Uncertainty in Scatterometer-Derived Vorticity

A more versatile and robust technique is developed for using QuikSCAT surface vectors to determine surface vorticity of tropical disturbances. The improved technique is discussed in detail as compared to two previous studies by Sharp et al. (2002) and Gierach et al. (2007). Three types of errors are explored: random observational error, truncation error and mismatches in the spatial averaging scale. Observational errors are random and are due to the instrument and natural variability. It is estimated as a worst-case scenario. Truncation error associated with the assumption of linear changes between wind vectors is more complicated, thus for accurate results it must be estimated on a case-by-case basis. An attempt is made to determine the approximate impact of the truncation error by using composites of tropical disturbances, though this calculation is an underestimation of the actual error. The third type of error discussed is due to the size of the area being averaged. If there is a vorticity maximum at the center of this area (away from the edges), it will essentially be “smoothed” over and the impact reduced. The same tropical disturbance composites mentioned above are used to examine this error. The discussion of these errors will benefit future projects of this nature as well as future satellite missions.