An analysis of HALOE observations in summer high latitudes using airmass trajectory and photochemical model calculations

Abstract

Stratospheric ozone, HCl, NO, and NO2 observed by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) in late summer at high latitudes show nonuniform distributions in their volume mixing ratios along a latitude circle, while the mixing ratios of the long‐lived tracers, such as CH4 and HF, are observed to be quite uniform. These anomalous distributions of the reactive gases are confined in approximately 20–35 km. It is found that in the summer high latitudes, the isosurfaces of long‐lived tracers nearly coincide with isentropes. On the basis of meridional distributions of these species, it appears that the HALOE‐observed features are the results of quasi‐isentropic transport across latitude lines with a timescale shorter than the chemical relaxation times of those HALOE observed ozone and other gases. A spectral analysis of the United Kingdom Meteorological Office meridional wind data indicates the existence of episodically amplified wave 1 that is quasi‐stationary in July and then slowly westward moving after mid‐August 1992. A quasi‐stationary wave 2 is also found to be important in late August. The isentropic trajectory calculations show that these planetary scale waves seem to pull air masses out of the polar region as far as 10°–15° latitude equatorward in a week. Midlatitude air is also pushed into the high summer latitude region. A photochemical box model is used to simulate the chemical evolution of HALOE‐observed species along air parcel trajectories which are initialized at HALOE positions. Reasonable agreement is found in comparisons between coincident HALOE measurements and the model results in later days. The characteristic structures in HALOE‐observed ozone and other gases in the low and middle stratosphere indicate that the chemical relaxation times taken for these species to relax to their new local mixing ratios under new sunlit conditions are comparable with or longer than the timescale of the meridional transport induced by waves in late summer.