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SUMMARY:The importance of numerical time-stepping errors - Williams\, P (R
 eading)
DTSTART:20101208T153000Z
DTEND:20101208T161000Z
UID:TALK28319@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:Comprehensive assessments of uncertainty in climate prediction
  models should in principle consider contributions from the discretised nu
 merical schemes\, as well as from the parameterised physics and other sour
 ces. The numerical contributions are often assumed to be negligible\, howe
 ver. This talk reviews the evidence for uncertainty arising from time-step
 ping schemes\, and suggests a possible avenue for progress to reduce it.\n
 \nThe context for the progress is that many climate models use the simple 
 leapfrog scheme in concert with the Robert-Asselin filter. Unfortunately\,
  this filter introduces artificial damping and degrades the formal accurac
 y\, because of a conservation problem. A simple modification to the filter
  is proposed\, which fixes the conservation problem\, thereby reducing the
  artificial damping and increasing the formal accuracy.\n\nThe Robert-Asse
 lin-Williams (RAW) filter may easily be incorporated into existing climate
  models\, via the addition of only a few lines of code that are computatio
 nally very inexpensive. Results will be shown from recent implementations 
 of the RAW filter in various models. The modification will be shown to red
 uce model biases and to significantly improve the predictive skill. 
LOCATION:Seminar Room 1\, Newton Institute
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