Summary
The Benefits of Ensemble Prediction for Forecasting an Extreme
Event: The Queensland Floods
In February 2019, northern Queensland
experienced an extreme weather system with cold temperatures, high rainfall, and
flooding. The event had serious impacts on agriculture including the mortality
of over 620,000 head of cattle and maximum inundation of around 200,000 km2
of land surface.
Researchers at the UK Meteorology Office
(Met Office) and the Bureau of Meteorology used a set of several forecasts (ensembles),
to investigate the predictability of this event. Ensemble forecasts are used to
understand forecast uncertainty and the likelihood of possible events. This
makes them valuable for identifying extreme events. The ensembles used in this
study are based on those from the Met Office’s numerical weather prediction (NWP)
system. There were 25 members in each ensemble, and they were used to
investigate the multiweek predictability of the event, in terms of precipitation
and temperature. The researchers also investigated forecast improvements gained
by increasing resolution, using a newer version with better representation of
physics, or using a model where the dynamics of the atmosphere and ocean are
connected.
The NWP had reasonable skill in
predicting the event, with the ensembles providing a strong indication of an
event at the limits of the observed record. Using a model with interactions
between the atmosphere and the ocean improved the prediction of the trajectory
of the low-pressure system. Thus, atmospheric-only models had greater biases in
the distribution of rainfall across northern Queensland. In contrast,
increasing the resolution and improving the physics of the atmospheric model
did not improve the forecast skill for this event. Further repeated analyses of
this technique over many cases with probabilistic evaluation is required before
this method can be used for regular forecasting.