Sensitivity of the Height Allocation of Thin Cloud Tracers to Errors in Satellite Calibration

The paper discusses the sensitivity of the height assignment of optically thin cirrus clouds to satellite calibration errors. Thin cirrus are often used as tracers for the derivation of high-level wind vectors. The height assignment is based on bi-spectral techniques, which in the case of Meteosat use both the IR (10.5 12.5 μm) and WV (5.7 7.1 μm) channels. Height allocation errors for typical errors in calibration are of the order of 500 m. It is shown that a high bias in the calibration, implying a lower height assignment, leads to better wind quality. This can be explained by the fact that wind shear through optically thin cirrus can be substantial over the typical cloud geometric thickness of a few kilometers. Thus cloud tracking of optically thin cirrus rather represents the displacement of volume and thus does not correspond to winds at cloud top.