An efficient hybrid method for estimating clear-sky surface downward longwave radiation from MODIS data

This paper proposes an efficient hybrid method for estimating 1 km instantaneous clear-sky surface downward longwave radiation (LWDN) from Moderate Resolution Imaging Spectroradiometer (MODIS) thermal infrared observations and the MODIS near-infrared column water vapor (CWV) data product. The LWDN was formulated as a nonlinear function of surface upwelling longwave radiation estimated from the MODIS top-of-atmosphere (TOA) radiance of channels 29, 31, and 32, as well as CWV and the MODIS TOA radiance of channel 29. Ground measurements collected at 62 globally distributed sites from six networks were used to develop and validate the proposed hybrid method. The validation results showed that the bias and root-mean-square error (RMSE) were 0.0597 W/m(2) and 21.008 W/m(2). These results demonstrate that the performance of our method is superior to that of other studies reported in the literature. The drawback of our method is that LWDN is overestimated over high-elevation areas with extremely low CWV (<0.5 g/cm(2)) and underestimated over regions with tropical climates that have extremely high CWV. A power function relating LWDN to CWV was derived and used as a complementary method to address these circumstances. The overestimation was overcome, and the bias and RMSE decreased from 9.407 W/m(2) and 23.919 W/m(2) to similar to 0.924 W/m(2) and 19.895 W/m(2). The underestimation was also alleviated.