Abstract
Abstract. Observations of the mesospheric semi-annual oscillation (MSAO) in the equatorial region have been reported dating back several decades. Seasonal variations in both species densities and airglow emissions are well documented. The extensive observations available offer an excellent case study for comparison with model simulations. A broad range of MSAO measurements is summarised with emphasis on the 80–100 km region. The objective here is not to address directly the complicated driving forces of the MSAO, but rather to employ a combination of observations and model simulations to estimate the limits of some of the underlying dynamical processes. Photochemical model simulations are included for near-equinox and near-solstice conditions, the two times with notable differences in the observed MSAO parameters. Diurnal tides are incorporated in the model to facilitate comparisons of observations made at different local times. The roles of water vapour as the "driver" species and ozone as the "response" species are examined to test for consistency between the model results and observations. The simulations suggest the interactions between vertical eddy diffusion and background vertical advection play a significant role in the MSAO phenomenon. Further, the simulations imply there are rigid limits on vertical advection rates and eddy diffusion rates. For August at the Equator, 90 km altitude, the derived eddy diffusion rate is approximately 1 × 106 cm2 s−1 and the vertical advection is upwards at 0.8 cm s−1. For April the corresponding values are 4 × 105 cm2 s−1 and 0.1 cm s−1. These results from the current 1-D model simulations will need to be verified by a full 3-D simulation. Exactly how vertical advection and eddy diffusion are related to gravity wave momentum as discussed by Dunkerton (1982) three decades ago remains to be addressed.
Highlights
The diurnal tides included in the simulation have been verified by comparison with a number of observed tidal signatures, in particular O, H2O and carbon monoxide (CO) diurnal variations
The analysis suggests that by constraining the model with the measured input parameters, namely the H2O mixing ratio and the O3 density, it is possible to derive unique values for the vertical advection rate and the eddy diffusion rate
For August, at the Equator and 90 km altitude, an eddy diffusion rate of approximately 1 × 106 cm2 s−1 and vertical advection of approximately 0.8 cm s−1 are inferred from the 1-D model
Summary
Seasonal oscillations in mesospheric atomic oxygen (O) were inferred by Sheese et al (2011) using observations from OSIRIS (Optical Spectrograph and InfraRed Imaging System) (Llewellyn et al, 2004) on Odin, and by Smith et al (2010) using TIMED/SABER data Both of these O studies showed diurnal variations. The present objective is not to address directly the complicated driving forces of the MSAO but rather to employ a combination of observations, in particular the measured vertical profiles of ozone and water vapour, in conjunction with model simulations to estimate the limits of two of the dynamical processes, namely vertical eddy diffusion rates and. The rationale for the final choice of the observed seasonal variations of ozone and water vapour vertical profiles is discussed The validity of these two “input constraints” is crucial to achieving the objective of determining the eddy diffusion rate and the vertical background wind. As an independent check on the MSAO model simulations the seasonal variation of model airglow emissions is compared against observed variations
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