Performance of weather models in the lead-up to Auckland Anniversary storm was poor, review finds
The MetService review found an urgent need to develop advanced forecasting methods for extreme rainfall events. A review by the MetService has found that the performance of weather models in the lead-up to the Auckland Anniversary Storm and Cyclone Gabrielle last summer was poor, causing four deaths and causing an estimated $9 billion to $14.5 billion in damages. The review found that weather models performed poorly for the Auckland storm, while the MetServivce severe weather team significantly improved their models. The storm was driven by a persistent north-easterly flow of sub-tropical air over the upper North Island, causing severe urban flooding, slips, and widespread damage to housing and infrastructure. Despite these issues, the review highlighted a need for advanced forecasting methods and proposed changes to the severe weather warning system.
Опубликовано : 2 года назад от Michael Daly в Weather
A MetService review of its performance during the Auckland Anniversary Storm and Cyclone Gabrielle last summer, has found an urgent need to develop advanced forecasting methods for extreme rainfall events.
Four people died as a result of the Auckland storm, and 11 during cyclone Gabrielle, while between them the two events caused an estimated $9 billion to $14.5 billion in damages.
The Auckland Anniversary Storm, on January 27, was caused by a persistent line of thunderstorms with inherently weak predictability, making the forecasting of the event “exceptionally challenging”, the review said.
The storm was driven by a persistent north-easterly flow of sub-tropical air over the upper North Island.
Within that moisture-laden flow, a line of thunderstorms formed over the Auckland region during the afternoon and persisted until mid-evening.
During the peak of the storm, over Auckland, rainfall rates of up to 80mm an hour, and up to 180mm in 3 hours, were recorded in some places, causing severe urban flooding, slips, and widespread damage to housing and infrastructure.
Coromandel Peninsula, Gisborne and Hawke’s Bay were particularly hard hit from February 12-14 by Gabrielle, which was likely to be regarded as one of this country’s worst storms, the review said.
It found weather models “generally performed poorly” for the Auckland storm, while the MetServivce severe weather team “significantly improved on the models and added significant value”.
Despite that, forecast rainfall amounts and intensities were lower than actual amounts, and extreme impacts were not anticipated before the storm started.
“Performance of all available weather models in the lead-up to the Auckland Anniversary Storm was poor,” the review said.
The storm was a “hybrid” event, with characteristics of both localised convective and broad-scale severe weather, and such hybrid events tend to be forecast poorly.
For Cyclone Gabrielle, the performance of the warning system in the Gisborne area was “very good”, and forecasters improved on model rainfall amounts, providing excellent lead time for northern Gisborne, the review said.
Overall performance of the warning system in the Hawke’s Bay area for Cyclone Gabrielle was moderate.
While lead time, start time and duration in Hawke’s Bay were well handled, the performance of both the models and the MetService severe weather team was poor with respect to the location of the extreme rainfall amounts near Esk Valley.
Conditions appeared to have set up a very strong low-level flow of moisture over Hawke’s Bay, but forecasters did not have access to the tools that could have made them aware of the difference between that flow, compared to the cooler and less moist flow over Gisborne.
Along with the limitations highlighted by the Auckland storm in forecasting capability for hybrid events, Gabrielle had highlighted a need for more guidance for dealing with rainfall processes within atmospheric rivers, the review said.
“There is an urgent need to develop advanced forecasting methods that address these challenges in the New Zealand environment,” the review said.
The Auckland storm also showed the limitations of the MetService warning system during complex hybrid weather systems.
A very large number of warning messages were issued, with sequences of overlapping watches and warnings from both the severe thunderstorm and broad-scale warning sub-systems.
“It is very unlikely that any user – even a user with specialist technical knowledge – would have been able to make full sense of the collective outputs of the warning system,” the review said.
Substantial changes were proposed, aimed at simplifying the severe weather warning system, increasing the clarity and reach of the message, and communicating the “rareness” of an event.
Research and development was also needed to improve the local weather models used by MetService meteorologists. Forecasts and warnings had suffered from inaccuracies in the predictions from those models, the review said.
The models were relied on for high-resolution details in rainfall predictions, related to complex terrain and small-scale atmospheric effects, which were critical for weather and subsequent flood warnings.