5 Apr 2001
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Comparison of ECMWF cloud fraction with radar derived valued
Robin Hogan, Christian Jakob and Anthony Illingworth
- In addition to the usual prognostic variable of cloud water
content, global circulation models carry a value for cloud fraction
which can be either diagnostic (e.g. UK Met Office Unified Model) or
prognostic (e.g. ECMWF model).
- This parameter is important for the model radiation budget but
previous validation attempts using satellite data have suffered from
poor vertical resolution, so only total cloudiness (i.e. vertically
integrated) could be validated, not the cloud fraction at every model
- The high vertical resolution of radar makes it the ideal tool
with which to validate the model cloud fraction. The principle is
simple - radar time height sections are divided up into boxes centred
on the model levels and 1 hour in duration. Cloud fraction is then
simply the fraction of pixels within the box that are cloudy.
- The lidar ceilometer is very sensitive to liquid water clouds and
in particular, is able to locate cloud base in the presence of rain
or drizzle, so is used to modify the cloud fraction derived from the radar.
- We use the 3 months of data taken by the 35 GHz Rabelais and 94
GHz Galileo during the Cloud
Characteristics campaign. Quicklooks of the radar and lidar data,
together with the retrieved and modelled cloud fractions for each day,
can be found here.
- An example of the comparison for a week of data is shown above.
- This is then used to compile a climatology for comparison. The
first panel below shows mean cloud fraction versus height for the
model and the observations - one can see that the model tends to
underestimate cloud fraction at mid-levels and overestimate it at
high levels. However when this is divided into a `frequency of
occurrence' (when cloud fraction is greater than 0.05) and an `amount
when present' we see that the model is good at saying when there will
be some cloud in a gridbox, but it tends to get the amount wrong.
- Comparison of ECMWF winter-season cloud fraction with
Hogan, R. J., C. Jakob and A. J.
Illingworth, 2001, J. Appl. Meteorol., 40(3), 513-525.
(Dowload from the Publications