Estimation and Diurnal Variation Analysis of Clear-Sky Surface Longwave Radiation Fluxes from Himawari08 Satellite

Based on infrared radiative transfer simulation of 446183 global atmospheric profiles and statistical regression analysis, two retrieval models relating the clear-sky surface downward and upwelling longwave radiation fluxes with the channel observations of Himawari08 satellite are established. By applying the models to the observation data of Himawari08, the two flux products from February 2016 have been processed. Several experiments are conducted to validate the processed products, and results show that the RMSE (Root Mean Squared Error) is 7.9 W/m² and the correlation coefficient R is 0.9399 when comparing the surface upwelling longwave flux product with that of AQUA/CERES; the RMSE is 14.5 W/m² and R is 0.9586 when comparing the surface downward longwave flux product with that of AQUA/CERES; the RMSE is 15.34 W/m² when the satellite estimated downward fluxes are compared with the empirically calculated fluxes by Brunt equation and ground air temperature and humidity observations collected at 2260 Chinese ground meteorological stations; the RMSE is 12.6 W/m² when comparing the upwelling flux product with the flux derived from land surface temperature product of Himawari08. Two months' products in February 2016 and June 2016 are selected to study the diurnal variation of the two fluxes. The results indicate that the diurnal cycles of the two fluxes are highly associated with the solar heating surface over land with the maxima appearing from 1200 LST to 1400 LST and the minima at around 0400 LST to 0700 LST in winter and summer. The diurnal variation of downward flux is either in the same phase with that of upward flux over most land, or one hour later in some locations. The variation patterns of both flux cycles can be approximated with a half-sine curve and two lines. But over ocean there are no distinct diurnal variation characters.