such a way to simulate the impact of small-scale encroachment on temperature observations. As expected, temperature observations were warmest for the site closest to the built environment with an average temperature difference of 0.31 degrees and 0.24 degrees C for aspirated and unaspirated sensors, respectively. Mean aspirated temperature differences were greater during the evening (0.47 degrees C) than during the day (0.16 degrees C). This was particularly true for evenings following greater daytime solar insolation (20+ MJ day(-1)) with surface winds from the direction of the built environment where mean differences exceeded 0.80 degrees C. The impact of the built environment on air temperature diminished with distance with a warm bias only detectable out to tower B ' located 50 m away. The experimental findings were comparable to a known case of urban encroachment at a U.S. Climate Reference Network station in Kingston, Rhode Island. The experimental and operational results both lead to reductions in the diurnal temperature range of similar to 0.39 degrees C for fan-aspirated sensors. Interestingly, the unaspirated sensor had a larger reduction in diurnal temperature range (DTR) of 0.48 degrees C. These results suggest that small-scale urban encroachment within 50 m of a station can have important impacts on daily temperature extrema (maximum and minimum) with the magnitude of these differences dependent upon prevailing environmental conditions and sensing technology.