storm_speed and storm_direction convention

[Keenan Fryer]

Hello, 

For the IBTrACS-algorithm-derived fields "STORM_DIRECTION" and "STORM_SPEED" I am wondering what convention is used in terms of "here to there" or "there to here". In other words, for a given valid time we have a single value for both of these variables - do those values represent the storm movement vector calculated using the previous position and the current position?

Thanks!

[Ken Knapp - NOAA Federal]

Hi Keenan

Great question on storm motion! 

In short, the motion / direction represent a mean state at the time of observation.

The longer explanation follows:

The problem with these types of calculations is that they generally represent a difference. So if you have two positions, then you can calculate one value from two values (one speed from two positions). This makes it difficult when trying to assign speed & motion for each point. For example, one could have speed at time N (v(n) be based on the vector from n-1 to n (V(n) = P(n)-P(n-1) / time). As you can see, this would leave one value (V(0)) with no value.

To combat this, we use midpoints. Consider a storm with 3 points (this will demonstrate the shortest example, from which you can see how we derive this for any storm): A, B and C. There are two midpoints: one between A and B and the other between B and C. You can see it this way:

A -----+----- B -----+----- C

      AB             BC

The midpoints between the points in the storm are interpolated. The speed at A is then calculated as [Position(AB) - Position(A)] / time (where time is actually half the time delta since we interpolated to the midpoint). So for the first position, the speed has some lag (because we don't know where it was before the first point. Then the speed at B is calculated as the difference of the mid-points: V(B) = (P(BC)-P(AB)) / time. This represents the mean speed and direction centered on B. So it is most representative at B. This can be done N times for a storm with N+2 points (meaning it can be done for each time excluding the end points). Then for the Last point (in this case, C), we use the midpoint to the end point: V(C) = (P(C) - P(BC)) / time. 

So to answer your question: the here to there points are the midpoints between the storm's points. This allows the values to be representative at the location. The effect, though, is that the velocities are calculated with slightly less information at the endpoints.

I hope this helps-

-Ken

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Ken Knapp,

NOAA/NCEI/CSDB

828-271-4339

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[Keenan Fryer]

Perfect! Thank you for the explanation!