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SUMMARY:Conservation in realistic simulations of composition beyond the mi
 xing timescale - Dr John Methven\, University of Reading
DTSTART:20101129T141500Z
DTEND:20101129T151500Z
UID:TALK25917@talks.cam.ac.uk
CONTACT:Dr Alex Archibald
DESCRIPTION:Much of our understanding of atmospheric chemistry has been pr
 opelled\nforward by attempting realistic simulations of composition and\nr
 elating the results (and errors) to measurements. Typically\,\nphotochemic
 al models are driven offline using wind and temperature\nanalyses obtained
  by data assimilation into a numerical weather\nprediction model. In this 
 talk\, deficiencies in tracer simulations\ndriven by analysed winds will b
 e identified using the global and\nmaterial conservation properties of flu
 ids. \n\nErrors in global conservation of tracers arise from discretisatio
 n of\nthe mass continuity equation\, even when the tracer advection scheme
  is\nconservative by design (given a flow obeying continuity exactly). For
 \nexample\, in ozone hole simulations the error in global ozone burden\nca
 n exceed the photochemical change by a factor of 3 when operational\nanaly
 ses are spatially truncated. The reasons behind the\nnon-conservation and 
 its effects on the global distribution are\nexplored. Recommendations are 
 given regarding wind field truncation\nand choice of vertical coordinate.\
 n\nAdvection schemes inevitably introduce numerical mixing but it is hard\
 nto quantify this effect with realistic wind fields. Simulations of the\nd
 ispersal of SO2 from the Sarychev volcano are compared with IASI\nsatellit
 e data for column amounts. It is shown that it is possible to\nback out bo
 th the implicit mixing rate in the tracer model and the\nmixing rate affec
 ting the plume dispersal in the real atmosphere. The\nmixing timescale (in
  the upper troposphere) is surprisingly long (>20\ndays)\, but is supporte
 d by independent evidence from a range of\naltitudes on the ICARTT Lagrang
 ian experiment. It exceeds the implicit\nmixing timescale at the resolutio
 n that it is currently feasible to\nachieve using a global chemical transp
 ort model.
LOCATION:Unilever Lecture Theatre\, Department of Chemistry
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