Shortwave (Solar) Radiation Model
An important component of the surface energy budget is the solar net radiation. The solar
radiation at the surface consists of direct and diffuse components. The relative magnitude
of these two components depends in a large part on the optical depth of the clouds. As the
cloud optical depth increases, the direct component will decrease, while the diffuse
component may increase initially if the optical depth is small enough. As the optical
depth increases beyond some threshold value, the diffuse component will decrease.
2.1 Downwelling Shortwave Radiation
Shapiro (1987, 1982, 1972) developed a simple model to determine the direct and diffuse
shortwave irradiance at the surface using only standard surface meteorological
observations. The Shortwave Radiation Module (SRM) developed for FASST uses this
model to determine the downwelling shortwave irradiance. The module also uses a
simple reflected shortwave irradiance model based on the downwelling shortwave
irradiance and knowledge of the surface albedo. Surface albedo is parameterized in terms
of the land surface type. For example, the albedo for grass is given as 0.20 and for sand
as 0.40. For each land type there will be an albedo value based on that land type. The
Solar Radiation Module requires cloud information that can be obtained from
observations, climatology or cloud conditions generated by mesoscale forecast models
such as the Army's Boundary layer Forecast Model (BFM).
The basic model approach involves dividing the atmosphere into k layers and assuming
Rk + Tk + Ak = 1 .
R, T, and A are the reflectance, transmission, and absorption of layer k. In the SRM R, T,
and A have been parameterized in terms of the solar zenith angle, φo, and the state of the
atmosphere/clouds using the very extensive SOLMET database (NOAA 1979). The
general form of the flux equations is
I sk↓ = Tk I sk↓-1 + Rk I sk↑
I sk↑ = Tk +1I sk↑+1 + Rk +1I sk↓ .
Layer k = 0 is the top of the atmosphere. For operational use, the atmosphere has been
divided into three layers consistent with the concept of low, middle, and high clouds. The
downwelling solar flux at the ground (bottom of layer 3) is given as
I s↓ = T1T2T3 / D2