Uses of satellite data in studies of stratospheric dynamics

Observations of the temperature structure of the stratosphere made by the selective chopper radiometer on the Nimbus 5 satellite are used to analyse the energetics of the sudden warming of January/February 1973. A method of retrieving vertical profiles of zonal Fourier coefficients of temperature from the Fourier coefficients of the measured radiances is described. Some retrievals are compared with conventional observations and quite good agreement is obtained. Comparison of independent estimates of the mean meridional circulation shows that the retrieved temperature and height fields are not sufficiently accurate to give precise values for the zonal mean vertical and meridional velocities. The implication of this result for the energy budget is demonstrated. The sudden warming is analysed first in terms of wave structure and changes in temperature and zonal wind. No evidence for an upward propagating temperature or geopotential disturbance is found. Wave amplifications are observed to occur simultaneously at all levels or to propagate downwards. Largest temperature changes occur in the upper middle stratosphere and maximum zonal flow accelerations in the upper stratosphere. The energy cycle of the lower middle stratosphere is found to be in agreement with observations of previous warmings except in that little increase in eddy energy occurs during the event. A marked baroclinic energy cycle below 10 mb in high latitudes is shown to enhance vertical energy propagation prior to the warming, leading to increased eddy available potential energy between 10 and 2 mb. The variation with latitude of the energetics during the warming is shown to be significant. In the upper stratosphere barotropic conversion from zonal to eddy kinetic energy dominates in mid-latitudes causing deceleration of the zonal flow there first. In high latitudes the deceleration of the zonal flow occurs through the action of the induced mean meridional circulation, while the major source of eddy kinetic energy here is convergence of the vertical eddy energy flux. Examination of the relationship between the latitudinal distributions of the vertical eddy energy flux and the zonal flow reveals that maximum upward propagation of energy is centred con- sistently to the north of the polar night jet until the onset of the warming when coincidence occurs. Some aspects of the observed warming are compared with numerical simulations of sudden warmings. Although the 1973 event is characterised by a wavenumber one disturbance some striking resemblances are found in two numerically simulated wavenumber two type warmings.