Surface wind fields of Antarctic mesocyclones derived from ERS 1 scatterometer data
The effectiveness of ERS 1 scatterometer data as a tool for studying the wind fields of Antarctic mesocyclones is analyzed by using 1 year's observations of these systems derived from satellite imagery encompassing the Antarctic Peninsula and Bellingshausen and Weddell Seas. It is shown that the scatterometer processing scheme utilized can affect the resultant wind fields, particularly the accuracy of the external forecast winds used for “meteorological ambiguity removal.” Very few of the mesocyclones (9%) were “captured” by the scatterometer data because (1) ∼60% of the vortices did not form over the open ocean (25% of those that did were included in the scatterometer data) and (2) the narrow scatterometer swath results in a poor temporal resolution that is less than ideal for observing small-sized, short-lived Antarctic mesocyclones over the Southern Ocean. Close agreement between the derived wind fields and those predicted by a simple model suggests that a mesocyclone may be considered as a vorticity anomaly superimposed on a uniform flow. Most systems had a surface circulation, varying from a slight deviation from the geostrophic flow to a closed circulation; however, 25% of mesocyclones did not exhibit a surface circulation. Four case studies are used to illustrate the different types of observed circulation. Derived maximum cyclonic relative vorticity values (those most negative in the southern hemisphere) varied between −100 and −400×10−6 s−1, an order of magnitude lower than that calculated for an Arctic mesocyclone. The scatterometer wind field data represent an important new tool for studying Antarctic mesocyclones but need to be used in conjunction with sensors having a higher frequency of coverage to allow multitemporal analyses of such systems throughout their development.