Two‐day wave structure and mean flow interactions observed by radar and High Resolution Doppler Imager

Abstract

Data obtained with four MF and meteor radars at equatorial and subtropical sites and with the High Resolution Doppler Imager (HRDI) instrument aboard the UARS satellite were used to examine the structure, wave‐mean flow interactions, and potential sources of the 2‐day wave in the middle atmosphere during three southern hemisphere summers. The three wave events were highly transient, having typical durations of 20 to 30 days and exhibiting modulation at shorter periods. Temporal variations were found to exhibit good correlations between radar and HRDI data. Radar and HRDI data were used to estimate those components of the Eliassen‐Palm flux that could be assessed with these data. Meridional fluxes of momentum and heat were computed using HRDI data and agree reasonably with the momentum fluxes computed from radar data at discrete locations. These fluxes were found to exhibit consistent latitudinal structures each year, suggesting systematic wave excitation and wave‐mean flow interactions. Meridional momentum flux gradients were seen to be anticorrelated with zonal wind accelerations in a manner consistent with wave forcing of the large‐scale circulation. The apparent wave‐mean flow interactions suggest that the 2‐day wave could be a transient response to baroclinic instability of the summer hemisphere mesospheric jet. A calculation of the meridional gradient of quasi‐geostrophic potential vorticity using HRDI winds and the COSPAR International Reference Atmosphere (CIRA 1986) temperatures exhibits a region of instability in the lower and middle mesosphere extending into subtropical latitudes and provides additional evidence of a possible source of this motion via baroclinic instability of the summer hemisphere jet structure.