E.J. Mlawer & S.A. Clough
Atmospheric and Environmental Research, Inc.

This work describes the second phase in the development of RRTM, a rapid and accurate radiative transfer model designed for climate applications. RRTM, which uses the correlated-k method for radiative transfer, now has the capability to calculate fluxes and cooling rates in the shortwave region for an arbitrary clear atmosphere. The initial phase of RRTM allowed the calculation of fluxes and cooling rates in the longwave region in clear-sky conditions. Based upon validations using the line-by-line model LBLRTM, RRTM has an accuracy in the spectral region 10-3000 cm-1 of 1.0 W/m2 in net flux, 0.07 K/day for cooling rates in the lower atmosphere, and 0.75 K/day for cooling rates in the upper atmosphere.

The current developmental phase of RRTM extends the capability for flux and cooling rate calculations to the shortwave region. All important shortwave spectral absorbers have been included, most notably the vibrational-rotational bands of H2O and CO2, the Hartley-Huggins bands of O3, and the Herzberg bands of O2. Validations performed using LBLRTM of the shortwave calculations of RRTM show substantial agreement. Results of calculations performed with RRTM are presented, including the dependence on solar zenith angle of both the solar flux absorbed by the water vapor continuum and the Rayleigh extinction of solar flux.

The longwave cloud approach that has been recently implemented in RRTM is also sketched in this work.