Cloud-top equivalent black-body temperatures and tropical cycloneforecasting in the Australian region

The detailed cloud analysis around a tropical cyclone, required to forecast the direction of motion of the cyclone centre on a short-term basis (see Lajoie and Nicholls 1974), is sometimes difficult to perform in visible or infrared satellite cloud imagery when a cirrus canopy masks the low cloud structure. The cloud analysis then becomes largely subjective and this can lead to appreciable forecasting errors. However, an objective analysis of the equivalent black-body temperature of cloud tops, as obtained from digitised infrared data, can be used to obtain a semi-objective cloud distribution of cirrus and penetrative convection around the tropical cyclone. From 46 consecutive three-hourly analyses the evolution of the outer cloudband around tropical cyclone Kerry is followed. It is shown that the cyclone changes its direction of motion twelve to fifteen hours after changes in the bearing from the cyclone centre of the downstream end of the outer cloudband, in agreement with Lajoie and Nicholls’s forecasting guidelines. One hundred other analyses for seven other tropical cyclones have been examined and show similar relationships between the bearing of the downstream end of the outer cloudband and the direction of motion of the cyclone.