A NEW PARAMETERIZATION OF SCALE-DEPENDENT RADIATIVE RATES IN THE STRATOSPHERE

Scale-dependent radiative dissipation rates, alpha, for finite-amplitu de temperature disturbances have been calculated with a narrowband (5 cm(-1)) radiative transfer model in the stratosphere (15-60 km). Resul ts are presented for the 15-mu m bands of CO2 and the 9.6-mu m bands o f O-3. The calculations update previous studies in several respects. F irst, more recent spectroscopic data were used. Second, the calculatio ns were performed for several reference temperatures, T-e(z), and ozon e profiles representative of various latitudes, enabling closer examin ation of the dependence on the background fields. Finally, the sensiti vity to the CO2 concentration was examined. For the 15-mu m bands of C O2, alpha(CO2) is influenced by the curvature of T-e(z) near the strat opause and tropopause. Doubling the atmospheric CO2 content from prese nt-day levels increases alpha(CO2) by a factor of around 1.4. At 9.6 m u m, alpha(O3) is extremely sensitive to the O-3 mixing ratio, which h as a latitudinal dependence: in the low stratosphere dissipation in th ese bands is much less effective in the Tropics than at high latitudes , but in the middle stratosphere (near 30 km) the opposite is true. Th ese dependencies have been included in a new numerical approximation, within the usual restrictions of the WKBJ theory, of the vertical scal e-dependent radiative dissipation rates. It is demonstrated that this parameterization yields accurate results throughout most of the strato sphere. For CO2 the error is never worse (and usually considerably bet ter) than 10%. Inaccuracies of 30%-40% sometimes occur for O-3 but the se are considerably smaller than in previous parameterizations, especi ally above the low stratosphere.