The problems of hurricane intensity Last week, the International Space Station made several passes over the storm that grew into Hurricane Arthur. Astronaut Reid Wiseman shared these images on his twitter account; in the first image, the center of the storm is diffuse, but in the 2nd image taken about 12 hours later, the storm has intensified and formed a well-defined eye. This process illustrates one of the details still confounding weather forecasts involving these storms. Over the last decade, the forecasting of hurricane tracks has massively improved; the weather service can now predict where a hurricane will be 48 hours into the future better than they could predict where a hurricane would be 24 hours into the future a decade ago. That massive improvement in accuracy gives much-needed time to prepare for evacuations. Arthur however did something different. Although I didn’t see every forecast, my impression was that most predictions had Arthur remaining a strong tropical storm or at best a weak category 1 storm when it brushed by North Carolina; instead, the storm strengthened into a much more powerful category 2 storm prior to landfall. This misfit in storm intensity is quite common. Although storm direction forecasts have significantly improved over the last decade, storm intensity estimates are no better than they were a decade ago. So why? A big part of this story is the dynamics of how these storms work. Large-scale features in the atmosphere, to some extent, control the paths of hurricanes; low and high-pressure systems push hurricanes around and satellites can track those fairly accurately. On the other hand, fine-scale features happening at the core of the storm control hurricane intensity. A single pocket of dry air, if it is pulled into the heart of the storm at the right time, can cause significant weakening, while a pocket of warm, wet air added at the right time will help strengthen a storm. Understanding intensity changes involves predicting very complicated, internal structures of the atmosphere and the storm, detecting pockets of air through the turbulence caused by the winds, and even estimating the complicated processes involved in eyewall replacement. On top of atmospheric dynamics, there’s another complication even harder to see. In the oceans there is a strong thermal gradient with depth; the top layers, heated by the sun, sit happily on top of colder layers at depth. This top layer is less dense than the layers below because it is warmer and mixed by the winds. Its pretty well understood that hurricanes passing over warmer waters will be likely to strengthen because they can take more energy from the ocean. However, the mixed layer plays a role also. The depth of the boundary between the cold waters at depth and the warm waters of the mixed layer can vary by a lot. Just for an example, if the mixed layer is twice as deep, then even if the water at the surface isn’t too warm, there will be a ton of extra energy stored in that deep mixed layer. The depth of the mixed layer is a consequence of the currents and the history of the water; warm waters pushed deeper by a major storm can have an impact on storm intensity if a hurricane comes by months later. The mixed layer boundary is dozens of meters below the ocean surface; that’s a hard thing to measure. It can also vary with time; the Gulf of Mexico occasionally forms isolated “loop currents” of water that break off from the main circulation pattern; those loop currents helped feed huge intensity increases observed in storms like Katrina and Rita in the last decade. Altogether, predicting hurricane intensity requires fine-scale measurements of the atmosphere and ocean, combined with highly-detailed computer models that are still being worked out. NOAA has a goal of improving hurricane intensity forecasts 50% by 2019. To make that happen will require improvements in both satellite measurements and measurements in place, perhaps using new tools including military-style drones to fly through hurricanes. Continued application of resources will eventually help make these problems tractable, but it will take time. -JBB Image credits: Astronaut Reid Wiseman https://twitter/astro_reid Read more: climatecentral.org/news/storm-intensity-forecasts-lag-putting-communities-more-at-risk/ hurricanescience.org/science/forecast/models/modelskill/ gfdl.noaa.gov/operational-hurricane-forecasting olympic.atmos.colostate.edu/AT741/papers/Houze_etal_2008_Science_article.pdf washingtonpost/blogs/capital-weather-gang/wp/2014/07/02/tropical-storm-arthur-nears-hurricane-intensity-a-guide-to-coastal-impacts-over-july-4-holiday/ web.mit.edu/12.000/www/m2010/finalwebsite/background/hurricanes/hurricanes-prediction.html Dear Readers, Most of our posts are not reaching you in your news feed due to fbs filtering system. If you wish to enjoy our posts more often, use the following for information on how to go about it: tinyurl/qgwac8k.
Posted on: Sun, 06 Jul 2014 21:45:01 +0000
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