The Winchelsea Convergence – using radar and mesoscale NWP to diagnose cool change structure

The forecasting of the passage of wind changes associated with dry cold fronts is vital to safe firefighting operations in southeastern Australia (Cheney et al. 2001), and the issuing of ‘Wind Change Forecast Charts’ is an important part of the Victorian, South Australian and New South Wales Regional Forecast Centres’ fire weather forecast product suite. While weather radars are most commonly used for monitoring hydrometeors, it has long been recognised that low-reflectivity echoes can be received in clearair environments, and that these are frequently associated with convergence lines. Wilson et al. (1994) discuss these echoes, and conclude that they are generally caused by reflections from insects trapped in the ascending air in these convergence lines, but may at times be a result of refractive index variance at the convergence line. Dickins and Cusworth (2001) showed several examples of low-reflectivity weather radar line echoes in the South Australian Gulfs region that were associated with significant dry cool changes, and suggested that these observations may be valuable in fire weather forecasting. The Western Australian Regional Forecast Centre has also shown the potential for this form of data during the westcoast trough passage during the Gin Gin fire of 20 December 2002. Aust. Met. Mag. 55 (2006) 47-58