Aerosol retrievals from METEOSAT-8 - CM SAF

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SAF on Climate Monitoring Visiting Scientists Report Doc. No: 1.0 Issue : 1.0 Date : 4 October 2006 way that the group will identify unambiguously specific cloudy or clear sky condition. Each group may contain tests aimed at detecting clouds or rejecting unphysical clear sky conditions. The different groups are then arranged in such a way that the independence between the tests is maximized. A different weight is assigned to each group of cloud detection tests. Additionally, groups have been implemented to specifically detect clear sky conditions and a similar weighting is applied. Finally, the results of all the tests and the sum of the weights are used to generate a summarized cloud mask that includes four confident levels: clear certain, clear uncertain, cloud uncertain and cloudy certain (similar to the cloud mask summary provided by the operational MODIS cloud mask product MOD35). Statistics on clouds and then on clear sky pixels available for aerosols detection are presented in this appendix based on results from this cloud detection scheme. Over the entire disk as observed by SEVIRI, the cloud mask of MODIS Aqua at 1kmx1km resolution (overpass at 14:30 UT) has been built from the composite of different orbits (see Fig. 1). In this map four classes of pixels can be found: clear confident in white, probably clear in red, probably cloudy in purple and confident cloudy in black. Only pixels flagged as clear confident are kept for aerosols retrievals. The histogram presented in Figure 2 shows the number of pixels which can be used for MODIS to derive aerosols properties. Almost 38% of the pixels are flagged as clear confident. Figure 3 shows the cloud mask from SEVIRI for the same day at 14:30 UT over the entire disk. Four classes are possible: clear certain (in light blue), probably clear (in yellow), probably cloudy (in red) and confident cloudy (black). Whereas MODIS AQUA observes only once a location the afternoon at 14:30UTC (MODIS TERRA overpass is morning at 10:30UTC), MSG/SEVIRI can observe every 15 minutes the same location. Taking into account the time period 12:00-16:00 UTC, 16 observations of the same location are then available. One can note that we can compare with MODIS TERRA using the period 08:00-12:00 UTC. Figure 4 shows the number of clear sky observations for each SEVIRI pixels. Essential information is the spatial coverage of the product. The spatial coverage of the aerosols products from MODIS-AQUA and SEVIRI can be compared in Figure 5. These figures also indicate how rich is the set of useful measurements available from SEVIRI to derive aerosols - 44 -
SAF on Climate Monitoring Visiting Scientists Report Doc. No: 1.0 Issue : 1.0 Date : 4 October 2006 properties. As an example, areas in green are areas found cloudy with MODIS where at least one clear sky observations is obtained with SEVIRI. For these cases, an aerosol product can be derived from SEVIRI but not from MODIS. It implies that the spatial coverage increases with SEVIRI compared to MODIS. Areas in light blue are areas where the retrieval of an aerosol product is possible with MODIS and SEVIRI but SEVIRI can offer better information because more than 7 clear sky observations can be used. Here SEVIRI is able to get a sample of the aerosol retrieval. In opposite, areas in purple are areas found always cloudy by SEVIRI whereas it is cloud free using MODIS. These areas are mainly at the border of the disk for large values of the viewing angles. It is more important for the east part because of the afternoon time (it will be the opposite corner -east border - when taking the morning period 08:00-12:00 UT). The difference in cloud contamination comes mainly from two reasons: 1 MODIS has a better spatial resolution and it can be easier to see through fields of clouds. In particular that is the case for the scattered clouds (fields of very small clouds) in the south hemisphere on the west of South Africa. 2 SEVIRI and its temporal resolution allow the observations of moving clouds. An area covered by clouds at one time can be then clear few minutes or hours later. Finally the cumulative histogram of Figure 7 shows the statistical distribution of clear sky observations available from SEVIRI for the entire disk. About 42% of the disk is always cloudy. So at least, one and more clear sky observations are available for 57% of the pixels. One can also see that 16 clear sky observations are available for about 7% of the pixels. Based on this comparison with MODIS-AQUA, the spatial coverage of an aerosol product would be about 57% for SEVIRI when it would be only 38% for MODIS. Moreover, targeting the same spatial coverage as MODIS (i.e. 38%), SEVIRI would be able to provide 5 times more observations than MODIS. As a preliminary conclusion (we only studied the 17 th August 2005), MSG/SEVIRI looks to be a real contribution for an aerosol product for the areas around the centre of disk whereas it looks tricky to use it for areas near the borders of the disk. - 45 -
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Aerosol retrievals from METEOSAT-8 - CM SAF

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