A THEORETICAL-ANALYSIS OF THE ENERGY BUDGET IN THE LOWER THERMOSPHERE

The University of Michigan's diagnostic post-processor (UM-DP) develop ed for use with the National Center for Atmospheric Research's-Thermos phere-Ionosphere-General Circulation Model (NCAR-TIGCM) has been exten ded to include a thermal term analysis capability. The upgraded proces sor calculates the magnitudes of the individual terms in the thermodyn amic equation solved by the TIGCM as a function of 3-D space and model time. In a first study using the new capability, the lower thermosphe ric heating and cooling terms have been examined for a diurnally repro ducible TIGCM run for moderate geomagnetic activity, solar maximum, De cember solstice conditions. Thermal terms calculated for geomagnetical ly quiet and active TIGCM runs have also been examined to investigate the geomagnetic activity dependence of the important nitric oxide (NO) radiational cooling term. Finally, the one-dimensional global mean mo del of Roble and Dickinson (1989) has been used to calculate the effec ts on the lower thermospheric thermal balance caused by the combinatio n of natural and anthropogenic forcings projected over the next 30 yr. The principal results of this study of lower thermospheric energetics are as follows. (1) Lower thermospheric heating and cooling terms hav e complex morphological dependencies on latitude, longitude, altitude, geomagnetic activity, and season. (2) For the highest altitudes consi dered (similar to 175 km), heating caused by minor species chemistry p lays the most important role in sunlit conditions, with direct solar E UV heating and Joule beating having secondary roles. The primary cooli ng terms at these altitudes are adiabatic expansion, NO cooling, and d ownward heat conduction. (3) At similar to 125 km altitude, direct sol ar insolation and Joule heating are the most important heating terms, with compressional heating also contributing significantly in the wint er hemisphere. The NO and CO2 radiational terms are roughly equal in m agnitude and together dominate the cooling, with adiabatic expansion b eing of significance at high summer latitudes. (4) At the lowest altit udes considered (similar to 103 km), direct solar insolation, heat con duction, and adiabatic compressional effects dominate the heating. The dominant cooling term here is caused by CO2 radiation, with heat adve ction and adiabatic expansion in the summer hemisphere playing minor r oles. (5) The important NO cooling rates can double globally for high levers of geomagnetic activity, with values at low latitudes rising fr om similar to 160 K/day to similar to 400 K/day at similar to 150 km a ltitude. (6) Solar-cycle-dependent changes in NO radiational cooling a nd EUV heating tend to cancel each other out near similar to 150 km al titude. In this altitude region, long-term temperature reductions caus ed by anthropogenic CO2 increases may become more readily measurable, owing to the smaller masking effects of solar activity variations. (C) 1997 Elsevier Science Ltd.