Water Vapor Structure Displacements from Cloud-Free Meteosat Scenes and Their Interpretation for the Wind Field

Abstract

The evaluation of water vapor (WV) images taken by satellite-borne radiometers has become an essential source of data in modern meteorology. The analysis of structure displacements within sections of WV images is an effective way to get horizontal components of wind vectors. In contrast to intermediate-level and high-level clouds, the height assignment of displacement vectors connected with cloud-free WV structures needs additional information from atmospheric profiles. Nevertheless, interpreting these motion vectors as independent wind vectors and not simply matching them to any model wind field is tried. This contribution reports on the evaluation of structure displacements and how it can be done efficiently in the case of smooth and shallow scenes by the use of appropriate filters. Then, a series of height-assignment methods is tested for altitude-sensitive parameters such as wind shear and brightness contrast of pixels within segments used for structure tracking. The results are verified by comparison with reference wind data from the analysis of a weather prediction model as well as from radiosonde ascents. From a statistical point of view and for cases of strong wind shear it is clearly revealed that methods that use the effective brightness temperature, in particular from the coldest pixels, lead to better height assignment than do others that are based on the contribution function explicitly. From a series of individual cases it is found that the relative minimum of the difference between reference wind and clear-air structure displacement is not related uniquely to either the coldest (or warmest) pixel of the segment or the effective brightness temperature of pixels leading the tracking procedure. A cloud-free WV structure consequently contains information from an atmospheric layer whose displacement vector is a weighted sum over the motions within its specific thickness. Then the individual situation will determine whether a structure displacement reflects the motion at a well-defined height or whether it should be taken as a collective motion within an atmospheric layer that is an essential part of the upper troposphere.