Molecular density retrieval and temperature climatology for 40–60 km from SAGE II

Abstract

The Stratospheric Aerosol and Gas Experiment (SAGE) II is a spaceborne solar occultation instrument that makes long‐term stable measurements of atmospheric transmission at seven wavelengths between the ultraviolet and the near infrared. It provides near‐global coverage (from about 75°S to 75°N latitude) and a data record spanning over 18 years starting in late 1984. Recently, a self‐consistent molecular density retrieval was developed which uses SAGE II version 6.10 transmission data above 40 km altitude. Since at this altitude attenuation of solar radiation due to aerosols can be neglected, there is enough independent information in five SAGE II channels (386–600 nm) to separate the absorption due to ozone and nitrogen dioxide from Rayleigh scattering. Vertical inversion to obtain profiles of these quantities from 40 to 60 km proceeds as in the standard SAGE II algorithm, using a matrix of refracted path lengths. The density gradient information for averaged profiles is converted to temperature. The data are sorted into 20° latitude bands from 70°S to 70°N latitude, and the time series are fitted to provide a mean climatology and long‐term trend estimates. The climatology agrees well with published climatologies for the northern midlatitudes and includes results for the other latitude bands as well. Analysis of long‐term variation finds no statistically significant effect from the solar cycle or quasi‐biennial oscillation and no statistically significant trend. An upper bound on a possible cooling trend of 3 or 4 K/decade is derived for high and middle latitudes between 40 and 45 km altitude.