A New K-Distribution Scheme For Clear-Sky Radiative Transfer Calculations In Earth'S Atmosphere. Part I: Thermal Infrared (Longwave) Radiation

A new k-distribution scheme of long wave radiation without the correlated-k-distribution assumption is developed. Grouping of spectral points is based on the line-by-line (LBL)-calculated absorption coefficient k at a few sets of reference pressure p(r) and temperature theta(r), where the cooling rate is substantial in a spectral band. In this new scheme, the range of k(p(r), theta(r)) of a band is divided into a number of equal intervals, or g groups, in log(10)(k(r)). A spectral point at the wave number nu is identified with one of the g groups according to its k(nu)(p(r), theta(r)). For each g group, a Planck-weighted k-distribution function H-g and a non linearly averaged absorption coefficient (k) over bar (g) (p, theta) are derived. The function H-g and the absorption coefficient (k) over bar (g) (p, theta) constitute the new k-distribution scheme. In this k-distribution scheme, a spectral point can only be identified with a g group regardless of pressure and temperature, which is different from the correlated-k distribution scheme. The k-distribution scheme is applied to the H2O, CO2, O-3, N2O, and CH4 absorption bands, and results are compared with LBL calculations. To balance between the accuracy and the computational economy, the number of g groups in a band of a given gas is chosen such that 1) the difference in cooling rate is <0.1 K day(-1) in the troposphere and <1.0 K day(-1) in the stratosphere and 2) the difference in fluxes is <0.5 W m(-2) at both the top of the atmosphere and the surface. These differences are attained with 130 g groups, which is the sum of the g groups of all five gases.