velocity folding.) A basic sampling problem that arises when the unambiguous velocity
sampling interval is less than the full range
of naturally occurring velocities, causing the erroneous appearance of higher velocities within the sampling interval.
This phenomenon occurs in Doppler velocity
measurements when the maximum unambiguous velocity
) is less than the full range of velocities being measured. Any true velocity, V
, appears within the interval from -Vmax
, with the value V
′, which is related to the true velocity by V
′ ± 2n'Vmax where n is an integer. Therefore a given measured velocity V′ may be caused by many values of the true velocity V. For example, suppose Vmax = 25 m s-1 and the measured velocity V′ = -15 m s-1. Then the values of true velocity that could account for this measurement are the following: -15 m s-1 (for n = 0); +35 or -65 m s-1 (for n = 1); +85 or -115 m s-1 (for n = 2); etc. In some instances the erroneous velocities can be recognized and ambiguities resolved by additional considerations, such as the requirement of spatial continuity of the velocity field.
See also aliasing, Nyquist frequency.
Copyright 2022 American Meteorological Society (AMS). For permission to reuse any portion of this work, please contact firstname.lastname@example.org. Any use of material in this work that is determined to be “fair use” under Section 107 of the U.S. Copyright Act (17 U.S. Code § 107) or that satisfies the conditions specified in Section 108 of the U.S.Copyright Act (17 USC § 108) does not require AMS’s permission. Republication, systematic reproduction, posting in electronic form, such as on a website or in a searchable database, or other uses of this material, except as exempted by the above statement, require written permission or a license from AMS. Additional details are provided in the AMS Copyright Policy statement.