The vector positions are calculated by interpolating the daily wind fields to one hour time steps, and integrating with a fourth order Runge-Kutta method. The Runge-Kutta technique uses an adaptive time step, with a first guess of ten minutes. Horizontal divergence for surface air is calculated at the grid points as a second order finite difference. The divergence field is bi-linearly interpolated to the central position of each moving vector. Divergence is shown by the color of the wind vector.
Blue vectors indicate convergent flow, and red vectors indicate divergent flow.
The relative vorticity field is also calculated as a second order finite difference. The vorticity contours highlight the central position of storms, and help show the extent of flow modifications due to orographic features. The animations are examined to find new and interesting features. The NCEP fields have a resolution of only 2 latitude and 2 longitude. The main difference between the figures is the flow over the North Sea.