An improved parameterization for estimating effective atmospheric emissivity for use in calculating daytime downwelling longwave radiation

An improved parameterization is presented for estimating effective atmosphe ric emissivity for use in calculating downwelling longwave radiation based on temperature, humidity, pressure, and solar radiation observations. The f irst improvement is the incorporation of an annual sinusoidal variation in effective clear-sky atmospheric emissivity, based on typical climatological variations in near-surface vapor pressure. The second is the continuous es timation of fractional cloudiness by taking the ratio of observed solar rad iation to a modeled clear-sky solar radiation. Previous methods employed ob server-estimated fractional cloudiness. Data from the Atmospheric Radiation Measurement (ARM) program were used to develop these improvements. The est imation of cloudiness was then used to modify the effective clear-sky atmos pheric emissivity in order to calculate 30-min averages of downwelling long wave radiation. Monthly mean bias errors (mbe's) of -9 to +4 W m(-2) and ro ot-mean-square errors (rmse's) of 11-22 W m(-2) were calculated based on AR M data over a 1-yr period. These mbe's were smaller overall than any of the six previous methods tested, while the rmse's were similar to the best pre vious methods. The improved parameterization was then tested on FIFE data f rom the summer of 1987. Although the monthly mbe's were larger, the rmse's were smaller. It is also shown that data from upper-air soundings can be used to calculat e the effective atmospheric emissivity rather than specifying the aforement ioned sinusoidal variation. Using ARM upper-air soundings, this method resu lted in larger mbe's, -7 to +11 W m(-2), especially during the summer month s, and similar rmse's. The success of the method suggests that it has appli cation at any observing site within reasonable proximity of an upper-air so unding, while removing the empiricism used to specify the annual sinusoidal variation in emissivity.