5 Apr 2001
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Deriving cloud overlap statistics from radar
Robin Hogan and Anthony Illingworth
- The predictions of climate models are sensitive to the assumed
overlap of clouds within a vertical column of model gridboxes, but
until now no reliable observations of the degree of cloud overlap
have been available.
- The three common approaches used by models are depicted in the
It can be seen immediately that the radiatively-important total
cloud cover predicted by the three schemes (indicated by the
vertical dotted lines) is very different, even though the cloud
fraction at each level is the same. Most models now use the
maximum-random overlap assumption, where clouds in adjacent
model levels are deemed to be maximally overlapped, while clouds
separated by an entirely cloud-free model level are deemed to be
- Observations by the 94 GHz
Galileo cloud radar have been used to quantify the extent of
cloud overlap for use in models. Quicklooks can be found here.
- The cloud enclosed by the red box in the figure above is an
example of a cloud that is `vertically continuous', yet is certainly
not maximally overlapped as would be predicted by most current
models. Note that the concept of overlap is meaningless when grid
boxes are entirely cloudy since all overlap assumptions must predict
the same total cloud cover: 100%.
- Seventy-one days of near-continuous radar observations such as
these have been analysed. Cloud overlap has been determined by
calculating the `combined cloud cover' of clouds in pairs of adjacent
and non-adjacent levels.
- This is compared with the overlap schemes used in models by
defining an overlap parameter which expresses the mean
overlap of layers a particular vertical distance apart, relative to
random and maximum overlap. For exactly random overlap the overlap
parameter is defined to be 0 and for exactly maximum overlap it is 1.
- The results are shown in the figure below:
- For vertically non-continuous cloud (right panel) we see that for
all level separations the overlap is very close to random on average,
in agreement with the assumptions made in current models.
- For vertically continuous cloud on the other hand (left panel),
overlap tends to move from near-maximum when the levels are adjacent
to near-random at separations of greater than 4 km. This is in
contrast to most current models which assume maximum overlap for
vertically continuous cloud.
- In this case the overlap parameter is seen to be well
approximated by a simple inverse exponential, which could be useful
for parameterising the behaviour in models. It is found that the
e-folding distance for the overlap is a function of the size of the
grid boxes used in the analysis, and therefore on the resolution of
- Deriving cloud overlap statistics from radar
R. J., and A. J. Illingworth, 2000, Quart. J. Roy. Meteorol. Soc.,
126(569A), 2903-2909. (Download from the Publications page)