Tm. Crawford et Ce. Duchon, An improved parameterization for estimating effective atmospheric emissivity for use in calculating daytime downwelling longwave radiation, J APPL MET, 38(4), 1999, pp. 474-480
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.