Sub-scale cloud detection using the HRV channel of MSG

Abstract

Methods and potentials for an improved detection of clouds and a better determination of their properties by means of the high resolution visible (HRV) channel of MSG/SEVIRI are explored. First, as a fundamental prerequisite for synergetic studies with other spectral channels, it is shown that a correction of the order of 1.5 HRV pixels has to be applied to the nominal position of the HRV in both North and West direction. Then, the positive contribution of the HRV to the detection of small orographic clouds is shown. This, on its turn, enables us to determine the cloud cover of each low resolution pixel. This extra information can be used to correct the estimation of the optical depth of cloudy pixels. Finally, the genesis of a small convective cell is examined. By means of the HRV its formation time can be assessed more accurately than by merely using the low resolution channels. Time series of low resolution reflectances resp. brightness temperatures show that signals that precede or accompany cloud genesis can be hardly detected in the low resolution spectral ranges. Important tools used are an MSG simulator for the computation of satellite radiances based upon the radiative transfer package libRadtran and a retrieval algorithm for cloud optical depths and effective particle radii adapted from Nakajima and King [1990].