An intercomparison of ground-based land surface temperature measurements

Land surface temperature (LST) is a key variable in the study of the exchange of energy and water between the land surface and the atmosphere, and it influences land surface and boundary layer process at local to global scales. Accurate measurements of LST are necessary for a number of applications including validation of satellite LST products or improvement of both climate and numerical weather prediction models. With the objective of assessing the quality of in situ measurements of LST and to evaluate the quantitative uncertainties in the ground-based LST measurements and methods, intensive field experiments were conducted at NOAA/ATDD in Oak ridge, TN from October 2015 to January 2016. The LST's retrieved by three narrow angle broadband infra-red temperature sensors (IRT), hemispherical longwave radiation (LWR) measurements by pyrgeometers, forward looking infrared camera were compared with direct LSTs by multiple thermocouples (TC), and near surface air temperature (AT), a proxy for LST. The in-situ brightness temperature (BT) measurements by IRTs were in good agreement with each other, with a correlation coefficient of ~1, bias <0.3 ^oC and root mean square error (RMSE) of <0.36 ^oC. The comparison of LSTs by IRTs, TCs and LWR showed better agreement during night time whereas the absolute difference between daytime LSTs by IRTs and TCs were small (<0.26 ^oC with RMSE <0.67 ^oC) compared with the difference between LST [LWR] with either LST [TC] or with LST [IRT] especially during clear-sky conditions (> 1.7 ^oC with RMSE >2 ^oC). The AT explained > 80% of the variance in LST [IRT] during nighttime conditions with a bias of 2.6 °C and RMSE of 3.63 °C. To evaluate the annual and seasonal differences in LST [IRT], LST [LWR] and AT, the analysis was extended to four grassland sites in the USA. The distribution of the difference between half-hourly LST [IRT] and LST [LWR] were site and season dependent with daytime difference exceeding 5°C in some sites, but with lower bias during night time periods. The monthly difference between LST [IRT] and LST [LWR] (or AT) reached up to 2°C (5°C) depending on the site characteristics and the season. For the annual dataset of LST, the bias between LST [IRT] and LST [LWR] was <0.7°C, except at the semiarid grassland (1.5 °C) whereas the absolute bias between AT and LST at the four sites were <2°C.