Operational Simulation and Validation of Slant-Path Atmospheric Transmittance at a High-Altitude Tibetan Site Using MERRA-2

Abstract

This study evaluates operational optical to near-infrared (NIR) band (400–1050 nm) atmospheric transmittance simulation at the Ali-CPT site (5250 m above sea level). Monthly mean profiles from a 20-year Modern-Era Retrospective Analysis for Research and Applications, Version 2 (MERRA-2) reanalysis were combined with precipitable water vapor (PWV) constraints from the POM-02 sun photometer and validated against DTF-8 sun photometer measurements under fixed aerosol parameters (visibility of 75 km, Clean Continental mode). Mode 1 used real-time lidar extinction profiles; Mode 2 used built-in aerosol modes scaled by aerosol optical depth (AOD). Both modes achieved correlation coefficients greater than 0.93. Mode 1 showed root mean square errors (RMSEs) of 0.034–0.040 in the 400–870 nm range, while Mode 2 exhibited a systematic negative bias (RMSE 0.040–0.052) due to the mismatch between the fixed visibility assumption and the much cleaner winter conditions. The errors in the 940 nm water vapor absorption band were dominated by the vertical structural deviation of the MERRA-2 water vapor profile during high-PWV summer conditions. The results confirm the feasibility of reanalysis-based operational transmittance simulation at data-sparse high-altitude sites.