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A 7-year (2010-2016) comparison study between measured and simulated longwave downward radiation (LDR) under cloud-free conditions has been performed at the Izaña Atmospheric Observatory (IZO, Spain). This analysis encompasses a total of 2062 cases distributed almost 50% between day and night. Results show an excellent agreement between Baseline Surface Radiation Network (BSRN) measurements and simulations with LibRadtran V2.0.1 and MODTRAN V6 radiative transfer models (RTM), similar for both models. Mean bias (simulated-measured) <1.1%, and root mean square error (RMSE) <1%, are within the instrumental error (2%). These results highlight the good agreement between the two RTMs, demonstrating to be useful tools for LDR measurement quality control and for detecting temporal drifts in field instruments. The standard deviations of the residuals, associated to the RTM input parameters uncertainties are rather small, 0.47% and 0.49% for LibRadtran and MODTRAN, respectively, at day-time, and 0.49% to 0.51% at night-time. The observed night-time difference between models and measurements is +5Wm-2, for precipitable water vapor (PWV)>10mm, indicating a scale change of the World infrared standard group of Pyrgeometers (WISG), which serves as reference for atmospheric longwave radiation measurements. [ABSTRACT FROM AUTHOR]