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The use of PV equivalent latitude for assimilating stratospheric tracerobservations is discussed - with particular regard to the errors in the equivalentlatitude coordinate, and to the assimilation of sparse data.  Some examplemeasurements are assimilated: they sample the stratosphere sporadically andinhomogeneously.  The aim was to obtain precise information about the isentropictracer distribution and evolution as a function of equivalent latitude.  Precisionis important, if transport across barriers like the vortex edge are to be detecteddirectly.  The main challenges addressed are the errors in modelled equivalentlatitude, and the non-ideal observational sampling.  The methods presented allowfirst some assessment of equivalent latitude errors and a picture of how good orpoor the observational coverage is.  This information determines choices in theapproach for estimating as precisely as possible the true equivalent latitudedistribution of the tracer, in periods of good and poor observational coverage.This is in practice an optimisation process, since better understanding of theequivalent latitude distribution of the tracer feeds back into a clearer pictureof the errors in the modelled equivalent latitude coordinate.  Error estimatesconstrain the reliability of using equivalent latitude to make statements like"this observation samples air poleward of the vortex edge" or that of moregeneral model-measurement comparisons.  The approach is demonstrated forground-based lidar soundings of the Mount Pinatubo aerosol cloud, focusing on the1991-92 arctic vortex edge between 475-520K. Equivalent latitude is estimated atthe observation times and locations from Eulerian model tracers initialised withPV and forced by UK Meteorological Office analyses.  With the model formulationchosen, it is shown that tracer transport of a few days resulted in an errordistribution that was much closer to Gaussian form, although the mean error wasnot significantly affected.  The analysis of the observations revealed a smallamount of irreversible transport of aerosol across the vortex edge during lateJanuary 1992, coincident with a strongly disturbed vortex

atmospheric chemistry and physics

Refinements in the use of equivalent latitude for assimilating sporadic inhomogeneous stratospheric tracer observations, 1: Detecting transport of Pinatubo aerosol across a strong vortex edge