Comparison of radiative fluxes at the top of the atmosphere from INSAT and ERBE

Abstract

An accurate knowledge of radiative fluxes at the Top of the Atmosphere (TOA) is necessary to study the variability of climate. Many geostationary satellites have been measuring radiative fluxes in a narrow spectral band in the infrared and visible regions during the past 30 years. This data will be useful for climate studies if it can be converted to total radiative fluxes. In this paper we demonstrate that the errors in monthly mean Outgoing Longwave Radiation (OLR) at the TOA obtained from the Indian geostationary satellite INSAT-1B is less than 15 W m^-2 in most of the regions when compared to the data from the Earth Radiation Budget Experiment (ERBE). This indicates that the conversion of INSAT narrowband flux to broadband flux does not result in large errors. This could be on account of high water vapour content in the Indian region. The error in INSAT OLR has been shown to be dependent on number of images available per month. INSAT albedo has a negative bias over ocean when compared to ERBE on account of the isotropic reflectance assumption. No such bias was noticed over land. This low albedo bias in INSAT was removed by adding a constant term equal to 2% over ocean. It has been shown that the effect of Sun Glint in clear sky albedo can be removed if two images per day are used for the calculation of albedo. The difference in net radiation at the TOA between INSAT and ERBE has been shown to be within 10 W m^-2. In some regions, such as Saudi Arabia (from December 1988 to March 1989) the difference between ERBE and INSAT estimated net radiation (~10 W m^-2) was found to be less than the difference between two ERBE satellite observations (~20 W m^-2). This indicates clearly that diurnal variation of net radiation can cause large errors in the estimates of monthly mean net radiation.