Velocity Determination of an Image Feature
It is useful to be able to measure the velocity of a meteorological feature. In satellite imagery such
measurements are possible when cloud features persist over multiple images. This example is based
on an 8-frame loop showing the thunderstorm which produced tornadoes in eastern CO and western
KS. In most cases, storm motion is complex. If one tracks the overshooting top, the resultant velocity
turns out to be from 32 degrees at 12 kts. In this case, the storm is backbuilding to the southwest along
the flanking line. However the overall cluster of storms is moving southeastward, from about 305 degrees.
In tornadic storms the most active area is usually where the flanking line joins the main storm (red dot in figure).
When this intersection point is tracked, the resultant motion is from 326 degrees at 31 kts. That is, the propogation
velocity of that point is much more rapid than certain individual storm features. Of course, these velocity measurements are estimates. Variations in cloud velocity and morphology will introduce uncertainty in the measurement. Other examples of features whose velocities may be estimated by satellite are shortwaves, fog/stratus cloud decks, and fronts.
Once a feature can be assigned a velocity, it is then possible to calculate the arrival time of that feature at a particular point, or to extrapolate the feature's location ahead over a given amount of time.
