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GDEST 2008 Conference Sessions

Theme 1 - Observing Africa
Cloud Height Determination and Comparison with Observed Rainfall by Using Meteosat Second Generation (MSG) Imageries
Peter Silla Masika, Kenyan Meteorological Department, Kenya

To obtain accurate estimates of surface and cloud parameters from satellite data an algorithm has to be developed which identifies cloud-free and cloud-contaminated pixels. Data from the Spinning Enhanced Visible and Infrared Imager (SEVIRI) on board Meteosat Second Generation (MSG) satellites have been available since February 2004. The data is accessible to National Meteorological and Hydrological Services (NMHSs).
This study attempts to utilize available MSG data for developing simple cloud mask and height algorithms and thereafter compare and determine the relationship between cloud height and observed rainfall on a ground station. A multispectral threshold technique has been used: the test sequence depends on solar illumination conditions and geographical location whereas most thresholds used here were empirically determined and applied to each individual pixel to determine whether that pixel is cloud-free or cloud-contaminated. The study starts from the premise of an acceptable trade-off between calculation speed and accuracy in the output data. For this reason, only three infrared channels of MSG satellite were used alongside climatological data provided by National Oceanographic and Atmospheric Administration (NOAA) and also land surface climatological data available from the WorldClim website.
The accurate measurement of spatial and temporal variation of tropical rainfall around the globe remains one of the critical unresolved problems in the field of meteorology. This study attempted to compare computed cloud height and observed rainfall on ground station (CGIS-Butare, Rwanda) and derived cloud height-total rainfall relationship from storms over the same station.
Results from the simple cloud mask algorithm were validated using EUMETSAT cloud mask products for a tropical region (≈ 11°N - 14°S and ≈ 6° - 51°E) over Africa. Overall accuracy of the simple cloud mask developed here was found to be 87% for four scenes which were during day- and night-time as well as twilight time as defined by sun elevation angles. Analysis of recorded rainfall at CGIS and comparison of the same with computed cloud height showed that rainfall mainly occurred when cloud heights were greater than 3000m. Further, deriving a relationship between the observed rainfall and the cloud height was found to follow a Gaussian model in which clouds at approximate heights between 4000m and 5000m produced higher amounts of rainfall. Below and above this height range, rainfall amounts were found to be generally low. The derived cloud height-total rainfall relationship was applied to other storms over this station. Initial results show low correlation between estimated and observed rainfall. More synoptic observations have to be used to evaluate the derived relationship. Next to this a better procedure to differentiate nimbostratus and cumulonimbus has to be incorporated. Different relations between height and observed rainfall for the two types of clouds may be derived which may improve the overall results.