The success of a GCM can be judged by its ability to reproduce features
and behaviour present in the atmosphere itself. The ECMWF ERA data set
represents the best available estimate of the state of the atmosphere during
the eighteen-year period that it covers. The ability of the UK Met. office
Unified Model (UM) to reproduce mid-latitude variability is investigated by
comparing data obtained using a tracking program developed by Dr. Kevin Hodges
with similar data derived from the ERA dataset. The version of the Unified
Model used in this study is called HADAM3. The model utilises the primitive
equations and is forced by prescribed sea surface temperatures. The data set
used is from a simulation, which covers an eighteen-year period from 1979 to
1996, only wintertime (DJF) data is investigated.
Previous studies have focused on the comparison of the mean fields or the
variability about this mean from an intrinsically Eulerian perspective. This
study takes the more system orientated approach of tracking individual synopticscale features through their lifetime and then compiling statistics to
investigate the atmospheric variability. The results are complementary to the
more standard view of looking at the variability at a point. Also they allow
further insight into the nature of this variability as it is possible to look
at the contribution to the variance of the cyclonic and anticyclonic systems separately.
Tracking results from a variety of diagnostics will be presented including MeanSea Level Pressure, meridional wind, temperature, geopotential and vorticity.
The implications of these results will then be discussed in terms of the
ability of UM to reproduce features observed in the ERA statistics.