A detailed evaluation of heating processes in the middle atmosphere

A fundamental problem in the study of the terrestrial middle atmosphere is to calculate accurately the local heating due to the absorption of solar radiation. Knowledge of the heat budget is essential to understanding the atmospheric thermal structure, atmospheric motions, atmospheric chemistry, and their coupling. The evaluation of heating rates is complicated (especially above the stratopause) by the fact that the heating is not a simple one-step process. That is, the absorbed solar energy does not all immediately appear as heat. Rather, substantial portions of the incident energy may appear as internal energy of excited photolysis products (e.g., O(1D) or O2(1 delta)) or as chemical potential energy of product species such as atomic oxygen. The ultimate disposition of the internal and chemical energy possessed by the photolysis products determines the efficiency and thus the rate at which the middle atmosphere is heated. In studies of the heat budget, it is also vitally important to consider transport of long lived chemical species such as atomic oxygen above approximately 80 km. In such cases, the chemical potential energy may be transported great distances (horizontally or vertically) before undergoing a reaction to release the heat. Atomic oxygen influences the heating not only by reactions with itself and with O2 but also by reactions with odd-hydrogen species, especially those involving OH (Mlynczak and Solomon, 1991a). Consequently, absorbed solar energy may finally by converted to heat a long time after and at a location far from the original deposition. The purpose of this paper is to examine the solar and chemical heating processes and to present parameterizations for the heating efficiencies readily applicable for use in numerical models and heat budget studies. In the next two sections the processes relevant to the heating efficiencies for ozone and molecular oxygen will be reviewed. In section 4 the processes for the exothermic reactions will be reviewed and parameterizations for the heating efficiencies for both the solar and chemical processes will be presented in Section 5.