Simulation of clear-sky outgoing longwave radiation over the oceans using operational analyses

Abstract

We have developed a system for the Simulation and Analysis of Measurements from Satellites using Operational a Nalyses (SAMSON). In this initial study, simulations of clear-sky outgoing longwave radiation (OLR) over the oceans are compared with data from the Earth Radiation Budget Experiment (ERBE). the calculations were performed with a Malkmus narrow-band radiation scheme, including the effects of all the radiatively important gases. Sea surface temperatures were taken from the National Meteorological Center's blended analyses and ozone data from the NIMBUS-7 Solar Backscatter Ultra Violet experiment. Atmospheric temperatures and humidities came from operational analyses. Parallel simulations were performed with the analyses produced both by the European Centre for Medium-range Weather Forecasts (ECMWF) and by the Meteorological Office. Simulations for the period February 1985 to October 1986 are compared with ERBE data. Differences between the humidity data in the analyses have a significant impact on the calculated clear-sky OLR. the changes to the ECMWF model in May 1985 reduce the differences compared with ERBE. After this time, the differences are consistent with two known systematic errors; a small overestimate in the ERBE data and a tendency for the ECMWF analyses to be too dry in the ITCZ and too moist in the sub-tropics. the simulations with the Meteorological Office's analyses generally show slightly larger differences compared with ERBE. Tests show the sensitivity of the ECMWF results to diurnal sampling of the analyses and to the time resolution employed to calculate the monthly means. These tests reveal instances of extremely low humidities in the ECMWF analyses in the upper troposphere over the tropical east Pacific. Over most of the domain, the simulations are within 5-10 W m−2 of the ERBE data. the clear-sky OLR can thus be computed from operational analyses with an accuracy comparable with that of ERBE. This indicates the potential of such simulations and has implications for the planning of future radiation-budget observations.