Diabatic Circulations in the Stratosphere

Three dimensional fields of radiative heating and cooling rates have been calculated from satellite observations of temperature, ozone and water vapour distributions in the stratosphere between November 1978 and May 1979. Error analysis shows that these fields are accurate to within <20% over the entire stratosphere. Zonal-mean cross-sections are broadly similar to other recent calculations. Net radiative heating rates have been obtained from these components; in accord with previous studies it was necessary to impose a constraint on the global heating field. The data were used to show that the mean pressure of each isentropic surface in the stratosphere changes by less than 1% each day, so that the assumption of no net flow of mass through each surface is very accurate; this constraint was applied to the heating fields. Corrected monthly, zonal-mean fields qualitatively agree with the broad range of previous calculations. Three-dimensional heating fields are analysed for the northern hemisphere winter and spring; comparison with the potential vorticity distribution reveals that the strongest net cooling tends to occur at the eastern edge of the displaced polar vortex, consistent with the dynamical situation of warm, tropical air being advected around the vortex by the cyclonic flow. The net radiative heating rates give the vertical component of the diabatic circulation in isentropic coordinates, since the 'vertical' (cross-isentropic) velocity is directly proportional to the heating rate. The horizontal component of the diabatic circulation is calculated from the vertical mass convergence, for both three-dimensional and zonal-mean cases, using the steady state continuity equation; the satellite data are used to validate the assumption that density changes can be neglected over long times, so that the diabatic circulation is the dominant part of the divergent flow; this is not the case over short periods. The zonal-mean diabatic circulation agrees well with the most convincing previous calculations; there is ascent in the tropics and descent at the winter pole, with ascent overlying weaker descent at the summer pole, with poleward flow which is strongest near the winter hemisphere stratopause. These circulations have been used with the eddy vertical velocity, determined from