1. An object-based analysis of cloud motion from sequences of METEOSAT satellite data
- Author
-
Newland, Franz Thomas
- Subjects
Astrophysics::Galaxy Astrophysics ,Physics::Atmospheric and Oceanic Physics - Abstract
The need for wind and atmospheric dynamics data for weather modelling andforecasting is well founded. Current texture-based techniques for tracking cloudsin sequences of satellite imagery are robust at generating global cloud motionwinds, but their use as wind data makes many simplifying assumptions on thecausal relationships between cloud dynamics and the underlying windfield. Thesecan be summarised under the single assumption that clouds must act as passivetracers for the wind. The errors thus introduced are now significant in light ofthe improvements made to weather models and forecasting techniques since thefirst introduction of satellite-derived wind information in the late 1970s. In thattime, the algorithms used to track cloud in satellite imagery have not changedfundamentally. There is therefore a need to address the simplifying assumptionsand to adapt the nature of the analyses applied accordingly.A new approach to cloud motion analysis from satellite data is introduced in thisthesis which tracks the motion of clouds at different scales, making it possibleto identify and understand some of the different transport mechanisms presentin clouds and remove or reduce the dependence on the simplifying assumptions.Initial work in this thesis examines the suitability of different motion analysistools for determining the motion of the cloud content in the imagery using afuzzy system. It then proposes tracking clouds as flexible structures to analysethe motion of the clouds themselves, and using the nature of cloud edges toidentify the atmospheric flow around the structures.To produce stable structural analyses, the cloud data are initially smoothed. Anovel approach using morphological operators is presented that maintains cloudedge gradients whilst maximising coherence in the smoothed data. Clouds areanalysed as whole structures, providing a new measure of synoptic-scale motion.Internal dynamics of the cloud structures are analysed using medial axis transformsof the smoothed data. Tracks of medial axes provide a new measure ofcloud motion at a mesoscale. The sharpness in edge gradient is used as a newmeasure to identify regions of atmospheric flow parallel to a cloud edge (jet flows,which cause significant underestimation in atmospheric motion under the presentapproach) and regions where the flow crosses the cloud boundary. The differentmotion characteristics displayed by the medial axis tracks and edge informationprovide an indication of the atmospheric flow at different scales.In addition to generating new parameters for measuring cloud and atmosphericdynamics, the approach enables weather modellers and forecasters to identify thescale of flow captured by the currently used cloud tracers (both satellite-derivedand from other sources). This would allow them to select the most suitabletracers for describing the atmospheric dynamics at the scale of their model orforecast. This technique would also be suitable for any other fluid flow analyseswhere coherent and stable gradients persist in the flow, and where it is useful toanalyse the flow dynamics at more than one scale.
- Published
- 1999