SUMMARYMonthly zonal mcan observations of H 2 0 and CH, made by the limb infrared monitor of the stratosphere (LIMS) and the stratospheric and mesospheric sounder (SAMS) instrumcnts on Nimbus 7 have been used to investigate whether the H 2 0 mixing ratios in the stratosphere are consistent with a source via the oxidation of CHI. While both sets of data show considerable seasonally varying structure, total hydrogen (neglecting molecular hydrogen) is relatively featureless with a mean value over the stratosphere of 6.020.35 p.p.m.v. (lo) for the five-month pcriod studied. The uniformity of the total hydrogen ficlds points to the validity of the CH, oxidation hypothesis.The derived fields of total hydrogen are used to deduce a mean H 2 0 mixing ratio for air as it enters the stratosphere of 2.720.35p.p.m.v. (lo) from which a desiccation temperature may be deduced (for example, -87.2"C at 60mb).
SIJMMARYSome interesting low latitude features of tracer distributions revealed by recent satellite experiments have been studied using a two-dimensional atmospheric model. It is found that the equatorial semi-annual oscillation, forced in the model by simply prescribing a suitable momentum convergence, is important in determining the tracer behaviour. In particular, the meridional circulation associated with the semi-annual oscillation leads to a successful model simulation of the 'double peak' in N,O and CH4 mixing ratios, observations of which show, at certain times of the year, maxima on a fixed pressure surface in the subtropics with a local minimum at the equator.
The ozone budget in the stratosphere of a modified diabatic circulation model is compared with traditional Eulerian calculations. The importance of the chemical contribution in comparison with net dynamical contributions, even in the lower stratosphere, is stressed. Furthermore the chemical dependence of the eddy transports is shown to introduce significant differences in the meridional transport and points to a potentially important source of error in two-dimensional simulations of tracer distributions in the stratosphere. Other aspects arising from photochemistry, the photochemical eddy term and the eddy coupling of different species. are also considered.
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