Impact of the Surface–Atmosphere Variables on the Relation Between Air and Land Surface Temperatures

Abstract

The energy, momentum and mass exchanges between the soil and the atmosphere take place in the surface layer. The difference between the screen temperature of the air at 2 m (T2) and the land surface temperature (LST) is a key parameter in their determination. Here an analysis of this difference is made with data from a site at the island of Mallorca, in the Western Mediterranean. While previous studies are often built for relatively short temporal intervals (from several days to few months), the current work analyzes 2-year-long series with a 30 min temporal resolution, in particular the diurnal and seasonal variability. The uncertainty of LST measured from near the ground is estimated to be at least 2 °C. The difference between T2 and LST (T2–LST) in the center of the day ranges between - 3 and - 10 °C, arriving at about - 18 °C for dry summer days. Statistically it is shown that these values are related to the net radiation heating of the surface transmitted upwards by thermal turbulence. The values in summer are much larger because the upper dry soil does not allow the heat to be transmitted efficiently into the ground. At night T2–LST is usually under 3 °C, with weak correlations of T2–LST with net radiation or ground flux, indicating that these processes are active and efficient transporting heat in most cases. The better correlated quantities at night with T2–LST are the upper soil temperature, the LST and the turbulent heat flux, reflecting the importance of the state of the surface. In summer nights T2–LST takes very small values, below 1 °C, even allowing unstable stratification. This is a result of a soil in which the first centimeters are very warm after sunset, not being able to send heat into the dry soil and releasing it into the atmosphere.