Surface radiation characteristics and downward cloud radiative forcing in southern Xinjiang during summer 2019

This study analyzed the surface radiation characteristics and evaluated downward cloud radiative forcing (CRF) in southern Xinjiang during summer 2019. Surface radiation fluxes show prominent day-to-day and diurnal variations, with the maximums of 915.6, 921.2, and 121.9 Wm−2, respectively, for total, direct, and diffuse shortwave radiations under cloudless conditions. Cloud layer or dust layer could significantly impede the incident solar irradiance and moderately reduce the emission of longwave radiation to space, and hence bring a strong downward negative radiative impact. A rigorous four-step screening algorithm was applied to effectively identify the clear-sky periods based on 1-min total and diffuse shortwave irradiances. The clear-sky shortwave (SW) radiation is empirically fitted using a power law equation and cosine of solar zenith angle, and the clear-sky longwave (LW) radiation is calculated via the average diurnal cycles of all datasets of detected 1-min LW radiation under cloudless conditions. The 24-h daily mean CRFs of SW are largely negative (− 204.1 to − 3.3 Wm−2), and corresponding LW are moderately positive (− 1.5 to + 70.4 Wm−2), which coincide with the overcast conditions or large cloud cover amounts. The overall averages of SW and LW CRFs and net cloud effect are − 69.7 ± 62.3, + 29.5 ± 22.2, and − 40.1 ± 42.1 Wm−2, respectively, implying that clouds have a net cooling effect on the regional climate. Our results are consistent with the summer mean values in northern China, but less than those in southern China, and greater than those of the global mean values. Such negative cloud radiative forcing would partly counteract the global warming induced by greenhouse gases and modulate the Earth’s radiation budget balance.