Determination of cloud top amount and altitude at high latitudes

Cloud identification over snow and ice has proved to be a difficult process to automate due to the similarity in their visible and thermal properties. A method is described here which utilises the nadir and forward views of the Along Track Scanning Radiometer to determine the height of the surface observed to a nominal accuracy of ±1000 m (pixel resolution). A digital elevation model allows the surface topography to be ascertained, with the remainder of high elevation features classed as cloud. Visual verification of the resulting cloud masks indicates that the stereo-matcher identifies 10-20 % more cloud than is recognised by the human eye, but nearly 100 % of cloud-free land identified by stereo-matching was confirmed by examination of the images. Further validation of the cloud top heights using radiosonde data, indicates 73 % of the stereo-matched heights to be within 500 m of the radiosonde predicted cloud tops.