I don't understand the following:
F = 1/tf * V
F... vector quantity, represents the friction
tf ... friction timescale
V ... velocity of an air parcel V= (u,v,w), also called wind velocity
In the Article "Ray tracing volume densities" Kajiya and Von Herzen (1984) writes:
"Frictional effects are approximated by a simple relation yielding an exponential decay of wind velocities with time"
(the wind velocities are described by a simplified equation of motion and also
change at every time step)
Why is this a exponential decay ?!
hope you can help me