Gravity waves in the equatorial MLT region

Abstract

Observations of MF radar winds made at Christmas Island ( 2 ∘ N , 157 ∘ W ) between 1990 and 1997 are used to produce daily values of gravity wave variances in the equatorial mesosphere and lower thermosphere (70–100 km). Gravity wave activity is modulated on number of time scales, ranging from diurnal through seasonal to intraseasonal and interannual. A semiannual variation is particularly evident. In this paper we examine the possibility of using the wave variances in the MLT to estimate the momentum flux spectrum near the source height in the troposphere. To test our ideas we focus on modeling observations in April 1993 and April 1994, months which had very different background winds and tides. A linear gravity wave model is used in combination with ray-tracing techniques to investigate the propagation of wave spectra through representative zonal wind and temperature fields, including the effects of tidal winds and temperatures. The magnitude and diurnal variation of observed wave variance are used to constrain the momentum fluxes as a function of height and time. A number of source distributions were tested using this technique. It was found that the observations impose rather strong constraints on the source-level spectrum. A spectrum that has a bias to toward eastward propagating waves gave the best correspondence with observations. Our results have significance for the design and constraint of gravity wave parameterization schemes used in climate models.