Radiative Effects on Turbulent Temperature Spectra and Budgets in the Planetary Boundary Layer

Abstract

The effects of radiative energy transfer on turbulent temperature fields are studied, and preliminary estimates show the infrared ?radiative dissipation? mechanism to be dominant. Spectral computations for the idealized homogeneous-isotropic case allow an explicit solution in terms of the radiative damping rate of harmonic temperature fluctuations. Using new data and methods, this rate is evaluated for realistic values of absorption and diffusion by water vapor, carbon dioxide and water droplets. Effective computations of radiative modifications in the temperature spectrum are then performed with the aid of several spectral transfer closures. Radiative dissipation is found to occur over a wide range of scales, and its global effect is shown to be of the same order as conductive dissipation when the turbulent energy dissipation rate is low. Approximate computations are then made for the case of the planetary boundary layer, using semiempirical representations of the spectral structure. The resulting change in the temperature variance budget is discussed, and evaluation formulas are given for an apparent radiative damping frequency to be used in numerical models. Radiative damping could be significant in nocturnal stable terrestrial cases and in certain planetary atmospheres.