WIIS - Cell Tracking and Extrapolation

User Perspective

After selecting the Extrapolation option cells in the current weather radar image are identified and displayed. Parameters for cell recognition such as intensity and minimum cell area can be modified and changes are displayed immediately.
Now the cell search is started on a over a set of images. For the identified cells parameters like position, size and intensity are extracted and time series are calculated and extrapolated into the future.
The cells are displayed for a selectable time in the past or a extrapolated position in the future.


Optional pre-processing of the weather radar image with different filter functions (average, median, erosion, dilation). The filters lead to a smooth outline of the cells (average, median) or increase (dilation) or reduce (erosion) the effective area of the cells.
Contiguous regions of image pixels exceeding an adjustable intensity form a cell. Regions with an area below a minimum size are not taken into account.
Calculation of position and expansion, contour and area as well as a simplified elliptic representation.


Finding matches between cells of consecutive images: with combinatorial methods the cell assignments are optimised for minimum distance and area difference. Plausibility test on cell movement and area differences are applied. Optionally a calibration with velocities measured by the weather radar is used.
Development of time series for the found cells: pair-wise matching cells are tracked over the selected set of images and the cell movement as well as size changes are analysed. Special cases due to cell splitting merging are handled. A missing radar image in the time series is replaced by a forecast from the preceding images.


From the time series movement vectors and area changes are determined for the actual image. A exponentially weighted averaging method is used, so that newer images contribute more to the result than older.
The forecast is made via linear extrapolation of the movement vectors and size changes. It is assumed that precipitation cells normally move along a straight line and size changes follow a linear trend (random departures from this behaviour occur).

Last update: May 2011