CLOUD-TOP PHASE DETERMINATION FROM THE FUSION OF SIGNATURES IN DAYTIME AVHRR IMAGERY AND HIRS DATA

An improved methodology for the retrieval of water vapour profiles fro m DMSP SSM/T-2 microwave sounder data has been demonstrated using clou d-top temperatures derived from NOAA AVHRR imagery as a constraint. Ho wever, the automated analysis of cloud-top temperature in AVHRR imager y is complicated by the presence of optically-thin cirrus clouds, sinc e a component of the upwelling radiation from below passes unattenuate d to space. Therefore, cloud-top phase must first be determined to ens ure the accurate specification of cloud-top temperature. In this paper , a new approach is presented for the specification of cloud-top phase in an operational environment. The methodology combines results from bi-spectral cloud tests for ice and water clouds in daytime AVHRR imag ery with cloud-top pressure analyses based upon the CO2 slicing of HIR S data. The accuracy of the automated cloud-top phase analyses is meas ured quantitatively against manual analyses of the AVHRR imagery. It i s concluded that the fusion of cloud signatures in AVHRR imagery and H IRS data improves the specification of cloud-top phase in the higher r esolution imagery and reduces the ambiguity inherent in analyses based solely upon bi-spectral techniques.