Uncertainty in scatterometer derived vorticity

Mark A Bourassa

doi:10.1109/igarss.2009.5417889

Abstract

A more versatile and robust technique is developed for determining area averaged surface vorticity based on vector winds from swaths of remotely sensed wind vectors. This technique is discussed in detail and compared to two previous studies that focused on early development of tropical systems. The error characteristics of the technique are examined in detail. Specifically, three independent sources of error are explored: random observational error, truncation error and representation error. Observational errors are due to random errors in the wind observations, and determined as a worst-case estimate as a function of averaging spatial scale. The observational uncertainty in vorticity averaged for a roughly circular shape with a 100 km diameter, expressed as one standard deviation, is approximately 0.5 × 10 -5 s -1 for the methodology described herein. Truncation error is associated with the assumption of linear changes between wind vectors. Uncertainty related to truncation has more organization in QuikSCAT data than observational uncertainty. On 25 km and 50 km scales, the truncation errors are very large. The third type of error, representation error, is due to the size of the area being averaged. This type of error is analogous to over-smoothing. Tropical and sub-tropical low pressure systems from three months of QuikSCAT observations are used to examine truncation and representation errors. Representation error results in a bias of approximately 1.5 × 10 -5 s -1 for area averaged vorticity calculated on a 100 km scale compared to vorticity calculated on a 25 km scale. The discussion of these errors will benefit future projects of this nature as well as future satellite missions.

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