Prediction of extreme precipitation associated with landfalling tropical cyclones

A major problem associated with landfalling tropical cyclones (TCs), whether or not they are low or high category TCs, is their potential for producing extreme rainfall amounts. Over much of Australia in the 1999/2000 season, record rainfall totals were generated by a series of landfalling tropical cyclones. These TCs ranged from Category 1 to Category 5, but the rainfall totals generally differed by less than a factor of two, averaged over the storms of interest here.

Clearly, aspects such as proximity to the coast, duration near the coast and additional rainfall enhancing factors such as orography, favorable SSTs, saturated watershed areas and mid-latitude interactions would all play a vital role in determining the rainfall intensities and amounts.

In this study we examine a total of 20 case studies and corresponding model simulations of landfalling TCs, to assess the significance of the model results in terms of QPF. It was found that for the majority of the storms the skill level was encouragingly high, based on a comparison of areal averaged forecast rainfall amounts with observed totals.

Finally, sensitivity experiments have been carried out on a subset of 5 tropical cyclones that produced extreme rainfall amounts. The approach adopted was to use an adjoint model to determine sensitivity patterns for a wide range of initial model variables in an attempt to assess the primary factors responsible for the extreme rainfall totals.