A New Parameterization of Scale-Dependent Radiative Rates in the Stratosphere

Abstract

Scale-dependent radiative dissipation rates. α, for finite-amplitude temperature disturbances have been calculated with a narrowband (5 cm−1) radiative transfer model in the stratosphere (15–60 km). Results are presented for the 15-µm bands of C02 and the 9.6-µm bands of O3. The calculations update previous studies in several respects. First, more recent spectroscopic data were used. Second, the calculations were performed for several reference temperatures, Te(z), and ozone profiles representative of various latitudes, enabling closer examination of the dependence on the background fields. Finally, the sensitivity to the C02 concentration was examined. For the 15-µm bands of C02, αCO2 is influenced by the curvature of Te,(z) near the stratopause and tropopause. Doubling the atmospheric CO2 content from present-day levels increases &alphaCO2 by a factor of around 1.4. At 9.6 µm, &alphaO3 is extremely sensitive to the O3 mixing ratio, which has a latitudinal dependence: in the low stratosphere dissipation in these bands is much less effective in the Tropics than at high latitudes, but in the middle stratosphere (new 30 km) the opposite is true. These dependencies have been included in a new numerical approximation, within the usual restrictions of the WKBJ theory, of the vertical scale-dependent radiative dissipation rates. It is demonstrated that this parameterization yields accurate results throughout most of the stratosphere. For C02 the error is never worse (and usually considerably better) than 10%. Inaccuracies of 30%–40% sometimes occur for O3 but these are considerably smaller than in previous parameterizations, especially above the low stratosphere.