The Mesospheric Diabatic Circulation and the Parameterized Thermal Effect of Gravity Wave Breaking on the Circulation

Abstract

A calculation of the monthly mean diabatic circulation of the mesosphere is performed by using the thermodynamic and continuity equations with observed climatological data. Non-LTE CO2 cooling, ozone infrared cooling, and solar heating from ozone and molecular oxygen absorption are included in the calculation of the diabatic heating rate. During the solstices, a distinctive summer-to-winter flow is produced throughout most of the mesosphere. During the equinoxes, the flow is from the fall to spring hemisphere in the upper mesospheric region but predominantly poleward in the rest of the mesosphere. An attempt was made to include the thermal effects of gravity wave breaking in the energy budget to investigate their effect on the mean circulation. Various unknown coefficients needed for the wave breaking parameterization made it difficult to assess their exact effect. However, the residual circulation was found to be relatively insensitive to the effective Prandtl number chosen. For large elective Prandtl numbers, the direct thermal effect of gravity wave breaking on the mesospheric residual circulation is very small. An independent evaluation of the overall impact of gravity wave breaking on the thermal budget comes from the requirement that the globally averaged net mass flux across a pressure surface be zero. This mass balance condition is violated for an effective Prandtl number of unity, and is more closely adhered to for large Prandtl numbers. The momentum residual can be used to check the compatibility of the Lindzen's parameterized wave drag with the diabatic/residual circulation. These comparisons can also provide an alternative way of determining the amplitude coefficients and breaking level coefficients needed to estimate the gravity wave drag. The pararmeterized wave drag can be tuned to agree very well with the structure of the momentum residual during the solstices, but not so during the equinoxes.