A New Bottom-of-Atmosphere (BOA) Radiance-Based Hybrid Method for Estimating Clear-Sky Surface Longwave Upwelling Radiation From MODIS Data

Surface longwave upwelling radiation (SLUR) is a critical parameter for studying the water-energy balance between the land surface and the atmosphere. In this study, we proposed a new hybrid method for estimating the clear-sky SLUR from the bottom-of-atmosphere (BOA) radiance (new BOA-hybrid method) of the Moderate Resolution Imaging Spectroradiometer (MODIS). There are two key parts in the developed new BOA-hybrid method. First, a physical four-channel algorithm was developed to estimate the atmospheric terms (atmospheric upwelling radiance and transmittance), which were used to calculate MODIS BOA radiance. Second, a linear model linking SLUR and MODIS BOA channel radiances was constructed using a large number of samples generated by extensive radiative transfer simulations. In situ measurements from 27 sites in four flux networks were collected to validate the new BOA-hybrid method. The average bias and root mean square error (RMSE) were 1.65 and 16.23 W/m(2) for the new BOA-hybrid method, which was superior to the original BOA-hybrid method whose bias and RMSE were 3.52 and 18.51 W/m(2), and slightly better than the classical hybrid method that estimates SLUR using the top-of-atmosphere (TOA) radiance (TOA-hybrid method) whose bias and RMSE were -1.64 and 17.44 W/m2, respectively. The performance of the new and original BOA-hybrid methods was similar when the total precipitable water (TPW) vapor was less than 1.5 cm. However, under a large TPW (i.e., TPW > 3 cm), especially in the case of a large viewing zenith angle (i.e., VZA > 45(?)), the accuracy of the original BOA-hybrid method decreased significantly, while the physical four-channel algorithm could effectively improve the accuracy of atmospheric correction, and the RMSE of the SLUR estimated by BOA-hybrid could be reduced by more than 10 W/m(2). In summary, the developed new BOA-hybrid method can estimate SLUR accurately and has great potential to produce a long-term high spatial resolution environmental data record of SLUR.

Automated summary

Surface longwave upwelling radiation (SLUR) is an important parameter for studying water-energy balance. A new hybrid method was proposed to estimate SLUR using MODIS data. The method utilizes a physical four-channel algorithm to estimate atmospheric terms and a linear model to relate SLUR to MODIS radiance. Validation results showed that the new method outperformed the original method and was slightly better than the classical hybrid method. The new method can accurately estimate SLUR and has the potential to provide long-term high-resolution environmental data.