Synoptic Density Maps of the Upper Atmosphere

Abstract

Knowledge of the horizontal variation of density is required in a number of problems, such as the calculation of aerodynamic heating rates of lifting reentry vehicles in near-horizontal flight. On constant-pressure surfaces the density field is given by the isotherms; but since the pressure surfaces may have great inclination in the stratosphere and mesosphere, compared with the troposphere, constant-height analyses are needed for a direct measure of the horizontal density variations. With suitable assumptions, previously analyzed constant-pressure maps can be converted hydrostatically to yield accurate constant-height maps. A computerized procedure is described for obtaining such maps from high-level constant-pressure analyses of the National Meteorological Center. Examples of synoptic density maps obtained for 30 and 40 km are shown and are discussed in relation to standard atmosphere values of the density. Good agreement is found between average values based on the maps and 1966 Supplemental Atmosphere values for 30 and 60N. The synoptic variability, however, is considerable, with strong horizontal gradients a common feature in high latitudes in winter. The summer atmosphere at the heights examined is found to be nearly barotropic, with relatively weak density gradients. For a hypothetical reentry trajectory, sample data for a specific day in winter are used to calculate the density changes along the trajectory.