January 13, 2009

Comparisons of POLDER-2 Rayleigh and Oxygen pressure with MODIS and ISCCP cloud top pressure

Comparisons of POLDER-2 Rayleigh and Oxygen pressure with MODIS and ISCCP cloud top pressure



Figure 1: Monthly mean of POLDER-2 Rayleigh pressure for June 2003


Figure 2: Monthly mean of POLDER-2 Oxygen pressure for June 2003


Figure 3: Monthly mean of MODIS cloud top pressure (day mean) for June 2003


Figure 4: Monthly mean of ISCCP cloud top pressure for June 1997


Figure 5: Monthly mean difference between POLDER-2 Rayleigh pressure and
MODIS cloud top pressure (day mean) for June 2003


Figure 6: Monthly mean difference between POLDER-2 Oxygen pressure and
MODIS cloud top pressure (day mean) for June 2003

The comparison between POLDER-2 Rayleigh pressure and MODIS cloud top pressure (fig. 5) exhibits high discrepancies:
Low clouds (West coast of America) are often retrieved lower (~150 hPa) with Rayleigh pressure method but it's also the case for some high clouds as around India. Almost everywhere else the contrary is observed: Rayleigh pressure is lower (the clouds are higher) than MODIS cloud pressure (~100 hPa over sea, ~200 hPa over land).
Cloud top pressure observed by ISCCP (for June 1997, fig. 4) looks more like Rayleigh pressure for land surfaces (except Sahara). Over sea ISCCP cloud tops are retrieved still higher than POLDER-2 Rayleigh pressure retrievals.

POLDER-2 Oxygen pressure is not a cloud top pressure retrieval but more a mean cloud pressure indicator (see comparisons with groundbased data). Comparison of this mean cloud pressure with MODIS cloud top pressure (fig. 6) is then quite surprising since the Oxygen pressure is often near and even lower than MODIS cloud top pressure (that's would mean a cloud middle upper than the cloud top).

In addition to high discrepancies between POLDER-2 Rayleigh pressure and MODIS cloud top pressure, these comparisons seem to show that MODIS cloud top pressure are often too high (see fig. 7 for a summary)


Figure 7: Scheme of the expected and the observed results for POLDER2
Rayleigh and Oxygen cloud pressure, MODIS and ISCCP cloud top pressure.


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