Eleven years of ground–air temperature tracking over different land cover types

Abstract

ABSTRACTWe have analyzed series of air, near‐surface and shallow ground temperatures under four land cover types, namely bare clayey soil, sand, short‐cut grass and asphalt; the samples were collected between 2002 and 2013 and monitored at the Geothermal Climate Change Observatory Sporilov, Prague (50°02.43′N, 14°28.54′E, 226 m a.s.l.). A comparison of all of the obtained temperature series revealed a strong dependence of the subsurface thermal regime on the respective surface cover material. The ground ‘skin’ temperature was generally warmer than the surface air temperature over all monitored surfaces; however, the temperatures over different land cover types differed significantly. Asphalt exhibited the highest temperatures, and temperatures below the grassy surface were the lowest. Special attention was paid to assessing the value of the ‘temperature offset’, the instant value of which sometimes varied dramatically, on both daily and annual scales, by up to 30+ K; however, on a long‐time scale, the temperature offset was generally constant and reflected the surface material. The characteristic 2003–2013 mean values for the individual covers are as follows: asphalt 4.1 K, sand 1.6 K, clay 1.4 K and grass 0.2 K. All four surface covers revealed typical daily and inter‐annual cycles, which were monitored and are discussed in detail. Incident solar radiation was the primary variable for determining the amount and temporal changes of the temperature offset values. A linear relationship between air–ground temperature differences and incident solar radiation was detected. The mean slope of the linear regression between both variables is clearly surface cover dependent. The greatest value, 3.3 K per 100 W m−2, was found for asphalt cover; rates of 1.0–1.2 apply to bare soil and sand cover, and a negative slope of −0.44 K per 100 W m−2 represents grass cover.