In Figure 3 the inter-comparison between the AATSR CTH and 
CALIOP CBL are presented. The same number of inter- 
comparisons were made. The CALIOP CBLs appear to show 
better agreement with the AATSR CTH measurements, though 
some under-estimation is still apparent. The bias between the 
measurements confirms this at -0.77 km. The RMSE is 1.36 km 
and the coefficient of determination is 0.71. 
AATSR CTH vs. CALIOP CBL 
  
  
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AATSR CTH (KM) 
Figure 3. This Figure presents the results of the 
inter-comparison between AATSR CTH and 
CALIOP CBL for the transect presented in 
Figure 1c. 
S. DISCUSSION 
The inter-comparison of the AATSR CTH measurements with 
the CALIOP CTL measurements appears to show that, in 
general, the AATSR stereo retrievals are underestimating the 
height of clouds. The bias demonstrates that CALIOP typically 
retrieves heights which are 2.5km above those retrieved by 
AATSR. Further doubt in the AATSR measurements is given 
by the RMSE (2.76km) and R? (0.54) scores, both of which 
show poor agreement between the height retrievals. The cause 
of the poor agreement is likely due to a number of different 
effects. Firstly, CALIOP is more sensitive to the actual top of 
the cloud; AATSR returns the height of the cloud where the 
optical thickness becomes sufficient to provide enough texture 
within the image for the stereo matcher to perform effectively. 
À second cause is likely to be wind displacement of the clouds 
in the along-track direction. This displacement can affect the 
accuracy of the stereo CTH retrieval by modifying the relative 
parallax between image acquisitions. To create noticeable 
effects at pixel level accuracy, the wind speed must be at least 
8m/s! (Prata and Turner, 1997), and previous studies have 
shown that the only region where these wind speeds occur with 
regularity in the meridional axis, which typically effects CTH 
estimation, is over Greenland (Muller er al., 2007). 
In order to assess whether winds are affecting the validation, 
coincident ECMWF single layer meridional wind profiles have 
been extracted and are presented in Figure 4. The collocated 
study region is approximated by the black transect running 
through Figure 4. The wind speeds are at most + 2 m/s”! in the 
meridional direction; therefore they are unlikely to be 
influencing the results of the validation. More likely is the 
higher sensitivity of CALIOP to the true cloud top. 
To assess whether AATSR is better able to detect the bulk of 
the cloud a second inter-comparison was made against the 
CALIOP CBL. The results for this are presented in Figure 3. 
The scatter plot shows much better agreement between AATSR 
CTH and CALIOP CBL with far fewer outliers present. The 
statistical analysis of the inter-comparison again shows a low 
bias present in the AATSR CTH retrievals, with the CALIOP 
CBL being around 0.8 km higher in general. The RMSE is still 
quite high and above 1km at 1.36km, but the R? value is much 
improved at 0.71, showing much better agreement between 
AATSR CTH and the CALIOP CBL. 
From this limited validation it appears that AATSR retrieves 
lower CTH heights than the CALIOP instrument, indeed 
retrieving heights which are below the base layer of the detected 
CALIOP clouds. The cause of this is undetermined currently. 
More extensive inter-comparisons are required to better under- 
stand the processes at work. Indeed this inter-comparison is 
limited, so is unlikely to be effectively representing the 
relationship between the AATSR and CALIOP CTH retrievals. 
noitOv 10 metre vind component (m s**-1) 
Mean 4.74846 Max 6.18825 Min 12.4446 
  
  
Source: British Atmospheric Data Centre 
Figure 4. This Figure shows the meridional 
wind speeds for the collocation region. The 
approximate path of the CALIOP instrument in 
Shoe by the black transect. All units are in 
m/s”. 
6. CONLCUSIONS 
This work has demonstrated the application of the Census 
stereo matching algorithm to a month of AATSR data over 
Greenland. Of the scenes processed, one was selected to be 
inter-compared with the CALIOP LiDAR instrument for 
preliminary validation purposes. Whilst the validation is very 
limited, some good agreement is found between the CALIOP 
cloud base layer and the AATSR cloud top height. Further 
work is required to more extensively validate the Census stereo 
outputs; however the initial results do show some promise. 
Once fully validated a processing chain is in place to process 20 
years worth of ATSR cloud top height data over Greenland for 
improving the understanding of clouds in this region, which is 
challenging for current retrieval methods. 
7. REFERENCES 
Ambach, W. 1974. The influence of fractional cloud 
cover on the net radiation balance of a snow surface with 
high albedo, J. Glaciol., 67, 73-84. 
Bhat, D., and S. Nayar. 1998. Ordinal measures for image 
correspondence,” IEEE Transaction on Pattern Analysis and 
Machine Intelligence, vol. 20, no. 4, pp. 415-423. 
   
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