Quantifying the Clear-Sky Bias of Satellite Land Surface Temperature Using Microwave-Based Estimates

Most available long-term databases of land surface temperature (LST) derived from space-borne sensors rely on infrared observations and are therefore restricted to clear-sky conditions. Hence, studies based on such data sets may not be representative of all-weather conditions and may be considered as biased toward clear sky. An assessment of the impact of this restriction is made using 3years of LST derived from passive microwave observations that are not affected by most clouds. A systematic analysis in space and time is performed of the clear-sky bias, defined as the difference between average clear-sky and average all-weather LSTs. The amplitude of the bias is closely related to the fraction of clear-sky days, and therefore, arid regions are associated to very low values of bias whereas midlatitudes present the highest values. During daytime, the input of solar radiation for clear-sky situations leads to higher LST values, and therefore, the bias is generally positive (e.g., 2-8K over the midlatitudes) whereas, during nighttime, the bias is generally negative although with lower amplitude (around -2K), because of the increased radiative cooling for clear-sky situations. The clear-, cloudy-, and all-sky LSTs are also compared with near-surface air temperature. Although LST is generally higher than air temperature, the contrast between the two may be strongly influenced by local weather conditions. Both the clear-sky bias and differences between LST and air temperature are also analyzed at the local scale taking into account the predominant cloud regime.