Radiation and aerosol measurements over the Tibetan Plateau during the Asian summer monsoon period

Abstract

Radiation and aerosols are essential to climate change but are not well reported or understood over the Tibetan Plateau (TP) due to the scarcity of observational data from this unique geographical location. This study analyzed the characteristics of radiation and aerosols and evaluated satellite products over the TP by using ground-based measurements from an atmospheric campaign at Qaidam during the Asian summer monsoon (ASM) period in 2019. Clouds decreased the surface downward solar irradiance (SSI) by a maximum of 295.8 W m-2 at 14:00 and increased the downward longwave radiation by 47.6 W m-2 at 7:00 when the lowest surface temperature (6.1 °C) occurred. The surface albedo was approximately 0.25 at Qaidam. The accuracies of the surface SSI products from the Advanced Geosynchronous Radiation Imager onboard the Fengyun-4 satellite and the Advanced Himawari Imager (AHI) onboard the Himawari-8 satellite were better under clear-sky conditions than cloudy conditions. The AHI tended to systematically overestimate surface UV radiation relative to ground-based measurements. The aerosol optical depth at 440 nm (AOD440) was <0.20 in most cases. A decreasing pattern of single scattering albedo with increasing wavelengths was revealed at Qaidam, which was 0.87 ± 0.04, 0.81 ± 0.07, 0.77 ± 0.09 and 0.76 ± 0.11 at 440, 675, 870 and 1020 nm, respectively. The average instantaneous aerosol direct radiative forcing at the bottom of the atmosphere was −41.6 ± 16.8 W m-2. A dust event with an AOD440 of 1.20 was recorded and analyzed with the aid of satellite aerosol measurements and backward-trajectory analysis.