Multichannel analysis of correlation length of SEVIRI images around ground-based cloud observatories to determine their representativeness

摘要

Images of measured radiance in different channels of the geostationaryMeteosat-9 SEVIRI instrument are analysed with respect to therepresentativeness of the observations of eight cloud observatories in Europe(e.g. measurements from cloud radars or microwave radiometers). Cloudysituations are selected to get a time series for every pixel in a300 km × 300 km area centred around eachground station. Then a cross correlation of each time series to the pixelnearest to the corresponding ground site is calculated. In the end acorrelation length is calculated to define the representativeness.It is found that measurements in the visible and near infrared channels,which respond to cloud physical properties, are correlated in an area with a1 to 4 km radius, while the thermal channels, that correspondto cloud top temperature, are correlated to a distance of about20 km. This also points to a higher variability of the cloudmicrophysical properties inside a cloud than of the cloud top temperature.The correlation length even increases for the channels at 6.2,7.3 and 9.7 μm. They respond toradiation from the upper atmospheric layers emitted by atmospheric gases andhigher level clouds, which are more homogeneous than low-level clouds.Additionally, correlations at different distances, corresponding to the gridbox sizes of forecast models, were compared. The results suggest thepossibility of comparisons between instantaneous cloud observations fromground sites and regional forecast models and ground-based measurements. Forlarger distances typical for global models the correlations decrease,especially for short-wave measurements and corresponding cloud products. Bycomparing daily means, the correlation length of each station is increased toabout 3 to 10 times the value of instantaneous measurements and also thecomparability to models grows.