Abstract:

Downward surface longwave radiation (DSLR) is indispensable for research about surface radiation balance. DSLR is one of the components of the surface radiation budget. Most of the DSLR inversion methods with remotely sensed data are on the conditions of clear sky. This method has a limited application range and their performances can be severely degraded in complex weather conditions as the effects of different aerosol types and levels are rarely considered. However, the arid and semi-arid area in northwest China is a frequent area of sandstorms, especially in spring each year, millions of tons of dust can enter the atmosphere, which not only causes significant damage to human life and production activities but also affects climate and the environment due to the physical and chemical effects of dust aerosol. It is difficult to study the optical properties of dust aerosols at home and abroad. Due to the complex mechanism of aerosol scattering and absorption, there is no mature DSLR algorithm that can be applied to dust sky, and this research is a tentative exploration in this field. For this issue, DSLR and top of the atmosphere (TOA) channel radiances under seven main land cover types are simulated to establish a simulation database. Secondly, the model coefficients are grouped by view zenith angle (VZA), water vapor content (WVC), and aerosol optical depth (AOD) to form a coefficient lookup table. Finally, based on the analysis of the effects of various four aerosol types and levels on atmospheric longwave radiative forcing and MODIS channel radiances, an improvement of the linear model using aerosol optical parameters is proposed to establish a DSLR estimation model constructed to apply to dust aerosol conditions, and four sites in Heihe River Basin, Gansu Province, China were used to verify the application ability of the improved model. Overall, the root mean square error of DSLR between the retrieved value and the site value is from 17.1 W·m^-2 to 20.4 W·m^-2, and the bias is from -12.3 W·m^-2 to -1.8 W·m^-2 for the four stations. Due to considering the dust aerosol optical depth and radiative forcing of dust aerosol, the improved model is able to significantly increase the estimating accuracy of DSLR under dust aerosol conditions and can reduce the uncertainty of DSLR in the applications of radiation. From the results, the inversion accuracy of this model can meet the requirements of instantaneous DSLR research and serve as a reference for subsequent research. However, more case studies and comprehensive analysis are needed before the model is used to estimate DSLR in dust sky actually.

Key words: downward surface longwave radiation, dust aerosol, radiation forcing, MODIS, Heihe River Basin