Trace constituent updates in the Marshall engineering thermosphere and global reference atmospheric model

Abstract

Global reference atmospheric model (GRAM-99) is an engineering-level model of Earth’s atmosphere. It provides both mean values and perturbations for density, temperature, pressure, and winds, as well as monthly and geographically varying trace constituent concentrations. From 0 to 27 km, GRAM thermodynamics and winds are based on National Oceanic and Atmospheric Administration Global Upper Air Climatic Atlas (GUACA) climatology. Above 120 km, GRAM is based on the NASA Marshall engineering thermosphere (MET) model. In the intervening altitude region, GRAM is based on middle atmosphere program (MAP) climatology that also forms the basis of the 1986 COSPAR International reference atmosphere (CIRA). Atmospheric composition is represented in GRAM by concentrations of both major and minor species. Above 120 km, MET provides concentration values for N 2, O 2, Ar, O, He, and H. Below 120 km, represented species also include H 2O, O 3, N 2O, CO, CH 4, and CO 2. At 34th COSPAR a comparison was made between GRAM constituents below 120 km and those provided by Naval Research Laboratory (NRL) climatology. No current need to update GRAM constituent climatology in that height range was identified. This report examines GRAM/MET constituents between 100 and 1000 km altitudes. Internal discrepancies are noted between GRAM/MET constituent number densities and mass density or molecular weight. Near 110 km altitude, there is up to about 25% discrepancy between MET number density and mass density (with mass density being valid and number densities requiring adjustment). Near 700 km altitude there is also up to about 25% discrepancy between MET number density and mean molecular weight (with molecular weight requiring adjustment). In neither case are MET mass density estimates invalidated. The discrepancies have been traced to MET subroutines SLV (which affects 90–170 km height range) and SLVH (which affects helium above 440 km altitude). With these discrepancies corrected, results are presented to illustrate GRAM/MET constituent mole fractions in terms of height-latitude cross-sections from 100 to 1000 km altitude, and latitude–longitude “maps” at 450 km. Plans are discussed for an update of MET and GRAM to correct these constituent inconsistencies, and to incorporate several new thermospheric model features.