Solar contribution to the variability of middle atmosphere solar tides in their interaction with zonal-mean-flow variations, planetary waves and gravity waves

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The purpose of the proposed project is a thorough understanding of the mechanisms controlling the variability of solar tides in the mesosphere/lower thermosphere (MLT) on different time scales, especially via their response to variability in the atmospheric layers below. A focus will be on the contribution of solar variability to the total signal above 50 km, mediated by variations in the atmospheric mean flow and in the solar forcing. Basis of the analyzes will be general-circulation-model (GCM) integrations for different seasons and various solar conditions, which shall be interpreted and explained with the help of a linear model for tidal propagation from the troposphere into the mesosphere and thermosphere. Major questions will be how much a solar effect is caused by related variations in the zonal-mean winds, planetary wave activity, direct thermal forcing, or gravity-wave activity. For the latter a special effort will be invested in an improvement of the description of gravity-wave propagation and breaking in the total background of steady mean atmospheric flow and temperature, and propagating tides.