A refinement of cyclone tracking methods with data from FROST

The Antarctic First Regional Observing Study of the Troposphere (FROST) project had three one-month Special Observing Periods (SOPs) during which the commitment was made to ensure that all additional data collected were passed on via the Global Telecommunication System (GTS) to operational centres for use in the construction of the analyses. These analyses can be regarded as the best available for these times of year, given the special effort to include additional data south of 50°S during these periods. The availability of these high-quality analyses has stimulated us to refine the Melbourne University numerical cyclone tracking algorithm, with additional synoptic guidance gained from a manual analysis of southern hemisphere cyclones in the winter SOP (July 1994). Using the refined scheme we have compiled and compared statistics of cyclone tracks obtained objectively from the Australian GASP (Global Assimilation and Prediction) system analyses and manually from semi-independent analyses. Our results show that the cyclones found by the numerical and manual approaches bear considerable similarity to each other, even for complex systems for which such unanimity might not have been expected. In general, the automatic algorithm tended to 'find' more systems than did the manual analyst, with these extra systems being predominantly those identified as weak and/or open. The results emphasise the difference in perception of what constitutes a low. The overall behaviour of cyclones revealed by the objective scheme in July 1994 was consistent with that identified in various climatologies in that many systems were generated in the western part of the ocean basins and moved to the east and, to a lesser extent, to the south. A concentration of tracks was found just to the north of the Antarctic continent. On the other hand, this specific month was anomalous in a number of respects; this was reflected in the nature and distribution of cyclone activity. The consistency of the findings with those of an experienced, practising synoptician means that the state-of-the-art numerical algorithm can be applied to numerical analyses and model output with confidence.