nethod 
In our 
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Since 
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So we 
ment. 
their 
frared 
of the 
the 12 
RK... is 
apest- 
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CTH, 
Ü cm 
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nt has 
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' one 
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(1) 
ectral 
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This 
ly - to 
te for 
cases. 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
/ N 
AT > -3 AT < -3 AT > -3 AT s -3 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Clear sky | |Probably Cloudy Cloudy 
cloudy or snow 
(a) (b) (c) (d) 
  
  
  
  
  
  
  
  
  
Figure 1: Cloud detection scheme. 
Where AT = T,2 - T9; T,2 and Ts - calibrated IR cloud 
brightness temperature at 12 UTC and 9 UTC 
respectively. Experiments show that & has different 
values within the year. 6 — 0.3 for period from May 
until September and & = 0.5 from October until April. 
Cloud top height assignment 
The cloud top height assignment is based on the IR cloud 
brightness temperature. The ECMWF forecasts are used 
as supplementary data to derive the CTH. The technique 
is based on finding the two neighbouring levels where the 
IR cloud brightness temperature takes place. The level, 
where the brightness temperature fits the forecast 
temperature, represents the cloud top level (height). The 
calculation is accomplished for cloudy [(c) and (d) cases 
in Fig 1.j or probably cloudy [(b) case in Fig. 1.] pixels. 
The assigned heights were compared with the maximum 
(Rad. Max) and the average (Rad. Avg) values of radar 
pixels in 3 km surrounding area. 
RESULTS AND DISCUSSION 
Radar data is characterised by high sensitivity in the 
locality of the radar which decreases depending on the 
distance (Fig. 2a-2d.) Based on our preliminary 
computations we took into account only the data not 
further than 60-100 km from the MRL-5 but skipped the 
data from the nearest neighbourhood. Comparing the 
CTHs derived from radar and satellite measurements we 
found comparable results when there was only one cloud 
type (Fig. 3. and Fig.4.). In case of multilayer cloudiness 
big differences were found between the two derived 
CTHs (Fig. 5.) due to the different measurement 
technique of satellite and radar. Thus, satellite measures 
from above the top of the cloud while the radar detects 
from the ground. Hence the radar detects the top of the 
lower cloud while the satellite measurements correspond 
to the top of the upper one. 
In general, the CTHs derived from satellite data are 
higher than the ones derived from radar data. 
Using predicted profiles in the CTH calculation scheme 
could cause systematic errors which may increase the 
inaccuracy of the results. The period used for the 
investigation (from January until April 1998) was too 
short to find systematic errors furthermore there were not 
enough days when only one cloud type was observed by 
MRL-5. Therefore this investigation requires data for 
much longer period According to the first results the CTH 
data derived from satellite and radar measurements have 
to be compared with other calculations. Thus, numerical 
weather prediction products or results of an inverse model 
could be used for this purpose. 
CONCLUSIONS 
The CTH data derived from satellite and radar 
measurements can be compared only in case when one 
cloud type is observed. 
To estimate the systematic error of the calculation scheme 
a longer observation period is required. 
It is recommended to compare the CTH data derived from 
satellite measurements -besides the radar data- with other 
calculations. 
ACKNOWLEDGEMENTS 
This study was partially supported by the National 
Comity for Technological Development (OMFB). 
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Derrien M., Lavanant L. and LeGléau H. (1988): 
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clouds with AVHRR. Proceedings of the IRS'88 
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Menzel W.P., Smith W.L. and Steward T.R. (1983): 
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Nieman SJ. et al. (1997): Fully Automated Cloud 
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American Meteorological Society, Vol.78, No.6, 
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Nieman S.J., Schmetz J. and Menzel W.P. (1993): A 
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Intemational Archives of Photogrammetry and Remote Sensing. Vol. XXXII, Part 7, Budapest, 1998 743