Thursday, September 2, 2010
Hurricanes and Uncertainty
Now that Northwest weather has returned to the more usual and benign September fare, lets turn to hurricane forecasting.
An interesting factoid: My profession has made huge progress in predicting hurricane tracks, but there has been little headway with hurricane intensity predictions.
Here are the official statistics for Atlantic hurricanes. First, for track (position) error. Big progress, particularly for the longer-range predictions. The 120h forecast today has roughly the same skill as the 48-h forecast in 1970. That's progress!
The intensity forecasts have gotten worse (since 1970) at 24-h and are only slightly better at longer time ranges.
Now why this difference?
For track or position forecasts, we need to predict accurately the large-scale conditions around the storm. You can think of a hurricane as a spinning top that is being steered by the larger scale atmospheric environment. With more observations and better computer models we have gotten very good in determining this environment and therefore the track forecasts have dramatically improved.
But for intensity forecasts one must predict the inner workings of the storm and that is MUCH harder. First, you need to have enough information to determine the detailed three-dimensional structure of the storm...something we generally don't have. Then you must simulate the storm evolution and it takes very high resolution to model the eyewall and rainbands realistically. Current hurricane models are not run with sufficient resolution. And one more thing...we also need to correctly predict the interactions of the atmosphere with the tropical oceans, and right now we are not doing a good job at that. For all those reasons, we just don't do well with the intensity predictions.
But this could change quickly if sufficient resources were provided--including more powerful computers for the National Weather Service, better observations in hurricanes, and other realizable technological improvements. Unfortunately, the National Weather Service is acutely lagging behind in computer technology and a few weeks ago their request for an update of their main computer system was rejected by the Office of Management and Budgets. Our national weather prediction effort is in danger of falling to third-class status....but that subject should wait for another blog.
Posted by Cliff Mass at 9:47 PM